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INTRODUCTION
Water stress is commonly attributed to situations where the water loss exceeds
sufficient absorption intensity causing a decrease in plant water content, turgor
reduction and consequently, a decrease in cellular expansion and alterations
of various essential biochemical processes that can effect growth or productivity.
Diethelm and Shibles (1989) had opined that the RuBisCo
content per unit leaf area was positivity correlated with that of the soluble
protein content. Drought causes reduction in ribulose-1, 5-biphosphate carboxylase/oxygenase
(RuBisCO) activity (Berkowitz and Wahlen, 1985; Pandey
et al., 2000). During drought, quality of chloroplast protein decreased
and electrophoretic spectrum of proteins changed in the tree plants. Many researchers
have reported alterations in the functioning and speed of enzymatic activity,
like amino acid synthesis (Andrews et al., 2004)
and decrease in protein levels (Xiong and Zhu, 2002),
as metabolically responses to water restrictions (Pimentel,
2004). The nitrate reductase (NRase) is the rate limiting enzyme in nitrogen
assimilation and is a key point of metabolic regulation (Eilrich
and Hageman, 1973) in crops. Thus, NRase is intimately associated with the
plant growth and development (Sinha and Nicholas, 1981).
The decrease in Nrase was accompanied by an increase in free amino acids and
a decline in protein synthesis. The plants submitted to water stress suffered
and decrease in the amounts of total protein casued by the decrease in their
synthesis and a fall in nitrate reduction activity caused by the low nitrate
flux were reported (Costa et al., 2008). Kaur
and Singh (1992) found that flower number and percentage of boll abscission
were decreased by water stress at flowering stage of cotton. Seed cotton yield
decreased as the allowable water deficit increased (Cudrak
and Reddell, 1988). Seed yield and yield components are severely affected
by water deficit.
The objective of the study was to evaluate and compare water stress effects
as well as to reveal that which genotypes better edopts water stress conditions
using parameters.
MATERIALS AND METHODS
The aim of this experiment was to investigate the responses caused by progressive
water stress and the necessary time for have biochemical and physiological changes
of Gossipium spp. during the vegetative, squaring and boll development
stages. For present investigation, twenty one genotypes including eight parents,
four F1 hybrids, five F2’s and four back crosses
along with parents were subjected for genetic diversity analysis using physiological
features. Field trials were conducted at Kharif 2008-2009 in the Department
of Cotton, Centre for Plant Breeding and Genetics, TNAU, Coimbatore.
Treatments:
T1: Control
T2: Stress at vegetative
T3: Stress at squaring
T4: Stress at boll development
Enzyme assay
Soluble protein content: Soluble protein content of the leaf sample
is a measure of indirect assessment of the photosynthetic efficiency of crop
plants. The content of soluble protein was estimated from the leaf samples following
the method of Lowry et al. (1951) and expressed
as mg g-1 fresh weight.
Nitrate reductase activity: Nitrate reductase activity in the leaves
was determined by adopting the method of Nicholas et
al. (1976) and the enzyme activity was expressed as μg of NO2¯
g-1 h-1.
Yield parameters: At final harvest flower number, boll number and seed
cotton yield per plant were determined.
Statistics: The data of three replications were statistically analyzed
by Factorial completely randomized design.
RESULTS AND DISCUSSION
Drought stress adversely affects multiple physiological and biochemical
pathways contributing to the growth and development and ultimately yield of
cotton. Although breeding programs have generally focused on yield as a cultivar
selection tool, there exists potential for the development of stress specific
screening tools for rapid identification of superior cotton cultivars. Water
stress caused a steep decline in soluble protein content irrespective of stages
and genotypes. The mean soluble protein content was found to be higher in KC
2xMCU 13 at boll development stage (Table 1). Among the F1,
F2 and F4 generations, KC 2xMCU 13 has shown higher values
irrespective of the treatment indicating that KC 2xMCU 13 is fairly tolerant
to drought situation than others.
| Table 1: Effect of drought on Soluble protein
content (mg g-1) and Nitrate Reductase activity (μg of NO2¯g-1
hr-1) at squaring stage of cotton in F1, F2,
back crosses along with parents |
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| Table 2: Effect of drought on yield components
of cotton in F1, F2, back crosses along with parents |
 |
Drought induced decrease in RuBisCO activity should be attributed not only
to proteolitic decomposition of enzyme protein but also to the partial inhibition
of its catalytic activity, because decrease in RuBisCO activity was more than
that in RuBPisCO content (Chernyad’ev and Monakhova,
1998). Higher value of NRase activity was observed at the boll development
stage for all the genotypes including control. Among the genotypes AS 2, KC
2, KC 2xMCU 13 and KC 2xJKC 770 (F1, F2 and F4,
respectively) have recorded the highest NRase activity at boll development stage
(24.47, 23.33, 25.37, 24.08 and 22.97). NRase activity was more in the control
than in stressed plants. The NRase, a substrate inducible enzyme, mediates conversion
of nitrate to nitrite. The reduction in the activity might be either due to
reduction in enzyme level (Bardzik et al., 1971)
or due to the inactivation of the enzyme (Nicholas et
al., 1976) caused by stress condition. Sivaramakrishnan
et al. (1988) studied the midseason drought indicating that there
is a sharp decline in NRase activity under water stress situation. NRase activity
was found to be more in KC 2 and AS 2 which may be tolerant irrespective of
the treatments. The stress imposed at squaring stage has shown a marked reduction
in seed cotton yield when compared to the control (Table 2).
The seed cotton yield recorded as 128.99 in KC 2xMCU 13 (F2) irrespective
of treatments. Significant differences were also observed between the genotypes,
treatments and their interactions. The genotypes KC 2 and AS 2 have the highest
value of seed cotton yield (120.28 and 110.22) than other genotypes at all stages
irrespective of the treatmental effects. Yield was remarkably reduced when stress
was imposed at squaring stage. Earlier report also indicated that the most critical
phenophase for water stress in cotton is flowering (Singh
and Sahay, 1992).
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