Abstract
Primordium initiation on the main stem of field-grown Japanese spring type wheat was studied with a scanning electron microscope to quantify the process determining the number of leaves, spikelets and florets. The number of leaves and spikelets was related to growing degree-days (GDD) with a three-piece linear-spline model. The rate of primordium initiation was 0.0181 [leaf (°C d)-1] and 0.0995 [spikelet (°C d)-1], and the duration of leaf and spikelet initiation was 374 °C d and 149 °C d, respectively. The relationship between the number of floret per spike and GDD was described with the Gompertz model and a linear regression model. The initiation of florets commenced at 494 °C d, and the number of florets reached the maximum (116 florets per spike) at 821 °C d. The number of florets per spikelet was closely correlated with GDD in all spikelet positions except the 15th spikelet. These significant linear equations made it possible to estimate the start of floret initiation, the rate of floret initiation and the time of initiation of the most distal floret per spikelet that formed grain at maturity (ISF). The number of grains per spikelet was maximum at the central spikelet and declined both toward the basal and distal spikelets. There were close relationships between the number of grains per spikelets and the start of floret initiation. The variations in the rate of floret initiation and ISF with the spikelet position were demonstrated and the significance of those traits to the distribution pattern of grain number per spikelet was discussed.
