Interaction of temperature with other environmental factors in controlling the development of plants.

Development is the ordered sequence of changes in plant form and phenology that occurs through time. As such it is distinguished from that of growth and includes the succession of ontogenetic stages as well as the initiation of leaves, shoots and roots. It is more useful to look at rates of development as functions of environmental influences rather than, for example, total number of days to a given stage. It is often found that linear, or close to linear, relationships are found between rates and temperature, photoperiod and the duration of cold temperatures. These ideas are treated theoretically and illustrated with data from an experiment where wheat was grown at ten sites in Britain. We conclude that temperature is the most important factor governing differences in developmental rates between sites and sowing dates, especially for later stages of development. Variation in the timing of earlier stages depends on either exposure to photoperiods of different lengths or on the degree of vernalization of the plants. Calculation is made of the maximum effectiveness of these environmental factors in shortening the vegetative phases in a range of wheat cultivars and the basis of their interaction is also explored. For wheat it seems that they act multiplicatively in their effect on developmental rate. The rate at which plants produce their leaves is important in canopy development. The sensitivity of the rate of leaf emergence, measured in thermal time, has been linked to the rate at which daylength changes (d phi/dt). A possible mechanism may involve plants responding to the ratio of light in the red (R, ca 660 nm) and far-red (FR, ca 730 nm) wavebands (R/FR). This ratio decreases when d phi/dt is becoming more negative and vice versa. Linkage can be found between the change in R/FR at twilight and commensurate changes in calculated phytochrome photoequilibria. Thus, the possibility exists that phytochrome may be involved in modulating the leaf production rate in response to different sowing dates. Alternatively, variation in rate of leaf production may be through changes in the base temperature, in response to the 'spectrum' of temperatures the plants experience during development. Thus, any involvement of photoperiod in this response may be spurious. Controlled environment studies of leaf emergence tend to support this latter interpretation as does analysis of experiments from three European sites.