Endogenous abscisic acid levels and induced heat shock proteins were measured in tissue exposed for 6 hours to temperatures that reduced their subsequent chilling sensitivity. Onecentimeter discs excised from fully expanded cotyledons of 11-day-old seedlings of cucumber (Cucumis sativus L., cv Poinsett 76) were exposed to 12.5 or 370C for 6 hours followed by 4 days at 2.5 or 12.50C. Ion leakage, a qualitative indicator of chilling injury, increased after 2 to 3 day exposure to 2.50C, but not to 12.50C, a nonchilling temperature. Exposure to 370C before chilling significantly reduced the rate of ion leakage by about 60% compared to tissue exposed to 12.50C before chilling, but slightly increased leakage compared to tissue exposed to 12.5 to chilling significantly reduced the chilling-induced increase in ion leakage, possibly by reducing the effects of chillinginduced water stress or the direct effect of low temperature (20). Rikin et al. (19) showed that exogenous application of ABA and pretreatments with physiological stresses, which increased the endogenous level of ABA, increased the tolerance of tissue to subsequent chilling. Stressful temperatures can alter the internal water status and thereby ABA production (25), or they can directly affect ABA levels (4). However, this conclusion was questioned by Eamus and Wilson (6) who showed that increased levels of endogenous ABA did not accumulate if leaves of Phaseolus vulgaris were chilled in a water-saturated atmosphere.Thermal stress also induces the synthesis of specific HSPs4 in a wide range of plants (1,3,10), and reduces the synthesis of some normally expressed proteins (1). The physiological basis by which these proteins confer thermal tolerance is still unclear, although their synthesis and accumulation appears to provide thermal tolerance, and their selective localization appears to be linked to the expression of this tolerance (10). Lin et al. (13) found that the synthesis of HSPs prevented heat shock-induced leakage from cell. However, the possible relationship between the induction of HSPs and the effect of temperature conditioning on the susceptibility of the tissue to chilling injury was not investigated. HSPs can also be synthesized in response to water stress and ABA (8, 10).We undertook to study the effect of prior temperature exposure on the susceptibility of cucumber plants to chilling injury and to determine the physiological basis for the observed changes. Since ABA may have a beneficial effect on reducing chilling injury, and ABA, water stress, and thermal stress elicit HSPs which reduce heat shock-induced ion leakage, we evaluated changes in ABA levels and induction of HSPs after exposure to temperatures in a water saturated atmosphere. This was done to determine the possible role of endogenous ABA and HSPs in increasing the chilling tolerance to plants previously subjected to various temperatures.The connection between ABA levels and HSPs was also studied.

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Water relations and growth of maritime pine (Pinus pinaster Ait.) were investigated in 2-year-old seedlings of French ('Landes'), Iberian ('Iberian') and Moroccan ('Tamjoute') origin raised for 67 days in a flowing solution culture system containing 0, 50, 150 or 250 mM NaCl. Height growth, and stem, needle and root dry matter were reduced by salinity with minor differences among geographic origins. Predawn needle water potential was decreased by salinity and corresponded approximately to the osmotic potential of the nutrient solution. Stomatal conductance was reduced according to the amount of salinity applied. Whole-plant hydraulic conductance was also reduced, even when expressed on a root dry weight basis. The osmotic potential of xylem sap was five- to sixfold lower than that of the nutrient solution. Seedlings of the most southerly origin (Tamjoute) exhibited a greater ability to decrease osmotic potential under saline conditions than seedlings of more northerly origin (Landes and Iberian) as a result of higher mineral cation transport to the shoot.

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