Handbook of Plant and Crop Physiology

(Steven Felgate) #1

clined under drought or saline conditions because of increased proteolysis and decreased protein synthe-
sis [12]. Muthukumaraswamy et al. [61] observed that the protein content decreased in root, shoot, and
leaf in Cicer arietinumseedlings under NaCl salinity. Venkatesalu et al. [132] and Chandrashekar and
Sandhyarani [133] reported an increase in protein content with increasing NaCl salinity in Sesuvium por-
tulacastrumandCrotalaria striata, respectively. Muthuchelian et al. [134] reported that salt stress in-
creased the protein content in Erythrina variegataseedlings. Thus, in many plants protein synthesis is
stimulated under saline conditions. Strogonov [9] reported higher protein content in maize under sulfate
salinity. According to him in general, salt-tolerant plants maintain protein synthesis under saline condi-
tions but salt-susceptible plants do not. Our findings revealed that the protein content of plants at both
saline and nonsaline sites was maximum during the rainy season, when plant water status was higher than
in winter or summer [23,122,123].


IX. STOMATAL BEHAVIOR


Water relations and stomatal behavior are important indices that reflect the ability of plants to economize
essential requirements under prevailing climatic and edaphic conditions [135–138]. In plants adapted to
dry environments, anatomical and morphological changes at the leaf and whole plant levels prevent
metabolic imblance and help to improve water relations [139]. Stomata show amazing versatility in their
reaction to the environment and respond to all factors that are of physiological importance. Stomata are
known to play a pivotal role in productivity of plants. Thus, it is necessary to study stomatal characteris-
tics. The plants require adaptive mechanisms to help their survival under saline stress. Leaves are the ma-
jor sites of transpiration and photosynthesis in higher plants. In relation to salinity-induced water stress,
one might expect the principal structural and metabolic modifications in leaves to be associated with a
tendency to minimize transpiration rate and the occurrence of photosynthetic pathways with high water
use efficiency.
According to Perera et al. [140], stomatal opening was suppressed by increasing NaCl concentration
inAster tripolium, which has no glands or cannot excrete salt, indicating that salt accumulation in cell
vacuoles increases. Na ions in apoplast around guard cells, causing partial closure, reducing transpiration
and increasing water use efficiency, reducing flow of salt to leaves, and not affecting new photosynthate
synthesis and growth. The increasing supplies of Ca^2 ions reduced the effect of salinity on stomatal con-
ductance in the whole plant (Aster tripolium) as well as in the isolated epidermis. This finding is consis-
tent with the well-established role of calcium in increasing resistance to salinity. In the presence of high
calcium, the plants can tolerate a greater salt intake, and hence there is a reduced need for transpiration to
be restricted by partial stomatal closure. Ayala and O’Leary [141] observed decreased stomatal conduc-
tance with increasing salinity that increased the transpiration rate at a low salinity level in Salicornia
bigelovii. Lakshmi et al. [142] reported a decrease in stomatal conductance in Morus albaunder saline
conditions.
Robinson et al. [143] noted that when saline plants were subjected to 200 mM NaCl, stomatal con-
ductance was reduced by 70%, which decreased actual photosynthesis. However, in white mangrove (La-
guncularia racemosa) the number of stomata and salt glands per leaf area was increased in high NaCl
[144]. The study of Gulzar and Khan [145] revealed that the water relations of perennial halophytes
showed similar patterns of variation in all parameters, and plants at the coastal locations appeared to be
more stressed than plants at inland locations in Pakistan. Plant physiological responses to high NaCl
included an increased chlorophyll a/bratio (to enable the plants to cover the high energy demands for
adaptation to salt stress), an increase in soluble protein contents with rising salt stress, a tendency for car-
bohydrates to accumulate in foliage, and a negative influence of high soil salinity on secondary compound
(phenol, hydrolyzed or condensed tannins) metabolism [144].
Villiers et al. [146] reported that the net leaf photosynthetic rate and leaf stomatal conductance de-
creased with increasing salinity, while the intercellular CO 2 concentration increased. Both stomatal clo-
sure and inhibition of biochemical processes probably caused the reduced leaf photosynthetic rates. The
stomatal indices suggested that the trend toward an increase in number of stomata per unit leaf area with
an increase in salinity was not due to decreased epidermal cell size.
The study of Bankar [64] revealed that the pattern of stomatal opening in C. tinctoriusis similar to
that of C 3 plants. The stomata remained closed when the heat was maximum in this plant. Thus, plants


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