Low irradiance modifies the effect of water stress on survival and growth-related parameters during the early developmental stages of buckwheat (Fagopyrum esculentum)

In order to characterize physiological modifications encountered by buckwheat plants exposed to both drought and low-light stresses, seedlings (cv. La Harpe) were exposed under controlled environmental conditions, to a progressive decline in soil volumetric water content under two light regimes: low irradiance (80 µmol m⁻² s⁻¹) or moderate irradiance (160 µmol m⁻² s⁻¹). Phenological evolution of the whole plant until the macroscopic appearance of the reproductive structure and physiological properties of leaves in relation to their position on the main axis were quantified. Water stress reduced net assimilation rate (NAR) before specific leaf area (SLA) and induced a decrease in stomatal conductance (g_l) and carbon isotope discrimination (Δ). Water consumption by stressed plants was similar under both light treatments. Water-stressed plants under moderate irradiance exhibited higher growth, NAR, osmotic adjustment, and lower SLA than plants maintained under low irradiance. However, the former died after 27 days of treatment while the latter still remained alive until the experiment was discontinued (40 days). We concluded that the physiological strategy adopted by the water-stressed plants maintained under moderate irradiance did not afford a long-term advantage in terms of survival. The effects of a combination of low-light and water stress on chlorophyll concentration and carbon isotope discrimination (Δ) are discussed in relation to growth parameters.