The Effect of Temperature on Photosynthesis and Carbon Fluxes in Sunflower and Rape

Abstract

Sunflower (Helianthus annuus L.) and oilseed rape (Brassica napus L.) were grown at constant temperatures of 30 °C (warm) and 13 °C (cold). Maximal rates of photosynthesis between 5 °C and 35 °C were at higher temperatures in sunflower than rape. Photosynthetic rate over 4 h at the growth temperature declined in warm-and cold-grown rape and cold-grown sunflower, but remained constant in warm-grown sunflower. The stimulation of photosynthesis by 2 0 kPa O2 compared to 21 kPa 02 declined with decreasing temperature. At 10 °C in warm-grown rape photosynthesis was insensitive to 2-0 kPa 02. However, sensitivity to low O2 continued at 10 °C in warm-grown sunflower. Carbohydrates accumulated in the cold, particularly fructose, glucose and sucrose in warm-grown sunflower transferred to 13 °C. By monitoring changes of 14C in leaves after the assimilation of 14C02, the rates of carbon export from leaves, pool sizes and carbon fluxes between them were estimated. The transfer of warm- and cold-grown rape to 13 °C and 30 °C, respectively, had little effect on these parameters over 22 h. However, export of carbon from sunflower leaves at 13 °C was markedly less than at 30 °C, irrespective of the growth temperature, due to slower export from the transport pool. The rapid suppression of carbon export at 13 °C in warm-grown sunflower may be due to inhibited translocation rather than reduced sink demand in the cold. It is concluded that assimilate utilisation is more depressed in the cold than is photosynthesis; this imposes a greater restriction on biomass production in sunflower than in rape.