Effects of Superimposed Temperature Stress on in vivo Chlorophyll Fluorescence of Vigna unguiculata under Saline Stress

Summary Cowpea [Vigna unguiculata (L.) Walp. var. IT 82 D-889] plants were exposed to a double stress whereby salinization (4–6 weeks at 100, 150 or 200 mM NaCl in the substrate) was the predisposing, and low and high temperatures were the additional constraints. Stress responses were assessed by measuring deviations from normal photosynthetic function by in vivo chlorophyll fluorescence. Salinization alone resulted in considerable changes in morphology, ion concentration and osmolality of the cellular sap of leaves, but had remarkably little effect on fluorescence characteristics at 20 °C. Decreasing and increasing temperatures caused marked changes in the fluorescence quenching kinetics and revealed that temperatures of 5–10 °C are suboptimal, and those around 40 °C, supraoptimal. In non-salinized plants, severe temperature stress at 3–6 °C and 45–50 °C was reflected by break-points in the temperature courses of the quenching coefficients at steady state, of Fv/FM, of (Fv)Mdark/(Fv′)Mlight, and of the heat-induced rise of Fo. Mild superimposed temperature stress enhanced the unfavourable effect of saline stress on the kinetics of qN, indicating an increased impairment of the photosynthetic process in response to the combined stress. With severe superimposed temperature stress, however, the salinized plants appeared to be less susceptible than the control plants, since the threshold temperatures for abrupt deviations of Fv/FM and (Fv)Mdark/(Fv′)Mlight, and for injury indicators (chilling symptoms, tissue freezing temperatures, Tc) shifted by 2–3K in the direction of the stressing temperature. We discuss possible mechanisms for these contrasting effects, i.e. the enhancement of disturbances at sub- and superoptimal temperatures but coadaptive adjustments at extreme stress temperatures. The results demonstrate the value of employing superimposed stressors for intensifying stress expression and for providing more information on interactions between environmental factor combinations as they exist in nature.