Sunflower evapotranspiration and water use efficiency in response to plant density

This study quantified sunflower (Helianthus annuus L.) water use efficiency for seed (WUEg) and for oil yield (WUEo) and their components (i.e., evapotranspiration [ET], seed and oil yields) in response to plant density increments and studied, in particular, the underlying processes relevant to the responses of crop ET to plant density (i.e. water uptake pattern, intercepted photosynthetically active radiation [iPAR], and soil water depletion profile). Sunflower was grown at 3, 6, and 9 plants m−2 in Season 1 and at 2, 3 and 6 plants m−2 in Season 2. Measurements included (i) soil water content and iPAR during the whole growing season, and (ii) seed and oil yield at physiological maturity. Crop ET was estimated by means of a water balance, and it was analyzed during three different periods (i.e. vegetative, critical period for seed set, and seed filling period). Increasing sunflower plant density from 2 to 9 plants m−2 did not modify seasonal ET, but it changed the water uptake pattern through the season; it increased ET during the vegetative period by depleting more water from deep soil layers (i.e., 41–140 cm), but it reduced ET during the seed‐filling period. Increments in plant density increased seed and oil yield in accordance with significant iPAR increments, whereas harvest index remained stable. Sunflower WUEg ranged from 4.8 to 9.4 kg ha−1 mm−1, WUEo ranged from 1.8 to 4.5 kg oil ha−1 mm−1 across plant densities, and they were positively and closely associated with seasonal iPAR.