Stomatal responses to changes in air humidity are not necessarily linked to nocturnal CO₂ uptake in the CAM plant Plectranthus marrubioides Benth. (Lamiaceae)

Abstract

Plants of the crassulacean acid metabolism (CAM) speciesPlectranthus marrubioides (Lamiaceae) were subjected to short-and long-term changes in air humidity in controUedenvironment experiments. Stomata of well-watered individuals of this all-cell leaf-succulent taxon responded directly, quickly and reversibly to variations of the water vapour gradient between leaf and air {Aw). Mean night-time leaf conductance to water vapour decreased curvilinearly with increasing Aw but linearly with lowered relative air humidity. Stomatal response was generally independent of the prevailing temperature and was not linked to CO2 uptake rates. Therefore, net night-time carbon gain, nocturnal malic acid accumulation and, thus, relative carbon recycling were not influenced by changes in air humidity in the temperature range tested. Mean nocturnal molar water use efficiency, however, decreased with decreasing air humidity because of the increased transpirational water loss. If watering was repeatedly withheld for several days during the experiments, employing a temperature regime of 35/30 °C day and night, stomatal conductance became low enough to inhibit CO2 uptake, but only at the highest Aw. The results suggest that drought stress was necessary to increase responsiveness of plants to the point where CAM was also inhibited by decreases in air humidity.

Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature

“…The lack of a temperature effect on assimilation in phase I at elevated CO 2 causes us to conclude that the decrease in CO 2 assimilation that occurred during phase I at ambient CO 2 when temperature was increased from 20 to 25°C was not due to an increase in the VPD at 25°C. Herppich (1997) found that while the stomata of the CAM plant Plectranthus marrubioides responded directly to variations in VPD, stomatal responses were not linked to CO 2 uptake rates. Herppich (1997) concluded that net carbon gain during phase I was not influenced by changes in air humidity in the temperature range tested.…”

“…Herppich (1997) found that while the stomata of the CAM plant Plectranthus marrubioides responded directly to variations in VPD, stomatal responses were not linked to CO 2 uptake rates. Herppich (1997) concluded that net carbon gain during phase I was not influenced by changes in air humidity in the temperature range tested. The interactive effect of CO 2 and temperature observed for A. comosus in this study does not appear to have been a The data are means ± SE for four leaves, each from a separate plant.…”

Seasonal photosynthetic gas exchange and water-use efficiency in a constitutive CAM plant, the giant saguaro cactus (Carnegiea gigantea)

“…Herppich (1997) showed that stomatal conductance of the CAM species Plectranthus marrubioides responded directly to variations in nighttime water-vapor partial-pressure gradient between the leaf and air, and when highly water limited, CAM was inhibited by low air humidity. Lange and Medina (1979) concluded that nighttime air humidity was responsible for changes in stomatal conductance for the CAM species Tillandsia recurvata, thereby aVecting net assimilation rate.…”

Stomatal Biology of CAM Plants

“…An apparent feed-forward response of transpiration to rising leaf-air vapor pressure deficit (VPD), in which stomata seem to respond directly to humidity rather than indirectly via leaf water status, has been observed in some CAM lineages, with important consequences for assimilation rates and water-use efficiency under contrasting humidity regimes (Lange and Medina, 1979;Osmond et al, 1979;Martin and Siedow, 1981;von Willert et al, 1985;Lüttge et al, 1986;Herppich, 1997). Epiphytic CAM species might be expected to show particularly high levels of stomatal sensitivity to VPD, given the special adaptive value this would have in highly water-limited epiphytic environments (see discussion of integrated leaf traits below).…”