Nocturnal Stomatal Conductance Research Articles

Physiological responses of Quercus ilex Leaves to Water Stress and Acute Ozone Exposure Under Controlled Conditions

The combined effect of water stress and ozone (O3) on stomatal O3 flux, damage to photosynthesis, and detoxification by biogenic volatile organic compounds (BVOC) in Quercus ilex leaves was studied. A 4-weeks O3 exposure (250 ppb, 4 h per day) caused a reduction of photosynthesis and stomatal conductance, which was fully recovered 1 week after the end of the treatment, in well-watered and water-stressed plants. Measurements of stomatal O3 flux revealed a low stomatal flux of the pollutant, which became minimal after stomatal closure caused by water stress. An induction of volatile monoterpenes, important compounds in the O3 scavenging system in Q. ilex, and a burst of lipoxygenase compounds (LOX), which are released as gaseous by-products of membrane peroxidation, was observed after 2–3 weeks of O3 fumigation. However, these compounds were also released in control leaves that were exposed to ozone only briefly, to determine stomatal O3 flux. The low stomatal flux that occurred in water stress conditions helped avoiding permanent damage to Q. ilex leaves, although during the O3 treatment photosynthesis was severely limited by stomatal closure. In well-watered plants, O3 fumigation caused a noticeable increase of nocturnal stomatal conductance. If confirmed on adult plants under field conditions, this effect can imply larger flux of O3 at night and possible detrimental effects of O3 on leaf functions in plants exposed to high nocturnal O3 levels.

Nocturnal stomatal conductance effects on the 18O signatures of foliage gas exchange observed in two forest ecosystems

We report field observations of oxygen isotope ((18)O) discrimination during nocturnal foliage respiration ((18)Delta(R)) in branch chambers in two forest ecosystems: a Sitka spruce (Picea sitchensis (Bong.) Carr.) plantation in Scotland; and a beech (Fagus sylvatica L.) forest in Germany. We used observations and modeling to examine the impact of nocturnal stomatal conductance on the (18)O/(16)O (delta(18)O) signatures of foliage gas exchange at night. We found that nocturnal stomatal conductance can influence the delta(18)O signature by affecting: (1) the bidirectional diffusion of CO(2) into and out of the leaf (with isotopic equilibration); and (2) the (18)O enrichment of the foliage water with which the CO(2) equilibrates. Both effects were manifest in high apparent (18)Delta(R) values and enriched delta(18)O signatures of foliage water at night. The effects were more pronounced for Sitka spruce because of its higher nocturnal stomatal conductance and higher specific leaf water content compared to beech. We found that taking the effects of nocturnal stomatal conductance into account may change the sign of the delta(18)O signature of nocturnal foliage respiration, generally thought to decrease the delta(18)O of atmospheric CO(2). We conclude that nocturnal stomatal exchange can have a profound effect on isotopic exchange depending on species and environmental conditions. These effects can be important when using delta(18)O signatures of canopy CO(2) to distinguish foliage and soil respiration, and when modeling the delta(18)O signature of CO(2) exchanged between ecosystems and the atmosphere.

Nocturnal transpiration causing disequilibrium between soil and stem predawn water potential in mixed conifer forests of Idaho

Soil water potential (Psi(s)) is often estimated by measuring leaf water potential before dawn (Psi(pd)), based on the assumption that the plant water status has come into equilibrium with that of the soil. However, it has been documented for a number of plant species that stomata do not close completely at night, allowing for nocturnal transpiration and thus preventing nocturnal soil-plant water potential equilibration. The potential for nighttime transpiration necessitates testing the assumption of nocturnal equilibration before accepting Psi(pd) as a valid estimate of Psi(s). We determined the magnitude of disequilibrium between Psi(pd) and Psi(s) in four temperate conifer species across three height classes through a replicated study in northern Idaho. Based on both stomatal conductance and sap flux measurements, we confirmed that the combination of open stomata and high nocturnal atmospheric vapor pressure deficit (D) resulted in nocturnal transpiration in all four species. Nocturnal stomatal conductance (g(s-noc)) averaged about 33% of mid-morning conductance values. We used species-specific estimates of g(s-noc) and leaf specific conductance to correct Psi(pd) values for nocturnal transpiration at the time the samples were collected. Compared with the unadjusted values, corrected values reflected a significantly higher Psi(pd) (when D > 0.12 kPa). These results demonstrate that comparisons of Psi(pd) among species, canopy height classes and sites, and across growing seasons can be influenced by differential amounts of nocturnal transpiration, leading to flawed results. Consequently, it is important to account for the presence of nocturnal transpiration, either through a properly parameterized model or by making Psi(pd) measurements when D is sufficiently low that it cannot drive nocturnal transpiration. Violating these conditions will likely result in underestimation of Psi(s).

Nocturnal stomatal conductance and implications for modelling δ18O of leaf-respired CO2 in temperate tree species.

Variation in the oxygen isotope composition of within-canopy CO2 has potential to allow partitioning of the ecosystem respiratory flux into above- and below-ground components. Recent theoretical work has highlighted the sensitivity of the oxygen isotope composition of leaf-respired CO2 (δRl) to nocturnal stomatal conductance. When the one-way flux model was tested on Ricinus communis L. large enrichments in δRl were observed. However, most species for which the isotope flux partitioning technique has been or would be applied (i.e. temperate tree species) are much more conservative users of water than R. communis. So, high stomatal conductance and very high enrichment of δRl observed may not be typical for temperate tree species. Using existing gas-exchange measurements on six temperate tree species, we demonstrate significant water loss through stomata for all species (i.e. statistically significantly greater than cuticular loss alone) at some time for some leaves during the night. δRl values predicted by the one-way flux model revealed that δRl might be very much more enriched than when the net flux alone is considered, particularly close to sunrise and sunset. Incorporation of the one-way flux model into ecosystem respiration partitioning studies will affect model outputs and interpretation of variation in the oxygen isotope composition of atmospheric CO2.

Nocturnal stomatal conductance and ambient air quality standards for ozone

Vegetation response to ozone depends on ozone conductance into leaves and the defensive action inside the leaf. Ozone parameters currently used for air quality standards do not incorporate conductance or defensive components. Nighttime flux has often been ignored in ozone metrics relating to plant response, since ozone concentration and conductance are considered to be minimal at night. However, ozone concentration can remain relatively high at night, particularly in mountainous areas. Although conductance is lower at night than during the day for most plants, nocturnal conductance can result in considerable ozone flux into plants. Further, plants can be more susceptible to ozone exposure at night than during the daytime, a result of lower plant defenses at night. Any ozone metric used to relate air quality to plant response should use a 24 h ozone exposure period to include the nighttime exposures. It should also incorporate plant defensive mechanisms or their surrogate.

Effects of different light treatments on the nocturnal transpiration and dynamics of stomatal closure of two Rose cultivars

In the cultivation of roses, supplementary lighting is thought to cause an increase in nocturnal transpiration and consequent low turgidity of the cut flowers, leading to problems in quality. Two cultivars with diverging tolerances to this phenomenon were subjected to high and low light intensities, followed by periods of supplementary lighting of different duration and with different spectral composition. Daily water consumption showed genetic variation. Supplementary lighting, both with respect to its duration and its spectral composition, had hardly any influence on water usage or nocturnal stomatal conductance, but increased stomatal opening during the day. The diurnal rhythm of stomatal movement, measured with a computer-modulated high-intensity light source, appeared to be determined by the integrated irradiance in the previous light period. High irradiance caused a faster stomatal opening during the next light period. Similar to this response, stomatal closing rates were also stimulated by high light intensity during the previous day. The observed effects were most pronounced for the cultivar reported to be the least sensitive to post-harvest damage by excessive transpiration. It is concluded that supplementary lighting does not have a negative impact on the water use efficiency of cut roses. In addition, the stomatal dynamics suggest that flower quality would benefit from being transferred to darkness following a period of several hours of high light intensity.

Some Characteristics of Crassulacean Acid Metabolism in Five Nonsucculent Scrub Species Under Natural Semiarid Conditions

Summary Leaves of five nonsucculent scrub species, Carissa spinarum, Maba buxifolia, Gymnosporia emarginata, Flacourtia sepiaria and Dodonaea viscosa exhibited some features of Crassulacean acid metabolism (CAM) under natural semiarid conditions. The occurrence of some characteristics of CAM in these species was exemplified by the nocturnal stomatal conductance, dark carbon assimilation and high activity of phosphoenolpyruvate carboxylase, resulting in the greater degree of dark acidification. The detection of ribulose bisphosphate carboxylase activity and the low δ13C values ranged from -24.5 to -29.1 %0 suggested that these five species also utilize reductive pentose phosphate cycle of photosynthesis. The five species exhibiting some CAM characteristics are suggested to be grouped under flexible CAM species which fix carbon dioxide in the light or darkness under natural semiarid conditions. Further, these five species belonged to five diverse dicotyledonous families which hitherto are not known to possess any CAM characteristics.