Effect of drought stress and different levels of nitrogen fertilizer on morphological and physiological traits and total dry matter in two rice cultivars (cvs. Hashemi and Gilaneh)

Drought stress is a limiting factor for growth in crops. Water and nitrogen relationships in the crop change in drought stress. The role of nitrogen in plant response during drought stress depends on the stress intensity and fertilizer level. Plants with relatively high nitrogen availability show better growth compared to plants with low nitrogen in drought stress. Therefore, the objectives of the present study were to investigate the effect of drought stress and different levels of nitrogen fertilizer on morphological and physiological traits in two rice cultivars.

The present study was conducted in Rasht in 2017 and 2018. The experimental design was split plot plot arrangement in randomized complete block design with three replications. The experimental factors were drought stress (main factor) at three levels (continuous submergence, 7 and 14 days irrigation interval), N fertilizer (sub factor) in three levels (50, 75 and 100 kg ha-1) and sub sub factor in two levels (cv. Hashemi, Gilaneh). The size of the experimental units was 9 m-2 (plot dimensions 3 × 3). To control weeds, butachlor herbicide (3 L ha-1) was used after planting and manual weeding. To calculate the maximum leaf area index, 10 days after transplanting, 7 sampling steps were performed every 10 days until the harvest stage and in each sampling, four mounds from each floor plot and total leaf area were determined. To determine the dry matter, sampling was done in the complete ripening stage after removing the margins from the middle rows of plots and all one square meter plants were harvested. The samples were placed separately in an oven at 75 ° C for 72 hours and then weighed and recorded.

Results showed that the impact of year and on shoot height, flag leaf length and width, RWC, SPAD and k were not significant but on LAImax, total dry matter were significant. Impact of drought stress × N fertilizer × cultivar on RWC, SPAD and k were significant. Impact of year × drought stress × N fertilizer × cultivar on LAImax and total dry matter were significant (p<0.01). Shoot height, LAImax, total dry matter, flag leaf length and width, RWC, SPAD were decreased by drought stress and were improved by increase of N fertilizer consumption under the three condition of drought stress. The lowest k (0.14) was obtained in continuous submergence and consumption of 100 kg ha-1 N fertilizer, which was same in two cultivars. The highest total dry matter accumulation (803.3 g m-2 in 2017 and 760.7 g m-2 in 2018) was obtained in Gilaneh with consumption of 100 kg ha-1 nitrogen fertilizer in continuous submergence. The highest number of SPAD in flooding with 51.8 with 100 kg of pure nitrogen per hectare in Gilaneh cultivar and the lowest with 50 kg of nitrogen fertilizer per hectare in 14 days irrigation cycle and in Gilaneh cultivar (27.65) was obtained. Hashemi cultivar showed the highest plant height (131.1 cm) in flood irrigation and the lowest plant height in 14 day irrigation (131.6 cm). Gilaneh cultivar, similar to Hashemi cultivar, showed the highest height (110.5 cm) in flood irrigation and the lowest height (100.6 cm) in 14 day irrigation.

Drought stress showed adverse effects on morphological and physiological traits, while the application of 100 kg ha-1 of nitrogen fertilizer compared to the other levels improve these traits in both cultivars stress and not-stress, and produce more dry matter in both years of study. Therefore, when there is not enough water for irrigation, it is possible to moderate the effects of water stress by using the optimal amount of nitrogen fertilizer. It can help to produce more dry matter and achieve higher yield.