Abstract
Among the tree crops in the tropics the coconut palm occupies an importantposition as a food, oil, and fibre crop. Productivity of crop species depends on thecombined effect of their genetic constitution, climate, nutrition and other bioticfactors. To increase the productivity of the coconut palm it is necessary to have abasic understanding of physiological processes such as assimilation of carbon andgrowth in relation to climatic factors.In-situ photosynthetic measurement of coconut seedlings using Li-COR 6500portable photosynthesis system revealed that their maximum photosynthetic capacityis attained at photosynthetic photon flux densities (PPFD) ranging from 1400-1700u moles photons m'2 S"1. The rate of apparent photosynthesis (A) of the coconutpalm varies between 2-15 a moi C02 m"2 S"1 depending on leaf age, nutritional status.and climatic factors. The rate of leaf respiration was closely similar in young andolder leaves ranging from - 2 to -1 fi mol C02m'2S"'. The measured light-useefficiency of the coconut palm was found to be 1.2 g MJ"1 and the conversionefficiency of solar energy was approximately 2.4%.Water deficit is the major constraint causing wide fluctuations in photosynthesis.Due to height of trees, the leaves in the canopy are exposed to a high wind velocityas well as water and thermal stresses. As a result, under soil water deficit conditionscoconut palms reduce excessive water loss and gaseous exchange by mid-day closureof stomata and maintain stomatal conductance at a lower level. However, the rateof apparent photosynthesis and the ratio of intercellular to ambient partial pressureof C 0 2 (P/PJ are not decreased proportionately with the stomatal conductance.
Highlights
IntroductionThe economic yield of a crop, depends on the solar radiation intercepted, conversion efficiency of intercepted solar radiation into chemical energy or dry matter, and the efficient partitioning of assimilated dry matter between vegetative and harvestable parts of the plant
Light saturated maximum rate of net photosynthesis of coconut seedlings was reached at photosynthetically active radiation (PAR) interception around 1400 u mol m'2 s'1 and which was about 60 % of the intercepted PAR
The results indicate that the gradual decline in photosynthetic activity of coconut fronds after 12 -14 months was accompanied by a decrease in the total nitrogen content while incident PAR decreased due to the self shading effect from the upper canopy
Summary
The economic yield of a crop, depends on the solar radiation intercepted, conversion efficiency of intercepted solar radiation into chemical energy or dry matter, and the efficient partitioning of assimilated dry matter between vegetative and harvestable parts of the plant. The highest conversion efficiency of photosynthetically active radiation (PAR) into dry matter in coconut has been estimated as 1.2 to 1.4 g MJ'1 (Corley, 1983; Jayasekara, 1992). This estimated efficiency of conversion of PAR in coconut plantations is found to be similar to the tropical rain forest species (Saldrarriaga and Luxmoore, 1991), but lower than many C3 crops as their .values generally falls in the order of 2.5 - 3.0 g MJ'1
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