Influence of Foliar Application of Some Chemicals on Gas Exchange, Water Relations and Photosynthetic Traits in Sunflower (Helianthus annuus L.) under Different Irrigation Regimes

Many solutions have been introduced to enhance drought tolerance. Exogenous applications of various chemicals such as compatible solutes, growth regulators and stress-messenger molecules have been considered during recent years. Glycine betaine (GB) is one of the most effective osmoprotectants. The physiological and antioxidant properties of selenium (SE) have become of higher importance for biologists. Salicylic acid (SA) plays an important role in the abiotic stresses tolerant. Nitric oxide plays an important role in many physiological and vegetative processes such as seed germination, stomatal closure, root development, aging and adaptation to several stress types. Sodium nitroprusside (SNP) is commonly used as a nitric oxide releasing agent in plants. Abscisic acid (ABA), as a messenger, plays a role in responding to drought and other environmental stresses, as well as in many physiological processes such as photosynthesis and regulation of stomatal opening and closure. The aim of this study was to compare the influence of some chemicals as foliar application in order to reduce the adverse effects of drought stress in sunflower. This experiment was conducted at the Agricultural Research Center of Hamedan, Iran, during 2015-2016. Three irrigation and six foliar application treatments were evaluated in a split plot experiment. Irrigation treatments consisted of 60%, 80% and 100% of crop water requirement and foliar application treatments included: abscisic acid 40 M, Selenium 20 mg L-1, Salicylic acid 500 M, SNP 100 μM, Glycine betaine 100 mM, and control. An infrared gas analyzer (IRGA, Lci, ADC Biosientific Ltd, Hoddeston, UK) was used in order to measure the photosynthesis rate, stomatal conductance, transpiration rate, leaf temperature and sub-stomatal CO2 concentration. Photosynthetic water use efficiency was calculated from photosynthesis rate divided by transpiration rate. The leaf relative water content (RWC) was calculated as the follows:RWC (%) = [(FW - DW.) / (TW – DW)] × 100
Where FW is the leaf fresh weight, DW is the leaf dry weight and TW indicates the leaf turgid weight.
Combined analysis of variance of two years data was done using SAS statistical software. The comparison of means was carried out by Duncan's multiple range test at 5% of probability level. Effects of irrigation and foliar application on stomatal conductivity and transpiration rate were significant. Stomatal conductivity and transpiration rate were decreased by decreasing irrigation amount. The lowest rate of stomatal conductivity and transpiration rate in all irrigation conditions were resulted from ABA. Leaf temperature and RWC were affected by irrigation and foliar application. The lowest leaf temperature was obtained under normal irrigation. The highest RWC (90.8%) was attributed to normal irrigation. Applying 80% and 60% of crop water requirement reduced RWC about 10% and 28.5%, respectively. Foliar application of chemicals increased RWC. The effects of irrigation and irrigation × foliar application on photosynthesis rate were significant. The photosynthesis rate decreased with decreasing the irrigation amount. Sub-stomatal CO2 concentration decreased under 80% of crop water requirement and it increased in 60% of crop water requirement. Under normal irrigation conditions, photosynthetic water use efficiency enhanced by foliar application of SA, but under water deficit conditions, foliar application with all compounds promoted photosynthetic water use efficiency. Decreasing of irrigation water had significant effects on gas exchange and photosynthetic properties of sunflower and caused a sharp decline in stomatal conductance and transpiration rate. Furthermore, RWC and photosynthesis rate were decreased, but leaf temperature were increased simultaneously. By decreasing irrigation, the sub-stomatal CO2 concentration initially reduced and continued to increase with more water limitation, while the photosynthetic water use efficiency was increasing gradually. Foliar application with chemicals affected on gas exchange and photosynthetic properties of sunflower. The lowest stomatal conductance and transpiration rate, and the highest amount of RWC were obtained by foliar application of ABA and SNP. In normal irrigation, SA improved photosynthesis rate, but ABA, SNP and SE reduced the sub-stomatal CO2 concentration and photosynthetic rate. Under low irrigation, foliar application of all chemicals increased photosynthesis rate and reduced the sub-stomatal CO2 concentration. SA had a more positive impact rather than other compounds.