Leaf gas exchange and radiation use efficiency of sunflower (Helianthus annuus L.) in response to different deficit irrigation strategies: From solar radiation to plant growth analysis

• RUE and photosynthesis under sunflower deficit irrigation management were evaluated.
• RUE was stable under different water supplies but decreased after flowering.
• Early water stress accelerated the decline of photosynthesis after the flowering.
• A curvilinear response was found between RUE and net photosynthesis.
• Sunflower irrigation up to heading ensured high water productivity.

Most studies on radiation use efficiency (RUE) have focussed only on the relationship between intercepted solar radiation and biomass accumulation, without considering the intermediate steps that underlie the development of the crop. The present study aims to estimate the RUE of sunflower (Helianthus annuus L.) under different deficit irrigation schemes, with the introduction of an intermediate step at the leaf scale, the net photosynthesis rate (PN), and the exploration of the relationship between RUE and PN. The linear relationship between intercepted photosynthetically active radiation (iPAR) and total plant dry matter was defined during the vegetative phase, and a single value of RUE (2.08 g MJ−1) was predictive of biomass accumulation; the curvilinear response of RUE with respect to PN explained one of the hierarchical processes that governs the smooth and stable conversion efficiency of iPAR to biomass. However, different RUE values should be used to estimate seed yield when irrigation is suspended at the heading (RUE, 0.29 g MJ−1), flowering (RUE, 0.35 g MJ−1) or milking (RUE, 0.44 g MJ−1) stages, because after the vegetative stage, synchronization of the processes underlying the late growth – grain filling phases was lost. Despite this, even if the yield performance followed the water treatment (2.69, 3.61 and 4.36 t ha−1 for increasing water supply), the productivity of sunflower with small (150 mm) or moderate (270 mm) amounts of water ensured satisfactory seed production and water saving (74% and 53% reduction, compared to well-watered treatment) in water-limited environments.