This study was conducted to evaluate the physiological indices in relation to the screening of cold tolerant rapeseed (Brassica napus L.) genotypes for drought tolerance and study yield and yield components under early and late season water deficit stress. Six rapeseed (Brassica napus L.) genotypes including Karaj 1, Karaj 3, Opera, Okapi, Licord and Modena were evaluated under non-stressed and water deficit conditions during rosette and seed filling stages.Correlations among above mentioned physiological indices at seed filling stage with each other and plant height, number of pods per plant, 1000-seeds weight and seed yield were significant. Among the genotypes, significant differences were observed for stomatal conductance at seed filling and RWC (Relative Water Content) at rossete stages, so that. Okapi and Licord genotypes having the highest seed yield.Therefore canopy temperature, stomatal conductance and RWC can be used to screen rapeseed genotypes for tolerance to water deficit stress especially during seed filling stage. Water deficit stress during the seed filling stage significantly decreased plant height, number of pods per plant, 1000-seeds weight and seed yield. Positive correlations among above mentioned traits also observed. They contributed to higher rapeseed yield than other yield components. All genotypes under study were found acceptable performance, but Okapi and Licord were more suitable for cultivation in normal and drought conditions.
Key words: Drought, physiological index, seed yield, yield components
Introduction
Among the different environmental stresses, drought is the major constraint reducing crop yields. Crop plants when subjected to this constraint, manifest a wide range of behaviors varying from great sensitivity to high tolerance. Rapeseed is no exception to the rule (El Hafid et al. 1998). Grewal (2010) indicated that rapeseed may be a better option for sustaining crop production and higher water use efficiency on sodic vertisols with high subsoil NaCl salinity. The most critical time for water supply in rapeseed, is reported to be during flowering and seed filling stages (Richards and Thurling 1978). Inter specific and intra specific variations were found in Brassica napus L. for response to drought (Richards and Thurling 1978; Rao and Mendham 1991). Kumar et al. (1984), Singh et al. (1985) and Kumar and Singh (1998) believe that there is a close association between osmotic adjustment and both stomatal conductance and canopy temperature in oilseed Brassica species. Singh et al. (1985) stated that transpirational cooling (canopy temperature minus air temperature) could effectively be used as a technique to screen Brassica genotypes for drought tolerance under receding soil moisture conditions. Pasban Eslam et al. (2000) reported that late season drought reduces rapeseed temperature stability (differences between daily minimum and maximum air to crop temperatures). They also indicated significant positive correlations of crop temperature stability with stomatal conductance, water potential, relative water content and seed yield. They suggested that this index may be more accurate than leaf temperature. Kumar and Singh (1998) showed significant correlations of seed yield with osmotic adjustment, transpirational cooling and stomatal conductance in oilseed Brassica species. Lehman et al. (1993) by studying the bentgrass clones suggested that relative water content would better predict growth maintenance under increasing water deficit than the simple measure of leaf water potential.Richards and Thurling (1978) found that late season drought leads to abortion of more than 50 percent of the pods in B. napus L. and B. rapa L., however, remaining pods produced more and heavier seeds. Jensen et al. (1996) reported that water deficit stress occurring during both vegetative growth and pod filling stages in rapeseed, decreased number of seeds per m2, oil percent, harvest index and seed yield. Irrigation after anthesis in rapeseed increased seeds per pod and harvest index and thus gave a higher seed yield (Rao and Mendham 1991). The main portion of seed yield variability in rapeseed can be related to the number of pods per plant, number of seeds in a pod and 1000-seed weight (Chen 1994). Peltonen-Sainio and Jauhiainen (2008) reported that environmental variation markedly affected seed yield, seed numbers per square meter and duration of flowering in rapeseed. Generally, seed weight in relation to environmental factors is less influenced than the number of pods per unit area and a number of seeds in pod in rapeseed (Keiller and Morgan 1988; Jensen et al. 1996). Pasban Eslam (2009) disclosed that fall season rapeseed cultivars (Okapi and SLM046) were more tolerant to late season water deficit stress. Evaluation of rapeseed genotypes in Mediterranean-type environment revealed that seed yield was strongly correlated with harvest index, the total number of pods per plant, 1000-seeds weight, the final height of plant, and number of primary branches. However, number of seeds per pod was not significantly correlated with seed yield (Gunasekera et al. 2006). Rapeseed genotypes with higher seed oil content produced higher dry matter in reproductive organs at the end of the seed filling stage (Hua et al. 2012). In spite of several reports about the effects of drought stress on B. rapa L. and B. juncea L. genotypes specially at temperate and warm areas, limited studies have been reported on cold tolerant rapeseed genotypes.
The objectives of this study were to evaluate the physiological indices related to the drought tolerance of cold season rapeseed genotypes, their yields and yield components under early and late season water deficit stress.