Plants under Climatic Stress

Mesophyll chloroplasts of the C(4)-pathway grasses Sorghum and Paspalum and of the C(3)-pathway legume soybean undergo ultrastructural changes under moderate light intensities (170 w.m(-2), 400-700 nanometers) at a tme when photosynthesis is much reduced by low temperature (10 C). The pattern of ultrastructural change was similar in these species, despite some differences in the initial sites of low temperature action on photosynthesis and differences in their mechanisms of CO(2) fixation. Starch grains in the chloroplasts rapidly reduce in size when chilling stress is applied. At or before the time starch grains completely disappear the membranes of the individual stromal thylakoids close together, reducing the intraspace between them while the chloroplast as a whole begins to swell. Extensive granal stacking appears to hold the thylakoids in position for some time, causing initial swelling to occur in the zone of the peripheral reticulum, when present. At more advanced stages of swelling the thylakoid system unravels while the thylakoid intraspaces dilate markedly. Initial thylakoid intraspace contraction is tentatively ascribed to an increase in the transmembrane hydrogen ion gradient causing movement of cations and undissociated organic acids from the thylakoid intraspace to the stroma. Chloroplast swelling may be caused by a hold-up of some osmotically active photosynthetic product in the chloroplast stroma. After granal unraveling and redilation of the thylakoid intraspaces, chloroplasts appear similar to those isolated in low salt hypotonic media. At the initial stages of stress-induced ultrastructural change, a marked gradient in degree of chloroplast swelling is seen within and between cells, being most pronounced near the surface of the leaf directly exposed to light.