Influence of Water and Temperature Stress on the Temperature Dependence of the Reappearance of Variable Fluorescence following Illumination.
The temperature dependence of the rate and magnitude of the reappearance of photosystem II (PSII) variable fluorescence following illumination has been used to determine plant temperature optima. The present study was designed to determine the effect of a plant's environmental history on the thermal dependency of the reappearance of PSII variable fluorescence. In addition, this study further evaluated the usefulness of this fluorescence technique in identifying plant temperature optima. Laboratory and greenhouse grown potato (Solanum tuberosum L. cv "Norgold M") plants had a thermal kinetic window between 15 and 25 degrees C. The minimum apparent K(m) of NADH hydroxypyruvate reductase for NADH occurred at 20 degrees C. This temperature was also the temperature providing maximal reappearance of variable fluorescence. Soybean (Glycine max [L.] Merrill cv "Wayne") plants had a thermal kinetic window between 15 and 30 degrees C with a minimum apparent K(m) at 25 degrees C. Maximal reappearance of variable fluorescence was seen between 20 and 30 degrees C. To determine if increasing environmental temperatures increased the temperature optimum provided from the fluorescence response curves, potato and soybean leaves from irrigated and dryland field grown plants were evaluated. Although the absolute levels of PSII variable fluorescence declined with increasing thermal stress, the temperature optimum of the dryland plants did not increase with increased exposure to elevated temperatures. Because of variability in the daily period of high temperature stress in the field, studies were initiated with tobacco plants grown in controlled environment chambers. The reappearance of PSII variable fluorescence in tobacco (Nicotiana tabacum L. cv "Wisconsin 38") leaves that had experienced continuous leaf temperatures of 35 degrees C for 8 days had the same 20 degrees C optima as leaves from plants grown at room temperature. The results of this study suggest that the temperature optimum for the reappearance of variable fluorescence following illumination is not altered by the plant's previous exposure to variable environmental temperatures. These findings support the usefulness of this procedure for the rapid identification of a plant's temperature optimum.
Thermal Dependence of the Apparent K(m) of Glutathione Reductases from Three Plant Species.
The thermal dependencies of the apparent K(m) of the glutathione reductases from spinach (Spinacia oleracea L.) corn (Zea mays L.), and cucumber (Cucumis sativus L.) were determined. The apparent K(m) of the enzymes were found to vary up to 9-fold between 12.5 and 45 degrees C. Values of the apparent K(m) in excess of 200% of the observed minimum are suggested to be detrimental to the normal function of the enzyme. We propose the term "thermal kinetic window" to describe to the range of temperatures over which the apparent K(m) of the glutathione reductase is within 200% of its minimum and suggest that it may be a useful indicator of the limits of thermal stress for a given species. The thermal kinetic windows determined in this study are: <16 degrees C for spinach, 23 to 32 degrees C for corn, and 35 to 41 degrees C for cucumber.
Glutathione reductase from germinated peas.
Since the discovery of glutathione reductase in plants (Mapson & Goddard, 1951; Conn & Vennesland, 1951) there has been little work on the nature and properties of the enzyme. In the work already published, there is some difference of opinion as to the nature of the prosthetic group of the enzyme prepared from both plant and animal sources. Langdon (1958) reported that the enzyme extracted from rat liver contained no flavin nucleotide, and Moustafa (1955) reported similarly on the plant en-