High vapour pressure deficit results in a rapid decline of leaf water potential and photosynthesis of carrots grown on free-draining, sandy soils

Two carrot (Daucus carota L.) genotypes (Nantes and Imperator) were grown in the field on a coarse-textured, sandy soil. Experiments were conducted over 2 consecutive seasons, one providing cool growing conditions and the other much warmer growing conditions during which the vapour pressure deficit was up to 2-fold higher than in the first season. Changes in growth, soil water content, and environmental conditions were monitored for both seasons, and diurnal measurements of leaf water potential and leaf photosynthesis were taken near maturity. Frequent irrigation maintained bulk soil water content above, or near, field capacity, with the sum of rainfall and irrigation exceeding potential evaporation by 1.4- and 1.3-fold during the cool and warm seasons, respectively. Even under such well-watered conditions, a large diurnal variation in leaf water potential (1200 and 1800 kPa for the cool- and warm-season crops, respectively) was recorded. During the cool season, withholding irrigation for up to 60 h resulted in further reductions in midday leaf water potential. However, there was no effect of withholding irrigation on leaf water potential during the warm season. During both seasons, leaf photosynthetic rate of well-watered plants peaked at around 20 mu mol/m(2).s in the early morning (0900 hours) and then decreased throughout the day, with the magnitude of the decline associated with the prevailing vapour pressure deficit. Under well-watered conditions, leaf water potential and photosynthesis were both negatively correlated with vapour pressure deficit, for both genotypes. Leaf water potential and photosynthesis were positively correlated with each other and we conclude that a high hydraulic resistance in the plant or soil results in a vapour pressure deficit-induced reduction in leaf water potential, which in turn reduces the rate of leaf photosynthesis.