ABSTRACT
Summary Productivity of white clover (Trifolium repens L.) is limited by plant characteristics that prevent efficient use of environmental inputs. In the temperate zone, low temperature often limits growth. Comparisons of ecotypes originating from sites with cool or warm climatic conditions (low or high altitudes) can show, when plants are grown in a common environment, the degree of adaptation of processes related to net photosynthesis and the distribution of dry matter. Our objectives were to determine the photosynthetic responses to temperature in relation to the thermal regime during growth and the effects of temperature and daylength on partitioning of dry matter and nonstructural carbohydrates. Vegetatively propagated plants from 600 m (valley type) and 2,000 m above sea level (alpine type) were grown in the field and in growth chambers. Photosynthetic rate and assimilate distribution were determined. The alpine genotype and plants grown at low temperatures showed relatively high photosynthetic rates at 3°C but limited photosynthesis at 24°C, probably due to intracellular shortage of CO2 and increased photorespiration. The valley genotype and plants grown at higher temperatures showed high photosynthetic rates at high temperatures but a relatively low rate at low temperatures, probably due to the decreased concentration of photosynthetic apparatus/unit leaf area. At low temperatures, pool sizes of metabolites of the carbon cycle suggest limitations by processes other than carboxylation. The high rate of photosynthesis of the alpine genotype was not associated with a high forage yield. The lower yield of the alpine genotype cannot be explained by leaf-area ratio but by leaf-area index and mean sward height. Growth temperature and particularly daylength modified the pattern of assimilate distribution more than did genotype. Plants grown at low temperatures showed increased content of nonstructural carbohydrates. This feature was associated with a greater decrease in rate of leaf appearance than in photosynthesis as temperatures were decreased. In addition, when daylength was reduced at low temperatures, 82% of all nonstructural carbohydrates were in the stolons and roots, compared with 55% to 65% in long days. Higher photosynthetic rate at low temperatures and a stronger shift to growth of storage organs of the alpine genotype are consistent with requirements of its ecological origin.
