Plants under Climatic Stress

Abstract

Patterns of radiocarbon exchange between photosynthetic intermediates of the chilling sensitive Sorghum bicolor were modified by exposure to a combined environmental stress of low temperature (10 C) and moderate light levels (170 w m-2, visible). Pulse chase experiments with"CO2 showed that this stress initially slowed the release of photosynthetically absorbed radiocarbon from malate. Further exposure caused an increased proportion of the radiocarbon to accumulate in aspartate. This trend continued, so that after 30 hours, some 80% of absorbed radiocarbon remained in aspartate after 1 minute of chasing and subsequent release of carbon into the C3 cycle was very slow. In Sorghum, chilling combined with light seemed to cause a restriction in an early step of the C pathway before ultrastructural changes could be detected in the mesophyll chloroplasts.Chilling temperatures combined with high levels of light have been shown (6,8) to cause a progressive reduction in the photosynthetic capacity of the leaves of several plants of tropical and subtropical origin. Chloroplasts, which were most exposed to light during such environmental stress, underwent marked and quite rapid ultrastructural changes before finally bursting and contributing to cell and eventual leaf necrosis (9). Many C4 photosynthetic pathway species (3) seemed to be particularly chilling-sensitive, and in these plants chilling under high light rapidly altered the level of amino acids formed from intermediates of the C4 pathway (10). This led us to suggest that some time-and temperature-dependent blockages were developing in the interconversion of C4 pathway intermediates and possibly in the flow of other intermediates to and from the sites of C4 photosynthesis.In the radiocarbon-free air of the main cabinet. A sequence of 14 mm 4 discs was punched from the main lamina of these leaves into methanol-chloroform-2 M formic acid (12:5:3) maintained at -70 C. CO2 supply was not limiting during this period of time, since leaves maintained continuously at 25 C used less than one-fifth of the available CO2 during the 20-sec pulse, while those at 10 C fixed less (see "Results").Leaf discs were ground first in the killing medium, then reextracted with 50% aqueous methanol, and the combined supernatants were separated into phases by adding water and chloroform. Aliquots of the aqueous layer were separated on thin layers of MN 300 cellulose after first being passed through Sephadex G25. Two-dimensional chromatography in watersaturated phenol followed in the second direction with a propyl acetate based solvent (1), or a combination of thin layer electrophoresis followed by chromatography (7) were both used. Developed plates were then autoradiographed and radioactive areas removed after coating the plates with collodion (2). RESULTS AND DISCUSSIONPhotosynthetic rates of Sorghum leaves prior to stress treatment were 20 to 22 X 10-' mg CO2/ cm' min at 25 C, corresponding to a radiocarbon uptake of 2.8 X 106 dpm/20 sec pulse in each leaf disc. Chilling progressiv...

Effect of Light and Chilling Temperatures on Chilling-sensitive and Chilling-resistant Plants. Pretreatment of Cucumber and Spinach Thylakoids in Vivo and in Vitro

“…The photosynthetic rates of thermophilic plants (Paspalum, Sorghum, Cucumis, and Glycine) decrease rapidly when the temperature is lowered from 25 to 10 C in the light (9, 25). Most work on the combined effects of light and chilling temperatures has concentrated on CO2 fixation, primarily in C4 grasses (3,24,26), although available data indicate that photochemical activities are altered when leaves are chilled in light (9). …”