Evaluation of Physiological and Agronomic Characteristics Related to Drought Tolerance in Spring Rapeseed

Among the different environmental stresses, drought is a major limitation in reducing crop yields. Rapeseed is a plant adaptable to areas with limited rainfall during winter and spring and dry air at flowering, grain filling and maturity stages. Water deficit stress during pod filling stage in rapeseed reduces the number of grains per m2, oil percent and grain yield. Positive and significant correlations were reported among grain yield with pod numbers per plant and grain number in a pod in winter genotypes of canola, under normal and drought stress conditions. Strong negative relationship between grain yield and canopy temperature during reproductive stage of Brassica napus L. genotypes have been reported. The results of studying rapeseed genotypes under drought stress indicated that the chlorophyll a and b content of all genotypes declined due to drought stress at flowering and grain filling stages, but greater reduction in grain yield was observed when stress was imposed at flowering stage. The objectives of this study were to recognize some of the physiological and agronomic characteristics related to drought tolerance in spring genotypes of rapeseed and to study the grain and oil yields and yield components relations under normal and water deficit stress conditions.
The experiment was carried out in the East Azarbaijan Agriculture and Natural Resources Research and Education Center with semi-arid and cold climate according to Koppen climatic classification system, during 2015 and 2016 cropping seasons. The experiment was conducted as split plot based on a randomized complete blocks design with three replications. The experimental factors were drought stress with three levels: non-stressed and drought stress from flowering and pod formation stages and genotype in 5 levels: RGS003, Zafar, Sarigol, Zarfam and Dalgan. Each plot consisted of 6 rows in 5 meters. Plants were harvested on the 5th and 17th of July during the first and second years of experiment respectively. During harvest time, in order to control boarder effects, plants from the sides of each plot were removed. Measured traits were leaf temperature, relative water content, stomatal conductance, leaf chlorophyll index, pod number per plant, grain number per pod, 1000- grains weight, grain yield and grain oil percent. Ten plants in each plot were used to determine grain yield components. Moreover, seed oil content was determined by NMR (nuclear magnetic resonance) method.
Occurring drought around flowering and pod formation stages led to a significant increase in leaf temperature and significant decrease in leaf relative water content, stomatal conductance, leaf chlorophyll index, pod number per plant, grain number per pod, 1000 grains weight, oil percent, grain and oil yields. But the effects of drought from flowering stage were too hard. Therefore, in case of water resources limitation, irrigation during flowering stage will be more important than pod formation stage. RGS003 genotype with higher relative water content, stomatal conductance and leaf chlorophyll index and lower leaf temperature, indicated the highest grain and oil yields. The highest grain and oil yields with 1120 and 466 Kg h-1 respectively were obtained from RGS003 under non-stress condition. Also RGS03 in all water conditions indicated higher yields and could be used to cultivate in areas with normal and limited irrigation water resources. The significant correlations among leaf temperatures, relative water content, stomatal conductance and leaf chlorophyll index with each other and grain and oil yields and yield components were seen. It seems that these traits can be used to select drought tolerant spring genotypes of rapeseed.
It can be concluded that spring rapeseed is more sensitive to occurring drought stress from flowering than from pod formation stage. Thus, water supply at flowering stage could be more effective in rapeseed yield production. Leaf temperatures, relative water content, stomatal conductance and leaf chlorophyll index can be used to screen high yielding spring genotypes of rapeseed for late season water deficit condition.