فتوسنتز جاري و انتقال مجدد مواد فتوسنتزي در اثر محلول‌پاشي تنظيم كننده‌هاي رشد در شرايط تنش خشكي در ذرت KSC 704

Current of photosynthesis and remobilization of assimilate affected spraying growth regulator under drought stress condition on maize cultivar KSC 704

آزمايش در سه محيط جداگانه، شامل محيط بدون تنش خشكي، تنش خشكي در مرحله رشد رويشي و تنش خشكي در مرحله رشد زايشي انجام شد. تنظيم كننده‌هاي رشد‌ سيتوكينين در سه سطح (شاهد، محلول‌پاشي در مرحله پنج تا شش برگي و هشت تا ده برگي) و اكسين در سه سطح (شاهد، محلول‌پاشي در مرحله ظهور ابريشم و 15 روز پس از ظهور ابريشم) در هر محيط درسه تكرار بصورت فاكتوريل در قالب طرح بلوك‌هاي كامل تصادفي در مزرعه مؤسسه تحقيقات اصلاح و تهيه نهال و بذر كرج طي سال‌ زراعي 1392 اجرا شد. تنش خشكي در مرحله رشد زايشي باعث كاهش ميزان فتوسنتز جاري و سهم فتوسنتز جاري در عملكرد دانه به ترتيب به مقدار 61/43 و 28/18 درصد گرديد و همچنين باعث افزايش انتقال مجدد و سهم انتقال مجدد در عملكرد دانه به ترتيب به مقدار 40/68 و 28/18 درصد گرديد. همچنين محلول‌پاشي تنظيم كننده رشد سيتوكينين در مرحله هشت تا ده برگي باعث افزايش ميزان فتوسنتز جاري و سهم فتوسنتز جاري در عملكرد دانه به ترتيب به مقدار 13/74 و 11/10 درصد گرديد و محلول‌پاشي تنظيم كننده رشد اكسين در مرحله ظهور ابريشم باعث افزايش ميزان انتقال مجدد و سهم انتقال مجدد در عملكرد دانه به ترتيب به مقدار 24/95 و 4/38 درصد گرديد. بنابراين محلول‌پاشي تنظيم كننده‌هاي رشد‌ سيتوكينين در مرحله هشت تا ده برگي در افزايش فتوسنتز جاري و اكسين در مرحله ظهور ابريشم در افزايش انتقال مجدد و نهايتا عملكرد دانه مؤثر خواهد بود.

Introduction: Drought is one of the major environmental conditions that adversely affects plant growth and crop yield. Drought is the most major restriction in maize production. The reduction in plant growth under drought stress conditions could be an outcome of altered hormonal balance and hence the exogenous application of growth regulators under stress conditions could be the possible means of reversing the effects of abiotic stress. Plant growth regulators such as auxin and cytokinin are known to be involved in the regulation of plant response to the adverse effects of stress conditions. Previous studies have shown that endogenous hormones are essential regulators for translocating and partitioning of photo-assimilates for grain filling in cereal crops (Yang, et al. 2003), and therefore could be involved in the regulation of grain weight and yield. This study was conducted to determine the effects of exogenous application of growth regulators on the current photosynthesis and remobilization of assimilates in maize cultivar KSC704 under drought stress conditions. Materials and Methods: The experiment was carried out in three distinct environments at Seed and Plant Improvement Institute (SPII), Karaj, Iran, in 2013. The environments included non-drought stress (irrigation after soil moisture reached 75% of field capacity), drought stress at vegetative stage (irrigation after soil moisture reached 50% of field 2016capacity from V4 to tasseling stage and irrigation after soil moisture reached 75% of field capacity from pollination to physiological maturity stage) and drought stress at reproductive stage (irrigation after soil moisture reached 75% of field capacity from V4 to tasseling stage and irrigation after soil moisture reached 50% of field capacity from pollination to physiological maturity stage). Cytokinin was applied at V5 –V6 and V8-V10 stages and auxin was sprayed at silk emergence stage and 15 days after that period. No usage of the growth regulators was served as control. The trial was laid out as a factorial scheme based on randomized complete block design with three replications. Auxin was used in the form of Indole-3-butyric acid and cytokinin was sprayed as N6-benzyladenin. Harvesting was done from 4.5 m2 at field maturity stage at 14 % grain moisture content and was used to estimate grain yield. Remobilization, efficiency of remobilization, contribution of remobilization, current photosynthesis, efficiency of current photosynthesis and contribution of current photosynthesis were measured by the equation of Uhart and Andrade (1995). Results and Discussion: The effect of drought stress on current photosynthesis, efficiency of current photosynthesis, contribution of current photosynthesis and remobilization was significant (P<0.01). Efficiency of remobilization was also affected by water stress (P<0.05) (Table 1). The difference in the rate of current photosynthesis and remobilization under drought stress and non- stress environments at vegetative stage was insignificant (Table 2). Cytokinin increased the rate of current photosynthesis by 13.74% as compared to control treatment at V8-V10 stage. It is possible that the application of cytokinin at V8-V10 stage resulted in enhanced cell division and leaf area, leading to increased absorption of photosynthetically active radiation by the plant. Contribution of current photosynthesis to grain yield was maximal under optimum irrigation and water stress treatments at vegetative stage. However, it experienced a 28.18% decline under drought stress at reproductive stage. The application of auxin was associated with increased contribution of remobilization to grain yield at silk emergence stage. It seems that auxin involvement in the conversion of sucrose to other hexoses such as starch in endosperm and its role in maintaining concentration gradient of assimilates from source to sink are the reasons for this increase. (Robert and John, 2004). Conclusion:Based on the results of this experiment, drought stress at vegetative stage (irrigation after decreasing soil moisture to 50% field capacity) can be effective in conserving irrigation water and increasing water use efficiency. Moreover, considering the fact that the higher rates of current photosynthesis and contribution of current photosynthesis to grain yield were achieved with cytokinin application at V8-V10 stage and spraying with auxin at silk emergence stage led to the increased rates of remobilization and contribution of remobilization to grain yield, the aforementioned growth periods were identified as the best application times for cytokinin and auxin, respectively.