Changes in photosynthetic apparatus in the juvenile rice canopy and a possible function of photosystem I in the bottom leaves

Changes in the photosynthetic apparatus and relative antenna sizes of photo system (PS) I and PS II were measured in the rice canopy. We used juvenile rice seedlings to examine light utilization and its absorption in the botto m leaves and obtained the following results: (1) When referred to chlorophy ll (Chl), levels of the electrochromic shift at 550 nm and cytochrome f dec reased from the sixth to the third leaves, but there was no loss of pigment (P)-700. As a consequence, the PS II/PS I ratio significantly decreased fr om 1.5 in the sixth leaves to 0.9 in the third leaves. (2) The electron tra nsport capacity in the sixth leaves was 1.5-times larger than that in the t hird leaves. (3) The levels of cytochrome b(6) referred to Chi were almost constant from top to bottom. (4) The photosynthetic performance of the leaf decreased concomitant with the depth, whereas the respiration was slightly increased. From these results, we hypothesize that there are maintenance m echanisms when the imbalances of light absorption and electron transport ca pacity occur in the bottom leaves.