Number of Seeds per Unit Area in Sunflower Correlates Well with a Photothermal Quotient

Potential yield in several crop species is affected by radiation and temperature when they are grown under unstressed conditions. This issue has been not explored for sunflower (Helianthus annuus L.), and the objectives of our experiments were (i) to analyze the dependence of seed number (SN, seeds m−2) on the thermal and light conditions during the period from floret initiation to the end of seed‐setting and (ii) to determine whether sensitivity of SN to radiation differs between pre‐ and post‐anthesis. Inter and intra‐annual variations in sowing dates and artificial shading were used to expose irrigated crops to different conditions of radiation (range 14–24 MJ m−2 d−1) and temperature (20–24°C) (temperature/radiation combinations = 18). Significant positive (P < 0.00001) relationships between SN (range 3499–11547 seeds m−2) and intercepted (r2 = 0.65) an incident (r2 = 0.56) short‐wave radiation during the floret initiation to seed‐setting period were found. A weaker negative association with temperature (r2 = 0.29; P < 0.05) was also found. The relationship between SN and a photothermal quotient [PQ = mean radiation/ (mean temperature‐base temperature), MJ m−2 d−1 °C−1], calculated with intercepted radiation was better than that achieved by either variable alone (SN = −436 + 9214 PQ; r2 =0.74; P < 0.000001). We conclude that PQ would be a better tool to predict SN in sunflower than the use of radiation or temperature alone. Crops subjected to shading after anthesis suffered significantly greater (P < 0.05) reductions in SN, at equivalent PQ, than those exposed to shading before anthesis.