Seasonal Patterns of CO2Exchange in the Shoot and Root of Loblolly Pine Seedlings

Abstract

Seedlings of 6 full-sib families of loblolly pine were grown outdoors in clay pots for 2 growing seasons. Dark respiration of shoot and root and CO2 exchange of the shoot in the light were measured periodically over a temperature range bracketing ambient conditions. Both shoot and root showed different physiological responses as seasonal temperature changed, but differences among crosses were not apparent. Optimum temperatures for net photosynthesis shifted with seasonal temperatures from a high of nearly 25.degree. C in midsummer to a low of 10.degree. C in midwinter. Root respiration was higher at 30.degree. C in the summer than at 30.degree. C any time after Aug. and higher at 0.degree. C in the winter than during summer. Dark shoot respiration was high during initial shoot elongation and 2nd-year bud break but fell thereafter. Rates of net photosynthesis remained high for a longer period of time before declining. A peak in root respiration occurred in Oct. of the 2nd year following a steady decline throughout the growing season. A period of rapid root growth in late autumn appeared to follow summer shoot growth.