Photoinhibition in plants depends on the extent of light energy being absorbed in excess of what can be used in photochemistry and is expected to increase as environmental constraints limit CO2 assimilation. Water stress induces the closure of stomata, limiting carbon availability at the carboxylation sites in the chloroplasts and, therefore, resulting in an excessive excitation of the photosynthetic apparatus, particularly photosystem II (PSII). Mechanisms have evolved in plants in order to protect against photoinhibition, such as non-photochemical energy dissipation, chlorophyll concentration changes, chloroplast movements, increases in the capacity for scavenging the active oxygen species, and leaf movement or paraheliotropism, avoiding direct exposure to sun. In beans (Phaseolus vulgaris L.), paraheliotropism seems to be an important feature of the plant to avoid photoinhibition. The extent of the leaf movement is increased as the water potential drops, reducing light interception and maintaining a high proportion of open PSII reaction centres. Photoinhibition in water-stressed beans, measured as the capacity to recover F(v)/F(m), is not higher than in well-watered plants and leaf temperature is maintained below the ambient, despite the closure of stomata. Bean leaves restrained from moving, increase leaf temperature and reduce qP, the content of D1 protein and the capacity to recover F(v)/F(m) after dark adaptation, the extent of such changes being higher in water-stressed plants. Data are presented suggesting that even though protective under water stress, paraheliotropism, by reducing light interception, affects the capacity to maintain high CO2 assimilation rates throughout the day in well-watered plants.

The yield of 24 commercial varieties and accessions of common bean (Phaseolus vulgaris) has been determined at different sites in Chile and Bolivia. Statistical analysis was performed in order to characterize whether a particular variety was more or less stable in yield under different environmental conditions. Amongst these, two varieties have been identified for more detailed study: one variety has a higher than average yield under unstressed conditions but is strongly affected by stress, and another has a reduced yield under unstressed conditions but is less affected by stress. The contrasting rate of abscission of the reproductive organs under drought stress was clearly consistent with these differences. The more tolerant genotype shows a great deal of plasticity at the biochemical and cellular level when exposed to drought stress, in terms of stomatal conductance, photosynthetic rate, abscisic acid synthesis, and resistance to photoinhibition. By contrast, the former lacks such plasticity, but shows an enhanced tendency for a morphological response, the movement of leaves, which appears to be its principal response to drought stress.

Changes in phenolic composition were determined in cv. Syrah-vines grape skins during ripening for two contrasting yields, resulting from cluster thinning at veraison. Treatments consisted on 16 and eight clusters per plant leading to approximately 8 ton/ha (T1) and 4 ton/ha (T2). In the grape skins of the samples analysed, 11 different low molecular weight phenolic compounds were identified, as well as 15 anthocyanins. Cluster thinning had a minimal effect on ripening time and weight of grape skins, however, clusters from low yield plants resulted in a lower total acidity and slightly higher pH. As for the phenylpropanoid pathway, the flavan-3-ol (+)-catechin, and the flavanols isorhamnetin-3- O-galactoside and isorhamnetin-3- O-glucoside, resulted in a higher concentration in berry skins from low yield plants. It is concluded that cluster thinning may result, from the oenological point of view, in an increased grape quality especially in compounds related to wine colour.

We studied the effect of increasing temperature on photosynthesis i n two bean (Phaseolus vulgaris L.) varieties known to differ in their resistance to extreme high temperatures, Blue Lake (BL), commercially available i n the United Kingdom, and Barbucho (BA), noncommercially bred in Chile. We paid particular attention to the energy-transducing mechanisms and structural responses inferred from fluorescence kinetics. The study was conducted in nonphotorespiratory conditions. lncreases in temperature resulted in changes i n the fluorescence parameters nonphotochemical quenching (qN) and photochemical quenching (qP) in both varieties, but to a different extent. I n BL and BA the increase i n qP and the decrease in qN were either completed at 30°C or slightly changed following increases from 30 to 35°C. No indication of photoinhibition was detected at any temperature, and the ratio of the quantum efficiencies of photosystem I1 (PSII) and O , evolution remained constant from 20 to 35°C. Measurements of 77-K fluorescence showed an increase in the photosystem I (PSI)/PSII ratio with temperature, suggesting an increase in the state transitions. I n addition, measurements of fast-induction fluorescence revealed that the proportion of PSII, centers increased with increasing temperatures. The extent of both changes were maximum at 30 to 35"C, coinciding with the ratio of rates at temperatures differing by 10°C for oxygen evolution.High temperature affects the photosynthetic functions of plants by its effect on the rate of chemical reactions and on structural organization. It has been previously reported that high temperatures are responsible for changes in the thylakoid membrane, altering not only its physicochemical properties, but also its functional organization (Berry and Bjorkman, 1980). PSII, particularly, is the most sensitive component of the photosynthetic system (Berry and Bjorkman, 1980;Mamedov et al., 1993). Extreme high temperatures affect the functioning of the O,-evolving system (Yamashita and Butler, 1968), resulting in the release of functional manganese ions from the complex (Nash et al., 1985). This release may be the result of reductions by peroxides or superoxides (Thomson et al., 1989). PSII also * Supported by a scholarship received by C.P. from the Ministry responds to the range of temperatures below those causing inhibition or destruction of the complex, with consequences for thylakoid organization and functioning. Separation of the LHCII from the core center induces destacking of the grana (Gounaris et al., 1984) and temperatureinduced migration of the reaction center (PSII,) or LHCII (state transition) to the nonappressed region, which would have consequences for the energy redistribution between PSI and PSII.Most of the information available on the effect of high temperature on photosynthesis, however, is either concerned with long-term responses by which plants are able to modify their photosynthetic functions, increasing both their tolerance and thermal optimum for net CO, assimilation, or wit...

The effect of high temperatures on CO, assimilation, metabolite content, and capacity for reducing power production in nonphotorespiratory conditions has been assessed in two different bean (Phaseolus vulgarus 1.) varieties, Blue Lake (commercially available i n the United Kingdom) and Barbucho (a noncommercially bred Chilean variety), which are known to differ in their resistance to extreme high temperatures. Barbucho maintains its photosynthetic functions for a longer period of time under extreme heat compared with Blue Lake. The CO, assimilation rate was increased by increases in temperature, with a decrease in ratio of rates at temperatures differing by 10°C. It is suggested that limitations to CO, assimilation are caused by metabolic restrictions that can be differentiated between those occurring in the range of 20 to 30°C and 30 to 35°C. It is likely that changes i n the capacity for Calvin cycle regeneration and starch synthesis affect photosynthesis in the range of 20 to 30°C. But following an increase in temperature from 30 to 35"C, the supply of reducing power becomes limiting. From analysis of adenylate concentration, transthylakoid energization, and, indirectly, NADPH/NADP+ ratio, it was concluded that the limitation in the assimilatory power was due to an oxidation of the NADPH/ NADP+ pool. In the range of 30 to 35"C, the photosystem I quantum yield increased and photosystem II maintained its value. We conclude that the reorganization of thylakoids observed at 30 to 35°C increased the excitation of photosystem I, inducing an increase in cyclic electron transport and a decrease in the supply of NADPH, limiting carbon assimilation.High temperatures affect photosynthesis by altering the excitation energy distribution by changing the structure of thylakoids (Berry and Bjorkman, 1980;Weis and Berry, 1988) and by changing the activity of the Calvin cycle and other metabolic processes such as photorespiration and product synthesis. Much of the attention has been focused on the former aspect, since thylakoids are highly sensitive to heat, whereas the restrictions imposed by high temperatures on carbon metabolism have been nearly exclusively interpreted as the effect on CO, ' Supported by a scholarship to C.P. from the Ministry of Planning of the Chilean Government.Present address: Facultad de Ciencias Agrarias y Forestales, Departamento de Producción Agrícola, Universidad de Chile, Casilla 1004, Santiago, Chile.* Corresponding author; e-mail cpasteneQabello.dic.uchi1e.cl; fax 562-678-5700.availability. The diffusion of CO, and O, and the affinity for carboxylation of the Rubisco enzyme have been proven to be affected by increasing temperatures (Hall and Keys, 1983;Jordan and Ogren, 1984; Brooks and Farquhar, 1985), but it has been argued that the stimulation of photorespiration cannot fully explain the effect of high temperature. It has been suggested that a restricted electron transport may limit RuBP supply (Berry and Bjorkman, 1980).It also has to be considered that changes in temperature induce variati...