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 µmol/m2.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.