Linking plant and soil indices for water stress management in black gram

Measurement of plant and soil indices as well as their combinations are generally used for irrigation scheduling and water stress management of crops and horticulture. Rapid and accurate determination of irrigation time is one of the most important issues of sustainable water management in order to prevent plant water stress. The objectives of this study are to develop baselines and provide irrigation scheduling relationships during different stages of black gram growth, determine the critical limits of plant and soil indices, and also determine the relationships between plant physiology and soil indices. This study was conducted in a randomized complete block design at the four irrigation levels 50 (I₁), 75 (I₂), 100 (I₃ or non-stress treatment) and 125 (I₄) percent of crop's water requirement with three replications in Urmia region in Iran in order to irrigation scheduling of black gram using indices such as canopy temperature (T_c), crop water stress index (CWSI), relative water content (RWC), leaf water potential (LWP), soil water (SW) and penetration resistance (Q) of soil under one-row drip irrigation. The plant irrigation scheduling was performed by using the experimental crop water stress index (CWSI) method. The upper and lower baseline equations as well as CWSI were calculated for the three treatments of I₁, I₂ and I₃ during the plant growth period. Using the extracted baselines, the mean CWSI values for the three treatments of I₁, I₂ and I₃ were calculated to be 0.37, 0.23 and 0.15, respectively, during the growth season. Finally, using CWSI, the necessary equations were provided to determine the irrigation schedule for the four growing stages of black gram, i.e. floral induction-flowering, pod formation, seed and pod filling and physiological maturity, as (T_c - T_a)_c = 1.9498 - 0.1579(AVPD), (T_c - T_a)_c = 4.4395 - 0.1585(AVPD), (T_c - T_a)_c = 2.4676 - 0.0578(AVPD) and (T_c - T_a)_c = 5.7532 - 0.1462(AVPD), respectively. In this study, soil and crop indices, which were measured simultaneously at maximum stress time, were used as a complementary index to remove CWSI constraints. It should be noted that in Urmia, the critical difference between the canopy temperature and air temperature (T_c - T_a), soil penetration resistance (Q), soil water (SW) and relative water content (RWC) for the whole growth period of black gram were - 0.036 °C, 10.43 MPa and 0.14 cm³ cm^-3 and 0.76, respectively. Ideal point error (IPE) was also used to estimate RWC, (T_c - T_a) and LWP as well as to select the best regression model. According to the results, black gram would reduce its RWC less through reducing its transpiration and water management. Therefore, it can be used as a low-water-consuming crop. Furthermore, in light of available facilities, the farmer can use the regression equations between the obtained soil and plant indices and the critical boundaries for the irrigation scheduling of the field.