Hysteresis in the response of photosynthesis to CO₂ and saccharide pools of wheat leaves grown at normal and enhanced CO₂

Wheat plants were cultivated in a growth chamber at 35 Pa (c₃₅ variant) and 70 Pa CO₂ partial pressure (c₇₀ variant) during the whole vegetation period. The response of net photosynthetic rate (P_N) of the flag leaf of both variants to successive increases in CO₂ partial pressure (step-up curve) showed hysteresis when the direction of the sequence was reversed (step-down curve) after 1.5 h at saturating CO₂ partial pressure and photosynthetically active radiation (PAR). Saccharose, glucose and fructose accumulated during the measurement of a step-up CO₂ curve for the c₃₅ and c₇₀ plants as the export rate was not able to keep pace with the rate of saccharide synthesis. Remaining 1.5 h at saturating CO₂ partial pressure and PAR, the saccharose pool increased further for both variants while glucose and fructose decreased reaching the values at growth conditions. The electron transport rate decreased after 1.5 h at saturating CO₂ partial pressure and PAR for the two variants due to end product feedback. Glucose and fructose contents fell 50% below the initial contents when partial pressure of CO₂ was lowered stepwise. The c₃₅ plants showed a double fold increase in the content of saccharose at the end point of the hysteresis curve. Contents of saccharose for the c₇₀ variant in contrast were similar to the initial values.