Photosynthetic capacity of the capsule wall and its contribution to carbon fixation and seed yield in castor (Ricinus communis L.)

Defoliation occurs in castor due to several reasons, but the crop has propensity to compensate for the seed yield. Photosynthetic efficiency in terms of functional (gas exchange and chlorophyll fluorescence) and structural characteristics (photosynthetic pigment profiles and anatomical properties) of castor capsule walls under light- and dark-adapted conditions was compared with that of leaves. Capsule wall showed high intrinsic efficiency of photosystem II (F-v/F-m, 0.82) which was comparable to leaves (F-v/F-m, 0.80). With increasing photon flux densities (PFD), actual quantum yields and photochemical quenching coefficients of the capsule walls were similar to that in leaves, while electron transport rates reached a maximum corresponding to about 118 % of the leaves. However, maximum net photosynthetic rate of the capsule walls (2.60 mu mol CO2 m(-2) s(-1)) was less than one-fourth of the leaves (15.67 mu mol CO2 m(-2) s(-1)) at the CO2 concentration of 400 mu mol mol(-1), and the difference was attributed to about 80 % lower stomatal density and the 75 % lower total chlorophyll content of capsule walls than the leaves. Furthermore, seed weight in dark-adapted capsules was 2.70-12.42 % less as compared to the capsules developed under light. The results indicate that castor capsule walls are photosynthetically active (about 15-30 % of the leaves) and contribute significantly to carbon fixation and seed yield accounting for 10 % photoassimilates towards seed weight.