Differences in physiological response of barley seedlings cultivated under low (LI, ~65 μmol m -2 s -1 ) and elevated irradiation (EI, ~450 μmol m -2 s -1 ) to upcoming drought were evaluated. After ten days of cultivation, drought stress was induced under LI by withholding water and was defined as: mild stress (MS), severe stress (SS), extreme stress (ES) and control (C, well watered). Decreased relative water content (RWC) in both LI and EI grown plants was associated with increased lipid peroxidation (TBARS) and electrolyte leakage (%EL) as well as with decreased total chlorophylls content at SS and ES. Antioxidative response to drought was, generally, indicated by higher accumulation of free proline, increased activities of catalase (CAT), guaiacol peroxidase (GPOD) and ascorbate peroxidase (APX). Even the majority of estimated physiological and biochemical parameters showed no differences between investigated light regimes before drought stress induction, the LI grown plants responded on drought by adjustment of the photosynthetic apparatus to prevent photoinhibitory oxidative damage. Photosynthesis downregulation in EI grown plants under ES was revealed due to reduced values of maximum quantum yield of photosystem II (Fv/Fm) and performance index (PI ABS ) showing that acclimatization to EI conditions lowered their tolerance to the following drought stress although it was applied under low irradiation (LI). Therefore, those plants had reduced capability to cope with the challenge of upcoming drought stress showing more pronounced cellular oxidative damage (SS and ES), less efficient and almost dysfunctional photosynthetic apparatus in extreme drought (ES).
Results of the studied photosynthetic efficiency parameters of wheat cultivars were also the good predictor for important agronomic traits, especially, when they were detected in the early stage of growth.
SUMMARYThe aim of this research was to investigate the variability of photosynthetic performance index (PI ABS ) and leaf temperature values measured in V6 development phase on 13 sunflower hybrids, grown in stressful conditions. The pot trial was made up of two treatments, one (T1) with 60% Field Water Capacity (FWC), and the other one (T2) with 80% FWC. Significant differences between T1 and T2 treatments were established for both of these parameters which prove their dependence on the water content in the soil, while the influence of hybrid was evident only in the case of PI ABS . Although in T1, as opposed to T2, all sunflower hybrids reacted by increasing leaf temperature, reaction to stress conditions measured with PI ABS parameter was not uniform. Some of the hybrids reacted by decreasing PI ABS values, while others reacted by increasing their PI ABS values. Therefore, it can be concluded that changes in parameters were independent of each other, which was confirmed by correlation analysis. Investigated parameters are suitable for determining the existence of undesirable environmental conditions that cause stress in plants and can be used in breeding of sunflower to withstand abiotic stress conditions, i.e. in selection of stress tolerant hybrids.
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