CONVERGENCE AND SYNCHRONY - A REVIEW OF THE COORDINATION OF DEVELOPMENT IN WHEAT

Although the ears, spikelets and florets of a wheat crop are initiated consecutively, all of the resulting grains of the crop ripen synchronously. Similarly, crops of winter wheat sown at widely different dates tend to ripen during the same period of a few days in the following summer. This review, which draws evidence from winter, spring and day-neutral varieties, and from a range of temperate and mediterranean environments, explores the mechanisms leading to this convergence of organ, plant and crop development. Central to these mechanisms are the roles of the environment (temperature, including vernalisation, and photoperiod) in determining the time of initiation of reproductive development (initiation of the collar) and the rates and durations of organ initiation (leaves, tillers, spikelets, florets); however, the mortality of organs, notably tillers and florets, caused by intra- and inter-plant competition also plays a major part in determining the duration of developmental processes and the number of initiated organs which survive. The observed synchrony of crop development cannot be achieved without the coordinated responses of a number of developmental processes (e.g. leaf and ear development) to the environment; how this coordination is achieved by the plant is largely unknown, and in spite of the widespread use of plant growth regulators, molecular aspects of the control of plant development are poorly characterised. In practice, the convergence of development rarely fails, and the few examples of failure of synchrony (e.g. in crops which have lodged or where an inappropriate variety has been used) are reviewed.