Characteristics of CO₂ exchange between peach stems and the atmosphere

Gas exchange by stems is dominated by respiratory CO₂ emission, but photosynthetic CO₂ uptake might also occur in stem bark. We show that light-dependent CO₂ uptake was present and often exceeded CO₂ release by respiration in illuminated current-year peach (Prunus persica L.) stems. Respiration of peach stems, as detected by ¹²CO₂ release into air in which the natural concentration of ¹²CO₂ was replaced with ¹³CO₂, was lower in the light than in the dark, but this accounted for only a fraction of the observed total CO₂ uptake by illuminated stems. Stem photosynthesis was saturated at low light and was negatively affected by elevated assay temperatures (30°C), especially when combined with light intensities above saturation. An inefficient mechanism of heat dissipation by transpiration in stomata-free stems might help explain this effect. Photosynthesis was rapidly stimulated and the electron transport rate was reduced when photorespiration was suppressed by exposure to low (2 kPa) oxygen. The time-course of these changes was closely associated with a transient burst of CO₂ uptake concurrent with a reduced inhibition of fluorescence yield. Photosynthesis was also stimulated by exposure to elevated (twice ambient) CO₂ concentration. These combined measurements of gas exchange and fluorescence suggested that (a) photorespiration may also be active in the bark of peach stems, (b) O₂ and CO₂ concentrations in the bark of peach stems may be similar to ambient concentrations, (c) a large amount of electron transport unrelated to photosynthesis and photorespiration may also be present in peach stems, and (d) stem photosynthesis may be enhanced under future atmospheric conditions.