Effect of irrigation on the relationships between leaf gas exchange related traits and yield in dwarf dry bean grown under Mediterranean conditions

► Higher irrigation regimes resulted in higher LAI and more photosynthetically active leaves. ► Harvest index and leaf-level WUE (A/E) were higher at the higher irrigation regimes. ► Early flowering cultivar showed higher yield than the late flowering cultivar. ► No difference was found at the physiological traits between the two cultivars. ► Tleaf−Tair can be used as a threshold indicator of water stress conditions in bean. Drought is a major yield constraint of crop productivity. Determining physiological traits involved in the response of common bean to drought stress could lead to improving drought tolerance. A two-year study (2008 and 2009) was carried out to determine the physiological traits that are associated with drought tolerance for maximum dry bean seed yield (SY), and water use efficiency (WUE) in northern Greece. Two cultivars (Borlotto and Green crop) were evaluated at five water levels based on crop evapotraspiration (ETm) (100% –I100, 75% –I75, 50% –I50, 25% –I25 and 0% –I0). Under higher irrigation regimes (I75, I100), higher seed yield (SY) was found due obviously to a denser canopy (higher LAI), with greener (higher SPAD), and more photosynthetically active leaves (higher net assimilation rate-A, transpiration rate-E and stomatal conductance-gs) compared with lower irrigation rates. In addition, under higher irrigation regimes (I75, I100), a higher partitioning of dry matter to seeds (higher HI) was evident. Instantaneous water use efficiency (A/E) increased with increasing irrigation levels. On contrary to negative correlations in C3 species, carbon isotope discrimination (Δ) was positively correlated to A/E doubting its reliability as a long-term assessment of water use efficiency in bean. However, Δ was strongly correlated with SY indicating that higher water availability contributed to higher yields. The early flowering cultivar (Borlotto) had higher SY compared with the late flowering one (Green crop), however, no significant difference was found at the physiological traits between the two cultivars. Leaf-to-air temperature difference (ΔT) showed to be a good threshold indicator of non-stress water conditions in bean during the post-filling period since a value higher than 2.95°C indicated a severe water stress.