بررسي اثر سطوح مختلف شوري بر برخي خصوصيات فتوسنتزي ارقام كلزا (Brassica napus L.)

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

شوری یكی از مهمترین عوامل محدودكننده تولید محصولات زراعی در مناطق خشك و نیمه خشك جهان است كه عملكرد گیاه زراعی را تحت تأثیر قرار می‌دهد. به منظور مطالعه تغییرات فرآیند تثبیت كربن و رنگدانه‌های فتوسنتزی سه ژنوتیپ كلزا تحت تنش شوری، آزمایشی در شرایط گلخانه به‌صورت فاكتوریل در قالب طرح پایه كاملاً تصادفی با چهار تكرار انجام شد. عامل اول شامل ژنوتیپ‌های مختلف كلزا (هایولا 401، RGS0003 و شیرالی) و عامل دوم تیمار آبیاری با آب شور دارای چهار سطح شامل شاهد (آب معمولی)، 50، 100 و 150 میلی‌مولار با استفاده از منابع كلریدسدیم و كلریدكلسیم به نسبت مساوی یك به یك بود. نتایج نشان داد كه آبیاری با آب شور باعث كاهش معنی‌دار هدایت روزنه‌ای در هر سه ژنوتیپ شد. میزان كلروفیل‌های a، b و كاروتنوئیدها در هر سه ژنوتیپ تا سطح شوری 100 میلی‌مولار افزایش و در 150 میلی‌مولار كاهش یافت. با افزایش شوری تجمع منیزیم نسبت به شاهد بیشتر شد. میزان نشاسته برگ در ژنوتیپ شیرالی تا سطح شوری 150 میلی‌مولار افزایش معنی‌دار داشت. روند تغییرات هدایت روزنه‌ای در سطح 50 میلی‌مولار تا اوایل گلدهی شبیه به شاهد بوده ولی در اواخر گلدهی كاهش شدید داشت. در سطوح شوری 100 و 150 میلی‌مولار هدایت روزنه‌ای كمتر از شاهد بود. به نظر می‌رسد ژنوتیپ شیرالی با استفاده از ساز و كارهای اجتناب از تنش مانند حفظ محتوای نسبی آب، همچنین افزایش محتوای كلروفیل‌های a و b تا سطح 100میلی‌مولار و نیز تجمع منیزیم تا سطح 150 میلی‌مولار، تنش شوری را تحمل كرده است.

<strong >Introduction </strong > 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. <strong >Materials and Methods </strong > 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. <strong >Results and Discussion </strong > 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. <strong >Conclusions </strong > 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.