Drought is the most important crop production limiting factor in the changing climate scenario and its intensity is predicted to increase in future. Sunflower is an important oilseed crop having 8% share in the world oilseed production. Although, it is a moderately drought tolerant crop, severe drought causes reduction in the seed and oil production. Therefore, to ensure sustainable sunflower achene and oil production, it is very important to understand the relationship among the physiological, biochemical, genetic and agronomic basis of drought for its sustainable management. Impact of drought stress on various aspects of sunflower has been reported earlier but there is not a single study describing the physiological, biochemical and genetic basis of drought in sunflower at molecular and crop level. In this review manuscript, influence of drought on sunflower achene yield and oil quality has been analyzed critically at both cell, plant and crop level, and the possible management options to mitigate the severity of the drought stress are proposed. Available literature describing the impact of drought stress on physiological and biochemical aspects (like, photosynthesis, water relations, nutrient uptake and oxidative damage), morphological and growth parameters and achene yield and oil quality has been discussed critically. Based on the discussion on the impact of drought stress, various management strategies, such as breeding for drought tolerance (conventional or biotechnological), exogenous application of hormones and osmoprotectants, seed treatment and soil nutrient management has been reviewed and discussed. It is concluded from discussion that sunflower responds to water stress by osmotic adjustments, turgor maintenance, carbon assimilation maintenance and hormonal regulations. A comprehensive research on integration of different management options, including agronomic management, conventional breeding and modern biotechnological advances, is needed for the sustainable improvement of sunflower achene yield and oil quality under drought stress. This may also contribute significantly under a climate change scenario.

Introduction

Sunflower (Helianthus annuus L.) is an annual oilseed crop globally cultivated on 24.77 million hectares with a production of 44.31 million metric tons and it has 8% share in world oilseed market (USDA, 2016). Sunflower contains 40–۵۰% oil and 17–۲۰% protein, thus have a fair potential to narrow the gap between production and consumption of edible oil and animal feed in the world. Actually, it is a crop of tropical and subtropical regions with semi-arid to arid climate, and frequently grown in dry lands or on supplementary irrigation. Therefore, the crop is affected by ambient environmental conditions like heat and drought (Pekcan et al., 2015; Robert et al., 2016). However, in a climate change scenario or/and with the onset of early droughts, the crop may be affected by drought stress (Debaeke et al., 2017). In addition to the hiking problem of water stress, the area devoted to irrigated food production systems is expected to decrease resulting in lesser food production (Alexandratos and Bruinsma, 2012; Farooq et al., 2012). Nonetheless, expansion of irrigated land is not possible because of water competition among domestic, industrial and agricultural users (Alexandratos and Bruinsma, 2012). Severe water competition among different users will predominately change the irrigated lands to rain-fed systems and ultimately crops have to suffer from periodic events of drought stress (Elliott et al., 2014). Hence, all of agricultural crops and even the moderately drought tolerant crops such as sunflower will be badly affected by drought stress. Severe drought events have been recorded in Asia and beyond, including the countries with arid and semiarid climates during the last decades (Miyan, 2015; Farooq et al., 2012, 2014), which made management of drought stress more challenging. Although, sunflower is moderately drought tolerant crop (due to drought escape behavior), it is highly sensitive to drought and heat stresses from early flowering to achene filling due to inefficiency in regulating the leaf expansion and transpiration rates under inadequate availability of soil moisture (García-López et al., 2014). The decline in soil moisture leads to leaf wilting, which results in substantial yield reduction in semi-arid areas receiving the low rainfall (Aboudrare et al., 2006). Several reports indicate that drought stress significantly reduces sunflower achene yield, oil yield and oil quality globally (Soleimanzadeh et al., 2010; Babaeian et al., 2011; Oraki and Aghaalikhana, 2012; Ibrahim et al., 2016).