ABOVEGROUND VEGETATIVE DEVELOPMENT AND GROWTH OF WINTER-WHEAT AS INFLUENCED BY NITROGEN AND WATER AVAILABILITY

Assessing the influence of nitrogen and water availability on developm ent 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 addin g nitrogen and water balances and response functions for seedling emer gence, tiller and leaf appearance, leaf and internode growth, and leaf and tiller senescence to the existing wheat development and growth mo del, 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 s everal factors describing the cultivar and soil conditions at planting . The model provides information on development and growth characteris tics of up to six cohorts of plants within the canopy (cohort grouping s are based on time of emergence). The cohort structure allows SHOOTGR O 2.0 to provide output on the frequency of occurrence of plants with specific features (tillers and leaves) within the canopy. The model wa s constructed so that only water availability limited seedling emergen ce. Resource availability (nitrogen and water) does not influence time of leaf appearance. Leaf and internode growth, and leaf and tiller se nescence processes are limited by the interaction of N and water avail ability. Tiller appearance is influenced by the interaction of N, radi ation and water availability. Predicted and observed dates of emergenc e and appearance of the first tiller had correlation coefficients of 0 .98 and 0.93, respectively. However, these events were, on average, pr edicted 3.2 and 5.2 days later than observed. SHOOTGRO 2.0 generally u nder-predicted the number of culms per unit land area, partially becau se the simulation is limited to a maximum of 16 culms/plant. Model out put 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 fro m emergence through boot. SHOOTGRO 2.0 can be used in evaluating alter native crop management strategies.