Plants under Climatic Stress

Abstract

An investigation has been made of the combined effects of low temperature and high light on the level of several photosynthetic products in the leaves of a group of plants differing widely in their tolerance to this stress. Starch levels in these plants after chilling are dependent on the time of day that temperatures are lowered and seem related to rates of C02 assimilation under this stress. Prolonged low-temperature, highlight treatment (10 C at 160 wmr') of Sorghum bicolor induced a rapid starch hydrolysis after a lag of some 24 hours. Differing rates of starch loss at the cellular level and a rapid migration of chloroplasts toward the base of upper mesophyll cells were also seen in legves of this stress-sensitive species.Chilling increased the level of almost all free amino acids in tolerant and in semi-tolerant species, while amino acids related to intermediates of the C4-pathway show a sharp or transitory decrease in Sorghum. These and other changes observed in Sorghum suggest that some time-and temperature-dependent blockages develop in the interconversion of C4-pathway intermediates and possibly in the flow of other intermediates to and from the sites of C4-photosynthesis.Levels of ATP in the leaves of Sorghum, Paspalum, and Amaranthus increased at night and following chilling and did not fall until pronounced necrosis of the leaves commenced.Previous reports (21,22) have shown that high light levels combined with low temperatures can cause necrotic lesions to develop on the leaves of several plants. Lower levels of light prevent or slow the onset of these lesions. Some species of both C3-and C4-photosynthetic pathway (7) Lolium multiflorum L. var Grasslands "Tama" Westerwolds.Potting media, plant nutrients, and controlled environment cabinets were as described previously (21). Plants were given three phases of treatment. They were first preconditioned at a constant day-night temperature of 25 C for 11 to 14 days. Temperatures were generally lowered at the start of a photoperiod to a constant 10 C day and night for 3 days and then returned to 25 C. Photoperiods were 12 hr with light intensities maintained at 160 wm-2 (400-700 nm).Soluble Sugar, Starch, and Malate Assays. Several leaves with most recently emerged ligules (youngest-mature) were harvested from different plants 3 hr before the completion of 12-hr photoperiods. Leaves were lyophilized and ground, and aliquots were used for subsequent analyses. Soluble sugars extracted in 80% (v/v) ethanol and starch extracted from the alcohol-insoluble residue according to Hassid and Neufield (6) were assayed with phenol-sulphuric acid (1). Malate was extracted from aliquots (250 mg) of the same dry leaf powder in 5 ml of 6% HCIO4 at 0 C and determined enzymically (8).Similar assays were also used to determine the levels of leaf starch at more frequent intervals before and during low-temperature treatment.Amino Acid Assays. Amino acids were extracted from 1.5 g wet weight of youngest-mature leaf lamina in methanol-chloroform (2:1) followed by 0.1 ...

Plants under Climatic Stress

“…Pyruvate (50 mM) was then added to the eluate used for pyruvate Pi dikinase measurements and subsamples equilibrated to the various temperatures. Dikinase activation was commenced by adding 2.5 mM Pi and aliquots removed at 10-min intervals over a 1-hr period for assay at 30 C. Dithiothreitol (20 mM) was added to temperature equilibrated subsamples to initiate the activation of NADP-malate dehydrogenase and aliquots removed at 5 min intervals for assay at 30 C. (24) showing that C,-grasses differing in chilling sensitivity show a build-up of one or other of these two amino acids when exposed to low temperatures and light. In addition, pulse chase type experiments on sorghum leaves revealed that 80% of radiocarbon from photosynthetically absorbed "CO., remained in aspartate after a 30 hr stress treatment, demonstrating a profound change in radiocarbon exchange between photosynthetic intermediates (6).…”

Carbon Metabolism and Photorespiration: Temperature Dependence in Relation to Other Environmental Factors

“…Thus, under extended cold stress the synthesis of malate appears to be impaired. When plants are transferred from 25 °C to 10 °C the pool size of alanine dramatically decreases while that of aspartate increases (Taylor et al, 1972). Thus, it appears low temperature has specific effects on the function of the cycle, although it is not possible from these studies to determine which steps are cold sensitive.…”

“…In cold-sensitive Sorghum bicolor under low temperature restrictions appear to develop in the interconversion of pathway intermediates and/or transport of metabolites (Taylor et al, 1972;Brookings and Taylor, 1973;Long, 1983). S. bicolor is a NADP-ME type species with malate as the predominant product of the cycle.…”

“…in crops early in the growing season well before a full canopy develops), chilling induced decreases in photosynthesis may be linked to a reduction in A reduction in indicates a decline in efficiency in producing assimilatory power. Since levels of ATP in the leaves of species increase dramatically during chilling stress prior to obvious cellular damage (Taylor et al, 1972), the production of NADPH through noncyclic electron flow may be more restricted than production of ATP via cyclic photophosphorylation.…”

Plants under Climatic Stress

“…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 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").…”