Validating an apple dry matter production model with whole canopy gas exchange measurements in the field

The simplified apple dry matter production model developed by Lakso and Johnson (1990) was modified by inputing tree-specific parameters from a study of seasonal growth and gas exchange of 4-year-old Empire/M.9 apple trees, and light and temperature response curves developed for Empire apple organs. Measurements of the seasonal trend of diurnal net C02 and canopy water vapor exchange were made at intervals on three four-year-old 'Empire'/M.9 slender spindle apple trees in the orchard from 10 days after bloom until 25 days after harvest. The tree canopies were enclosed in clear plastic "balloon- type" chambers (similar to Corelli and Magnanini, 1993) that was monitored continuously for more than 40 days with an automated control/datalogging system. The measurements over the season under different weather conditions and with late-season reductions in leaf photosynthesis due to pests gave a good range of values with which to test the model. In general, the model simulations showed the same seasonal patterns of gas exchange as the measurements, and gave actual values quite close to those measured. Variation in the canopy gas exchange rates after pest damage were not adequately reflected in the sampled single leaf gas exchange rates that were used as model inputs, suggesting that leaf sampling patterns should be adjusted for non-uniform pest damage.