survey of the effect of different irrigation levels on yield and yield components of sensitive and tolerant cotton cultivars

introduction: more plains of iran are located in arid and semi-arid regions and so agricultural production systems depend heavily on water. recently, the reduction of water resources has become a serious threat for crop production such as cotton planting. therefore, application of low irrigation methods can be an appropriate method to cope with mentioned condition. in addition, it is vital to identify the cotton cultivars reaction to water deficiency. sensibility of some cotton cultivars is lower than others. sensitive cultivars seed cotton yield decreases more than tolerant cotton cultivars. moreover, some of growth stages in cotton plant are more sensitive to water deficiency stress. for example, flowering stage is more sensitive than vegetative growth stage, and boll number per plant is more effective parameter on yield than boll weight. ulla and et al (2) showed that there are genetic variations for drought stress toleration in cotton plant. afshar and mehrabadi (3) indicated that low irrigation on the basis of 50% and 75% of cotton water requirement had no significant effect on vegetative growth of cotton plant. however, it caused the increase of flower and boll shedding per plant. application of tolerant cultivars compared with sensitive cultivars can increase seed cotton yield under drought stress condition. consequently, the aim of this study is to survey yield and related morphological traits reactions in sensitive and tolerant cotton cultivars to different water deficiency levels. materials and methods: two tolerant cotton cultivars (armagan and varamin) and two sensitive cotton cultivars (coker349 and nazili84) as a subplot at three irrigation levels (as a main plot) using split plot design based on complete block design with three replications were carried out at agricultural research station of kashmar in 2011. three levels of water consumption based on penman-montith method and using cotton kc coefficients were [33% (i33%), 66% (i66%) and 100% (i100% of water requirement)] that it were take placed using drip irrigation method. yield and yield components traits such as height, the number of boll per plant, boll weight, flower and boll shedding percent, seed cotton yield, biological yield, earliness percent and harvest index were determined at the end of experiment. data analysis was carried out using excel and mstat-c software. results and discussion: the results showed that although water deficiency stress decreased vegetative components, the amounts of decreasing were higher in tolerant cultivars than sensitive cultivars but, by contrast, the reproductive components of tolerant cultivars indicated lower decreasing than sensitive cultivars under water deficiency stress. for example, varamin cotton cultivar had more number of bolls per plant than coker349 as a sensitive cultivar. the results also showed that more retention of boll number per plant was the main factor of cultivars difference as for seed cotton yield. the highest number of boll and flower per plant retention belonged to sensitive cultivars such as coker349 and nazili (71.2 and 69 percent, respectively) at 66 percent of water consumption and the lowest number of boll and flower per plant retention were 92.3 percent belonging to varamin as a tolerant cotton cultivar at full water irrigation treatment. the results indicated that all yield components except boll weight were significantly affected by low irrigation levels. in addition, the results revealed that low shedding of flower and boll and accordingly more retention of boll number per plant and also biomass preservation under drought stress were the main factor in yield of tolerant cultivars in comparison with sensitive cultivars. biological yield reduction was higher than seed cotton yield under water stress condition. high and significant correlation was observed among yields with boll number per plant, biological yield and harvest index under drought stress. moreover, there was a significant correlation between yield with plant height and biological yield only in full irrigated treatment. furthermore, harvest index decreased significantly under drought stress. harvest index value for coker349 was significantly lower than other cultivars. coefficient correlation between harvest index and yield and its components showed that harvest index was more dependent with seed cotton yield to biological yield. therefore, tolerant cultivars had higher seed cotton yield and also biological yield in comparison with sensitive cultivars. while there was a significant correlation between yield with plant height and biological yield only in full irrigated treatment.