Seasonal variation in the gas exchange characteristics of Primula species

Seasonal dynamics of CO₂ uptake are presented for three species of Primula native to the British Isles. Leaf photosynthetic rates were estimated in the field from the assimilation of14CO2. Maximum photosynthetic rates showed a steady decline as the year passed in all three species but P. vulgaris displayed a peak in photosynthetic rate prior to canopy expansion which was not observed in the other species. Overwintered leaves of P. vulgaris photosynthesised at comparable rates to those of leaves at the end of the previous growing season. The carbon gain due to these overwintered leaves together with that from utilization of the high, spring light phase by the first formed leaves could result in a greater allocation of resources to flowering. Information from a comparison of light response curves measured at a similar developmental stage, in the field and the laboratory, allowed the species to be ranked according to their degree of shade tolerance; P. vulgaris greater than P. elatior greater than P. veris.