Effects of sowing date and cultivar on spike weight and kernel number in durum wheat

In wheat, spike weight is associated with kernel number. The response of spike weight to photoperiod and the amount of radiation available during the period of spike growth and the associated changes in spike : stem ratio were investigated through field trials involving three durum wheat cultivars with different flowering time over two seasons and three sowing dates. Across the three cultivars spike and stem weight differed in response to the photoperiod and to the photothermal quotient, i.e. the ratio between intercepted radiation and temperature; this reflected the sensitivity of the spike : stem ratio to the environmental conditions induced by sowing date, which affected the allometry of the ratio. The photothermal quotient (0.14–1.70 MJ m–2 day–1 °C–1) explained most of the variation in both spike weight (83–270 g m–2) and kernel number per m2 (2638–13 993), across all the environments sampled. The phenology explained a significant portion of the variation in spike weight, but its influence was minor compared with the combined effects of the quantity of intercepted radiation and the temperature. Therefore, the correlation between kernel number and the photothermal quotient before anthesis was more sensible to the environmental variation induced by sowing date beyond its conventional window.