Chloroplast Ultrastructure, Chlorophyll Fluorescence, and Pigment Composition in Chilling-Stressed Soybeans

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Abstract
Shoots of 16-day-old soybeans were chilled to 10.degree. C for 7 days and monitored for visible signs of damage, ultrastructural changes, perturbations in fluorescence of chlorphyll (Chl) and quantitative changes in Chl a and b and associted pigments. Precautions were taken to prevent the confounding effects of water stress. A technique for the separation of lutein and zeaxanthin was developed utilizing a step gradient with the high performance liquid chromatograph. Visible losses in Chl were detectable within the 1st day of chilling, and regreening did not occur until the shoots were returned to 25.degree. C. Ultrastructurally, unstacking of chloroplast grana occurred and the envelope membranes developed protrusions. The lipids were altered to the point that the membranes were poorly stabilized by a glutaraldehyde/osmium double-fixation procedure. Chl fluorescence rates were greatly reduced within 2 h after chilling began and returned to normal only after rewarming. The rapid loss of Chl that occurred during chilling was accompanied by the appearance of zeaxanthin and a decline in violaxanthin. Apparently a zeaxanthin-violaxanthin epoxidation/de-epoxidation cycle was operating. When only the roots were chilled, no substantial changes were detected in ultrastructure, fluorescence rates or pigment levels.