Sink-Regulation of Photosynthesis in Relation to Temperature in Sunflower and Rape

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Abstract
Stimulation of the rate of photosynthesis at 2·0 kPa O2 in comparison with 21 kPa O2 and carbohydrate accumulation over 4h were measured during exposure of sunflower (Helianthus annuus L.) and rape (Brassica napus L.), grown at 30 °C and 13 °C, to temperatures between 7 °C and 35 °C. The effect of reducing source: sink ratio by shading on the response of photosynthetic rate to temperature was also determined. Stimulation of photosynthesis by 2·0 kPa O2 in comparison with 21 kPa O2 decreased over 4 h at cool temperatures in sunflower plants grown at 30 °C but not in rape grown at 30 °C. Stimulation did not decrease over 4 h in plants grown at 13 CC. Sucrose was the main carbohydrate accumulated over 4 h; its accumulation increased with decreasing temperature. Starch accumulation either decreased or remained the same with decreasing temperature. In plants grown at 30 °C more carbohydrate accumulated between 8 °C and 21 °C in sunflower than in rape, but more carbohydrate accumulated at 30 °C in rape than in sunflower. In plants grown at 13 °C much less carbohydrate accumulated between 13 °C and 23 °C than in plants grown at 30 °C. Photosynthetic rate in plants grown at 30 °C exposed to between 20 °C and 35 °C over 32 h (14 h light-10 h dark-8 h light), declined over 32 h at 20 °C and 25 °C in sunflower and at 20 °C in rape. This fall over 32 h, especially at 20 °C in sunflower, was significantly reduced by shading the rest of the plant. Shading had little effect on photosynthetic rate above 25 °C. The work confirms that low temperature imposes a sink-limitation on photosynthesis which occurs at higher temperatures in sunflower than in rape. This limitation may be relieved by decreasing the source:sink ratio.

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