Water deficit index to evaluate water stress status and drought tolerance of rainfed barley genotypes in cold semi-arid area of Iran

Drought and heat are major stresses that adversely affect crop production in arid and semi-arid regions. This study assessed the water stress status of rainfed barley genotypes and identified their critical drought stress threshold in the cold semi-arid area of Iran. The field experiments were conducted at the Dryland Agricultural Research Institute, Maragheh, Iran in 2015–16 and 2017–18. The experimental design was a split-plot arrangement in a randomized complete block design with three replications. The main plots included two water regimes, rainfed (stress) and supplemental irrigation, and the subplots included 15 barley genotypes. Data were collected for grain yield, and its components, surface temperature (Ts) taken at six crop growth stages from flag leaf to soft dough, and normalized differences vegetative index (NDVI) at seven crop growth stages from the first node to the soft dough. The water deficit index (WDI) was determined using NDVI and the difference between Ts and air temperature (Ta). Based on the WDI, the critical water stress and temperature thresholds for genotypes were 0.59, and 24.3 °C, respectively. The identified temperature threshold was equivalent to 7.2 mm day⁻¹ evapotranspiration (ET0) and 4.3 kPa vapor pressure deficient. The genotypes experienced a maximum drought stress when the WDI, Ts, and ET0 reached 1.01, 34.6 °C (upper limit), and 11.6 mm day⁻¹, respectively. The critical drought threshold happened 254 days after the sowing date. WDI had strong negative correlation with both grain and biological yields. According to WDI, Ghara Arpa, Kuban-06, and Ansar genotypes were placed in the drought-tolerant group and are suitable for rainfed conditions, whereas ARM-ICB, Sahand, Sararood1, and Ste/Antares//YEA762 genotypes were more suitable for supplemental irrigation conditions. It can be concluded that WDI could effectively discriminate genotypes into tolerant and susceptible groups, and identify the starting time of water stress under rainfed conditions.