Modeling Concord Grapes with “VitiSim”, a Simplified Carbon Balance Model: Understanding Pruning Effects

A simplified grape carbon balance model, called VitiSim, was adapted from the simplified apple carbon balance model developed earlier. The model uses a daily time step, a big-leaf daily canopy photosynthesis light response and respiration of organs based on mass and specific respiration rates. Weather inputs are only daily max and min temperatures and radiation. Partitioning is based on the balance of total supply to total demand with relative sink strength partitioning coefficients if the carbon supply is less than total demand. Root growth and respiration and berry set submodel have been initially developed and are being tested. Validation studies indicate that simulated total dry matter production and seasonal dry matter patterns are very realistic in behavior and amount. Seasonal dynamics of simulated carbon supply to demand suggests that the period of greatest carbon deficit is around or shortly after bloom. The period of greatest positive carbon balance appears to be just before veraison when the canopy is complete, but the crop growth is slow. The model is used to simulate the seasonal variation of carbon supply to demand in minimally and balance pruned Concord vines. In field experiments minimally-pruned vines had more stable year-to-year yields, yet could not ripen any more crop than balance-pruned vines. Simulations suggest that earlier and more rapid completion of canopy development of the minimally-pruned vines gave more positive carbon supply during the early fruit set and flower bud development period. However, later during ripening, the carbon supply was no better.