Modelling effect of temperature on area expansion at the leaf, the shoot and the whole plant level

In this study we examine validity of the thermal time (DegreeDays) concept for modelling effects of variable temperature on leaf area expansion in kiwifruit vines. The study is based on controlled environment experiments where vines were grown under a number of variable temperature sequences having the same value of accumulated DegreeDays, and on previous experiments with sets of constant day/night temperatures (Seleznyova and Greer, 2001 Annals of Botany 88: 605-615). The data demonstrated that the total leaf area of a kiwifruit shoot could not be expressed as a function of thermal time, mainly due to a non-linear response of leaf appearance rate, positional effects on patterns of expansion of individual leaves, and an implicit effect of temperature on final area of leaves. We propose alternative methods for modelling effects of variable temperature on area expansion at the leaf and the shoot levels. In particular, we show that at the shoot level leaf area can be expressed in terms of two variables: the number of leaves that had reached their point of inflection (maximum absolute growth rate) n(I)(t), related to temperature via phyllochron concept, and the final area of individual leaves along the shoot A(n,t), depending on the leaf position and the timing of expansion. Our data suggest that temperature also affects the total number of leaves per shoot; in particular, low temperatures after budbreak result in early cessation of shoot growth.