Abstract
CO2 assimilation and plant water status were measured in sand-culture-grown amputatedcoconut seedlings treated with three levels ofN, K, and CI and subjected to different soil waterdeficit conditions. Application of increased levels of N and K enhanced the assimilation of C02while individual and combined effects of N,K, and CI improved the water economy of coconutseedlings. Under the soil water deficit conditions, adequate supply o fN helped to maintain highleaf water potential by the accumulation of solutes like proline and sugars. Whereas, adequatesupply of K and Cl themselves acted as osmotica by increased accumulation of these nutrients inleaf 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 thecoconut seedling through improved gaseous exchange, CO2 assimilation and better partitioningof assimilated carbon into shoot and roots, potassium and C1 are further important for maintenanceof water status of coconut seedlings by improved stomatal regulation, water uptake and osmoticadjustment of tissues under water deficit conditions.
Highlights
Productivity of perennial tree crops mainly depends on climatic factors, availability of soil nutrients and fertilizer usage in addition to their genotypic characters
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)
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
Summary
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). 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. 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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