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

Abstract Changes in the Chl a / b ratio, electron transport, electron carriers, and photosystem stoichiometries were examined in rice leaves in the present study. The Chl a / b ratio is known to decrease gradually from the top to the bottom leaves, indicating a increase in the abundance of LHC II relative to the reaction center complexes of the two photosystems. We used juvenile rice canopy and obtained the following results: (1) the photosynthetic activity and Chl content per leaf area decreased from the top to the bottom leaves, the Chl a / b ratio also declined from 3.7 to 3.0; (2) when determined on the basis of Chl content, C-550 and Cyt f content decreased, but there was no loss of P-700, Consequently, the PS II/PS I ratio significantly decreased; (3) on the basis of mmol Chl, the levels of Cyt f dramatically decreased and, therefore, no loss was observed for whole chain electron transport per Cyt f ; and (4) the percentage abundance of PS IIα increased, but the rate constants of Q A photoreduction and P-700 photooxidation gradually decreased. From these results, we hypothesize that there is a compensatory relationship between the decline in the Chl a / b ratio and that in the PS II/PS I ratio in the lower leaves in rice seedlings.