The responses of guard and mesophyll cell photosynthesis to CO2, O2, light, and water stress in a range of species are similar (original) (raw)

2003, Journal of Experimental Botany

High resolution chlorophyll a¯uorescence imaging was used to compare the photosynthetic ef®ciency of PSII electron transport (estimated by F q ¢/F m ¢) in guard cell chloroplasts and the underlying mesophyll in intact leaves of six different species: Commelina communis, Vicia faba, Amaranthus caudatus, Polypodium vulgare, Nicotiana tabacum, and Tradescantia albifora. While photosynthetic ef®ciency varied between the species, the ef®ciencies of guard cells and mesophyll cells were always closely matched. As measurement light intensity was increased, guard cells from the lower leaf surfaces of C. communis and V. faba showed larger reductions in photosynthetic ef®ciency than those from the upper surfaces. In these two species, guard cell photosynthetic ef®ciency responded similarly to that of the mesophyll when either light intensity or CO 2 concentration during either measurement or growth was changed. In all six species, reducing the O 2 concentration from 21% to 2% reduced guard cell photosynthetic ef®ciency, even for the C 4 species A. caudatus, although the mesophyll of the C 4 species did not show any O 2 modulation of photosynthetic ef®ciency. This suggests that Rubisco activity is sig-ni®cant in the guard cells of these six species. When C. communis plants were water-stressed, the guard cell photosynthetic ef®ciency declined in parallel with that of the mesophyll. It was concluded that the photosynthetic ef®ciency in guard cells is determined by the same factors that determine it in the mesophyll. Abbreviations: C a , ambient CO 2 concentration; F¢, chlorophyll¯uorescence in the light-adapted state; F m ¢, chlorophyll¯uorescence when PSII centres are maximally closed in the light-adapted state; F o ¢, chlorophyll¯uorescence when PSII centres are maximally open in the light-adapted state; F q ¢, difference between F¢ and F m ¢; F q ¢/F m ¢,¯uorescence parameter that provides an estimate of the operating ef®ciency of PSII photochemistry; F q ¢/F v ¢, factor relating the operating and maximum ef®ciencies of PSII photochemistry; F v ¢, variable chlorophyll¯uorescence (F m ¢±F o ¢); F v ¢/F m ¢,¯uorescence parameter that provides an estimate of the maximum ef®ciency of PSII photochemistry (when all PSII centres are open); PPFD, photosynthetic photon¯ux density; Rubisco, ribulose 1,5-bisphosphate carboxylase oxygenase; VPD, vapour pressure de®cit.