Response of Stomatal Conductance to Leaf Water Potential, and Canopy Temperature in Almond Trees under Drought and Salinity Stress

Large areas of Iran are affected by salinity and drought. Due to the tolerance of almond (Prunus dulcis) to water stress, this tree is mainly grown in arid and semi-arid regions, where salinity is commonly another agricultural problem at such areas. Sensitivity of almond trees to salinity calls for special attention to the integrated effect of salinity and water stress on its water relations. This study aimed to evaluate the combined effect of salinity and drought stress on almond water status. The trial was conducted during 2014 based on randomized complete block design with three replications on a loamy sand soil at horticultural station of the Agricultural Research Center of Azarbaijan province. Treatments comprised three irrigation salinity levels viz. 2 (T₁), 4 (T₂), and 5 (T₃) dSm^-1. Leaf (T_c) and air (T_a) temperatures, air relative humidity, leaf water potential (LWP) and integrated volumetric soil water content (SWC) at three depths (0˗20, 0˗40, and 0˗70) were measured at midday (12˗14) during the growing season. Results indicated that salinity had significant effect (p<0.0001) on stomatal conductance (g_s), T_c, LWP and available water depletion (AWD). Seasonal averages of g_s for treatment T₁ to T₃ were 0.86, 0.59 and 0.44 cm.s^-1, respectively, and the corresponding LWP were ˗1.90, ˗1.93 and ˗2.16 MPa. Also, significant correlation was found between g_s, LWP, T_c and AWD. Based on the regression equations, threshold value of g_s for initiating stress was obtained to be 0.73 cm s^-1. Corresponding LWP and AWD for this g_s value were ˗1.85 MPa and 64%, respectively. Optimum T_cfor highest g_s was determined to be 28.2 ^oC. Highly significant correlation between T_c and other water status indicators showed that midday canopy temperature may be a useful tool for assessment of water status and irrigation scheduling of almond orchards.