Effects of Mannose on Photosynthetic Gas Exchange in Spinach Leaf Discs

When mannose is provided in the transpiration stream to spinach ( Spinacia oleracea ) leaf discs, a series of specific and nonspecific changes occur in CO 2 and H 2 O vapor exchange as a function of feeding time. The initial increases in apparent photosynthesis and transpiration are nonspecific effects due to osmotic changes leading to passive stomatal opening. The mannose-specific effects are: (a) time-dependent changes in the CO 2 concentration required for saturation; (b) complex kinetics of the inhibition of CO 2 assimilation dependent on CO 2 and O 2 concentrations and the duration of feeding (high CO 2 and low O 2 lead to rapid inhibitions of photosynthesis); (c) elimination of the capacity of 2% O 2 to stimulate photosynthesis; and (d) oscillations in the CO 2 exchange rate following transitions from 20% to 2% O 2 . The mannose-specific effects are reversible by orthophosphate. The mannose-dependent changes in gas exchange are attributed to altered [ATP]/[ADP] ratios.