Water‐stress‐induced heat tolerance in geranium leaf tissues: A possible linkage through stress proteins?

Evidence is accumulating in favor of a linkage at the cellular level between various abiotic stresses. We conducted a study to evaluate the effect of water stress on the heat tolerance of zonal geraniums, Pelargonium×hortorum cv. Evening Glow. Water stress was imposed by withholding irrigation until pots reached 30% (by weight) of well-watered controls, and by maintaining the pot weight by additions of water for another 7 days. Leaf xylem water potential (XWP, MPa), relative water content (RWC. %), and heat-stress tolerance (HST; LT50, defined as the temperature causing half-maximal % injury based on electrolyte leakage) were measured in control, stressed, and recovered plants. Proteins were extracted from the leaves following the above treatments, and SDS-PAGE and immunoblotting were performed by using standard procedures. Immunoblots were probed with antibodies to dehydrin and 70-kDa heat shock cognate (HSC70) proteins. Data indicate that XWP and RWC, respectively, were −0.378 MPa and 92.3% for control plants and −0.804 MPa and 78.6% for stressed plants. Water-stressed plants exhibited a significant increase in HST compared to control (LT50 of 55°C vs 51°C). Water-stress-induced HST was not due to heat acclimation (leaf warming in stressed plants). Data also indicate that water-stress treatment did not increase freezing tolerance of geranium leaves. Increased HST was associated with the accumulation of several heat-stable, dehydrin proteins (25–60 kDa), and both cytosolic and ER luminal (BiP) HSC70 proteins. Leaf XWP, RWC, and HST reversed to control levels concomitant with the disappearance/reduction of dehydrins and HSC70 proteins in water-stress-relieved plants. The possibility of a cellular linkage between water stress and heat-stress tolerance is discussed.