Stomate-Dependent and Stomate-Independent Uptake of NOX, and Effects on Photosynthesis, Respiration and Transpiration of Potted Plants
Cultivars of the most commonly grown potted plants in Danish commercial greenhouses were exposed to CO2, NO, NO2 gases, alone or in combination. Gas uptake and effects on net photosynthesis, dark respiration and transpiration were measured before, during and after the exposures. Carbon dioxide stimulated net photosynthesis (41%) and subsequent dark respiration (24%), while transpiration was reduced (10%). The NO exposure of 1 μL.L−1 NO reduced photosynthesis (20%) and transpiration (18% at high (CO2), but did not affect respiration. NO2 rarely had significant effects. All the effects of the gases on photosynthesis and transpiration were reversible and had independent mechanisms, while effects on respiration were non-reversible in clean air.
NO2 uptake by the eight cultivars was related to transpiration, i.e., open stomates, while NO uptake was not dependent on stomatal opening. NO was taken up at a constant rate throughout the light period, while NO2 uptake decreased towards the end of the day, as did transpiration. Uptake of NO2 in the dark was reduced as much as transpiration.
At the same concentration, NO was four times more inhibitory to photosynthesis than NO2. Relative to uptake, NO was 22 times more toxic than NO2. 60 must therefore have a potent mechanism of toxicity, different from that of NO2.
The higher NO-dose, which reduced transpiration because the stomata closed, reduced photosynthesis less than a lower NO-dose. Since total NO uptake was not dependent on stomatal aperture, but stomatal closure reduced the NO-effect on photosynthesis, it could be suggested that stomata regulate the effective NO uptake (i.e. the small fraction affecting photosynthesis).
The difference in the sensitivity of the cultivars to NO and NO2 did not correlate with the uptake of either pollutant gas.
Short-term effects (four days) of CO2 + NO on photosynthesis were significantly correlated (p < 0.05) with previously observed long-term effects (four to five months) on the dry weight ofthe same cultivars in similar treatments.
KeywordsDark Respiration Stomatal Opening Stomatal Aperture Hedera Helix Assimilation Chamber
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