Effects of temperature acclimation pretreatment on the ultrastructure of mesophyll cells in young grape plants (Vitis vinifera L. cv. Jingxiu) under cross-temperature stresses

Leaves from annual young grape plants (Vitis vinifera L. cv. Jingxiu) were used as experimental materials. The ultrastructural characteristics of mesophyll cells in chilling-treated plants after heat acclimation (HA) and in heat-treated plants after cold acclimation (CA) were observed and compared using transmission electron microscopy. The results showed that slight injury appeared in the ultrastructure of mesophyll cells after either HA (38 degrees C for 10 h) or CA (8 degrees C for 2.5 d), but the tolerance to subsequent extreme temperature stress was remarkably improved by HA or CA pretreatment. The increases in membrane permeability and malondialdehyde concentration under chilling (0 degrees C) or heat (45 degrees C) stress were markedly inhibited by HA or CA pretreatment. The mesophyll cells of plants not pretreated with HA were markedly damaged following chilling stress. The chloroplasts appeared irregular in shape, the arrangement of the stroma lamellae was disordered, and no starch granules were present. The cristae of the mitochondria were disrupted and became empty. The nucleus became irregular in shape and the nuclear membrane was digested. In contrast, the mesophyll cells of HA-pretreated plants maintained an intact ultrastructure under chilling stress. The mesophyll cells: of control plants were also severely damaged under heat stress. The chloroplast became round in shape, the, stroma lamellae became swollen, and the contents of vacuoles formed clumps. In the case of mitochondria of control plants subjected to heat stress, the outer envelope was digested and the cristae were disrupted and became many small vesicles. Compared with cellular organelles in control plants, those in CA plant cells always maintained an integrated state during whole heat stress, except for the chloroplasts, which became round in shape after 10 h heat stress. From these data, we suggest that the stability of mesophyll cells under chilling stress can be increased by HA pretreatment. Similarly, CA pretreatment can protect chloroplasts, mitochondria, and the nucleus against subsequent heat stress; thus, the thermoresistance of grape seedlings was improved. The results obtained in the present study are the first, to our knowledge, to offered cytological evidence of cross-adaptation to temperature stresses in grape plants.