Several models of apple tree carbon balance have been developed, including a simplified model by our lab. Tree photosynthesis and total dry matter production is the best characterized except for root growth and root respiration. Once dry matter is produced and partitioned to the different organs (another key problem for modeling), the effects of carbon availability to the fruits on their growth and abscission needs to be modeled. Our approach is based on an observed relationship between increased abscission with decreased fruit growth rate of populations of fruit. From several empirical studies of fruit growth and abscission during chemical thinning or imposed stress early in the season, a relationship was found between % abscission and classes of fruit growth rates. It appears to be best if the fruit growth rate is expressed as a percent of the growth rate of the fastest growing group of fruits in each study. Thus in the model the fruit growth allowed by the available carbon each day is compared to a pre-determined maximum growth rate for the cultivar. The percent-of-maximum growth rate then determines how much abscission will occur. Then the growth rate of the remaining fruit is calculated. Additional parameters of the model allowed for a multiple-day buffer of carbon availability, an imposed fruit number reduction (i.e. equivalent to hand thinning), and temperature effects. Although there are more improvements planned, the initial tests have been promising with the simulations showing realistic patterns of fruit abscission and fruit growth.
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