干旱区生态环境知识资源中心

Spinach (Spinacia oleracea) and jojoba (Simmondsia chinensis), two plants adapted to contrasting domains of temperature, were compared for their short term reactions to heat shock. The limit temperatures acceptable for efficient photosynthesis determined by the oxygen evolution of isolated plastids after a 5 min-long heat shock: 47-degrees-C for spinach, and 52-degrees-C for jojoba, were further used for the heat-shock treatment in leaves or isolated plastid fractions. In heat-treated leaves of both plants the amounts of lipids and their specific radioactivities increased during the first 5 min, then decreased with time below the control value (25-degrees-C) in spinach while they remained higher than the control in jojoba. After a 5 min-heat treatment the amount of carotenoids decreased in spinach while beta-carotene increased in jojoba. During a 1 h-heat treatment a gradual labelling of violaxanthin and neoxanthin from beta-carotene occurred in jojoba, while spinach leaves accumulated label only in beta-carotene and lutein. These results suggest that, under a 5 min-long heat stress, a rapid modification of the lipid composition, specially marked by an increase in the monogalactosyldiacylglycerol (MGDG)/digalactosyldiacylglycerol (DGDG) ratio, and a quick labelling of beta-carotene in plastids, occurred in both types of plants. This might correspond to a trigger mechanism for a first structural and functional protection of the reaction centres of photosystems. After a 60 min-long heat treatment, the synthesis of carotenoids by the beta-pathway was stimulated and the efficient structural proportions of MGDG and DGDG were maintained in jojoba plastids, while in spinach the degradation of membrane components occurred.