Evaluation of the RUE and Growth Indices of Pinto Bean (Phaseolus vulgaris L.) Genotypes Deficit Irrigation Condition

Pinto bean (Phaseolus vulgaris L.) is one of the most important bean types in Iran. Cultivation area of pinto bean is about 50% of the total bean cultivation area and more than half of the grain bean production belongs to this type of bean. Drought is the most important environmental stresses that affected agricultural production in arid and semiarid areas and reduced crop productions. About 90 percent of Iran located in arid and semi-arid areas where water stress in the plants is inevitable. In addition, 60% of bean cultivation area in the world are faced with drought. Quantitative analysis of crops growth under water limitation condition is a good way to identify genotypes differences in response to drought stress.
In order to study the impact of deficit irrigation on pinto bean genotypes, an experiment was conducted as split plot based on randomized complete block design, at Shahrekord University, during 2013. Main plot was irrigation regime (supply of 100, 80 and 60 percent of crop water requirement) and sub plot was pinto bean genotype (Taylor, Sadri, C.O.S.16, KS21193 (Koosha193) and KS21486). Seeds were disinfected with benomyl fungicide before planting. Planting was conducted in 30 plant m-2 density. Crop water requirement was calculated by FAO Penman-Monteith equation. Irrigation regimes were applied at V4 stage (4th three foliate leaf has unfolded) and continued until the end of the growing season.
Results showed that there is genotypic variation in stem, leaf and seed dry weight, leaf area index (LAI), leaf area duration (LAD), crop growth rate (CGR), relative growth rate (RGR), net assimilation rate (NAR) and radiation use efficiency (RUE) in response to deficit irrigation. The dry matter accumulation of bean genotypes was sigmoidal shape and reduction in water use causes degradation of dry matter both in terms of the amount of dry matter and dry matter accumulation rate.
The highest RUE was recorded in KS21486 in supply of 80 percent of crop water requirement (3.21 g MJ-1). RUE changes to the reduction of irrigated water vary from genotype to genotype. So that, RUE of Taylor, Sadri and C.O.S.16 reduced and RUE of KS21193 and KS21486 increased in supply of 80 percent of crop water requirement condition. Moreover, RUE of all genotypes reduced in supply of 60 percent of crop water requirement condition. In fact, the amount of light absorption and biomass production reduced due to deficit irrigation. Under drought conditions, reduction in water availability decreases cell growth and subsequent reductions in leaf area index. Thus ratio of dry matter produced by the absorbed light is low and radiation use efficiency will be less. Drought stress reduced total dry weight. KS21486 had more rapidly seed dry weight enhancement rather than the other genotypes in the early stages, however Taylor and KS21193 had highest seed dry weight in all three levels of irrigation. Deficit irrigation decreased leaf area index in supply of 60 and 80 percent of crop water requirement condition compared to supply of 100% of crop water requirement condition. Maximum leaf area index in supply of 100, 80 and 60 percent of crop water requirement conditions were 3.1, 2 and 1.5, respectively. Average reduction of LAD in supply of 80 and 60 percent of plant water requirement conditions were 27 and 42 percent respectively. C.O.S.16 showed the highest CGR and maximum NAR and RGR, in supply of 100 percent of plant water requirement. In supply of 80% and 60% of plant water requirement conditions, KS21486 had the maximum NAR and RGR.
Biomass duration (BMD), LAD, LAI showed the highest correlation with grain yield, thus they are appropriate indicators to estimate grain yield under different moisture conditions. With the increase in leaf area index, leaf area duration and biomass duration, seed yield increased. To obtain more seed yield, the area of photosynthetic levels (leaves) and stay green must be increased to produce more photosynthetic products and allocated to seeds.