THE EFFECT OF COOLING ON PHOTOSYNTHESIS, AMOUNTS OF CARBOHYDRATE AND ASSIMILATE EXPORT IN SUNFLOWER

Sunflower plants (Helianthus annuus L.) grown at 30-degrees-C were cooled to 13-degrees-C in the light in atmospheric CO2 or low CO2, or in darkness. Photosynthetic rate at 30-degrees-C after cooling whole plants in atmospheric CO2 for 12 h during a photoperiod was significantly lower than at the start of the photoperiod compared to plants cooled at low CO2, those cooled in the dark and those maintained at 30-degrees-C. Amounts of sucrose, hexoses and starch in leaves at 13-degrees-C increased throughout a 14 h photoperiod to levels higher than in leaves at 30-degrees-C, where amounts of sucrose and hexoses were stable or falling after 4 h. Carbohydrate accumulation at 13-degrees-C during this photoperiod was more than twice that at 30-degrees-C. After three photoperiods and two dark periods at 13-degrees-C carbohydrate levels in leaves were still as high as at the end of the first photoperiod, but less carbohydrate accumulated during the photoperiods than during the first photoperiod, and more was partitioned as starch. Amounts of soluble carbohydrate in roots were greater after 14 h at 13-degrees-C than in roots of plants at 30-degrees-C. Loss of C-14 from leaves at 30-degrees-C as a proportion of (CO2)-C-14 fixed by them at 30-degrees-C, decreased after exposure of plants to 13-degrees-C in the light for 30 min prior to (CO2)-C-14 feeding. Results indicate an effect of cold on the transport process that was light-dependent. It is inferred that the reduction in the proportion of C-14 lost from leaves after 10 h cooling was due to reduced sink demand, whereas the rise in the proportion of C-14 lost from leaves after 24 h reflects reduced photosynthetic rate. The coincidence of reduced photosynthetic rate with raised carbohydrate levels in leaves maintained at 30-degrees-C throughout, whilst the rest of the plant was cooled to 13-degrees-C in the light implies feedback inhibition of photosynthesis. This may reduce the imbalance between source and sink in sunflower during the first days of long-term cooling.