SEASONAL-CHANGES IN CO2 ASSIMILATION OF CRANBERRY LEAVES

CO2 assimilation rates of current season and 1-year-old leaves were me asured under field conditions in 1991 and 1992 on cranberry (Vaccinium macrocarpon Ait. cultivars 'Stevens' (high yielding) and 'Searles' (l ow yielding)) uprights. One-year-old cranberry leaf CO2 assimilation r ates increased from May to early June to a peak of about 22 mu mol CO2 kg(-1) dry weight s(-1). Most 1-year-old leaves were retained on upri ghts into the second growing season until May, after which there was a steady decline in leaf area per upright due to abscission. The early positive CO2 assimilation rates measured on 1-year-old leaves suggest that they can provide photoassimilates for early growth. Current seaso n leaves reached a maximum CO2 assimilation rate of 45 mu mol CO2 kg(- 1) dry weight s(-1) almost immediately upon expansion. In both types o f leaves, the CO2 assimilation rate declined from August through Octob er when leaves turned visibly red. During the entire growing season, t he CO2 assimilation rate of current season leaves was almost twice tha t of 1-year-old leaves. The significantly higher CO2 assimilation rate in current season leaves than 1-year-old leaves and the continuing ab scission of 1-year-old leaves suggest that during fruit set and develo pment, current season photoassimilates from new leaves are the primary source of carbohydrates for fruit set and growth. 'Searles' cranberri es had higher CO2 assimilation rates than 'Stevens', but 'Stevens' had greater leaf area per upright and more uprights per ground area than 'Searles' and as a result, 'Stevens' has higher yields than 'Searles'.