Incorporating a chronology response into the prediction of leaf appearance rate in winter wheat

The prediction of leaf appearance rate (LAR) is an important part of many crop simulation models. Most wheat simulations models assume that LAR is affected by temperature and photoperiod. This assumption ignores the fact that seed reserves contribute to a greater LAR of the first two leaves and that the LAR of subsequent leaves decreases as a result of an increase in the distance that each leaf primordium must extend before it appears. The objective of this study was to develop a generalized LAR chronology response function [f(C)] for wheat that takes into account seed reserves and the increasing distance from the meristem to the whorl for later appearing leaves. This chronology response function was then incorporated into an existing LAR model [Wang and Engel (WE) model; Wang and Engel, 1998, Agricultural Systems 58: 1-24]. This function varied from 0 to 1, being equal to 1 for the first two leaves due to seed reserves, and decreasing (taking the form of a power law) for subsequent leaves. Data from a growth chamber (two cultivars) and several field experiments (four cultivars, two years and eight sowing dates) at Lincoln, Nebraska, USA, were used as independent data to test three LAR models (Miglietta model, Miglietta, 1991, Climate Research 1: 145-150; WE model; and modified WE model). Predictions of the main stem Haun stage, both in the growth chamber and in the field, were greatly improved by incorporating f(C) into the Wang and Engel model. The root mean square error for the field data was 1.1, 0.7, and 0.3 leaves for the Miglietta model, the Wang and Engel model, and the modified Wang and Engel model, respectively.