SEASONAL-CHANGES IN NET CARBON-DIOXIDE EXCHANGE-RATES OF AUTUMN-BLISS, A PRIMOCANE-FRUITING RED RASPBERRY (RUBUS-IDAEUS L.)

Seasonal changes in leaf net carbon exchange rate (NCER), stomatal con ductance (g(s)), and intercellular CO2 (ci) were determined for 2-yr-o ld potted Autumn Bliss (Rubus idaeus L.) plants grown under field cond itions. NCER varied inconsistently between leaves which subtended frui ting lateral branches (laterals) and those that did not. in leaves wit h fruiting laterals, it was lower on three dates, similar on three oth er dates and once greater than in leaves without fruiting laterals. Ev idence of nonstomatal inhibition of photosynthesis was also apparent a s leaf NCER and g, fluctuated during the season while ci remained rela tively constant. The leaf chlorophyll content increased when fruiting laterals were present, but this did not produce a consistently higher leaf NCER. The SPAD-501 meter provided a rapid and accurate, nondestru ctive estimate of leaf chlorophyll content for Autumn Bliss red raspbe rry leaves. Under uniform environmental conditions, all healthy, fully expanded leaves along the primocane had the same photosynthetic poten tial. Also, leaf age did not influence leaf gas exchange until the lea ves started to senesce. Therefore, good light interception throughout the canopy is needed to optimize dry matter production in primocane-fr uiting red raspberries.