Abstract
This study examines intraspecific variability among three sources (KSA, SD5.1 and SD6.2) of Balanites aegyptiaca in Saudi Arabia in their response to different watering frequencies and seasonal changes in vapour pressure deficit (VPD) and air-to-leaf temperature difference (Delta T) under field condition. Irrigation was done once a week, once every two weeks or once every three weeks. Traits measured include: tree height, diameter (DM), relative monthly height (RMHI) and diameter (RMDI) increments, stomatal resistance (Rs), specific leaf weight (SLW). VPD and Delta T were measured during the same time of Rs and SLW measurements. Both Rs and SLW directly responded to irrigation treatment and seasonal variation in Delta T and VPD. Interactive effects of hot weather and water stress increased leaf temperature, resulting in less Delta T and more VPD that induced higher Rs and SLW values. SD5.1 accounted for better responses under water stress, due to its higher Rs and SLW in the same time maintained better growth. DM and RMDI were more responsive to watering stress and varied among the sources. Early seedlings root-toshoot ratio was associated with better growth performance later in the field. The results highlight the role of hot weather and water stress in producing large changes in Delta T and VPD that have a major impact on Rs and SLW. In addition, there is large intraspecific variation in field growth and adaptive responses among seeds brought from different provenances.