PHOTOSYNTHETIC RESPONSES OF BEANS TO WATER STRESS IN THE FIELD Pastenes C, Porter VH, Baginsky C. Facultad de Ciencias Agrarias y Forestales, Universidad de Chile. Casilla 1004 Santiago. Chile. Horton P. Robert Hill Institute, University of Sheffield, Sheffield S 10 2TN, UK. Key words : drought stress, environmental stress, fluorescence, gas exchange, photoinhibition 1. Introduction Under water stress plants decrease their growth rate, first because of a decrease in their assimilatory surface and, second, because the photosynthetic rate is inhibited (Lawlor and Uprety, 1993). CO2 reaches the carboxilation sites through stomata, the same than water lost, therefore as an attempt to save water under drought conditions, the stomatal closure affects photosynthesis by limiting CO2 availability (Mansfield 1990). Also, water stress restricts photosynthesis through "non-stomata!" effects, among which PSII and ATP synthase inhibition in thylakoids are thought to be important (Boyer, 1977 ; Boyer and Younis, 1983). Non-stomatal effects are believed to occur mainly because CO2 shortages would induce an over-excitation of PSII complexes, which in time, and depending on the capacity for absorbed energy dissipation as heat, would induce inhibition of photosynthesis. This water induced inhibition of PSII activity is light dependent, and therefore recognised as photoinhibition. In the present study, the effect of water stress on photosynthesis has been assessed in two bean varieties in the field in summer: Orfeo Inia (OI) and Arroz Tuscola (AT), the former believed to be resistant and the later to be non-resistant to water stress. Water stress was imposed by withholding irrigation, while control plants were watered once a day by means of drip irrigation. Measurements of fluorescence, gas exchange, leaf temperature and angle, and Fv/Fm recovery were carried out from 8 AM to 18 PM. 2. Results and Discussion Water potential for control plants was -0.5 MPa and for stressed plants -1.3 MPa in four leaves old plants measured at midday. Such leaf water potential for stressed plants was achieved by withholding water for 10 days. Maximum CO2 assimilation was observed in the morning, in control and stressed plants (Fig. I) decreasing through the afternoon, however in stressed plants such a decrease in gas exchange occurred faster than control. Control plants achieved not only a maximum rate, from IO AM to midday, but were able to maintain a high CO2 assimilation, in the two varieties, for a longer period of time. In 2561 G. Garab (ed.), Photosynthesis: Mechanisms and Effects, Vol. IY, 2561-2564. © 1998 Kluwer Academic Publishers.