Changes in photosynthetic characteristics and photosystem stoichiometries in the lower leaves in rice seedlings

Changes in the Chi a/b ratio, electron transport, electron carriers, and ph otosystem stoichiometries were examined in rice leaves in the present study . The Chi alb ratio is known to decrease gradually from the top to the bott om leaves, indicating a increase in the abundance of LHC II relative to the reaction center complexes of the two photosystems. We used juvenile rice c anopy and obtained the following results: (1) the photosynthetic activity a nd Chi content per leaf area decreased from the top to the bottom leaves, t he Chi alb ratio also declined from 3.7 to 3.0; (2) when determined on the basis of Chi content, C-550 and Cyt f content decreased, but there was no l oss of P-700, Consequently, the PS II/PS I ratio significantly decreased; ( 3) on the basis of mmol Chi, the levels of Cyt f dramatically decreased and , therefore, no loss was observed for whole chain electron transport per Cy t f; and (4) the percentage abundance of PS II alpha increased, but the rat e constants of Q(A), photoreduction and P-700 photooxidation gradually decr eased. From these results, we hypothesize that there is a compensatory rela tionship between the decline in the Ch1 alb ratio and that in the PS II/PS I ratio in the lower leaves in rice seedlings. (C) 1999 Elsevier Science Ir eland Ltd. All rights reserved.