Photosynthetic Acclimation to Temperature in the Desert

The response of photosynthetic electron transport and light-harvesting efficiency to high temperatures was studied in the desert shrub Larrea divaricata Cav. Plants were grown at day/night temperatures of 20/15, 32/25, or 45/33 C in rough approximation of natural seasonal temperature variations. The process of acclimation to high temperatures involves an enhancement of the stability of the interactions between the light-harvesting pigments and the photosystem reaction centers. As temperature is increased, the heat-induced dissociation of these complexes results in a decrease in the quantum yield of electron transport at limiting light intensity, followed by a loss of electron transport activity at rate-saturating light intensity. The decreased quantum yield can be attributed to a block of excitation energy transfer from chlorophyll b to chlorophyll a, and changes in the distribution of the excitation energy between photosystems II and 1. The block of excitation energy transfer is characterized by a loss of the effectiveness of 480 nm light (absorbed primarily by chlorophyll b) to drive protochemical processes, as well as fluorescence emission by chlorophyll b. Larrea divaricata Cav., a predominant desert shrub of the southwestern United States, maintains photosynthetic activity throughout the year, tolerating the great extremes of temperature which occur in warm deserts (4, 13, 14, 17, 18, 23). Field studies of the photosynthetic activity of L. divaricata Cav. performed in Death Valley, California demonstrated that shifts of the photosynthetic thermal optimum are correlated with seasonal temperature variations (14). When plants were grown in the laboratory under controlled environmental conditions, the ability of L. divaricata to acclimate to a wide range (20/15 to 45/33 C.day/night) of growth temperatures was confirmed ( 14). In addition, these studies demonstrated that the differential heat stability of photosynthetic activity of plants grown under different temperature regimes was attributed to changes at the chloroplast level (14). Bjorkman and co-workers (5, 6) and Pearcy and co-workers (19) have found that photosynthesizing leaves of heat-adapted desert plants tolerated 10 C higher temperature without suffering damage to photosynthetic functions than did leaves of coldadapted coastal plants. The irreversible decrease in the quantum yield of photosynthesis by intact leaves, resulting from exposure ' This work was supported by National Science Foundation Energyrelated Postdoctoral Fellowship SM176-17963 to P. A. A. and National Science Foundation Grant DEB 75-020640 to 0. B. C.I.W.-D.P.B. Publication No. 608. 2 Present address: Biophysical Laboratory of the State University, Wassenaarseweg 78, Leiden, The Netherlands. of these leaves to high temperature, in each species was accompanied by a closely matching inhibition of the quantum yield of PSII-driven electron transport and an uncoupling of photophosphorylation in chloroplasts isolated from similarly treated leaves. Schreiber and Berry (21) showed that these inhibitory effects were coincidental with increases in Chl a fluorescence. These workers suggested that the observed inhibition of photosynthesis by heat stress is closely related to a perturbation of the thylakoid membranes, affecting both associated enzymes and the pigment system. These results, indicating that thylakoid membrane reactions (especially PSII activity and photophosphorylation) are some of the most heat-sensitive components in the intact leaf are in general accordance with the results of other studies in which isolated chloroplasts were subjected to heating (8, 12, 15, 16, 20). However, such comparisons must be made with some caution since heat damage of isolated chloroplasts generally occurs at considerably lower temperatures than is the case with intact leaves. To determine what changes in the photosynthetic apparatus of L. divaricata occur during acclimation, as well as to investigate the effect of heat damage on the light reactions, an analysis of photosynthetic electron transport and Chl fluorescence properties of chloroplasts isolated from L. divaricata plants grown under different temperature regimes was performed. It will be demonstrated that a substantial part of the heat-induced damage to the photosynthetic apparatus is at the pigment system level, and that the process of acclimation results in an increased stability among the pigment-protein complexes in the chloroplast membrane. MATERIALS AND METHODS L. divaricata plants were grown at day/night temperatures 20/15, 32/20, or 45/32 C. Chloroplasts were isolated by grinding chilled, washed leaves in a solution of 0.4 M sorbitol, 0.1 M NaTricine (pH 7.8), 0.05 M Na-ascorbate (pH 7.8), 5 mM MgCl2, and 2.5 mg/ml BSA. The brei was filtered through two layers of Miracloth and then loaded on a two-step (0.8 M and 1.6 M) sucrose gradient containing 25 mm Na-ascorbate. Following centrifugation at 20,000g for 30 min, the chloroplasts were removed from the gradient interface, washed once with the isolation medium, and resuspended in 0.4 M sorbitol, 20 mm Na-Tricine (pH 7.8), 10 mM Na-ascorbate (pH 7.8), 5 mM MgCl2, and 5 mg/ml BSA. Chl concentration was determined according to the procedure of Arnon (2). Whole chain electron transport (H20 -methyl viologen) was monitored in 4 ml of a reaction medium containing: 1 mm methyl viologen, 1 mm sodium azide, 6 mM NH4Cl, 4 x 10-2 M sodium phosphate (pH 6.7), 5 mM MgCl2, chloroplasts containing 20 ytg of Chl. The oxidation of the methyl viologen was monitored with a polarographic 02 electrode (Rank Bros., Bottisham, England). The reaction medium was allowed to equilibrate at temperatures from 15 to 50 C prior to the introduction of the chloroplasts. The 411 www.plantphysiol.org on October 1, 2017 Published by Downloaded from Copyright © 1978 American Society of Plant Biologists. All rights reserved. ARMOND, SCHREIBER, AND BJORKMAN chloroplast aliquots (5-10 Al, depending upon the Chl concentration of the stock suspension) were injected into the reaction vessel and were temperature-equilibrated for 15 sec prior to onset of illumination. In order to facilitate comparison of thermal stability, all of the rates expressed in the figures are those measured during the first 15 sec of illumination. The apparatus for measuring the heat-induced fluorescence changes and the fluorescence induction kinetics has been previously described (21). The changes in the Fo level of Chl fluorescence which occurred upon slow heating (approximately I C/min) of the sample were monitored with an excitation light of extremely low intensity which by itself did not cause any changes in the Chl fluorescence yield. The Chl fluorescence induction curves were measured in the presence of 3 mM NH20H and 20 /IM DCMU (3, 7) and were recorded on either a strip chart recorder or a storage oscilloscope. Chl fluorescence emission spectra at 77 K were measured in a Perkin-Elmer fluorescence spectrophotometer MPF-3L. RESULTS AND DISCUSSION The temperature dependence ofthe rate of light-saturated whole chain electron transport (H20 --*methyl viologen; Fig. 1) is similar to the temperature dependence of net photosynthesis measured under saturating CO2 conditions described previously (14). The differences in the thermal optima for chloroplast samples isolated from plants, grown under different temperature regimes, are less distinct than those observed for net CO2 fixation under saturating CO2 conditions, and are shifted to somewhat higher temperatures. The threshold temperature at which electron transport becomes inhibited appears to be a function of the growth temperature and is similar (less than 5 C difference) to the temperatures at which C02-saturated photosynthesis becomes inhibited (14). PSI activity (Fig. 2) in contrast, remained stable for the temperature range indicated in the figure. The site of inhibition of whole chain electron transport under saturating light conditions must therefore be prior to the site of donation by reduced dichloroindophenol (DCIPH2). The maximal rates obtained for light-saturated electron transport in the chloroplast samples isolated from L. divaricata, grown at the indicated temperatures, are somewhat higher than the rates of C02-saturated photosynthesis would predict, when both are expressed on a Chl basis. The methyl viologen reduction is, however, uncoupled from photophosphorylation and reflects only