Plant and Cell Physiology, 1995, Vol. 36, No. 1 9-17
© 1995
Changes in the Number and Size of Chloroplasts during Senescence of Primary Leaves of Wheat Grown under Different Conditions
1 Department of Biology, Faculty of Science, University of Tokyo Hongo, Bunkyo-ku, Tokyo, 113 Japan
2 Department of Biology, College of Arts and Sciences, University of Tokyo Komaba, Meguro-ku, Tokyo, 153 Japan
Changes in the number and size of chloroplasts in mesophyll cells were investigated in primary leaves of wheat from full expansion to yellowing under different growth conditions. The number of chloroplasts per cell decreased slowly, although the decrease was steady and statistically significant, until the last stage of leaf senescence, when rapid degradation of chloroplasts took place. Rates of leaf senescence, or the decline in the number of chloroplasts, varied greatly among plants grown at different seasons of the year, but about 20% of chloroplasts always disappeared during the phase when steady loss of chloroplasts occurred. The area of chloroplast disks also decreased gradually but significantly, with a rapid decrease late in senescence. Thus, the total quantity of chloroplasts per mesophyll cell decreased substantially during leaf senescence. Yellowed leaves contained numerous structures that resemble oil drops but no chloroplasts. Decreases in rates of photosynthesis that occur during senescence may, therefore, be largely due to decreases in the quantity of chloroplasts. However, a better correlation was found between the decrease in the maximum capacity for photosynthesis and the degradation of RuBP carboxylase. When plants had been grown with a sufficient supply of nutrients, the number of chloroplasts decreased steadily but at a reduced rate and the reduction in the area of chloroplast disks was strongly suppressed. Thus, the quantitative decrease in chloroplasts in senescing leaves appears to be regulated by the requirements for nutrients (nitrogen) of other part of the plant.
3Present address: Department of Biology, Faculty of Science, Toho University, Miyama, Funabashi, Chiba, 274 Japan
(Received July 7, 1994; Accepted October 3, 1994)
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