The thermal interval for leaf tip appearance (phyllochron) is a critical variable for modelling plant development and growth. Phyllochron varies across environments, but is generally constant for a species grown in specific environments. For example, the phyllochron in maize is lower in temperate environments than in tropical and subtropical environments. The limitation of existing data is that each experiment has been evaluated in a narrow range of environments, and underlying mechanisms have not been adequately examined. Consequently, no method is available to model the variation across environments. Models use constant values that are fixed for particular environments. This situation is unsatisfactory, as model users must have values that have been determined for their locality. A method of adjusting the value of phyllochron according to genotype adaptation groups or environmental conditions is required. Of these two options, the latter appears most promising as there is little, if any, variation among genotypes when grown in specific environments. Recent information shows that phyllochron in maize is related to light intensity. The data also suggests that phyllochron depends on the adequacy of current photosynthesis (source) to meet the demands of the plant for growth (sink), one aspect of which is the production of new leaves. This paper reports on concepts and early progress in relating phyllochron to both irradiance and thermal time
