R. W. Rickman

Above-ground vegetative development and growth of winter wheat as influenced by nitrogen and water availability

Abstract Assessing the influence of nitrogen and water availability on development and growth of individual organs of winter wheat (Triticum aestivum L.) is critical in evaluating the response of wheat to environmental conditions. We constructed a simulation model (SHOOTGRO 2.0) of shoot vegetative development and growth from planting to early boot by adding nitrogen and water balances and response functions for seedling emergence, tiller and leaf appearance, leaf and internode growth, and leaf and tiller senescence to the existing wheat development and growth model, SHOOTGRO 1.0. Model inputs include daily maximum and minimum air temperature, rainfall, daily photosynthetically active radiation, soil characteristics necessary to compute soil N and water balances, and several factors describing the cultivar and soil conditions at planting. The model provides information on development and growth characteristics of up to six cohorts of plants within the canopy (cohort groupings are based on time of emergence). The cohort structure allows SHOOTGRO 2.0 to provide output on the frequency of occurrence of plants with specific features (tillers and leaves) within the canopy. The model was constructed so that only water availability limited seedling emergence. Resource availability (nitrogen and water) does not influence time of leaf appearance. Leaf and internode growth, and leaf and tiller senescence processes are limited by the interaction of N and water availability. Tiller appearance is influenced by the interaction of N, radiation and water availability. Predicted and observed dates of emergence and appearance of the first tiller had correlation coefficients of 0.98 and 0.93, respectively. However, these events were, on average, predicted 3.2 and 5.2 days later than observed. SHOOTGRO 2.0 generally under-predicted the number of culms per unit land area, partially because the simulation is limited to a maximum of 16 culms/plant. Model output shows that the simulation is sensitive to N and water inputs. The model provides a tool for predicting vegetative development and growth of the winter wheat with individual culms identified and followed from emergence through boot. SHOOTGRO 2.0 can be used in evaluating alternative crop management strategies.

The physiological life cycle of wheat: Its use in breeding and crop management

Efforts to model winter wheat (Triticum aestivum L.) crop growth and yield have required scientists to define processes in wheat physiology and to link knowledge of physiology with detailed descriptions of phenology. The use of precise visual plant staging to express different phenological phases of wheat has allowed modellers to define such features as: the specific organs available for growth; resource allocations to tiller, root, and leaf growth; above versus below ground distribution of dry matter; and timing of phenological events. Use of cumulative heat units (growing degree days) to drive cereal development has simplified the expression of some of these relationships. This paper presents phenological aspects of wheat development used for modelling and will discuss how this information might be of use to breeders and crop managers concerned with selection and management of plant materials. Cultivar properties such as stress avoidance, timing of critical stages, and harvest ratios as well as management factors including spray schedules, fertilisation timing and amounts, and planting geometry can all be examined for their impacts on production by use of models. These are the types of properties that breeders can use to improve yields.