Interactive Effects of Photosynthetic Photon Flux Density and Drought on Growth and Physiological Responses of Barley Seedlings

Environmental factors, such as light and drought, affect plant growth and functioning both independently and through their mutual interactions. The aim of this study was to evaluate the effects of three levels of photosynthetic photon flux density (PPFD: 400, 800, and 1200 μmol·m−2·s−1) on the growth and physiological responses of spring barley (Hordeum vulgare L.) seedlings of the Ella and Kucyk cultivars grown under control conditions or PEG 6000-induced osmotic stress (−0.5 MPa), applied to simulate water-deficit conditions. After 28 days of the experiment, the fresh and dry mass of shoots and roots, SPAD index, electrolyte leakage (EL), chlorophyll fluorescence parameters, and delayed chlorophyll luminescence parameters were determined. PEG-induced osmotic stress significantly reduced shoot and root fresh mass in both cultivars, whereas its effect on dry mass accumulation was less pronounced. Greater shoot and root dry mass was observed at higher PPFD levels under both the control and stress conditions. Most measured traits varied among PPFD treatments, whereas osmotic stress primarily affected biomass-related parameters. Significant PPFD × drought interactions were detected for root fresh mass and Ft/F0 in cv. Ella, and for root fresh mass, root dry mass, SPAD, EL, Y(II), and Ft/F0 in cv. Kucyk. SPAD values were generally higher under stress conditions in cv. Kucyk, while remaining relatively stable in cv. Ella. Most chlorophyll fluorescence parameters, including Fv/Fm, Y(II), and Rfd, remained relatively stable across treatments. Ft/F0 and the NDS parameter of delayed chlorophyll luminescence showed the greatest variation among the applied PPFD treatments. Overall, treatments characterized by higher PPFD were associated with greater biomass accumulation, while physiological performance remained largely stable under PEG-induced osmotic stress. However, the magnitude and nature of the responses depended on both cultivar and trait.