Changes in Photosynthetic Oxygen Evolution and Chlorophyll Fluorescence in Some Cool Season Grasses and Zoysiagrasses (Zoysia spp.) from Autumn to Winter
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Photoinhibition under low temperature conditions is considered to be one of the major factors for the reduction of the photosynthetic activity of leaves during winter in the temperate climate zone in chilling-sensitive plants (Oguist et al.1987). Photoinhibition has been manifested by the changes in CO_2-dependent O_2 evolution rate and fluorescence intensity in intact leaves and isolated chloroplasts (Powles and Bjorkman 1982; Powles et al.1983; Somersalo and Krause 1989). Although short term inhibition and/or recovery has been observed in chilling-sensitive plants (Cleland et al.1986; Greer et al.1986; Terashima et al.1991a, b), the role of photoinhibitory reduction under natural chilling conditions in the photosynthetic capacity grasses has not been clarified yet. The means of daily maximum and minimum temperatures record in a local weather station (Chiba) were 26.8℃ and 19.4℃ in September, 1992 and they decreased to 13.1℃ and 5.3℃ in December, 1992. As a result, plants grown outdoors were subjected to chilling temperatures (below 15-10℃, Levitt 1980) during the daytime in winter. The lowest daily minimum temperature was not lower than 0℃ during the experiment and freezing stress was not considered to be the main factor determining the growth plants in the location of the present experiment. In this study, the changes in both CO_2-dependent O_2 evolution rate and leaf fluorescence at room temperature from autumn to winter were recorded to estimate the occurrence of the damage caused by photoinhibition at the time of the application of the chilling temperature stress. The CO_2-dependent O_2 evolution rate affects the photosynthetic activity of the leaf under high light intensity and also provide the photon yield at a low light intensity where the linear relationship with PFD is maintained. Chlorophyll fluorescence of the intact leaves provides information about the damage caused to PSII and consequent photochemical reactions (Krause and Weis 1984, 1991). As a comparison, warm and cool season grasses were used in this study. Zoysiagrasses are commonly used warm season grasses and are widely distributed in Japan including various species and ecotypes. Our preliminary results showed that they were sensitive to chilling temperatures (below 15-10℃) and brief exposure to chilling temperatures with moderate light intensity severely impaired the photon yield of leaves. Three cool season grasses, Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.), and tall fescue (Festuca arundinaceae Schreb.) show a good performance during cool seasons and a comparison was made to estimate the photoinhibition under chilling temperature conditions in zoysiagrasses.