Gas-exchange responses of rose plants to CO2 enrichment and light

This paper describes the response of gas exchange rates and water use efficiency of rose plants, by means of the characterization in situ and the analysis of the response of photosynthesis, transpiration and water use efficiency of whole plants to CO2 enrichment under the irradiance conditions prevailing in greenhouses of southern France. Net CO2 assimilation (A(n)) and transpiration (E) of whole rose plants (Rosa hybrida, cv. Sonia) were measured during winter and spring periods. The response of A(n) to light and CO2 were fitted to a double hyperbola function (r(2) = 0.84). Maximum net assimilation rate (A(nmax)), light and CO2 utilization efficiencies (alpha(1), alpha(c)) as well as light and CO2 compensation points (Gamma(1), Gamma(c)) were calculated for the whole plant and compared with leaf and canopy data in the literature. The whole-plant characteristics generally had values intermediate between those related to leaf and canopy. Light saturation at subambient air CO2 concentration (C-a) was reached for relatively low PFFD values (300 mu mol m(-2) s(-1)), whereas at ambient and enriched C-a light saturation occurs for PPFD approximate to 1000 mu mol m(-2) s(-1). Doubling C-a from 350 to 700 mu mol mol(-1) increased A(nmax) and alpha(1) by respectively 40% and 30%, while reducing Gamma(1) by 27%. A threefold increase of C-a from 350 to 1050 mu mol mol(-1) induced a reduction of 20% of E. Instantaneous transpirational water use efficiency, WUE (=A(n)/E), is relatively insensitive to PPFD, although a slight decrease with PPFD is observed at high CO2 concentration, but shows marked variations with C-a and leaf to air vapour pressure deficit (D-1). Increase of C-a from 350 to 1000 mu mol mol(-1) gave about 50% increase in WUE. Increase of D-1 from 0 to 2 kPa induced 30% decrease in WUE at ambient C-a and 50% decrease at 1000 mu mol mol(-1).