Mediterranean semi-arid regions in northwestern Iran receive precipitation below potential evapotranspiration and are characterised by excessive heat and terminal drought stress. Exogenous application of some nanoparticles and growth regulators appears to alleviate the adverse effects of abiotic stress and improve plant performance. In the present paper, an attempt is made to evaluate the effects of foliar application of salicylic acid (SA), glycine betaine (GB), ascorbic acid (AA), nano-silica (nSiO2) and nano titanium dioxide (nTiO2) suspensions on growth and yield components of sunflower (cv. Azargol) in the semi-arid highland region of Maragheh, Iran. Plants were grown under rain-fed conditions with supplemental irrigation applied three times during the reproductive growth stages of growth. The results revealed that foliar application of SA considerably improved head diameter, chlorophyll content of plant leaves, and days to flowering, while it delayed physiological maturity for a fairly long time. Evaluation of the achene yield component indicated that application of GB significantly increased the weight of achenes when compared with other treatments. The percentage of empty achenes was considerably affected by treatments, the lowest percentage of hollow achenes being recorded for plants sprayed with GB, followed by ones sprayed with AA and nTiO2. The percentage of oil in sunflower achenes was substantially increased by foliar application of nano particles, the highest oil percentage being recorded in plants treated with nTiO2. The results also showed that the highest number of achenes per head, greatest kernel weight, and largest achene size were obtained by spraying with nanoparticle suspensions. Although all treatments improved the achene yield, the effect of nTiO2 was most prominent. The results of the present experiment support the conclusion that foliar application of nanoparticles may alleviate adverse environmental factors and improve the performance of
