Agriculture Reference
In-Depth Information
in maximum daily stomatal conductance than to variations in the most
commonly used water status parameters, like leaf water potential or rela-
tive water content. The drought-tolerant species control stomatal function
to allow some carbon fixation at stress, thus improving WUE, or open
stomata rapidly when water deficit is relieved. In fact stomatal conduc-
tance can be used as an integrative parameter to reflect the severity of
water stress. It is well established that in conditions of moderate water
deficit, the photosynthetic apparatus is not damaged but continue to func-
tion, however, under severe water deficit, the photosynthetic capacity is
reduced which could be reflected in enhancement of the internal CO
2
.
Mesophyll conductance (g
m
) and biochemical limitation (bl)
l
) (often
termed as nonstomatal limitations) to photosynthesis mainly under severe
water stress has also gained importance in the recent years and their rela-
tive importance to photosynthesis limitation has been subjected to long-
standing debate. (Keenan et al., 2010) In drought stress, solute potential
(g
s
) has been shown to relate well and exhibit a specific pattern over al-
most all the important photosynthetic parameters similarly (Rouhi et al.,
2007). Earlier, Srinivasa Rao et al. (1999) also reported that cultivar Arka
Meghali has better ability to cope with water stress at various crop stag-
es exhibiting better osmotic adjustment, photosynthesis, RWC and other
physiological traits under mild and severe drought stress.
9.3.2 OSMOTIC ADJUSTMENT AND MAINTENANCE OF
CELL TURGOR
Osmotic adjustment (OA) has been considered as an important physio-
logical adaptation character associated with drought tolerance and it has
drawn much attention during the past years. Osmotic adjustment (OA)
is defined as the active accumulation of organic solutes in plant tissues
in response to an increasing water deficit. It is considered as useful pro-
cess for maintaining cell turgor when tissue water potential declines. OA
has been shown to maintain stomatal conductance and photosynthesis at
lower water potentials, delayed leaf senescence and death, reduced flower
abortion, improved root growth and increased water extraction from the
soil as water deficit develops (Turner et al., 2001). OA involves the net
accumulation of solutes in a cell in response to fall in water potential of
the cell's environment. As a consequence, the cell's osmotic potential is
