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Fifteen years of Michigan harvest data for highbush blueberry (Vacciniun corymbosum L.) were used in conjunction with daily maximum and minimum temperatures to determine appropriate heat-unit models for first-picking dates of 13 cultivars. For each cultivar, an optimal heat-unit model was chosen after evaluating the performance of a standard method with 72 combinations of three variables: a) starting date for the heat-unit accumulations (SDATE), b) low-temperature threshold (TLOW), and c) high-temperature threshold (THIGH). The optimal model sought to include the most important criteria values with respect to model performance and to minimize the average square of the prediction error (days) and the range in that error. Compared with a strict calendar-day method of estimating harvest dates, the heat-unit models reduced the standard deviation of the prediction error from 22% to 69%, depending on cultivar.
Abstract
A 2nd order equation relating net photosynthesis (Pn) to photosynthetic photon flux density (PPFD), temperature, and CO2 was determined from single leaf CO2 depletion measurements made with an open gas analysis system. From this information, a photosynthetic optimization equation was used as the basis for computer regulation of greenhouse environment control using 2 strategies. In strategy 1, both temperature and CO2 setpoints were reset every 15 min based on the PPFD in the greenhouse. In strategy 2, only the temperature setpoint was reset, CO2 was ambient. The calculated setpoints represented temperature and/or CO2 values, where predicted Pn was maximized at the particular PPFD. Both strategies were compared to a typical commercial chrysanthemum environment of 16/20/24°C (night/day/vent) with ambient CO2. Chrysanthemum morifolium Ramat. (‘Bright Golden Anne’) grown in the temperature and CO2 optimized environment had significantly greater leaf, stem, and total dry weight at flowering compared to the other 2 environmental strategies. The percentage of stem dry weight and the stem length also were increased. For all 3 planting dates the percentage of flower dry weight was reduced but statistically significant on 1 date only. Flowering date was not affected. No consistent statistical differences in plant development were observed between the temperature optimized environment and the traditional environment.