evaluation of morphophysiological traits of cotton cultivars (gossypium hirsutum) under water deficiency stress at seedling stage

introduction major cultivated cotton regions of iran are located in dry and semiarid climates, therefore water deficiency or drought stress is inseparable part of cotton production systems in these regions. so identification and introduction of drought tolerant cotton genotypes is crucial. showed that water stress decreased plant growth rate, leaf area and finally photosynthesis in cotton. in addition plant height reduction is a primary effect of water stress. according to results, net photosynthesis, stomatal conductance and transpiration decreased simultaneously with an increase in drought stress intensity. physiologic traits monitoring were notified as a proper gadget for selection and improvement of germplasm. whereas investigation of many genotypes in field conditions under drought stress is difficult and it is not accurate enough, also good correlation has observed between the results of drought tolerance at seedling stage and field experiments, therefore this research has tried to evaluate some of the morphologic traits in cotton genotypes at seedling stage under drought stress and none stress conditions. materials and methods 22 cotton cultivars were grown under none water stress (field capacity) and drought stress conditions (-1mpa) using a factorial arrangement of treatments based on randomized completely design with three replications at college of agriculture, ferdowsi university of mashhad in 2011. then each genotype (10 seed) was sown at a pot (1.5 l-1). at two true leaf stages, pots were tinned to one seedling. soil moisture content was kept up to two true leaf stages at field capacity and at the end of experiment, it was conserved about -1mpa using weight method. the measured parameters were: plant height, leaf area, dry weight of leaf, stem, root and whole plant and also decreasing percent of each parameter under stress in comparison with control in any genotype was determined at the end of experiment. stomatal resistance and leaf temperature were measured with leaf porometer set (model decagon devices, inc) on three leaf stage of cotton seedling. relative water content (rwc) of leaf was also measured. variance analysis, comparison of trait means and correlation between traits were carried out using sas and excel and least significant difference (lsd). results and discussion interaction between water deficiency stress and cotton cultivars were significant (p<0.01). drought stress decreased the number of leaf per plant significantly (p<0.01) by about 43.2%. leaf area decreased significantly (p<0.01) under drought stress, but there was not significant correlation between leaf area and total dry mater or leaf dry weight. this object showed that effect of variations of leaf area on the amount of drought tolerance in relation with dry matter production was low. overall drought stress decreased dry weight of stem (29.1%), leaf (59.9%) and root (61.5%) significantly at the all cultivars in comparison with well-watered condition. according to the correlation coefficient, leaf dry weight altered with changes in leaf surface (r=0.82**) and leaf number (r=0.76**). under conditions (stress and none stress), maximum dry matter had allocated in leaves. in addition to cultivars such as varamin and 43347 that had the lowest decrease in leaf area and leaf and stem dry weight, root dry weight showed the lowest decrease of. the most decrease of plant dry weight after drought stress belonged to 43259 (80.7%), asj2*349 (72.5%) and nazili84 (72.1%) cultivars and the least decrease of plant dry weight after drought stress belonged to varamin cultivar (39.7%). according to the correlation coefficients, changes of leaf dry weight (r=0.95**), stem dry weight (r=0.94**) and leaf number per plant (r=0.80**) had the most effect on changes of plant dry weight. the results showed that cultivars with the maximum dry weight necessarily had not the lowest decrease of dry weight in comparison with the control. according to the result of leidi and et al, (1999) drought stress decreased relative water content of leaf in cotton cultivars significantly (p<0.01), tolerant cotton cultivars showed lower decreasing of rwc under drought stress in comparison with susceptible cultivars and also plants with higher rwc had higher plant dry weight. drought stress had significant effect (p<0.01) on stomatal resistance and amount of stomatal resistance reached 4.8 fold under drought stress. cotton cultivars showed obvious difference in stomatal resistance. 43347 and nazili84 cultivars had the highest and varamin and irma had the lowest stomatal resistance, respectively. in addition stomatal resistance increased more in susceptible cotton cultivars in comparison with tolerant cotton. stomatal resistance had negative effect on other parameters. drought stress increased leaf temperature significantly (p<0.01) and also difference of leaf temperature among cotton cultivars was significant (p<0.01)