Evaluating salinity tolerance at different growth stages in rapeseed (Brassica napus) cv. Talaye

Environmental stresses represent a major constraint to meeting the world food demand. The estimation of potential yield losses by individual environmetnal stresses are estimated 17% by drought, 20% by salinity, 40% by high temperature and 15% by low temperature. Salt stress is among the most important abiotic stress, depended to severity and stage, reducing growth and changing accumulation and distribution of ions in rapeseed (Brassica napus). Rapeseed sensitivity to soil salinity continually changes during the growing season. Most crops are tolerant during germination, but the young developing seedlings are susceptible to injury during emergence from the soil and during early juvenile development. Once established, plants generally become increasingly tolerant during later stages of growth. Althought effect of salt stress was well docucmented on rapseed growth; however, sensitivity of different growth stages of this plant has not fully studied. So, the main objective of this research was to examine salinity tolerance at different growth stages in rapeseed.
In this research the effect of different saline waterat different growth stageswas evaluated on some morpho-physiological traits on rapeseed (cv. Talaye) in controlled greenhouse in College of Agriculture, Shiraz University at 2014. The treatments included 0.4 (tap water as control), 4, 7 and 10 dS.m-1imposed at different growth stages consisted of 5 leaves, stem elongation and flowering. The pots (5 L size) were filled with field soil, sand and humus as 2:1:1 ratio. The used soil texture was sandy clay with pH = 7.05 and EC= 0.52 dS m-1.Saline solutions were achieved using 1:1 weight ratio of NaCl: CaCl2 and controlled by a portable EC-meter.In this research, plant height, leaf number per plant, leaf area index, chlorophyll content index, dry weight of shoot and root, water content of shoot and root, sodium concentration of shoot and root as well as potassium concentration of shoot and root were measuered. Relative salinity tolerance at different growth stage of rapeseed for dry matter production also was assessed based on Van Genuchten and Hoffman method.
The result generally showed that salt stress reduced plant height, leaf number, leaf area index, shoot and root dry weight, shoot and root water content and shoot and root potassium concentration. On the other hand, chlorophyll content index and shoot and root sodium concentrations were enhanced as affected by salinity. These changes were intensified in parallel with increased salinity levels. In the most cases, the negative effects of salt stress in all levels were more at 5 leavesstage, and delay in salinity impose decreased these negative effects. For example, salt stress in the highest level (i.e. 10 dS.m-1)imposed at 5 leaves, stem elongation and flowering stages were associated with 78.0%, 50.7% and 26.5% reductions in shoot dry weight and with 61.8%, 41.2% and 21.0% reductions in root dry weight, respectively. Salinity thresholds (50% reduction) for rapeseed cv. Talaye were determined by 1.2, 2.8 and 7.5 dS m-1 for 5 leaves, stem elongation and flowering stages, respectively. Therefore, it could be concluded that this plant was sensitive, mild tolerance and tolerance at 5 leaves, stem elongation and flowering stages, respectively.
The results of this experiment revealed that salt stress limited growth of shoot and root as well as changed potassium and sodium accumulation and distribution to the detriment of rapseed plants. It was also showed more sensitivity of rapeseed to salinity at early growth; which salt stress tolerance was enhanced with plant aging.