CHANGES IN THE NUMBER AND SIZE OF CHLOROPLASTS DURING SENESCENCE OF PRIMARY LEAVES OF WHEAT GROWN UNDER DIFFERENT CONDITIONS

Changes in the number and size of chloroplasts in mesophyll cells were investigated in primary leaves of wheat from full expansion to yellow ing under different growth conditions. The number of chloroplasts per cell decreased slowly, although the decrease was steady and statistica lly significant, until the last stage of leaf senescence, when rapid d egradation of chloroplasts took place. Rates of leaf senescence, or th e decline in the number of chloroplasts, varied greatly among plants g rown at different seasons of the year, but about 20% of chloroplasts a lways disappeared during the phase when steady loss of chloroplasts oc curred. The area of chloroplast disks also decreased gradually but sig nificantly, with a rapid decrease late in senescence. Thus, the total quantity of chloroplasts per mesophyll cell decreased substantially du ring leaf senescence. Yellowed leaves contained numerous structures th at resemble oil drops but no chloroplasts. Decreases in rates of photo synthesis that occur during senescence may, therefore, be largely due to decreases in the quantity of chloroplasts. However, a better correl ation was found between the decrease in the maximum capacity for photo synthesis and the degradation of RuBP carboxylase. When plants had bee n grown with a sufficient supply of nutrients, the number of chloropla sts decreased steadily but at a reduced rate and the reduction in the area of chloroplast disks was strongly suppressed. Thus, the quantitat ive decrease in chloroplasts in senescing leaves appears to be regulat ed by the requirements for nutrients (nitrogen) of other part of the p lant.