Seasonal patterns of CO2 assimilation (ACO2), leaf water potential (ψ1) and stomatal conductance (g1) were studied in three clones (`Augusta', `Brunswick', and `Chignecto') of lowbush blueberry (Vaccinium angustifolium Ait.) over two growing seasons. Plants were managed in a 2-year cycle of fruiting (year 1) and burn-prune (year 2). In the fruiting year, ACO2 was lowest in mid-June and early September. Rates peaked between 10 and 31 July and declined after fruit removal in late August. Compared with the fruiting year, ACO2 in the prune year was between 50% and 130% higher in the early season, and between 80% and 300% higher in mid-September. In both years, however, mid-season maximum ACO2 for each clone was between 9 and 10 μmol·m–2·s–1CO2. Assimilation of CO2 increased with increasing photosynthetic photon flux (PPF) to between 500 and 600 μmol·s–1·m–2 in `Augusta' and `Brunswick', and to between 700 and 800 μmol·s–1·m–2 in `Chignecto'. Midday ψ1 was generally lower in the prune year than in the fruiting year, reflecting year-to-year differences in soil water content. Stomatal conductance (g1), however, was generally higher in the prune year than in the fruiting year over similar vapor pressure deficit (VPD) ranges, especially in June and September when prune year g1 was often twice that observed in the fruiting year. In the fruiting year, g1 declined through the day in response to increasing VPD in June, but was quite constant in mid-season. It tended to be higher in `Augusta' than in the other two clones. Stomatal closure imposes limitations on ACO2 in lowbush blueberries, but not all seasonal change in C-assimilative capacity can be explained by changes in g1.
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