STOMATAL‐DEPENDENT AND STOMATAL‐INDEPENDENT UPTAKE OF NO x

SUMMARY NO 2 uptake of eight pot plants (Ficus elastica ‘Robusta’, Ficus benjamina, Hedera helix ‘Anne Marie’, Hedera canariensis ‘Montgomery’, Hibiscus rosa‐sinensis ‘Red’. Hibiscus rosa‐sinensis ‘Moesiana’, Dieffenbachia maculata ‘Compacta’ and Nephrolepis exaltata ‘ Bostoniensis ’) was shown to correlate with transpiration, while NO uptake had no significant dependence on stomatal opening. NO was taken up at a constant rate throughout the light period, while the uptake of NO 2 decreased towards the end of the day in the same manner as transpiration. Uptake of NO in the dark was as high as in the light, while uptake of NO 2 in the dark was reduced by as much as transpiration. Relative to total pollutant uptake, the toxicity of NO towards photosynthesis is 22 times higher than that of NO 2 . NO must therefore have a mechanism of toxicity which is different from that of NO 2 . A sufficiently high dose of NO (4 ml 1 −1 for 5 hours) to reduce transpiration (close stomata) decreased photosynthesis less than a lower dose (1 μ1 1 −1 for 12 h). Since total NO uptake was not dependent on stomatal aperture, but stomatal closure reduced the effect of NO on photosynthesis, it is suggested that stomatas regulate the effective NO uptake, i.e. the small fraction of NO uptake that affects photosynthesis. The larger ineffective portion of the uptake is thought to be absorbed on the outer leaf surface. The sensitivity of cultivars or species to NO or NO 2 (i.e. effects on photosynthesis and transpiration) did not correlate with the uptake of either pollutant gas.