Changes in the levels of chlorophyll and light-harvesting chlorophyll a/b protein of PS II in rice leaves aged under different irradiances from full expansion through senescence
Hidema, J. | Makino, A. | Kurita, Y. | Mae, T. | Ojima, K.
Effects of irradiance on changes in the amounts of chlorophyll (Chl) and light-harvesting chlorophyll a/b protein of PS II (LHCII) were examined in senescing leaves of rice (Oryza sativa L.). Results of treatments at two irradiances (100% and 20% natural sunlight) were examined after the full expansion of the 13th leaf throughout the course of senescence. With 20% sunlight, the Chl content decreased only a little during leaf senescence, while with 100% sunlight it decreased appreciably. Similarly, the amount of LHCII protein during treatment with 20% sunlight remained almost constant. However, the ratio of Chl a/b during the shade treatment decreased significantly and the rate of decrease was greater than during the full-sunlight treatment. The ratio of Chl a/b for Chl a and b bound to LHCII was about 1.2, irrespective of leaf age or irradiance treatment. When the amounts of Chl bound to LHCII were calculated from the total leaf content of Chl and the ratio of Chl a/b, assuming a ratio of Chl a/b bound to LHCII of 1.2, they were well correlated with the amounts of LHCII protein. Changes in the amounts of LHCII synthesized during the two irradiance treatments were examined using an 15N tracer. Incorporation of 15N into LHCII declined dramatically during both treatments from full expansion through senescence, suggesting that there was little synthesis of LHCII protein during that time. In addition, the amount of LHCII synthesized during senescence was lower during the shade treatment than during the 100% sunlight treatment. These results indicate that the absence of an apparent change in levels of LHCII with shade treatment during senescence was caused by the very low rate of turnover of LHCII protein.Show more [+] Less [-]