No abstract

Chilling temperatures (5°C) and high irradiance (1000 microeinsteins per square meter per second) were used to induce photooxidation in detached leaves of cucumber (Cucumis sativus L.), a chilling-sensitive plant. Chlorophyll a, chlorophyll b, ,B carotene, and three xanthophylls were degraded in a light-dependent fashion at essentially the same rate.Lipid peroxidation (measured as ethane evolution) showed an 02 dependency. The levels of three endogenous antioxidants, ascorbate, reduced glutathione, and a tocopherol, all showed an irradiance-dependent decline. a-Tocopherol was the first antioxidant affected and appeared to be the only antioxidant that could be implicated in long-term protection of the photosynthetic pigments. Results from the application of antioxidants having relative selectivity for '02, 02-, or OH indicated that both '02 and 2-were involved in the chilling-and light-induced lipid peroxidation which accompanied photooxidation. Application of D20 (which enhances the lifetime of '02) corroborated these results. Chilling under high light produced no evidence of photooxidative damage in detached leaves of chilling-resistant pea (Pisum sativum L.). Our results suggest a fundamental difference in the ability of pea to reduce the destructive effects of free-radical and '02 production in chloroplasts during chilling in high light.

Steady-state flow rates and exudate osmotic potentials were measured from complete root systems from warm-(28/23 C) or cold-(17/11 C) grown soybean or broccoli (Brassica oleracea) plants at various pressures or different temperatures.In warm-grown soybean roots systems, a break occurred at 14.7 C in the Arrhenius plot of total flow at constant pressure. When plants were grown at lower temperatures, the break point shifted to 8 C. Broccoli, a chileing-resistant species, showed no break for the temperature range used, but cooler growth temperatures decreased the activation energy for water flow through the root system from 18 kilocalories per mole to 9 kilocalories per mole. In both broccoli and soybean, cold-grown plants had lower exudate potentials and greater flow rates at low hydrostatic pressures than the warm-grown plants.These observations indicate that the rate-limiting site for passive water transport is a membrane which may be modified as the plant acclimates to varying growth temperatures. An additional part of the acclimation process is an increase in activity of root ion pumps.Since the time of Hales (1727) it has been known that cold soil reduces the absorption of water by plants. Differences among species in reaction to water absorption at low temperatures were reviewed by Kramer (1 1). Kuiper (13) was among the first to note that at a "critical" temperature, which varied with the temperature at which roots were grown, a sharp drop occurred in water absorption. Some investigators attributed anomalies such as this to discontinuities in properties of surface water at 15, 30, 45, and 60 C (7, 15, 18).A more likely hypothesis attributes sharp breaks in Arrhenius plots of biological phenomena to phase transitions in membrane lipids in which the hydrophobic core changes from a liquid to a solid or crystalline state at some specific temperature. Such changes alter membrane permeability and enzyme activity (8,10,19). Arrhenius plots were used by Clarkson (2) to study exudation rates and ion concentration of the xylem exudate from excised barley and rye roots preconditioned at various temperatures. With rye roots grown at 20 C he found a break in the Arrhenius plot of exudation rate versus temperature at 10 C, but for roots preconditioned at 8 C for 3 days the break occurred at 5 C.

Chilling-induced photooxidation was studied in detached leaves of chilling-sensitive (CS) cucumber (Cucumis sativus L.) and chilling resistant (CR) pea (Pisum satirum L.). The rates of photosynthesis and respiration, measured as 02 exchange, were found to be comparable in the two species over a temperature range of 5 to 35°C. Chilling at 5°C for 12 hours in high light (1000 microeinsteins per square meter per second) decreased CO2 uptake 75% in detached pea leaves whereas CO2 uptake by cucumber was reduced to zero within 2 hours. Respiration was unaffected in either species by the chilling and light treatment. Although ultrastructural alterations were apparent in chloroplasts of both species, cucumber's were affected sooner and more severely. The mechanism of photooxidative lipid peroxidation was investigated by following the production of ethane gas under a variety of conditions. Maximum ethane production occurred in the CS cucumber at low temperature (5°C) and high light (1000 microeinsteins per square meter per second). Atrazine, an inhibitor of photosynthetic electron transport, almost completely halted this chilling-and light-induced ethane production. These data, taken with those reported in an accompanying article (RR Wise, AW Naylor 1986 Plant Physiol 83: 278-282) suggest that the superoxide anion radical is generated in cucumber chloroplasts (probably via a Mehler-type reaction) during chilling-enhanced photooxidation. Parallel experiments were conducted on pea, a CR species. Detached pea leaves could only be made to generate ethane in the cold and light if they were pretreated with the herbicide parquat, a known effector ofO2 production. Even so, pea showed no lipid peroxidation for 6 hours, at which time ethane production began and was at a rate equal to that for the chilled and irradiated cucumber leaves. The results indicate that pea has an endogenous mechanism(s) for the removal of toxic oxygen species prior to lipid peroxidation. This mechanism breaks down in pea after 6 hours in the cold, light, and the presence of paraquat.Low temperatures and high light can cause photooxidation (a light-and oxygen-dependent bleaching [18]) in the leaves of chilling-sensitive plants (24,30). In addition to chlorosis (24), other symptoms of chilling injury in the light include the rapid appearance ofphotosynthetic dysfunction (9,19,20,23,28), altered chloroplast ultrastructure (22,27,31), and cellular lipid degradation (5, 26).It is reasonable to assume that chloroplasts are a primary site for photooxidative injury because these organelles absorb roughly '

The effects of light and water stress upon chilling injury of chloroplasts have been assessed by electron tnicroscopy in seedlings of three species known to differ in their chilling susceptibility. Chilling injury to chloroplasts was first manifested by distortion and swelling of thylakoids, reduction in starch granule size, and the fortnation of stnall vesicles of the envelope, called the peripheral reticulum. More prolonged treatment produced accumulations of lipid droplets, increased staining of the stroma, disintegration of the envelope, and mixing with cytoplastnic contents. Cotton, a notably chilling-sensitive plant, and bush bean, a somewhat less sensitive plant, showed damage within 6 h when exposed to both light and water sttess at chilling temperatures (5 C). Even collard, a chilling-resistant species, exhibited signs of chilling itijury to chloroplasts after 6 h when exposed to both light and water stress but the plastids retnained intact throughout the 48 h ol^ treattnent. Cotnparable chilling injury does not occur in cotton until around 72 h if the plants are exposed to water stress or light separately. Bush bean was affected less by separate treatments of light and water stress. The least chilling injury occurred in all three species when they were kept in the dark at a high hutnidity.