Changes in Transpiration Rate, Leaf Diffusion Resistance and Leaf Water Potential for Satsuma Mandarin (Citrus unshiu MARC.) Trees during Prolonged Water Stress and Subsequent Recovery

Changes in transpiration rate, leaf water potential (ψ) and leaf diffusion resistance (RL) were studied in potted satsuma mandarin (Citrus unshiu MARC.) trees subjected to a drought-rewatering cycle. Results follow:1. ψ(ψmax) measured before sunrise at which a decline in relative transpiration rate (drought tree/well-watered control tree) occurred was dependent on Δe (difference in vapor pressure between ambient air and saturated atmosphere at leaf temperature). Namely, under high Δe, the reduction in relative transpiration rate took place at a higher ψmax (about -4 bars). When Δe was low, relative transpiration rate did not decline appreciable until ψmax fell to a lower value (-7 to -10 bars).2. Transpiration rate had high positive correlation with 7.15×10-7Δe/RL (r=0.984) and VPD/RL (r=0.920). The ratio, VPD/RL may be used as an estimation of transpiration rate in citrus orchards, because VPD (vapor pressure deficit) and RL can be easily measured in the field.3. ψmin measured in the daytime recovered rapidly after rewatering in time of desiccation to -24.5 bars of ψmax and exceeded more about 2 bars than that of the well-watered control tree for several hours, because recovery in RL of the drought tree was incomplete. Both RL and ψmin returned completely to the control level at the same time.