Alessio Giorgio A, Pietrini Fabrizio, Brilli Federico, Loreto Francesco
CNR - Istituto di Biologia Agroambientale e Forestale Via Salaria Km. 29300-00016 Monterotondo Scalo (Roma), Italy.
Funct Plant Biol. 2005 Sep;32(9):787-795. doi: 10.1071/FP05070.
Gas exchange by stems is dominated by respiratory CO emission, but photosynthetic CO uptake might also occur in stem bark. We show that light-dependent CO uptake was present and often exceeded CO release by respiration in illuminated current-year peach (Prunus persica L.) stems. Respiration of peach stems, as detected by CO release into air in which the natural concentration of CO was replaced with CO, was lower in the light than in the dark, but this accounted for only a fraction of the observed total CO 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 CO uptake concurrent with a reduced inhibition of fluorescence yield. Photosynthesis was also stimulated by exposure to elevated (twice ambient) CO concentration. These combined measurements of gas exchange and fluorescence suggested that (a) photorespiration may also be active in the bark of peach stems, (b) O and CO 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.
茎的气体交换以呼吸作用释放二氧化碳为主，但茎干树皮中也可能发生光合作用吸收二氧化碳的过程。我们发现，在光照下的当年生桃树（Prunus persica L.）茎中，存在依赖光的二氧化碳吸收现象，且这种吸收量常常超过呼吸作用释放的二氧化碳量。通过检测向空气中释放二氧化碳（空气中自然浓度的二氧化碳被替换为二氧化碳）来测定桃树茎的呼吸作用，结果表明，光照下的呼吸作用比黑暗中弱，但这仅占光照下茎干观察到的总二氧化碳吸收量的一小部分。茎光合作用在低光照强度下达到饱和，且受测定温度升高（30°C）的负面影响，尤其是当温度与高于饱和点的光照强度同时存在时。在没有气孔的茎中，通过蒸腾作用进行的低效散热机制可能有助于解释这种影响。当通过暴露于低氧（2kPa）环境抑制光呼吸时，光合作用迅速受到刺激，电子传递速率降低。这些变化的时间进程与二氧化碳吸收的短暂爆发密切相关，同时荧光产量的抑制作用减弱。暴露于升高（两倍于环境浓度）的二氧化碳浓度也会刺激光合作用。这些气体交换和荧光的综合测量结果表明：（a）光呼吸在桃树茎的树皮中也可能很活跃；（b）桃树茎树皮中的氧气和二氧化碳浓度可能与环境浓度相似；（c）桃树茎中可能还存在大量与光合作用和光呼吸无关的电子传递；（d）在未来的大气条件下，茎光合作用可能会增强。
