Foliar application of cytokinin hormone on morphophysiological and yield traits of sunflower cultivars in late sowing date

Introduction
Crop yields in tropical regions is threatened by high temperatures during reproductive phase which causes a severe change in physiological and biochemical behavior of the major crops (Salehi et al., 2023). A suitable sowing dates provide adequate growth and development for crops and altered sowing dates is beneficial for crop productivity as it helps the critical growth stages avoid high temperature stress. Manipulating the sowing date is one of the the most important management practices for improving crop yield and resource use efficiency (Srivastava et al., 2018). It has been reported that the highest grain and biological yield of safflower were observed on early sowing date, compared to late sowing dates (Sahu and Thakur., 2016)). Delay in sowing date causes shortening of the growing season and distance of the plant from suitable growing conditions. Cytokinins are effective in response to environmental stresses by triggering a complex signaling network (Verma et al., 2016). To further improve plant growth and production, exogenous cytokinin has been applied to enhance the heat stress tolerance of plants. Exogenous application of cytokinin reduced the inhibitory impact of heat stress on photosynthetic properties, and increased antioxidant system activity (Xu et al., 2010). Hence, the current study aimed to evaluate the effect of cytokinin foliar on morphophysiological traits and quantitative yield of sunflower cultivars in winter planting dates in Ahvaz conditions.
Materials & Methods
A field experiment carried out in a split plot factorial in randomized complete block design with three replications at the research farm of Shahid Chamran University of Ahvaz in 2022. Main plots consisted of two sowing dates; 3th February and 5th March (Normal and late sowing dates, respectively), and sub plot consisted of factorial arrangement of different concentrations of cytokinin (0 and 50 μm l-1, Kinetin), and sunflower cultivars (Progress, Lakomka, Shams, Oskar and Qasem). The cultivation was performed on 3th February as normal sowing date plants experienced optimum temperature during growth stages. On 5th March as late sowing date, the flowering and grain filling periods were exposed to severe heat stress at the end of growing season
Results & Discussion
Terminal heat stress in late sowing dates caused a significant decrease in in biological yield, grain yield, grain number per head, 1000-grain weight, oil yield, chlorophyll index, relative water content and stomatal conductance. There was a significant difference among cultivars in terms of all traits. Late sowing date was caused a significant reduction in grain yield of Lakomka, Progress, Shams, Oskar and Qasem cultivars by 32, 30, 32, 37 and 33%, respectively, compared to normal sowing date. It was equal to 1.1, 1.03, 1.1, 1.27 and 1.13% for each day of sowing delay, respectively. The concentration of 50 μm l-1 kinetin improved the studied traits in both sowing dates. In late sowing date, foliar application of cytokinin enhanced grain yield by 24, 29, 14, 25 and 21%, in Lakomka, Progress, Shams, Oskar and Qasem cultivars respectively, when compared to cytokinin-deficient plants. Lakomka and Progress had the highest grain and oil yield in both sowing dates. In optimum and late sowing dates; cytokinin application induced an increase in grain yield (by 11 and 24%, respectively) and oil yield (by 27and 34%, respectively) for Lakomka, when compared to cytokinin-deficient plants. Lakomka cultivar identified as the most tolerant to heat stress due to higher grain and oil yield in both sowing dates and both cytokinin levels compared with other cultivars.
Conclusions
We focused on foliar application of cytokinin on morphophysiological traits and quantitative yield of sunflower cultivars in response to terminal heat stress to highlight the function of cytokinin in heat stress response and it is potential to increase heat stress tolerance. It appears the delay in optimum sowing date leads to synchronize the plant's reproductive stages with a high temperature at the end of the season. The results of this study provides evidence that exogenous cytokinin (50 μm l-1) can be an effective strategy in improvement of heat tolerance of sunflower under terminal heat stress conditions in tropical regions such as Khuzestan.
Acknowledgements: We gratefully acknowledge funding support from Shahid Chamran University of Ahvaz grant number SCU.AA1401.96.
Key words: Chlorophyll index, grain yield, oil percentage, stomatal conductance