Agriculture Reference
In-Depth Information
be used as screening parameters to develop drought resistant varieties are
discussed in the following subsections.
9.3.1 STOMATAL CONDUCTANCE AND PHOTOSYNTHESIS
One of the basic mechanisms for reducing the impact of drought is early
stomatal closure at the beginning of the period of water deficit. Stomatal
closure not only reduces water loss, but also reduces the gas exchange
between the plant and the ambient air. The reduced CO
2
intake then results
in reduced photosynthesis (Chaves et al., 2002). As plant water potential
falls due to water deficit, the sensitivity of stomatal conductance and pho-
tosynthesis rate reduced. Water deficit causes reduction in photosynthesis
mainly due to decreased stomatal conductance. Stomatal closure has been
reported in tomato at leaf water potential (Ψ
leaf
) between −0.7 to 0.9 MPa
(Duniway, 1971), in pepper −0.58 to −0.88 MPa (Srinivasa Rao and Bhatt,
1988), however eggplant can withstand a greater drought than the most
other vegetables. Srinivasa Rao and Bhatt (1990) observed that drop in
photosynthesis of eggplant with decreasing Ψ
leaf
was less than tomato and
capsicum. Bahadur et al. (2009) observed significant reduction in pho-
tosynthesis rate and stomatal conductance in spring-summer okra when
water stress was imposed for 10 or 12 days. Stomatal conductance is the
major limitation to photosynthesis under drought conditions in cowpea;
however, a pronounced nonstomatal limitation can occur under severe
drought stressed conditions that may also lead to impairment of photosyn-
thetic activity (Singh and Reddy, 2011). If perpetual decline in photosyn-
thesis is more than the transpiration, then nonstomatal factors contribute
more to the reduction of photosynthesis than stomatal effects. This is be-
cause stomatal resistance accounts for a smaller portion of total resistance
in CO
2
pathway. During this nonstomatal control of photosynthesis inter-
cellular resistance for CO
2
from the intercellular space of the chloroplasts
plays an important role. Thus, a decrease of the photosynthesis rate under
water deficit condition can be attributed to both stomatal and nonstomatal
limitations. Non-stomatal photosynthesis limitation has been attributed to
the reduced carboxylation efficiency, ribulose-1, 5-bisphospate (RuBP)
regeneration, amount of functional Rubisco, or to the inhibited functional
activity of PSII. Flexas et al. (2002) have shown that drought induced
changes in many photosynthetic parameter are more related to variations
