COCOS (1991 -1993), 9, 30-39 
Printed in Sri Lanka 
EFFECT OF N, K, CI ON PHOTOSYNTHESIS AND WATER 
RELATIONS OF OPEN POLLINATED TALL (typica) 
COCONUT SEEDLINGS 
C. JAYASEKARA, R. G. MUDALIGE, and C. S. RANASINGHE 
Coconut Research Institute of Sri Lanka. 
ABSTRACT 
C 0 2 assimilation and plant water status were measured in sand-culture-grown amputated 
coconut seedlings treated with three levels ofN, K, and CI and subjected to different soil water 
deficit conditions. Application of increased levels ofN and K enhanced the assimilation of C 0 2 
while individual and combined effects of N,K, and CI improved the water economy of coconut 
seedlings. Under the soil water deficit conditions, adequate supply ofN helped to maintain high 
leaf water potential by the accumulation of solutes like proline and sugars. Whereas, adequate 
supply of K and CI themselves acted as osmotica by increased accumulation of these nutrients in 
leaf tissues, although other solute concentrations such as those of sugars and proline were reduced. 
Our data show that maintenance of sufficient levels of N and K contribute to the growth of the 
coconut seedling through improved gaseous exchange, C 0 2 assimilation and better partitioning 
of assimilated carbon into shoot and roots, potassium and C1 are further important for maintenance 
of water status of coconut seedlings by improved stomatal regulation, water uptake and osmotic 
adjustment of tissues under water deficit conditions. 
INTRODUCTION 
Productivity of perennial tree crops mainly depends on climatic factors, availability of soil 
nutrients and fertilizer usage in addition to their genotypic characters. Nitrogen is an important 
nutrient for coconut, specially during the seedling stage. It is essential because if combines with 
organic molecules to form biologically active compounds like amino acids, proteins, nucleic 
acids, phyto hormones etc. Nitrogen deficiency in coconut palms manifests itself by yellowing of 
leaves, reduced vegetative growth, diminished female flower production and yield. 
It has been reported that potassium and magnesium are the two most limiting nutrients 
affecting the productivity of coconut palms in Sri Lanka with about 80% of the coconut plantations 
showing low leaf nutrient levels (Jayasekara, 1989). Potassium ion is found to be the only ion 
which has a major effect on the diffusion of C O 2 to the chloroplast in mesophyll cells. Peaslee and 
Moss (1968) and Terry and Ulrich (1973) reported that decline in rate of photosynthesis of maize 
and sugar beet leaves as potassium become deficient. Potassium further influences protein 
synthesis (Hsiao et al., 1970), water uptake by roots (Mangel and Kirkby, 1980), cell turgor 
maintenance (Leigh and Wyne Jones, 1984), water transport and enzymatic reactions. Potassium 
in coconut is mainly stored in the husks, nut water, leaves and petioles. The average annual 
removals of potassium from those crop components were found to be approximately 0.75 to 0.9 
kg palm - 1 y"1 (Nathaniel, 1969). 
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Until recently, the importance of chlorine as a plant nutrient in coconut was not known. 
However, recent studies have shown that chloride ions are essential for certain genera of plants 
including palmae (Von Uexkull, 1984). Potassium and chloride ions in plant cells do not complex 
with organic molecules but function as free ions. Potassium in plantcell exists compartmentalized 
as cytoplasmic and vacuolar K +. These subcellular compartments have different K + concentrations 
(Leigh and Wyne Jones, 1986). Cytoplasmic potassium is essential for biochemical processes 
such as photosynthesis, glycolysis, starch and protein synthesis, whereas vacuolar potassium 
mainly act as an osmoticum. Stomatal regulation for gaseous exchange and transpiration depend 
on the turgor and solute potential of guard cells. 
In this paper importance of N,K, and CI for two main physiological processes namely (a) C 0 2 
assimilation and (b) water relations of the coconut seedlings are discussed. The processes of 
transpiration (E) and of C 0 2 assimilation (A) both are crucial in plant productivity and depend on 
C 0 2 and water vapour diffusion through stomata. Stomatal regulation of gaseous exchange takes 
place by opening and closing of stomatal aperture, which is stimulated by turger mediated process 
within guard cells and potassium play a key role as the osmotic agent for turgor changes. Variation 
in stomatal aperture in relation to the rate of transpiration or C 0 2 assimilation are quantified at the 
leaf or whole plant level as changes in either stomatal diffusive resistance (g) or conductance 
(Reciprocal of g). Measure of these parameters in the Held or glass house level could be used as 
an indicator of either water status, photosynthesis or transpiration rates of plants. Similarly water 
status of plants could be measured as water content (WC), relative water content (R WC), and water 
potential (V|/) of tissues. Eventhough, WC and RWC quantify the amount of water present in a plant 
tissues they do not imply the driving force for water within tissues. In simple terms water potential 
(y) is the driving force for water movement in liquid phase within the soil-plant system. Presently 
pressure chamber and psychrometers are widely used to meassure plant and soil water potentials. 
In this study pressure chamber has been used to measure leaf water potential of coconut leaves. 
MATERIAL AND METHODS 
M a t e r i a l : 
Open pollinated typica coconut seednuts were obtained from the Bandirippuwa estate, 
Lunuwila and seedlings were grown from dehusked seednuts in nursery beds. Plastic containers 
(30 cm in diameter and 34 cm in height) were filled with washed sand. Seedlings were amputated 
from the seednuts, eight weeks after sprouting. Any damaged roots were cut and dipped in a 5% 
benlate solution (fungicide) for 10 minutes. Cut ends were covered with a panel dressing Tb -192 
(CIC Ltd) and seedlings were transferred into the containers of washed sand. 
Exper imenta l : 
Experiment was a 3 4 factorial confounded block design with nine blocks and each block with 
nine replicates. Three levels each of N,K, and CI as 0 mM, 5 mM, and 10 mM were used as the 
nutrient treatments. Half strength Hoagland solution was used to supply other macro and micro 
nutrients. In 0 mM N, K, and CI treatments ionic strength of the Hoagland solution was balanced 
by substitution of Na and S 0 4 ions. Nutrients were applied three times a week and seedlings were 
watered to the field capacity with deionized water for ten months. Twodrying cycles were imposed 
to simulate drought conditions. One set of seedlings was dried up to six weeks and the other set 
was dried for eight weeks by witholding application of water. A third set was maintained at the 
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field capacity by regular watering throughout the study. At the end of each drying cycle, seedlings 
were rewatered up to the field capacity. Whenever necessary few leaflets were harvested from last 
fully expanded leaf for \ |/ | t chlorophyll, and amino acid and sugar assay. 
METHODS 
Chlorophyll concentra t ion: 
Chlorophyll in fresh leaf samples was extracted homogenizing in 20 ml of 80% acetone 
(v/v) in an ultra turrex. After centrifugation (1500 rpm, 5 min) at room temperature, absorbance 
of the supernatant fraction was measured at 645 and 668 nm in a UV/VIS spectrophotometer 
(Shimadzu UV 160- A, Japan). 
C 0 2 assimilation : 
The rate of C 0 2 assimilation under planthouse conditions was measured using a portable ADC 
photosynthesis meter (Analytical Development Co. Ltd., Hertfordshire, U.K.). The instrument 
was operated in the differential mode at the normal environmental conditions. 
Leaf water potential and stomatal diffusive resistance : 
Water potential of leaves was measured using a commercial type pressure chamber (Soil 
Moisture Corporation, California, U.S.A.). Stomatal diffusive resistance and rate of transpiration 
of leaves were measured using a Li-1600 steady state porometer (LI-COR INC., Nebraska, USA) 
Sugars and p ro l i ne : 
Dried ground leaf samples (0.5 g) were homogenized in an ultra turrex wilh 20 ml of 80% 
ethanol (v/v) and with the use of chloroform-water, chlorophyll was separated from the aqueous 
fraction. Aqueous fractions were then centrifuged (1500 rpm, 5 min) and supernatant fractions 
were then centrifuged (1500 rpm, 5 min) and supernatant fractions were concentrated to a constant 
volume of 5 ml. Total sugars in the concentrated fractions were determined by the phenol-
 sulphuric assay method (Dubois et al., 1956). 
Reducing Sugars in the aqueous fractions were determined by the 3-5 dinitrosalicylic acid 
method as described by Sumner (1924). 
Fresh leaf samples (5 g) were homogenized with 80% ethanol in an ultra turrex and the 
soluable fractions were extracted as described above and proline concentration was determined 
by the method of Troll and Lindsay (1955) without using permutit-T resin. 
RESULTS 
Effect of N,K, and CI on Chlorophyll content: 
The effects of different treatment levels of N,K, and CI on leaf chlorophyll concentration as 
well as the chlorophyll a/b ratio are given in the Table 1. Analysis of leaf nutrients revealed that 
N,K, and CI exceeded the critical levels when the treatment nutrient levels were increased from 
5 mM to 10 mM, High N levels increased the chlorophyll a & b concentrations. Nevertheless, 
chlorophyll a/b ratio was decreased significantly. Potassium has no effect on leaf chlorophyll 
32 
concentration. Increased levels of chlorine reduced the chlorophyll concentration possibly by 
reducing synthesis. 
The effect on C 0 2 assismilation and assimilate partitioning : 
C 0 2 assimilation, stomatal conductance and inter cellular C 0 2 concentration of the second 
expanded leaf of the individual seedlings were determined at field capacity when the seedlings 
were 8,9, and 10 months old. Fig. 1 shows the relationship between rate of photosynthesis and 
transpiration of coconut seedling at field capacity and with different levels of N,K, and CI for 
averaged data for three consecutive measurements. Increased supply of N and K enhanced the rate 
of C 0 2 assimilation, whereas chlorine showed a negative response by reducing assimilation of 
C 0 2 . Increased supply of K and reduced supply of CI increased the leaf water potential (less 
negative) of seedlings with concomitant increase in the rate of C 0 2 assimilation. Increased supply 
of N increased the assimilation of C 0 2 of leaves but reduced the leaf water potential of coconut 
seedlings. 
The effect on water relations : 
Leaf water potential (\|/ 2), rate of transpiration (E), and stomatal diffusive resistance (g) of the 
same leaves used for C 0 2 assimilation studies were measured concurrently. Fig. 2 shows the 
relationship between \|/, and E with different levels of N,K, and CI. Nitrogen and K have shown 
linear and contrasting effects on leaf water potential, whereas transpiration was increased with 
increase in the supply on N and K. Tables 2 ,3 , and 4 describe the effect of interaction between of 
K and CI, N and CI, and N and K on leaf water pqtential, transpiration, and stomatal diffusive 
resistance of seedlings. All three nutrient combinations displayed significent interactive effects 
either enhancing or decreasing their response to these parameters depending on the levels of 
nutrients. 
The effect of N,K, and CI on osmotic ad jus tment : 
Some plants have the ability to maintain tissue turgor at higher levels under soil water deficit 
conditions by accumulation of solutes within the tissues. Sugars, sugar acids, cations and anions 
like k, Na, CI, malate, imino acids like proline and glycinebetaine are some such substances 
accumulated within tissues to maintain turgor balance. Table 5 shows the accumulated levels of 
sugars, proline, K, and CI under water deficit conditions. It was very clear that in the presence of 
adequate supply of N, leaf sugar and proline concentrations were increased at soil water deficit 
conditions, whereas in the presence of adequate supply of K and CI, proline concentration was 
decreased but concentrations of these ions themselves were increased in leaves. These results 
suggest that K and CI help to maintain turgor balance under water deficit conditions. 
DISCUSSION 
Nitrogen is an important nutrient for plant growth. As in many other plants N has increased 
chlorophyll content and photosynthetic activity of coconut seedlings. Nitrogen deficiency results 
in reduction of stomatal conductance (Balton and Brown, 1980) or mesophyll conductance 
(Wong, 1979) or both (Navin and Loomis, 1970; Brown and Wilson, 1983). Jayasekara (1986) 
reported that the reduction in leaf water potential, stomatal conductance, and photosynthetic 
activity under nitrogen deficiency and water deficit conditions in Chamaedorea elegans palms. 
Similarly nitrogen deficiency reduces the stomatal conductance and photosynthetic activity in 
coconut seedlings. Nitrogen metabolism is inhibited under reduced water potentials. Synthesis of 
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1.2 1.3 1.4 1.5 1.6 
Rate of transpiration (U g r[p m * s-1) 
1.7 
Figure 1: Relationship between rates of transpirations 
and rate of photosynthesis with different treat?
 ment levels of N (???),K and CI (?-?). 
34 
Figure 2 : Relationship between rate of transpiration and 
leaf water potential with different treatment 
levels of N (?-*), K ( a - ^ ) , and CI (?-?). 
35 
3.4 
3.2 
E 
M 
1 2.8 
o 
2.6 
2.4 
2.2 
12 
Co 
C2 KO 
14 16 18 
Stomatal diffusive resistance s cm-1 
i CI 
? N 
*K 
20 
Figure 3 : Relationship between rate of photosynthesis and 
stomatal diffusive resistance under different 
levels of N (?-?), K and C / ( M ) . 
36 
proteins inhibited and hydrolysis of existing proteins is enhanced, consequently increasing the 
concentration of free amino acids or orther nitrogen compounds. This is specially so in the case 
of the iminoacid proline. In this study it became apparent that under water deficit conditions, total 
leaf amino acids and proline concentrations were increased with increase in the nitrogen supply. 
Gaseous exchange of water vapour and C 0 2 in leaves largely depend on the movement 
stomatal guard cells. During the opening and closure of stomata, K plays a pivotal role as an 
osmotic agent for turgor changes in the guard cells. Humble and Raschke (1971) provided 
convincing evidence for the requirement of K +for stomatal movement by electrone probe analysis. 
Very recently by the same method, Serge Braconnier and Jean d'Auzac (1990) reported, that on 
stomatal opening, increase of K + and CI' in guard cells of coconut and oil palm leaves and their 
concentration variation percentage of K + and CI' were quite similar for both species of palms. In 
general stomata open less under K-deficiency, but this may occur only at an advanced stage of 
deficiency in many plants (Hsiao, 1975). As presented in Fig. 3, higher levels of K showed a 
marked increase in assimilation of C 0 2 probably through increased stomatal conductance for 
gaseous exchange and maintenance of high water potential within leaves. Further, requirement of 
K for the photophosphorylation process within chloroplasts (Trebst, 1980) and promotion of 
RuBp-carboxy lase enzyme synthesis (Peoples and Koch, 1979) has been reported for other plants. 
However, such effects were not investigated as they were beyond the scope of this study. Uptake 
of K + by root cells depresses the osmotic potential of the cells, thereby inducing uptake of water 
from soil. Water transport into the xy lem vessels is also an osmotic process, function of K + in which 
is very important for upward movement of water. Therefore, K + is the important osmolicum which 
drives water flux from the surrounding cells into the xylem vessels (Baker and Weatherly, 1969). 
In this study it was observed that K and CI accumulated in leaf tissues have possibly acted as 
osmolica to maintain the turgor balance under water deficit conditions. In the treatments where 
high concentration of potassium, high transpiration rates and \p*, were maintained. Thus from these 
observations it was clear that K in coconut seedlings has helped to regulate turgor potential within 
tissues, maintain water balance or water economy by influencing water uptake, upward movement 
and transpiration through stomata. At the same time K + helps to maintain high photosynthetic 
activity in coconut seedlings. These beneficial effects of K in coconut seedlings are particularly 
important for crop production if they hold also for native palms. 
Chlorine deficiency in coconut leaves manifests itself by drying of leaves at the base as a result 
of inadequate supply of water (Ollagnier, 1985). Several authors have reported that the beneficial 
effects of CI on yield, drought tolerance induced by K and CI, and improvement of assimilation 
o f C 0 2 (Magat etal., 1975; Von Uexkull, 1984; Ollagnier, 1985). Increased yield and growth of 
coconut and oil palm were observed when leaf CI concentration was brought up to the critical 
levels of 0.65% of dry weight. However, in this study we found that CI concentration in the zero 
chlorine treatments was in the range of0.40 - 0.90% depending on the N, K, and water treatments. 
High concentrations of chlorine in the zero treatments probably resulted from the tissue CI 
levels at the time of amputation of seedlings and also from CI received from the atmosphere (wind 
borne from the coast). Therefore, the effect of low levels of CI supply (below critical level) on 
physiological processes of coconut palm seedlings was not well reflected in this study. High tissue 
CI concentrations (above criticial level) resulted in undesirable effects on growth of coconut 
seedling by reducing chlorophyll content and assimilation of C O r However, interactive effects of 
K and CI gave beneficial effects by improving osmotic adjustment by turgor balance and leaf water 
potential under water-limiting conditions. 
37 
ACKNOWLEDGEMENTS 
The author$ wish to thank Mr. D. T. Mathes, Head/Biometry Division for statistical designing 
the experiments and for the analysis of data and also Miss P. S. A. de Saram and Mr. R. D. N. 
Premasiri, Technical Assistant for their help in the experimental work. 
These experiments were carried out under the project awarded to C. J. by Canadian 
International Development Agency (CIDA). Assistance of CIDA and Natural Resources Energy 
and Science Authority of Sri Lanka (NARESA) is greatfully acknowledged. 
REFERENCES 
Baker D. A. and Weatherly P. E. (1969) Water and solute transport by exuding root system of 
Ricinus cummunis. J Exp. Bot., 20,485-95. 
Bolton, J. K. and Brown, R. H. (1980) Photosynthesis of grass species differing in carbon dioxide 
fixation pathways. V. Responses of Panicum-maximum, Panicum mitiodse and tall fescue 
to nitrogen nutrition. Plant Physiol., 66: 97 - 100. 
Braconnier, S and Jeand'Auzec (1990) Chloride and stomtal conductance of coconut. Plant 
Physiol. Biochem., 28 ( 1 ) : 105 - 111. 
Brown, R. H. and Wilson, J.R. (1983) Nitrogen response of panicum species differing C 0 2 
fixation pathways. I I C 0 2 exchange characteristics. Crop Sci., 23 : 1154 - 1159. 
Dubois, M, Gilles, K. A. Hamilton, J. K. Rebers, P. A., and Smith, F (1956) Colorimetric method 
for determination of sugars and related substances. Anal. Chem., 28 ( 3 ) : 350 - 356. 
Hasiao, T C, Hagemen R H and E H Tyner (1970) Effects of potassium nutrition on protein and 
total free amino acids in Zea Mays. Crop Sci., 10 : 78 - 82. 
Hsiao, T C (1975) Variables affecting stomatal opening - complicating effects. In : Measurements 
of stomatal apearture and diffusive resistance. E T Kenamasu Ed. Washington Stale Univ. 
Bull., 8 0 9 : 2 8 - 3 1 . 
Humble, G D and Raschke, K (1971) Stomatal opening quantiti vely related to potassium transport. 
Evidence from electron probe analysis. Plant Phyiol., 48,447 - 53. 
Jayasekara, C (1986) Ecophysiological studies in relation to photosynthesis and assimilate 
partitioning in two species of palms - Ph D Thesis, Uni. of QLD. 9 6 - 1 0 4 . 
Jayasekara, K S (1989) Report of the Soils and Plant Nutrition Division - Annual Report of the 
Coconut Research Institute of Sri Lanka for 1989, 88 - 118. 
Leigh, R A and Wyn Jones, R G (1986) Cellular compartmentation in plant nutrition: the selective 
cytoplasm and promiscuous vacuole. In Advances in Plant Nutrition Vol 2 (B Tinker and A 
Lauchli Ed) pp 249 - 279, Praeger, New York. 
38 
Magat, S S, Margate, R Z and R L Prundente (1975) Yield improvement of coconut in elevated 
inland area of Davao by K G fertilization. Oleagineux : 413-448. 
Mangel, K and Kirkby, E A (1980) Potassium in crop production. Academic Press. 59 - 110. 
Nathaniel, W R N (1969) The application of fertilizers to adult coconut palms in relation to 
theoretical concepts, fertilite., 35 (11/12): 11 - 17. 
Navin, D J and Loomis, R S (1970) Nitrogen nutrition and photosynthesis in sugar-beat (Beta 
ulgaris L.) Crop Sci., 10 :21 - 25. 
Peaslee, D E and Moss D N (1968) Photosynthesis in K and Mg deficient (Zea mays L) leaves. 
Soil. Sci. Soc. Amer. Proc , 30 (2 ) : 220 - 223. 
Peoples T R and Koch D W (1929) Role of potassium in carbon dioxide assimilation in Medicago 
sativa L. Plant Physiol., 63: 878 - 881. 
Sumner, J B (1924) The estimation of sugar in diabetic urine, using dinitrosalicylic acid. J. Biol. 
Chem., 62: 287 - 290. 
Terry, N and Ultrich A (1973) Effects of potassium deficiency on the photosynthesis and 
respiration of leaves of sugar beet under conditions of low sodium supply. Plant physiol., 
51: 1099-1101. 
Trebst, A (1980) Inhibitation in electron flow: Tools for the functional and structural localization 
of carriers and energy conserving sites. In: San Pietro A (Ed) Methods in enzymology Vol 
69. Academic Press, London New York, pp 675 - 715. 
Troll, W and Lindsley, J (1955) A photometric method for the determination of proline. J Biol. 
Chem., 215 :655-660 . 
Von Uexkull, H R (1984) Chlorine in the nutrition of palm trees. ICOSANP, Kuala Lumpur 
1 3 - 1 5 Aug 1989. 
Wong, S C (1979) Elevated atmospheric partial pressure of C 0 2 and plant growth. Interactions 
of nitrogen inhibition and photosynthesis capacity in C } and C 4 and plants. Oecologia, 
44: 68 - 74. 
39