VIII. PHYSIOLOGY OF METABOLIC PRODUCTS
Plants regulate various aspects of their growth in a synchronized form with a high degree of organization
involving coordination of many components. Regulation of various metabolic processes has direct con-
trol with respect to regulation of catalysis, action, and transport. To understand the metabolic activities,
it becomes necessary to study multienzyme systems because metabolic processes in plant systems occur
due to specific enzyme activity [64].
In living cells, the intense chemical activity is due to the activity of numerous specific enzymes,
which leads to consideration of the interdependence of physiological processes [113]. Under saline con-
ditions, growth is related not only to osmotic and nutritional effects but also to the disturbances in their
normal physiological and metabolic processes [114]. Also, under salt stress, the salt induces a decrease
or increase in enzyme activity [115], which in turn reflects several metabolic processes.
The biochemical processes inside the leaf cells generally regulate the growth and development of
plants. But toxicity influences early metabolic changes, such as enzyme synthesis, to a greater extent
[116]. The adaptation of glycophytes to saline soil in adverse conditions is possible mainly because of
changeability of their metabolism and chemical properties of the protoplasm [114]. Oxidative, photosyn-
thetic, and photorespiratory enzymes are important because of their various interrelationships in the pro-
cess of growth and development. Physiologists and biochemists have tried to correlate the possible role
of these enzymes with relative metabolic processes of the plants under saline conditions.
Ribulose 1,5-bisphosphate Carboxylase (RuBP-Case) is the main enzyme of CO 2 fixation in C 3
plants, and Phosphoenolpyruvate Carboxylase (PEP-Case) and RuBP-Case are important enzymes of C 4
plants. Popova et al. [117] reported that NaCl stress imposed through the root medium for 8 days de-
creased the activity of RuBP-Case in Hordeum vulgare. Sudhakar et al. [118] studied the response of a
few Calvin cycle enzymes to salinity shock in vitro and observed a decline in RuBP carboxylase activity
in 10-day-old seedlings of Dolichos uniflorussubjected to NaCl or Na 2 SO 4 treatment, indicating that
RuBP-Case was more sensitive to salt shock than other enzymes and NaCl was more toxic than Na 2 SO 4.
Bankar [64] reported that when plants of Carthamus tinctoriuswere grown at ECe 5.0 to 15 mS cm^1
of NaCl, the activity of RuBP-Case decreased with increasing concentrations of NaCl in the growth
medium. Saha and Gupta [87] reported an increase in peroxidase activity with increasing NaCl salinity in
sunflower seedlings. Sankhla and Huber [119] studied the effect of NaCl on the activities of photosyn-
thetic enzymes in wheat, Lemna minorandPennisetum typhoides, and reported that the salt tolerance of
RuBP-Case and PEP-Case varies with species.
According to Poljakoff-Mayber [120], the enzymes do not behave identically under saline condi-
tions, and enzymes located in certain places or on certain membranes in the cell may be salt tolerant,
whereas others may be salt sensitive.
Peroxidase is an oxidative enzyme and it is also essential for the conversion of H 2 O 2 to H 2 O and O
in photorespiration. According to Strogonov [9], peroxidase plays an important role in adaptation of
plants to saline conditions by regulating toxic accumulation of H 2 O 2.
It was reported by Seemann and Sharkey [121] that salinization lowered the RuBP pool size in
Phaseolus vulgaris. The biochemical basis for this reduction under salt stress is unknown. One reason
may be inhibition of ATP synthesis under saline conditions. In addition, the rate of photosynthesis at any
given pool size was lower for leaflets from the salinized plants than the control leaves. Thus, it can be
concluded that salinity reduces the photosynthetic capacity of leaves by reducing the pool of RuBP as an
effect on the RuBP regeneration capacity and secondly by reducing the activity of RuBP-Case by an un-
known mechanism when RuBP is in limited supply.
Salinity is known to affect almost all the aspects of plant metabolism. The leaves of plants subjected
to water stress often showed a decrease in starch, which is usually followed by an increase in sugar con-
tents [45,122,123]. A similar trend was also observed by these investigators in halophytes of the Indian
desert [122,124]. These investigators observed that plant species from site II, which is less saline, showed
a maximum sugar content during the summer, when plant water stress was higher than in winter or rainy
seasons. The plant species at site I, which is extremely saline, had a higher sugar content during the rainy
season, followed by the winter, and the sugar content was least in summer (Table 5) [124]. These vary-
ing observations of sugar content may be due to the higher salinities at a particular site. The level of sol-
uble sugars decreased with increased salinity levels, as observed by Gill and Singh [125] in different va-
rieties of paddy (Oryza sativa). Naidoo and Naidoo [59] found that in Sporobolus virginicus, CO 2
572 SEN ET AL.