Sink–Source Balance and Down-Regulation of Photosynthesis in Raphanus sativus: Effects of Grafting, N and CO2

To clarify whether excessive accumulation of total non-structural carbohydrate (TNC) causes down-regulation of photosynthesis in Raphanus sativus, we manipulated sink-source balance to alter TNC levels in source leaves and examined its effects on photosynthetic characteristics, whole-plant biomass allocation and anatomical characteristics of leaves and petioles. Comet and Leafy varieties with large and small hypocotyls were reciprocally grafted to change hypocotyl sink strength. They were grown at high or low nitrogen (N) availability and at elevated or ambient CO2. Maximum photosynthetic rate, which was highly correlated with Rubisco and leaf N contents, was hardly correlated with TNC across the grafting combinations and growth conditions. Biomass allocation to petioles and hypocotyls and accumulation of TNC in each organ were significantly higher at low N. TNC and structural carbohydrates such as cellulose and hemicellulose were higher and the proportion of intercellular air space in source leaves was lower at low N and elevated CO2. We conclude that excess TNC does not cause severe down-regulation of photosynthesis, and cell walls and petioles are also major carbohydrate sinks responding to changes in sink-source and carbon-nitrogen balances, which contribute to alleviating further accumulation of TNC to avoid its negative effects in source leaves.