Effect of Different Salinity levels on some Photosynthetic Characters of Canola (Brassica napus L.) Cultivars

Salinity is one of the most important factors limiting crop production in arid and semiarid regions of the world that affects crop yield. Salt tolerance of Brassica species are very complex due to genetic relationships. Because of low erucic acid (less than 2% of total fatty acids) and glucosinolates contents (less than 3 µmol g-1), oil of Canola has many consumers around the world. Because Canola have tolerance potential against toxicity of salinity and its minerals, its growth can be successful in saline condition. According to the recent ongoing drought and the need to use low quality irrigation water for crops such as Canola, aim of this experiment was to evaluate the effect of salinity on changes in carbon fixation process and photosynthetic pigments of three Canola genotypes under salinity as well as determine most salt tolerant genotype for use in saline regions.
An experiment was conducted in the greenhouse of Shahid Chamran University during 2007-2008 growing season in factorial test based on a completely randomized design with four replications. The first factor (genotype) included Hayola 401, RGS0003 and Shiraly and the second factor (salinity levels) had four levels of salinity (50, 100 and 150 mM NaCl) as well as distilled water as a control. Sources of salinity were NaCl and CaCl2 with equal ratio as most resembles to lower water quality resources in the region. Date and time of stress were considered four weeks after planting (four-leaf stage). A Stepped irrigation method using saline water was done every 12 days over three steps period. To perform this study 10 liters volume pots were used. Three pots per each treatment, and totally 144 pots were used. SAS (version 9.1), Excel and MSTAT-C software's was used for statistical analysis. The comparison of means was done by Duncan method.
The results showed that content of chlorophyll a, b and carotenoids in all three genotypes increased to 100 mM, while decreased at 150 mM. Health and invulnerability of main photosynthetic pigments at 100 mM NaCl, can be considered as an important reason for salt tolerance in the evaluated canola cultivars. Reducing the absorption of water due to stress caused by salinity reduced nutrient uptake. Toxicity of chlorine and sodium also leads to inhibition of plant growth and dry matter. Magnesium concentration raised with increasing salinity compared to control. It seems that the magnesium concentration in stomatal guard cells decreased osmotic potential and play a role in the biosynthesis of organic compounds such as glutamic acid. So, despite ABA-induced stomatal closure, the stomatal openings, even in small amounts remain photosynthesis activities. Probably, magnesium has a key role in the continuing photosynthesis of Canola under salinity. The leaf starch in the genotype Shiraly significantly increased to 150 mM salinity level. Reduction of starch in high levels of salt stress interferes with enzymes efficiency involved in the starch biosynthesis, such as starch synthase and ADP- glucose pyrophosphorylase. The trend of variation in stomatal conductance was similar to 50 mM in the early flowering, but a sharp decrease was observed at late flowering compare to control. When stress started on the four-leaf stage, the plant for some time kept stomatal conductance at maximum level, but continuation stress period led to close stomata. Thus, reducing the amount of leaf relative water content can be considered as the reason of different stomatal conductance among the different levels of salt stress. A significant positive correlation between relative water content and stomatal conductance confirms this results. On the other hand to avoid stress and better use of the limited amount of available water, the plants closed stomata to prevent more water loss.
It seems that genotype Shiraly using some mechanisms to avoid stress, such as maintaining relative water content, as well as increase the content of chlorophyll a and b up to 100 mM. Moreover, the concentration of magnesium up to 150 mM, has endured salinity.