Pot Culture Experiment to Determine Desirable Leaf Nutrient Levels for Maximized Growth of Acacia mangiam

Pot Culture Experiment to Determine Desirable Leaf Nutrient Levels

for

Maximized Growth of Acacia mangium

Amitabha Guha

Agricultural Research & Advisory Bureau [ARAB]

1988

ABSTRACT

Acacia mangium plants were grown in a pot culture medium of fine sand supplied with Complete and Minus Nutrient Solutions to study the nutrient status of the plants and their effect on growth under different nutrient treatments.

The nutrient status of the plants were determined by analyzing the 6^th leaf from the bud-tip of secondary branches in the lower canopy which was found to show relatively stable macro-nutrient levels from one plant part to another within a particular plant.

The result obtained from this experiment indicates that the optimum 6^th leaf nutrient levels for the various plant nutrients are: P: 0.20-0.23 %; Mg: 0.17-0.22 %; Mn: 140-190 ppm.

N and K levels fluctuated at two distinct ranges at different times, but were always above 2.60 % N and 1.00 % K in plants where these nutrients were provided.

Ca, S, Fe and B results are such that no inferences could be made with a reasonable degree of certainty. However, Ca levels mainly fluctuated between 0.35-0.55 %, S between 0.20-0.35 %, and Fe between 40-80 ppm.

The Minus Boron treatment resulted in the leader shoots of the plant dying off and caused the deformation of new leaf buds to seriously affect growth. This may occur at levels less than 10 ppm but will definitely occur at levels below 5 ppm. In treatments where Boron was provided, their levels generally fluctuated between 20-70 ppm.

A linear growth rate pattern, as shown by Relative Volume calculation from height and girth measurements, were observed for those plants treated with Complete Nutrient Solution (the Control) once optimum nutrient status had been reached. Before reaching this nutritionally optimal status, the Relative Volume showed an exponential growth pattern. The growth pattern of the various Minus Nutrient Solution treatments was similar to the control, although in some cases, a slight deviation was observed for a short period, indicating a change in the growth rate.

INTRODUCTION

Acacia mangium is a legume tree species native to the marshy areas of tropical Australia. It is naturally hardy and fast growing (even on poor soils), and is therefore being grown in logged over areas to combat deforestation and erosion in addition to providing a future source of timber and pulp.

It is important to identify nutrient deficiencies of this species as it is being planted on such a large scale requiring high amounts of capital inputs. Thus nutrient deficiencies affecting its growth (and thus its benefits) need to be identified, quantified and subsequently corrected through manuring as economically as possible.

For the purpose of manuring the species economically, the nutrient status of the plant needs to be identified by both visual and foliar analysis, and its nutrient status related to the growth and yield of the species.

This paper describes and discusses the results obtained from a pot culture experiment set up at Agricultural Research and Advisory Bureau to determine optimum foliar nutrient levels for maximized growth of A. mangium. Nutrient deficiency symptoms observed while carrying out the experiment were also recorded.

It is envisaged that fertilization of A. mangium in the field will be targeted to achieve such nutrient levels if the economics of doing so prove viable.

MATERIALS & METHODS

Establishment of Pot Culture

Acacia mangium was grown in 50 twenty-five liter clay pots with glazed inner and outer surfaces and filled with fine sand. A sand particle size of 0.6 - 1.6 mm was used to provide free drainage as well as suitable water holding capacity (12.2%). The pots had a basal outlet in the form of a 2 cm drainage hole covered with a thin weft of glass wool. The holes were stoppered with a single hole rubber bung through which a short glass tube was inserted to facilitate the inflow / outflow of the nutrient solution from / to 2 liter bottles.

Prior to transplanting of the A. mangium seedlings from polybags (filled with well fertilized soil), the sand in the pots was treated with acid and subsequently flushed with deminneralized water to remove any nutrients from within the pots. The roots of the 6 month old seedlings were then cleaned gently of polybag soil with running deminneralized water and transplanted into the pots.

At the start of the experiment (during Phase 0), all 50 pots were treated daily for 1 month with deminneralized water only, until the plants all appeared stunted, sickly and chlorotic.

Complete Nutrient Solution was then applied daily (during Phase 1) to all the 50 pots for 2 months to bring the nutrient levels in the plants to optimum or above optimum levels. A modified Bolle-Jones nutrient solution was used, some changes being made to adjust the pH to about 6.

It was then that actual nutrient solution treatments were begun (Phase2) where every lot of 5 pots underwent a different treatment such that 5 pots continued to be treated with Complete Nutrient Solution (as a control), another 5 pots being treated with all nutrients except N (-N Nutrient Solution), another 5 pots being treated with all nutrients except P (-P Nutrient Solution), and so on for K, Ca, Mg, S, Fe, Mn, and B. Thus a total of 10 treatments (including the Control) was effected without replicates.

The composition of the various Nutrient Solutions and their pH values are presented in Tables 1 & 2. It should however be noted that due to selective nutrient uptake by the plants at various times within the pots, these compositions were continually altering, thus also affecting the pH values.

To limit such variations in the composition of the nutrient solution within the pots, the pots were weekly flushed with deminneralized water and refilled with nutrient solution to restore the desired nutrient concentrations. Throughout the duration of the experiment, the concentration of the nutrient solution within the pots was not allowed to vary more than 10% of the concentration applied for a particular nutrient.

Establishment of Foliar Sampling Technique

In order to identify the particular type of leaf tissue (or more accurately, the phyllode) to be sampled for analysis, the effects of leaf age, the leaf's exposure to various micro-environments, the age of the tree and the position of the leaf within the tree structure, were considered.

This had to be done because the analytical values of the leaf for the various plant nutrients have to represent, as accurately as possible, the nutrient status of the whole tree. Thus, the variation in the nutrient content (due to the above mentioned factors) of various leaves within a tree had to be studied to find a leaf that shows minimal variation in nutrient content. Such a leaf would best reflect on the nutrient status of the tree, and should thus be sampled.

For this purpose, leaf samples were taken from forest areas of Acacia mangium, where the Acacia mangium trees were divided into 3 age groups:-

1. Young Plantings, below one year in age, with no branching and fully sun-exposed.

2. Young Plantings, approximately between one and two years of age, with first and/or second series branching, but all fully sun-exposed.

3. a> Plantings approximately more than two years old in the field with a developed canopy that does not converge in the tree row / interrow.

b> Plantings approximately more than two years old in the field with a developed canopy that does converge in the tree row / inter row.

Foliar samples were collected from different positions on the trees under the various age groups. They were then analyzed to study the variation in nutrient content on the following basis:

- the position of the leaf within a stem / branch.

- the position of the branch.

- the effect of sunlight / shade.

All leaf types (the bud + leaf upto the 15^th leaf) were sampled individually.

The foliar samples were then analyzed for five macro - nutrients i.e. N, P, K, Ca, Mg.

The levels for each of the above macro-nutrients in the various plant parts were then graphed and used to identify a leaf that shows the most stable nutrient levels for these macro-nutrients. From these plots it appears that generally, the 6^th leaf lies in a position of most stability for the various macro-nutrients on a particular branch. As such, the 6^th leaf of the second series branching in the lower canopy was chosen as a standard for sampling purposes.

The 6^th leaf nutrient levels were thus tracked for these pots during Phase 1 & 2 of the experiment as it was this leaf that showed relatively stable macro-nutrient levels from one plant part to another within a particular plant in samples collected from the field. This was especially true for all the primary macro-nutrients (N,P,K) and for Mg. Levels of Ca were comparatively less stable, the former probably reflecting age variation of the sampled 6^th leaf.

Monitoring Nutritional Status of Treatment Plants

During Phase 1, 6^th leaf sampling was carried out on a weekly basis from all 50 plants to make 1 composite sample.

During Phase 2, 6^th leaf sampling was carried out on a weekly basis for 2 months and then later, on a fortnightly basis.

Sampling was carried out on each treatment lot of 5 pots to make10 composite samples.

Growth measurements (Height and Girth) were taken from all 50 pots during Phase 1; and from each treatment lot of 5 pots during Phase 2.

Girth was measured 20 cm from the base (sand level) while Height was measured up to the bud-tip from the base.

In the process of daily maintaining the sand medium with its water holding capacity, the evapotranspiration rate was measured. This value was found to be approximately 0.1 cm³ water/cm² leaf surface area in 24 hours on a 'hot' day.

Chemical Methods used for Foliar Analysis

Standard laboratory methods were used to analyze nutrient contents in plant tissue samples collected. N and P levels in foliar tissue was measured by an Autoanalyzer, while the cation nutrients, K, Ca, Mg, Fe and Mn were determined by atomic absorption.

The determination of N is based on a colorimetric method in which an emerald-green colour formed by the reaction of ammonia, sodium salicylate, sodium nitroprusside and sodium hypochlorite (chlorine source) in a buffered alkaline medium at a pH of 12.8 - 13.0. The ammonia salicylate complex is read at 660 nm. The classical Kjeldahl digestion of leaf dry ash was used before passing the samples through the Autoanalyzer.

The determination of P is based on the colorimetric method in which a blue colour is formed by the reaction of ortho phosphate, molybdate ion and antimony ion followed by reduction with ascorbic acid at an acidic pH. The phosphomolybdenum complex is read at 660 nm.

S was determined with Barium Chloride after dry ashing and read by a photoelectric colorimeter set as 425 mu.

B levels were measured by colorimetric determination based on the quantitative reaction of B with carmine solution in concentrated sulphuric acid which results in a blue carmine - boron complex being formed. (The interference from nitrates was removed by the addition of hydrochloric acid).

RESULTS

Fig. 1a.– i. illustrates the nutrient levels in the 6^th leaf of Acacia mangium during Phase 1 & 2 for the Complete nutrient solution treatments. Fig. 2a.– i. does the same for the various Complete and Minus nutrient solutions during Phase 2.

Fig. 3a illustrate the Growth pattern of Acacia mangium as indexed by Height and Girth during Phase 1 & 2 for the various nutrient treatments. Fig. 3b illustrate the calculated Volume Growth Index pattern (which is a function of both Height and Girth).

Findings on the respective leaf nutrient levels and their deficiency symptoms are as follows:

Phase 1 - All pots treated with Complete Nutrient Solution

(after being starved nutritionally).

N - Leaf N levels increased at a relatively fast rate in a near linear manner without slowing down (curving out). However, the curve dipped temporarily at the 3.0% and the 3.1% level when new leaf flushes appeared, caused by a possible temporary dilution effect.

N deficiency symptoms, as evidenced by general chlorosis of the whole plant, seem to be evident at levels below 2.4% N.

P - Leaf P levels decreased for the most part from a high of 0.30%, but leveled off at 0.20% and then rose towards the end of this phase.

This indicates that leaf P levels had decreased to the critical level when it leveled off. As this occurred at the 0.20% level, the critical level is likely to be thereabout.

K - Leaf K kevels rose during the first half of this phase to 1.80% but leveled off and fell gradually during the second half. It again rose at 1.60% towards the end of this phase.

This indicates that the critical level may be at the minimum point of the drop i.e. at 1.60%.

Ca- Leaf Ca levels were very variable and followed no distinguishable pattern, perhaps partly reflecting a variation in the age of the sampled 6^th leaf.

Mg- Leaf Mg levels rose quite sharply during the middle portion of this phase, fell suddenly, and then rose again at the 0.20% mark, thus indicating this to be the critical level for optimum growth. Excessive Mg levels could be about 0.23% as this was the point at which Mg levels began to drop.

Mg deficiency symptoms were observed to manifest itself as sharp interveinal leaf chlorosis at levels below 0.15%.

S - Leaf S levels rose but fell sharply during the third quarter of this phase. As no leveling off was observed, it is difficult to identify a critical level for this nutrient.

S deficiency appeared similar to N deficiency symptom (general chlorosis/paling of the whole plant).

Fe- Leaf Fe levels rose sharply for the most part of this phase, but also fell sharply towards the end without leveling off. Thus, a critical level could also not be identified for this nutrient.

Mn- Leaf Mn levels fell rather slowly and then sharply during this phase. However, it leveled off at the 140 ppm mark which could well be the critical level.

B - Leaf B levels rose sharply and fell sharply towards the last quarter of this phase. However, as no leveling off was observed, it was difficult to specify a critical level during this phase.

For almost all nutrient levels tracked (i.e. N, P, K, Ca, Mg, Mn, but not S, Fe and B), the drop and rise effect occurred towards the end of this phase when nutrient levels were more than adequate for maximum growth and when new leaf flushes were forming. The short term drop in nutrient levels is likely to be due to the dilution effect that occurs when new leaves form and when nutrients are translocated from older leaves to younger ones.

From the 50 plants measured for Growth, a rather linear height and girth growth rate was noted during this phase. This resulted in an exponential Relative Volume growth rate.

Phase 2 - Pots treated with Complete and various Minus Nutrient Solutions.

N - Leaf N levels fluctuated within the 3.0% - 3.4% range for the Complete Nutrient Solution treatment (the control). But as the canopy developed and after the start of the dry season, this range was lowered to 2.6 - 3.0%.

However, for the various Minus Nutrient Solution treatments (except the -N treatment), the lower limit of the fluctuation was 2.7%, and later, this dropped to 2.4%.

For the -N treatment, leaf N levels dropped below 2.4% - when N deficiency symptoms (general plant chlorosis) was exhibited - to as low as 2.0%. However, this level rose to become comparable to treatments where Nitrogen was applied during the last 60 days of the experiment. Also, the colouration of the leaves turned greener. This is due to good nodule development during this time. The nodules, when sliced open, were brownish-pink in colour, thus indicating that leghemoglobin, which is found in active nitrogen fixing nodules, was present.

Nodulation occurred in all treatments to various degrees except in the -S treatment. Nodulation in the -N, -K and -B treatment plants were quite good. In the -Fe treatment plants, nodulation was poor and the reddish-pink colouration of leghemoglobin was absent. Nodulation was also poor in the -Mn treatment.

P - Leaf P levels fluctuated between the 0.20% and 0.32% range for the Complete Nutrient Solution treatment although this was largely concentrated between 0.20 and 0.23%.

However, for the various Minus Nutrient Solution treatments (except the -P treatment), the upper limit generally reached was 0.28%. But for the -Ca treatment, the upper limit reached was 0.38%, thus suggesting some interaction between Ca and P uptake. Milder interaction effects in such manner were also noted with the -K and -N treatments.

For the -P treatment, leaf P levels fluctuated between 0.10% -0.19%. The plants all appeared as dark bluish-green. In some leaves, lamina scorching was noted. There was also some retardation in the development of roots. However, the aerial portions of the plants showed good growth and had a good canopy.

K - Leaf K levels fluctuated between the 2.0% - 2.5% range for the Complete Nutrient Solution treatment. But like nitrogen, as the canopy developed and the dry season set in, this range was lowered gradually to 1.5 - 2.0%. However, for the various Minus Nutrient Solution treatments (except the -K treatment), the lower limit generally reaches 1.5%. This was later lowered to 1.0%.

For the -K treatment, leaf K dropped to below 1.5% (when temporary general chlorosis of the plant was observed), to as low as 0.5%. As the plants grew older, only the tips and margins of leaves turned yellow, then necrotic, as other parts of the leaf showed a healthy colouration. Later, yellow spottings with enlarging necrotic centres were observed.

Ca- Leaf Ca levels mainly fluctuated between the 0.35% - 0.55% range for the Complete Nutrient Solution treatment.

However, for the various Minus Nutrient Solution treatments (except the -Ca treatment), the range was wider: 0.22% - 0.62%.

For the -Ca treatment, leaf Ca levels dropped to 0.20% and the leaves of these pots were initially large and wide as well as 'papery' in feel at levels below 0.30%. The leaves of these pots were also more susceptible to insect damage.

Mg- Leaf Mg levels fluctuated between the 0.17% - 0.22% range for the Complete Nutrient Solution treatment.

However, for the various Minus Nutrient Solution treatments (except the -Mg treatment), Mg levels in the -Mn, -S and -Fe treatments were found to be below the lower limit of 0.17% while the other treatments had Mg levels above the upper limit of 0.22% (especially the -Ca treatment).

For the -Mg treatment, the Mg level fell to below 0.15% (where very marked interveinal chlorosis of individual lower leaves were noted), to 0.08%. The leaves of the plants undergoing this treatment were quite small and narrow. Root development seemed to be somewhat retarded in plants undergoing this treatment.

S - Leaf S levels generally fluctuated between the 0.20% - 0.35% range for the Complete Nutrient Solution treatment.

However, for the various Minus Nutrient Solution treatments (except the -S treatment), S levels in the Mg, Fe, and Mn treatments fell below the lower limit of0.20%, while the other treatments fell above the upper limit of 0.35% (especially the -Ca treatment).

For the -S treatment, S levels fell to 0.05% giving these plants a pale colouration (general chlorosis). Root development was also retarded in plants not provided with S.

Fe- Leaf Fe levels mainly fluctuated between the 45 - 75 ppm range for the Complete Nutrient Solution treatment.

However, for the various Minus Nutrient Solution treatments (except the -Fe treatment), the fluctuation was very variable.

For the -Fe treatment, Fe levels in the lower canopy were not much lower or higher than that of the Complete Nutrient Solution treatment (the control) at any time. However, their leaves in the upper canopy showed diffused interveinal chlorosis.

Mn- Leaf Mn levels fluctuated between the 140 - 190 ppm range for the Complete Nutrient Solution treatment.

However, the Mn level in all the Minus Nutrient Solution treatments (other than the -Mn treatment), generally fell within the above range of 140 - 190 ppm, except in the -K treatment where Mn levels exceeded the 300 ppm mark.

For the -Mn treatment, Mn levels in the lower canopy fell to as low as 63 ppm although at various times their levels were similar to that of the Complete Nutrient Solution treatment (the control). Like the -Fe treatment plants, their leaves in the upper canopy showed diffused interveinal chlorosis. However, these leaves later scorched, starting from the tip and moving down the margins (usually more on one side).

B - Leaf B levels fluctuated between the 30 - 70 ppm range for the Complete Nutrient Solution treatment but as the canopy developed this range dropped to 10 - 30 ppm.

The B levels of the various Minus Nutrient Solutions (except the -B treatment), also fluctuated in a similar manner as the above control treatment.

For the -B treatment, B levels fell to below 10 ppm to as low as 3ppm to exhibit very marked deficiency symptoms: dying off of the leader shoots and the appearance of crinkled/blackened new leaf buds that were closely spaced with other leaves. The severe defoliation that resulted contrasted very sharply with the dense canopy that the plants of this treatment showed earlier.

With respect to the Growth of the plants undergoing various treatments during this phase, no treatment showed a linear growth rate in both height and girth. However, the Relative Volume growth pattern was linear for the Complete Nutrient Solution treatment - the control, while that of the other treatments deviated from this slightly at various times, particularly that of the -K, -Ca, and -Mn treatments. The plants of the -B treatments however, stopped growing as soon as their deficiency symptoms appeared.

Statistical Analysis of Leaf Nutrient Levels

Table 3 provides Summary Statistics of Nutrient Concentrations in plants undergoing Complete Nutrient Solution Treatment during Phase 2 i.e. when the plants had achieved a balanced nutrient status.

Nutrient Removal

Leaf samples of old senescing leaves just about to drop indicate that the following approximate amounts of plant nutrients are exported in leaf litter:

Nutrient Removal through Leaf Litter by Acacia mangium
Plant Nutrient		Levels in Leaf Litter
Macronutrient:
N	%	1.60 - 1.80
P	%	0.28 - 0.38
K	%	0.85 - 0.90
Ca	%	0.50 - 0.55
Mg	%	0.25
S	%	0.48
Micronutrient:
Fe	ppm	85
Mn	ppm	353
B	ppm	70 - 90

The nutrient amounts removed through fruit drop was not determined as at the closing of the experiment, the plants had not yet matured.

DISCUSSION

Assuming that the plants undergoing Complete Nutrient Solution treatments have an optimal and balanced nutritional status, it can be inferred that the optimum concentration of the various plant nutrients inthe 6^th leaf of Acacia mangium would lie within their fluctuation range in this control treatment (once such an optimum status has been reached).

This is because each nutrient will have a different fluctuation capacity / range owing to its individual chemical and physical properties and physiological roles affecting its mobility within the plant environment. Such fluctuation effects are especially pronounced when 6^th leaves of different ages are sampled, when new leaf flushes appear (dilution effect) or when senescence occurs. Seasonal changes also might affect the fluctuation range of certain nutrients, especially with regard to N and K.

As it is obviously out of the scope of this experiment to monitor the plants biochemical processes to identify when each of the above processes begins and ends, the 'critical' nutrient concentration of the 6^th leaf of Acacia mangium is defined as the lowest nutrient concentration reached by plants receiving all nutrients (the Com treatment) during Phase 2. It is not likely that physiological processes are being limited in any significant manner above these levels.

However, as noticeable differences in the N and K fluctuating ranges were observed at different times, interpretation methods should take this into consideration. This could partly to be due to the fact that less active (shaded) photosynthetic tissue translocates some mobile nutrients to the more photosynthetically active (sun-exposed) leaf tissues.

It appears however, that differences in N and K levels due to shading would not alone account for the large differences observed in the fluctuating ranges - between approximately 2.6-3.0 % and 3.0-3.3 % for N; and 1.5-2.0 % and 2.0-2.5 % for K. As the dry season set in at about the same time that leaf N and K levels dropped during Phase 2 (Day 150 - 180), it is suspected that seasonal temperature changes, but not water stress, somehow affects N and K levels in the species. Then again, age or nodule formation and the plants symbiotic relationship with rhizobia might be a factor.

But, as leaf levels did not drop below 2.6 % for N and 1.0 % for K, these levels can, for the time being at least, be considered as critical values.

It is therefore desirable to identify the critical levels in the 6^th leaf of Acacia mangium as the lower limit of the fluctuation of a particular nutrient in the control Complete Nutrient Solution treatment, especially after the levels of all the other nutrients have stabilized within their optimum range.

The critical and optimum levels for the 6^th leaf on secondary branches of Acacia mangium in the lower canopy would then be as shown in the table below.

Critical & Optimum Nutrient Levels of Acacia mangium (6^th Leaf on Secondary Branches)
Plant Nutrient		Critical Level	Optimum Range	Levels below which Deficiency Symptoms might appear

Macronutrient:
N *	%	2.60	3.00 - 3.40
		2.60	2.60 - 3.00	2.40
P	%	0.20	0.20 - 0.23	0.15
K *	%	1.50	2.00 - 2.50
		1.00	1.50 - 2.00	1.00
Ca	%	0.30	0.35 - 0.55	0.20
Mg	%	0.17	0.17 - 0.23	0.15
S	%	0.15	0.20 - 0.35	0.10

Micronutrient:
Fe	ppm	35	40 - 80	30
Mn	ppm	120	140 - 190	100
B	ppm	10	20 - 70	5

*Note: Two rather distinct ranges in N and K nutrient levels were noted at different times. However, levels above 2.60 % N and 1.00 % K are not likely to affect growth significantly.

Keeping the above value range of the various nutrients in mind, leaf analytical results of some nutrient treatments are interesting in the sense that it illustrates some nutrient interactions with respect to their uptake.

The strong nutrient interactive effects observed are as follows:

· P, Mg and S levels were abnormally high in the -Ca treatment.

· Mn levels were abnormally high in the -K treatment, while

· S levels were somewhat low in the -Mg treatment.

This suggests that Ca inhibits the uptake of P, Mg, and S; while K inhibits the uptake of Mn. It also indicates that Mg increases the uptake of S.

To explain these interactive nutrient effects that were observed, an understanding of the physical and chemical nature of these nutrients and their role in plant physiology is quite essential.

The roles of these various plant nutrients in the plant physiology is very briefly reviewed here (Salisbury and Ross, 1985):

NH₄⁺ / NO₃^-: These forms by which N is taken up into the plant are converted by biochemical processes to form chlorophyll (with Mg), amino acid components of proteins, as well as the nuclear proteins of DNA and RNA. Excess N increases shoot growth (at the expense of root development) and delays maturity. When deficient, plants turn pale as the chlorophyll is stripped of their N's; and growth is stunted as mitosis is retarded - the N supply for nucleic acid duplication being limited.

H₂PO₄^-: P is taken up in this form to generate energy rich ATP (Adenosine Tri-Phosphate) from ADP (Adenosine Di-Phosphate) to drive the biochemical processes. It is also used as the phosphoric acid component of nucleic acids and is found in phospholipids in membranes. Excess P (in contrast to excess nitrogen), promotes root development at the expense of shoot growth and leads to early maturity. Lack of P tends to cause plants to appear dark green and perhaps slightly purplish as anthocyanin pigments accumulate.

K⁺: K is used as a catalyst of many enzymes that are essential for photosynthesis and respiration, and it also activates enzymes needed to form starch and proteins. This element is so abundant that it is also responsible (with Na) for maintaining the osmotic stability of plant cells, a mechanism very important in the stomatal function of leaf tissue. When deficient in dicots, leaves initially become slightly chlorotic, especially close to dark necrotic lesions that soon develop. In many monocots, cells at the tips and margins of leaves die first.

Ca²⁺: Ca is used to form Calcium Pectate which is found outside the plasma membrane of cells as the rigid cell wall. The cell wall is less permeable to larger and mobile nutrient ions (such as H₂PO₄^-, SO₄^2-, Mg²⁺) than it is to smaller mobile ones (NH4+ and K+). Ca is also essential for normal plasma membrane functions in cells, probably as a binder of phospholipids to each other or to membrane proteins. Ca also has a role in the cell division process (in spindle formation during mitosis).

Mg²⁺: Mg chelates with many N atoms to form the chlorophyll porphyrin. The N's surrounding the central Mg atom are capable of trapping sunlight energy and is what gives phosynthesizing tissue its green colour. In its absence, chlorosis of the older lower leaves is the first symptom. This chlorosis is usually interveinal, because the mesophyll
cells next to the vascular bundles retain chlorophyll for longer periods than the palisade cells between them. Also, Mg is essential because it combines with ATP (thereby allowing it to function in many reactions) and because it activates many enzymes needed in photosynthesis, respiration and formation of RNA / DNA.

SO₄^2- : S is used as a component of proteins, specifically in the amino acids cysteine and methionine that are building blocks of proteins. Other essential compounds that contain S are the vitamins thiamine and biotin and coenzyme A, a compound essential for respiration and for the synthesis and breakdown of fatty acids.

Fe²⁺ / Mn²⁺: Fe is essential because it forms part of certain enzymes, and part of numerous proteins that carry electrons during photosynthesis and respiration. It undergoes alternative oxidation and reduction between the Fe/Mn²⁺ and Fe/Mn³⁺ states as it acts as an electron carrier in proteins.

Zn²⁺ / Cu²⁺: These play an important role in enzyme action and electron transfer in both the photosynthetic and respiratory processes, similar to that played by Fe and Mn. For instance, Cu is present in cytochrome oxidase, a respiratory enzyme in mitochondria and in plastocyanin, a chloroplast protein.

H₃BO₃: The role of this nutrient has not been completely elucidated as yet but its deficiency has led to the failure of root tips to elongate normally and inhibition of DNA and RNA synthesis. Cell division in the shoot apex is also inhibited. B also plays an undetermined but essential role in the elongation of pollen tubes.

Cl^-: Cl is used in splitting water to liberate electrons in the photosynthetic process.

MoO₄^2-: The only well documented functions of Mo is as part of nitrate reductase, which is the enzyme that reduces NO3- to NO₂^-. Mo also plays an important role in the formation of leghemoglobin, the compound responsible for giving the reddish-pink colour in sliced open active Nitrogen fixing nodules of rhizobia in leguminous plants including Acacia mangium. (Note that Cobalt is also required to support high rates of Nitrogen fixation by legumes as it is essential for most bacteria, including inoculated rhizobia).

Bearing in mind that Ca forms the cell wall that encases protoplasts (naked cells), a plant deficient in Ca would have larger protoplasts / cells with thinner walls which more readily accommodates the entry of larger nutrient ions - such as Mg²⁺, H₂PO₄^2-, and So₄^2- that are too big to squeeze through the cell wall by faster passive (osmotic) transport mechanisms.

Ca is also connected with the lipid based plasma membrane which allows ions with a smaller hydrated radius to enter the cytoplasm faster. Monovalent, single element ions which have a smaller hydrated radius, such as K⁺ will thus cross the membrane faster than divalent ions, such as Mg²⁺ (Clarkson, 1974).

This appears to occur in plants not provided with Ca in this experiment which were initially observed to have large papery leaves very susceptible to insect damage. Large leaves would be expected as the size of individual leaf cells are not being restricted by
a rigid cell wall. Later on the size of the leaves in the -Ca treatment was comparable with that of the control Com treatment.

K, which along with Na, maintains the osmotic balance of cells and controls stomatal opening appears to be replaced by Mn²⁺ as higher Mn levels were noted in the -K treatment.

Low S levels in the -Mg treatment, could perhaps be due to the inactivation of enzymes that are required for photosynthesis and respiration.

Clear deficiency symptoms were observed in the -N and -S treatments as general plant chlorosis, leaf tip necrosis and spottings with enlarging necrotic centres in the -K plants, and as sharp interveinal chlorosis of lower leaves in the -Mg treatment plants.

However, N levels in the -N treatment plants rose to above 2.6 % once nodulation occurred and N-fixation took place. As this level is comparable to that of plants provided with N, it appears that the plants N requirement can be satisfied by N-fixation alone without additional application of N fertilizers. Nodulation was also observed in all treatments to various degrees, except for the -S treatment.

Fe and Mn deficiency appeared towards the conclusion of the experiment as diffused interveinal chlorosis of leaves in the upper canopy. In Mn deficient treatment plants however, this progressed to leaf scorching along the margins of the leaf.

B deficiency, like Fe and Mn deficiency, although taking a much longer time to manifest itself, was present for quite some time as it caused the leading shoots on the plants to die off and the crinkling / twisting of young leaf buds. The complete death of leading shoots and upper branches seriously reduced the canopy, thus effectively stopping growth.

The functions and mobility of the various plant nutrients also serve to explain deficiency symptoms observed, at least to some extent.

N and Mg, the main constituents of chlorophyll that give photosynthesizing tissue its green colour will lead to paling / yellowing of green leaves if deficient. Likewise, but to a much rarer extent, if the micro-nutrients Fe, Mn, (which are involved in chlorophyll formation and its associated processes) were deficient, some chlorotic symptoms would also result, as was observed. N and Mg, being mobile within the plant environment, their deficiency symptoms occur in the lower canopy. In contrast, deficiencies of these micronutrients are observed in the upper canopy.

The long time period for the appearance of deficiency symptoms of Boron (which is in the form of Boric Acid) is because it is only slowly translocated out of organs in the phloem once it arrives there in the xylem (Raven, 1980).

B, required for cell division, particularly in the meristematic tissues of the shoot and root apex, led to shoot dieback (starting in the upper canopy) and reduced root elongation when deficient. This was very vividly observed in the plants undergoing -B treatment. The young leaf buds that formed were deformed, twisted and brittle.

Plant Growth

With regard to the Growth of Acacia mangium, both height and girth measurements indicated a rather linear Relative Volume increase, once the plants had achieved optimal nutrient status for all nutrients - as in the control treatment. This is illustrated in Graphs 3a.

The Relative Volume Growth rates of all the various Minus Nutrient treatments are comparable to this. However, towards the end of the experiment, the Relative Volume Growth rate of the -K, -Ca and the -Mn treatments seem to have slowed down. Refer to Graphs 3b for an illustration.

Biomass Growth is relatively more accurately represented by Volume rather than by Height or Girth per se, and therefore a calculation of the relative volume index to study the growth rate from these two measured parameters was made:

As Girth (the circumference) = 2 π r (i.e. r = G / 2 π ),

and Vol = π r² . H,

then... Vol = π (G / 2 π )² . H also.

It can therefore be seen that while Vol is only directly proportional to Height, it is proportional to the exponential of the Girth.

Thus Girth is a relatively more critical Growth parameter than is Height. The Relative Volume Index was therefore calculated using the following formula:

Rel. Vol (cu. cm) = (G / 2 π)² . H

π = 3.1428

G (cm) = Girth at 20 cm from base

H (cm) = Height from base to bud-tip

The Role of Potassium in Plant Water Balance

It is highly likely that the growth of Acacia mangium may be affected by water stress as its transpiration rate is very high. It should be borne in mind that the ecological origin of this species is from a marshy environment and it is thus accustomed, genetically and physiologically, to wet ground conditions.

Also, as water plays an important role in the uptake of nutrients, the nutrient status of the plants depends on the availability of adequate amounts of water to dissolve the cation / anion nutrients. As the divalent ions require more water to dissolve, in periods where water availability is limited, these nutrient ions are not easily taken up by the plants. Furthermore, the monovalent ions have preference in plant uptake (Clarkson, 1974) as shown in this experiment. Thus, in the field, sampling should take place towards the end of the wet season once the plants have had enough opportunity to satisfy itself nutritionally.

As it appears that A. mangium is a species sensitive to water stress, care should be taken to ensure that its potassium status is adequate. This is because K increases turgidity and helps to maintain the internal water balance and the hydration of the protoplasm. (Tisdale et al., 1985). A reduction in turgidity through K deficiency is accompanied by a decreased ability of stomatal guard cells to conserve water. As such, transpiration rate is increased with K deficiency.

Stomatal opening is controlled by active transport of K into the guard cells. The guard cells of the closed stomata contain less K than those cells of the open stomata. If K were deficient in the guard cell, water would not be able to enter it by osmosis to inflate it and thus close the stomatal opening in order to conserve water.

Hence, the fixation of CO₂ by photosynthesis in K deficient plants is more likely to be limited by the lack of water.

Also, a high ion concentration in the cell increases osmotic pressure of the cell solute and consequently the plants ability to withstand high water tension in the soil during the dry season, especially where fine clay is concerned.

It should be remembered that water deficits within the plant affect all processes of cell growth, including division, enlargement and maturation. It also reduces the rate of photosynthesis, which is related to the decreased supply of CO₂ caused by the closed stomata. The proportion of starch to sugar is often reduced because of greater hydrolysis of the starch.

CONCLUSION

The data obtained from this experiment indicates that Acacia mangium is a species that can still show a reasonably good growth rate even when one nutrient is very deficient, at least for a few months. But, the plant does not seem to be able to tolerate acute B deficiency and possibly prolonged K, Ca, and Mn deficiency without affecting growth significantly. Growth could still be affected by N, P, Mg and S deficiency (although this was not immediately observable during the short duration of this experiment), as these nutrients also play a critical role in the plants physiology.

Acknowledgment

On behalf of ARAB, the author would like to express his appreciation to the Sabah Forestry Development Authority (SAFODA) for their cooperation in conducting this experiment and their permission to publish this paper. The assistance of Mr. Li Kai Yuan (SAFODA Research Dept) and Dr. Donald J. Mead (Forest Nutrition Specialist at the Univ. of Canterbury) in providing literature and other useful information / suggestions is also gratefully acknowledged. The support and assistance of ARAB staff (both in the field and the lab) proved invaluable in the actual running of the experiment and to them are expressed our thanks. Special thanks are due to my colleagues Eddie Vettivel and Unni Krishnan for briefings on the field / silvicultural aspects of the species and their valuable suggestions.

References ____________

Bolle-Jones, E.W. 1954. Nutrition of Hevea brasiliensis. Experimental Methods. J. Rubber Research Institute of Malaysia.

Clarkson D.T. 1974. Ion Transport and Cell Structure in Plants. Mc Graw Hill Book Company, New York.

Raven, J.A. 1980. Short- and long- distance transport of boric acid in plants. New Phytologist 84: 231-249.

Salisbury F.B. and Ross C.W. 1985. Nutrient Deficiency Symptoms and Some Functions of Essential Elements. Plant Physiology. Wadsworth Publishing Company, Belmont, California. 106-113.

Shorrocks, V.M. 1964. Mineral deficiencies in Hevea brasiliensis and associated cover plants.

Tisdale, S.L., Nelson, W.L., Beaton, J.D. 1985. Physiological Effects of Plant Nutrients Related to Water Economy. Macmillan Publishing Company, New York. 705.