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

Abstract

Shoots of 16-day-old soybeans (Glycine max L. Merr. cv Ransom) were chilled to 10°C for 7 days and monitored for visible signs of damage, ultrastructural changes, perturbations in fluorescence of chlorophyll (Chl), and quantitative changes in Chi a and b and associated 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 first day of chilling, and regreening did not occur until the shoots were returned to 25°C. Ultrastructurally, unstacking of chloroplast grana occurred, and the envelope membranes developed protrusions. Furthermore, 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 hours 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.A large number of crop plants originated in the tropics or subtropics and begin to show deleterious responses as the temperature is lowered below 20°C. If there is physiological and structural damage, it is referred to as 'chilling injury.' The phenomenon has been well described (5,12,13).Among chilling sensitive plants such as Gossypium, Paspalum, Phaseolus vulgaris, and Glycine max, the chloroplast is the first of the organelles to show ultrastructural damage from chilling (1,11,29,35). Changes in chloroplast function have been tabulated for five chilling-sensitive species (26) and decline in photoreductive activity is common (10,14). Since changes in photosynthetic electron transfer activity can be monitored fluorometrically in intact leaves (20,26), changes in fluorescence of Chl have the potential for serving as a very early sign of chilling injury. This expectation is tested herein.Aside from the light energy transfer role of the carotenoids in photosynthetic membranes, they are known protectants of Chl against photooxidation. Apparently under chilling conditions, the equilibrium is shifted in the direction of excessive photooxidation (16, 29

Chilling-Enhanced Photooxidation

“…Ultrastructural injury was restricted to the chloroplast (Figs. 5 and 6) which is in agreement with longer term, lower irradiance studies involving other chilling-sensitive plants (15,22,27,31). Of possible significance in the present study is the finding that pea chloroplasts showed signs of ultrastructural alterations (Fig.…”

“…5a), are intriguing. Such changes have been reported previously in chilling-sensitive plants such as Phaseolus vtulgaris and Gossypium hirsutum given a moderate stress of 5°C, 100%RH, and 500 uE-m-s' (31) and Glycine max given 10°C, 85% RH, and 500 ,gE-m-2s' (15). The peripheral vesicles are similar in appearance to components of the chloroplast peripheral reticulum, a system of anastomosing tubules connected to the inner envelope membrane and thought to be involved in metabolite transport (10).…”

Variation among Species in the Temperature Dependence of the Reappearance of Variable Fluorescence following Illumination

“…Smillie and Hetherington (17) showed that the maximal rate of the light induced F, rise in heat treated leaves from different species was a useful method for determining relative heat tolerance. F, increases during chilling in the dark have previously been reported (5,6,9); however, chilling in the light often results in a decline in F, (6,10). The temperature dependence of the recovery of PSII F, following illumination in cucumber and pea leaves has been reported by Peeler and Naylor (1 1 …”

Reduction of photosynthesis in sun leaves of Gossypium hirsutum L under conditions of high light intensities and suboptimal leaf temperatures

“…At these relatively high temperatures it is unlikely that phenomena like membrane disruption (Rikin et al, 1979;Powles, 1984) and changes in the chloroplast ultrastructure (Taylor and Craig, 1971;Musser et al, 1984;Wise and Naylor, 1987) (table II).…”