Dc. Percival et al., GAS-EXCHANGE, STEM WATER POTENTIAL AND LEAF ORIENTATION OF RUBUS-IDAEUS L. ARE INFLUENCED BY DROUGHT STRESS, Journal of horticultural science & biotechnology, 73(6), 1998, pp. 831-840
Rubus idaeus L. cv. Heritage raspberry plants were placed in controlle
d environment chambers (25 degrees C, 14 h photoperiod, 2.0 kPa satura
ted vapour pressure deficit, CO2 concentration of 380 mu l l(-1)) to s
tudy the effects of drought stress on leaf gas exchange and stem water
potential. Whole-plant photosynthesis (Pn) and transpiration were sen
sitive to drought stress and gradually decreased from the second day o
f the study until rehydration. Stomatal aperture regulation occurred d
uring the initial 48 h of the study with transpiration rates dropping
in response to a decrease in stem water potential. Spatial differences
in response to drought stress were found with values of leaf Pn, and
water vapour and CO2 conductance in the younger, distal (i.e. closer t
o the apex) leaves decreasing at a faster rate than the older, proxima
l leaves (i.e. close to crown). Evidence of increased mesophyll resist
ance to drought stress was apparent with intercellular CO2 concentrati
on (c(i)) remaining either constant or increasing, while Pn and carbox
ylation efficiency simultaneously decreased. Therefore, an optimum bal
ance between water loss and uptake of c(i) existed, and an alteration
in these rates represented an adjustment in stomatal conductance to ma
tch the intrinsic photosynthetic capacity rather than a causal relatio
n. Protection of the underlying photochemistry was evident with parahe
lionastic leaf movements which resulted in a reduction in the effectiv
e leaf area and associated heat loads. Despite stem water potential of
the stressed plants returning to control levels after re-watering, Pn
of the stressed plants never attained control values, presumably as a
result of damage to photochemistry.