Time and amount of supplemental irrigation at different distances from tree trunks influence on morphological characteristics and physiological responses of rainfed fig trees under drought conditions

Abstract

This study was carried out to identify the effects of time and amount of supplemental irrigation (SI) at different distances from tree trunks on morphological characteristics and physiological responses of mature rainfed fig trees in Estahban County, Fars Province, Iran. The two-year experiment was conducted in a split-split plot statistical design with four replications. SI treatments consisted of 1) irrigation position treatments in a micro-catchment close to tree trunks, 1–1.1 m from tree trunks inside of canopy, and 2.1–2.2 m from tree trunks outside of canopy 2) irrigation time treatments in early spring and in mid-summer, and 3) irrigation water amount treatments of no supplemental irrigation (control), 1000 liters, and 2000 liters of irrigation water per tree. The results showed a positive effect of SI on some morphological traits of rainfed fig trees. SI increased the leaf gas exchange parameters, including net photosynthesis rate (An), transpiration rate (Tr), and stomatal conductance (gs). The highest An and gs occurred in leaf water potential (LWP) around -1 MPa and the air temperature of 20 °C. The LWP equal to −2 MPa could be considered as a threshold for stomatal closure of rainfed fig trees, which put this plant among drought-tolerant trees. The higher correlation coefficient between An and gs compared with that obtained for An and Tr implied the higher dependence of the An variation on the gs than that of Tr. The highest leaf scale water use efficiency for trees irrigated with 2000 liters of water, out of the canopy, indicated higher increase of An compared with Tr after irrigation. Applying SI with 2000 liters of water per tree (200 m3 ha−1), outside of canopy in early spring could improve some of the morphological traits and physiological responses of rainfed fig trees, under drought conditions. However, under water resources limitation, 1000 liters of irrigation water per tree (100 m3 ha−1) could be recommended for rainfed fig orchards.