Crop yield response to water - Cra

genotypes to photoperiod is variable, exhibiting a long-day response for floral initiation inmost sunflower genotypes. Water and N supply are secondary factors in determining thetiming of floral initiation. Cultivar differences for maturity date are usually associated withchanges in the length of the vegetative period before the head becomes visible, and longerperiods usually are associated with higher leaf numbers per plant. Therefore, the length ofthe vegetative period depends on the cultivar, temperature and photoperiod (Rawson et al.,1984). As the head enlarges and the two or three leaves subtending the head approach theirfinal size, anthesis begins and lasts 8 to 12 days. The number of grains per head are determinedin a period lasting 30 to 40 days centred around anthesis. Post-anthesis effect on grain numberis mostly the result of abortion of the youngest developing grains, apparently as determinedby the amount of available assimilate to fill them. Canopy senescence starts with the oldestleaves soon after anthesis, but the upper part of the canopy stays green until a couple ofweeks before physiological maturity.Sunflower has a deep and aggressive root system and, as already mentioned, is capable offully depleting the water present in subsoil layers (Bremner et al., 1986; Sadras et al., 1989).Rooting depth is one of the highest among annual crops, with some studies reporting arooting depth beyond 3 m in easily penetrable soils. Rooting depth depends on life cycle lengthof the cultivar; long-season cultivars may reach 3 m while the root system of short-seasoncultivars in the same soil would not extend beyond 2.3 m (Gimenez and Fereres, 1986). Therate of root deepening of the sunflower is also very high, averaging 3.5 cm/day in the abovementionedstudy. In the very open soils of the experimental farm at University of California,Davis, apparent deepening rates of sunflower of about 4.5 cm/day have been observed in afield where maize and sorghum root systems deepened at about 3 cm/day (Berengena, 1976,unpublished).Water Use and ProductivityThe combination of season length and different climate generates a wide range of consumptivewater use (ET) by sunflower. ET for short-season cultivars may be less than 450 mm, while forlong-season cultivars it may exceed 800 mm in some situations (Gimenez and Fereres, 1986).Typical values between 500 and 650 mm are normally found. For a C 3 species sunflower hasa high photosynthetic rate, and its efficiency in the use of transpired water for biomassproduction (WP B/et ) is between 2.5 and 3.5 kg/m 3 (25 to 35 kg/ha per mm). WP B/et decreasesafter anthesis because of the high energy requirements for the biosynthesis of sunflowerseeds high in oil content (Villalobos et al., 1996; Steduto et al., 2007).Responses to stressesSunflower is often reported as a drought-tolerant crop given its high capacity to extract waterfrom the subsoil. Water deficits have differential effects on leaf expansion and stomatalconductance, and hence on transpiration of sunflower genotypes (Connor and Jones, 1985;Connor et al., 1985). No impact has been observed on leaf expansion until the fraction of totalavailable water (TAW) in the root zone declined below 0.85, but it has to decline below 0.4to induce stomatal closure. The threshold for leaf expansion rate depends on the evaporativedemand (Sadras et al., 1993). Prior to anthesis, transpiration is largely dependent on canopysunflower 167