Acetaldehyde emission by the leaves of trees – correlation with physiological and environmental parameters

The leaves of trees emit significant amounts of acetaldehyde which is synthesized there by the oxidation of ethanol. In the present study, we examined plant internal and environmental factors controlling the emission of acetaldehyde by the leaves of young poplar ( Populus tremula × P. alba ) trees. The enzymes possibly involved in the oxidation of ethanol in the leaves of trees are catalase (CAT; EC 1.11.1.6) and alcohol dehydrogenase (ADH; EC 1.1.1.1), both expressed constitutively in the leaves of poplars. Inhibition of ADH in excised leaves caused a significant decrease of acetaldehyde emission accompanied by an increased ethanol emission. Since inhibition of CAT by aminotriazole did not affect acetaldehyde and ethanol emission, it is concluded that the oxidation of ethanol in the leaves is mediated by ADH rather than by CAT. Further studies indicated that aldehyde dehydrogenase (ALDH; EC 1.2.1.5) seems to be responsible for the oxidation of acetaldehyde. The present results demonstrate that acetaldehyde emission is clearly dependent on its production in the leaves as controlled by the delivery of ethanol to the leaves via the transpiration stream. Environmental factors that control stomatal conductance seem to be of less importance for acetaldehyde emission by the leaves.     

Metabolic origin of acetaldehyde emitted by poplar (Populus tremula x (P. alba) trees

The metabolic origin and emission by the leaves of the tropospheric trace gas acetaldehyde were examined in 4-month-old poplar trees (Populus tremula x P. alba) cultivated under controlled environmental conditions in a greenhouse. Treatments which resulted in increased ethanol concentration of the xylem sap caused significantly enhanced rates of acetaldehyde and ethanol emission by the leaves. Leaves fed [¹⁴C]-ethanol via the transpiration stream emitted [^<14C]-acetaldehyde. These findings suggest that acetaldehyde in the leaves is synthesized by a metabolic pathway that operates in the opposite direction of alcoholic fermentation and results in oxidation of ethanol. Enzymatic studies showed that this pathway is mediated either by alcohol dehydrogenase (ADH; EC 1.1.1.1) or catalase (CAT; EC 1.11.1.6), both constitutively present in the leaves of poplar trees. Labelling experiments with [¹⁴C]-glucose indicated that the ethanol delivered to the leaves by the transpiration stream is produced in anaerobic zones of submersed roots by alcoholic fermentation. Anoxic conditions in the rhizosphere caused by flooding of the root system resulted in an activation of alcoholic fermentation and led to significantly increased ethanol concentrations in the xylem sap. These results support the hypothesis that acetaldehyde emitted by the leaves of trees is derived from xylem transported ethanol which is synthesized during alcoholic fermentation in the roots.Keywords: Acetaldehyde, emission, ethanol, anaerobiosis, Populus tremula x P. alba      

Photosynthetic responses of citrus trees to soil flooding

Continuous soil flooding reduced leaf photosynthetic rate, stomatal conductance to water vapor, chlorophyll concentration and activity of ribulose bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39) of sweet orange [ Citrus sinensis (L.) Osbeck cv. Hamlin] trees, grafted onto rough lemon (RL; C. jambhiri Lush.) and sour orange (SO; C. aurantium L.) rootstocks. After 24 days of waterlogging, trees showed senescence, wilting and abscission of leaves, and these symptoms were more evident with flooded Hamlin/SO than Hamlin/RL. Reduction of leaf photosynthetic rate at day 24 was ca 94%, of stomatal conductance, 71%, of chlorophyll, 38% and of Rubisco, 62% for flooded Hamlin/SO, compared with 22, 5, 18 and 33%, respectively, for flooded Hamlin/RL. For both Hamlin/RL and Hamlin/SO, leaf photosynthetic rate and stomatal conductance to water vapor were closely correlated (r²= 0.87). Leaf internal CO₂ concentration of flooded trees, however, was not decreased by reduced stomatal conductance. Dark respiration rates of fibrous roots of flooded trees were greatly reduced, but not in leaf tissues. Total nonstructural carbohydrate concentrations were higher in leaves (50 and 80% increases for Hamlin/SO and Hamlin/RL, respectively), but drastically reduced in roots (60 and 45% reductions for Hamlin/SO and Hamlin/RL, respectively), as a result of flooding. The data indicate that Hamlin grafted onto RL rootstocks was more tolerant to soil flooding than Hamlin grafted onto SO rootstocks.     

提高CO2浓度对两种亚热带树苗叶片水分状况的影响

鼎湖山季风常绿阔叶林的主要优势乔木树种黧蒴和荷木的幼苗,盆栽于自然光照和人工调节CO₂浓度为500μl·L^-1或空气CO₂(340μl·L^-1)的气罩中3个月.在各自生长条件下测定,高CO₂下生长的黧蒴和荷木叶片平均气孔导度分别降低13%和20%,蒸腾速率下降20%和18%,水分利用效率提高1倍以上,不同CO₂浓度下的植物叶片气孔导度和蒸腾速率日进程曲线也有明显差异.处理后将幼苗置于自然条件下观测其后效应,第7d时处理间的气孔导度和蒸腾速率皆无明显差异     

Physiological effects of long-term exposure to low and moderate concentrations of atmospheric NH3 on poplar leaves

Abstract. Poplar shoots ( Populus euramericana L.) obtained from cuttings were exposed for 6 or 8 weeks to NH₃ concentrations of 50 and 100 μgm⁻³ or filtered air in fumigation chambers. After this exposure the rates of NH₃ uptake, transpiration, CO₂ assimilation and respiration of leaves were measured using a leaf chamber. During the long-term exposure also modulated chlorophyll fluorescence measurements were carried out to obtain information about the photosynthetic performance of individual leaves. Both fluorescence and leaf chamber measurements showed a higher photosynthetic activity of leaves exposed to 100 μg NH₃ m⁻³. These leaves showed also a larger leaf conductance and a larger uptake rate of NH₃ than leaves exposed to 50 μg m⁻³ NH₃ or filtered air. The long-term NH₃ exposure did not induce an internal resistance against NH₃ transport in the leaf, nor did it affect the leaf cuticle. So, not only at a short time exposure, but also at a long-term exposure NH₃ uptake into leaves can be calculated from data on the boundary layer and stomatal resistance for H₂O and ambient NH₃-concentration. Furthermore, the NH₃ exposure had no effect on the relation between CO₂-assimilation and stomatal conductance, indicating that NH₃ in concentrations up to 100 μg m⁻³ has no direct effect on stomatal behaviour; for example, by affecting the guard or contiguous cells of the stomata.     

Interaction of flooding with carbon metabolism of forest trees

Waterlogging and flooding cause oxygen deprivation in the root system of trees. Since oxygen is essentially for mitochondrial respiration, this process cannot be maintained under anoxic conditions and must be replaced by other pathways. For the roots it is therefore a matter of survival to switch from respiration to alcoholic fermentation. Due to the low efficiency of this process to yield energy equivalents (ATP), energy and carbon metabolism of trees are usually strongly affected by oxygen deprivation, even if a rapid switch from respiration to fermentation is achieved. The roots can compensate for the low energy yield of fermentation either (1) by decreasing the demand for energy by a reduction of energy-dependent processes such as root growth and/or nutrient uptake, or (2) by consuming more carbohydrates per unit time in order to generate sufficient energy equivalents. In the leaves of trees, flooding and waterlogging cause a decline in the rates of photosynthesis and transpiration, as well as in stomatal conductance. It is assumed that, due to reduced phloem transport, soluble sugars and starch accumulate in the leaves of flooded trees, thereby negatively affecting the sugar supply of the roots. Thus, root growth and survival is negatively affected by both changes in root internal carbon metabolism and impaired carbon allocation to the roots by phloem transport. In addition, accumulation of toxic products of fermentation in the roots, such as acetaldehyde, can further impair root metabolism. A main feature of tolerance against flooding and waterlogging of trees seems to be the steady supply of carbohydrates to the roots in order to maintain alcoholic fermentation; in addition, roots of tolerant trees seem to avoid accumulation of fermentation-derived ethanol and acetaldehyde. From studies with flooding tolerant and non-tolerant tree species, it is hypothesized that (1) the transport of ethanol produced in the roots under hypoxic conditions into the leaves via the transpiration stream, (2) its conversion into acetyl-CoA in the leaves, and (3) its use in the plant's general metabolism, are mechanisms of flooding tolerance of trees.     

Stomatal responses of pearl millet (Pennisetum americanum [L.] Leeke) to leaf water status and environmental factors in the field

Abstract. Factors affecting stomatal conductance (g₁) of pearl millet ( Pennisetum americanum [L.] Leeke), cultivar BJ 104, were examined in the field in India during the dry season.
Diurnal changes in g₁ were evaluated for upper expanded leaves at flowering on two occasions using plants subjected to varying degrees of water stress. Except for the most severely stressed treatment, diurnal changes in g₁ closely matched changes in irradiance ( I ), the promotive effect of which largely overcame opposing influences on g₁ of increasing atmospheric vapour pressure deficit, and decreasing leaf water and turgor potentials (Ψ, Ψ_p).
Two main effects of water stress on g₁ were evident: (i) a decrease in the amplitude of the mid-day peak in g₁, and (ii) a decrease in the time over which high g₁ was maintained, resulting in early (mid-day) closure and hysteresis in the relationship between g₁ and I .
Leaf conductance was greatest for upper leaves and decreased down the canopy. At equivalent depths in the canopy g₁ was higher in flowering than in photoperiodically-retarded plants of the same age. The magnitude of water stress-induced stomatal closure increased down the plant, and was more marked in retarded than in flowering plants.
Within individual stress treatments Ψ of upper leaves decreased linearly as transpiration flux increased. It is concluded that stomatal behaviour of upper leaves of pearl millet at flowering largely operates to maximize assimilation rather than to minimize water loss.     

Quantitative method for pheromone delivery in studies of sensory adaptation of moth antennae

Abstract.  A pheromone sprayer and an electroantennogram (EAG) are used to study sensory adaptation in the antennae of male obliquebanded leafrollers, Choristoneura rosaceana and oriental fruit moths, Grapholita molesta , to the main pheromone compounds ( Z )-11-tetradecen-1-yl acetate ( Z 11-14:Ac) and ( Z )-8-dodecen-1-yl acetate ( Z 8-12:Ac), respectively. The atomization of 0.125, 0.25, 0.5 or 1 μL ethanol min⁻¹ into the EAG air delivery tube at an airflow rate of 2 L min⁻¹, with resultant concentrations of 6.25, 12.5, 25 or 50 × 10⁻⁵μL ethanol mL air⁻¹, respectively, does not affect the EAG response of C. rosaceana or C. molesta after a 30-min exposure period. The atomization of 0.125 μL min⁻¹ of a solution of 8 mg Z 11-14:Ac mL⁻¹ ethanol into the EAG air delivery tube at an airflow rate of 2 L min⁻¹, with a resultant concentration of 0.5 ng pheromone mL⁻¹ air, reduces the EAG response of C. rosaceana by approximately 70% after a 15-min exposure period. An additional 15 min of exposure to pheromone does not result in increased sensory adaptation. Antennae recover 32% of the lost responsiveness when exposed to pheromone-free air for 15 min. The atomization of 0.125 μL min⁻¹ of a solution of 8 mg Z 8-12:Ac mL⁻¹ ethanol into the EAG air delivery tube at an airflow rate of 2 L min⁻¹, with a resultant concentration of 0.5 ng pheromone mL⁻¹ air, reduces the EAG response of C. molesta antenna by approximately 80% after a 15- or 30-min exposure period. The antennae of this species do not recover responsiveness when exposed to pheromone-free air for 15 min.     

Stomatal and photosynthetic responses to shade in sorghum, soybean and eastern gamagrass

We studied photosynthetic and stomatal responses of grain sorghum ( Sorghum bicolor [L.] Moench cv. Pioneer 8500), soybean ( Glycine max L. cv. Flyer) and eastern gamagrass ( Tripsacum dactyloides L.) during experimental sun and shade periods simulating summer cloud cover. Leaf gas exchange measurements of field plants showed that short-term (5 min) shading of leaves to 300–400 μmol m⁻² s⁻¹ photosynthetic photon flux density reduced photosynthesis, leaf temperature, stomatal conductance, transpiration and water use efficiency and increased intercellular CO₂ partial pressure. In all species, photosynthetic recovery was delayed when leaves were reilluminated, apparently by stomatal closure. The strongest stomatal response was in soybean. Photosynthetic recovery was studied further with soybeans grown indoors (maximum photosynthetic photon flux density 1 200 μmol m⁻² s⁻¹). Plants grown indoors had responses to shade similar to those of field plants, except for brief nonstomatal limitation immediately after reillumination. These responses indicated the importance of the light environment during leaf development on assimilation responses to variable light, and suggested different limitations on carbon assimilation in different parts of the soybean canopy. Photosynthetic oxygen evolution recovered immediately upon reillumination, indicating that the light reactions did not limit soybean photosynthetic recovery. While shade periods caused stomatal closure and reduced carbon gain and water loss in all species, the consequences for carbon gain/water loss were greatest in soybean. The occurrence of stomatal closure in all three species may arise from their shared phenologies and herbaceous growth forms.     

Stomatal limitation of photosynthesis and reduced growth of the halophyte, Plantago maritima L., at high salinity

Abstract. Plantago maritima L. was grown at three levels of salinity, 50, 200, 350 mol m⁻³ NaCl, and the effects on growth, ion content and photosynthetic capacity were studied. Shoot and root dry weight, leaf production and leaf length were all substantially reduced in plants grown at high salinity. Total leaf area of plants grown at 350 mol m⁻³ NaCl was only 20% of that in plants at low salinity. Both the Na⁺ and K⁺ content of leaves and roots increased with external salinity. There was no change in the Na⁺/K⁺ ratio of leaves or roots at different salinity levels. Despite the large reductions in growth and high accumulation of Na⁺ ions, leaf photosynthetic rate was only slightly reduced by salinity stress. The reduction in photosynthesis was not caused by reduced biochemical capacity as judged by photosynthetic response to intercellular CO₂ and by ribulose-1,5-bisphosphate carboxylase activity, but was due to reduced leaf conductance and low intercellular CO₂ concentration. The increased stomatal limitation of photosynthesis resulted in higher water-use efficiency of plants grown at high salinity.     

Structural and physiological changes in the roots of tomato plants over-expressing a basic peroxidase

Previous studies on the tomato ( Lycopersicon esculentum Mill.) peroxidase TPX1, including the development of transgenic tomato over-expressing this gene, supported an involvement of this peroxidase in the synthesis of lignin and suberin. The transgenic plants showed a wilty phenotype at flowering, but the relationship between this role in ligno-suberization and this phenotype was not clear. In the present study a histological approach and the measurement of water-related parameters have been performed in order to obtain an insight into the origin of this phenotype. Clear differences between transgenic and non-transgenic roots were observed in the cross-sections of the basal root zones where secondary growth was evident. The diameter of the xylem vessel was diminished in the transgenic plants. Total area corresponding to xylem in the basal cross-sections decreased 3.9 fold in the transgenic roots. In addition, the radial and outer tangential walls of the exodermis cells were more ligno-suberized in transgenic than in non-transgenic plants. After fruit set, predawn and midday water potentials were lower in transgenic than in-non-transgenic plants. At midday, the stomatal conductance was also lower in the transgenic plants, 494±69 versus 594±60 mmol m⁻² s⁻¹. Root hydraulic conductances of the transgenic and non-transgenic plants were 1.4±0.38 and 3.47±0.19 g water min⁻¹ MPa⁻¹, respectively. The results obtained support that the phenotype is caused by the anatomical differences found in the transgenic roots. These differences would be the cause of a increased resistance to water flow in the roots that would negatively affect the water supply to the shoot and, as a consequence, resulted in a decreased water potential in the leaves.     

Effects of leaf wetting and high humidity on stomatal function in leafy cuttings and intact plants of Corylus maxima

When rooted cuttings of Corylus maxima Mill. cv. Purpurea are moved from the wet and humid conditions of the rooting environment, the leaves frequently shrivel and die. Since the newly formed adventitious root system has been shown to be functional in supplying water to the shoot, stomatal behaviour in C. maxima was investigated in relation to the failure to prevent desiccation. Stomatal conductance (g_s) in expanding leaves (L3) of cuttings increased almost 10-fold over the first 14 days in the rooting environment (fog), from 70 to 650 mmol m⁻² s⁻¹. In contrast, g_s of expanded leaves (L1) changed little and was in the region of 300 mmol m⁻² s⁻¹. Midday leaf water potential was much higher in cuttings than in leaves on the mother stock-plant (−0.5 versus −1.2 MPa) even before any roots were visible. Despite this, leaf expansion of L3 was inhibited by >50% in cuttings and stomata showed a gradual reduction in their ability to close in response to abscisic acid (ABA). To determine whether the loss of stomatal function in cuttings was due to severance or to unnaturally low vapour pressure deficit and wetting in fog, intact plants were placed alongside cuttings in the rooting environment. The intact plants displayed reductions in leaf expansion and in the ability of stomata to close in response to dark, desiccation and ABA. However, in cuttings, the additional effect of severance resulted in smaller leaves than in intact plants and more severe reduction in stomatal closure, which was associated with a 2.5-fold increase in stomatal density and distinctively rounded stomatal pores. The similarities between stomatal dysfunction in C. maxima and that observed in many species propagated in vitro are discussed, as is the possible mechanism of dysfunction.     

Effect of high external NaCl concentrations on ion transport within the shoot of Lupinus albus. I. Ions in xylem sap

Abstract. Xylem sap was collected from individual leaves of intact transpiring lupin plants exposed to increasing concentrations of NaCl by applying pneumatic pressure to the roots. Concentrations of Na⁺ and Cl⁻ in the xylem sap increased linearly with increases in the external NaCl concentration, averaging about 10% of the external concentration. Concentrations of K⁺ and NO₃⁻, the other major inorganic ions in the sap, were constant at about 2.5 and 1.5 mol m⁻³, respectively. There was no preferential direction of Na ⁺ or Cl⁻ to either young or old leaves: leaves of all ages received xylem sap having similar concentrations of Na⁺ and Cl⁻, and transpiration rates (per unit leaf area) were also similar for all leaves. Plants exposed to 120–160 mol m⁻³ NaCl rapidly developed injury of oldest leaves; when this occurred, the Na⁺ concentration in the leaflet midrib sap had increased to about 40 mol m⁻³ and the total solute concentration to 130 osmol m⁻³. This suggests that uptake of salts from the transpiration stream had fallen behind the rate of delivery to the leaf and that salts were building up in the apoplast.     

Decoupling the influence of leaf and root hydraulic conductances on stomatal conductance and its sensitivity to vapour pressure deficit as soil dries in a drained loblolly pine plantation

The study examined the relationships between whole tree hydraulic conductance ( K _tree) and the conductance in roots ( K _root) and leaves ( K _leaf) in loblolly pine trees. In addition, the role of seasonal variations in K _root and K _leaf in mediating stomatal control of transpiration and its response to vapour pressure deficit ( D ) as soil-dried was studied. Compared to trunk and branches, roots and leaves had the highest loss of conductivity and contributed to more than 75% of the total tree hydraulic resistance. Drought altered the partitioning of the resistance between roots and leaves. As soil moisture dropped below 50%, relative extractable water (REW), K _root declined faster than K _leaf. Although K _tree depended on soil moisture, its dynamics was tempered by the elongation of current-year needles that significantly increased K _leaf when REW was below 50%. After accounting for the effect of D on g _s, the seasonal decline in K _tree caused a 35% decrease in g _s and in its sensitivity to D , responses that were mainly driven by K _leaf under high REW and by K _root under low REW. We conclude that not only water stress but also leaf phenology affects the coordination between K _tree and g _s and the acclimation of trees to changing environmental conditions.     

Low carbon dioxide concentrations can reverse stomatal closure during water stress

Leaf water potentials below threshold values result in reduced stomatal conductance (g_s). Stomatal closure at low leaf water potentials may serve to protect against cavitation of xylem. Possible control of g_s by leaf water potential or hydraulic conductance was tested by drying the rooting medium in four herbaceous annual species until g_s was reduced and then lowering the [CO₂] to determine whether g_s and transpiration rate could be increased and leaf water potential decreased and whether hydraulic conductance was reduced at the resulting lower leaf water potential. In all species, low [CO₂] could reverse the stomatal closure because of drying despite further reductions in leaf water potential, and the resulting lower leaf water potentials did not result in reductions in hydraulic conductance. The relative sensitivity of g_s to internal [CO₂] in the leaves of dry plants of each species averaged three to four times higher than in leaves of wet plants. Two species in which g_s was reputed to be insensitive to [CO₂] were examined to determine whether high leaf to air water vapor pressure differences (D) resulted in increased stomatal sensitivity to [CO₂]. In both species, stomatal sensitivity to [CO₂] was indeed negligible at low D, but increased with D, and low [CO₂] partly or fully reversed closure caused by high D. In no case did low leaf water potential or low hydraulic conductance during drying of the air or the rooting medium prevent low [CO₂] from increasing g_s and transpiration rate.     

Effects of chloride and sodium on gas exchange parameters and water relations of Citrus plants

Gas exchange parameters, water relations and Na⁺/Cl^- content were measured on leaves of one-year-old sweet orange ( Citrus sinensis [L.] Osbeck cv. Hamlin) seedlings grown at increasing levels of salinity. Different salts (NaCl, KCl and NaNO₃) were used to separate the effects of Cl⁻ and Na⁺ on the investigated parameters. The chloride salts reduced plant dry weight and increased defoliation. Accumulation of Cl⁻ in the leaf tissue caused a sharp reduction in photosynthesis and stomatal conductance. By contrast, these parameters were not affected by leaf Na⁺ concentrations of up to 478 m M in the tissue water. Leaf water potentials reached values near −1.8 MPa at high NaCl and KCl supplies. This reduction was offset by a decrease in the osmotic potential so that turgor was maintained at or above control values. The changes in osmotic potential were closely correlated with changes in leaf proline concentrations. Addition of Ca²⁺ (as calcium acetate) increased growth and halved defoliation of salt stressed plants. Furthermore, calcium acetate decreased the concentration of Cl⁻ and Na⁺ in the leaves, and increased photosynthesis and stomatal conductance. Calcium acetate also counteracted the reductions in leaf water and osmotic potentials induced by salinity. In addition, calcium acetate inhibited the accumulation of proline in the leaves which affected the reduction in osmotic potential. These results indicate that adverse effects of salinity in Citrus leaves are caused by accumulation of chloride.     

Responses of apple leaf stomata: a model for single leaves and a whole tree

Abstract. An empirical model of stomatal response to environmental factors was developed from measurements of stomatal conductance ( g _s) made in a leaf chamber under controlled conditions. Results presented in a companion paper (Warrit, Landsberg & Thorpe, 1980) indicated that the model could be written in terms of only two factors, photon flux density ( Q _p) and leaf to air vapour pressure gradient ( D ). The response of Q _p was hyperbolic and that to D linear; combining these the equation of the model is where g _r is a reference conductance, α is the slope of the response to D and β indicates the sensitivity of g _s response to Q _p. Values of α were 0.20 and 0.30 kPa⁻¹ in June and August; the corresponding values of β were 59 and 79 μmol m⁻² s⁻¹.
The model was tested against mean values of g _s obtained with a porometer in the field, using environmental measurements as inputs. Correspondence between measured and calculated values was good. Transpiration rates were calculated from the Penman-Monteith equation, with stomatal resistance values calculated from the model, and compared with gravimetric measurements of tree water use. It was shown that transpiration could be calculated with acceptable accuracy. The effects of variations in stomatal resistance on transpiration rates under a range of conditions were explored using the model and the Penman- Monteith equation.     

Leaf Gas Exchange Characteristics in Relation to Leaf Canopy Position of Myrica faya in its Native Environment (Tenerife, Canary Islands)

Abstract: Diurnal courses of gas exchange were measured throughout one year in fully expanded current-year leaves in the uppermost canopy (sun leaves, 18 m above ground) and in the lower canopy (shade leaves, 12 m above ground) of Myrica faya Ait., a dominant component of the Canarian laurel forest in Tenerife, Canary Islands, Spain.
M. faya showed large differences between sun and shade leaves in gas exchange characteristics (about 50 % of maximum carbon assimilation rate (A_max) reduction in shade leaves, but this reduction can be higher on specific days) that were modulated by strong light attenuation and high leaf area index (LAI) of the stand. This species presented low A_max, about 10 μmol m^-2 s^-1, high maximum transpiration (E, 8 mmol m^-2 s^-1) and stomatal conductance (g_s, 750 mmol m^-2 s^-1) and very low instantaneous water use efficiency (WUE, mean maximum 1.1 mmol mol^-1) and A/g_s (mean maximum 23.5 μmol mol^-1). M. faya responded to high air vapour pressure deficit (VPD), decreasing its g_s but maintaining relatively high values of A and E during the studied period. Stomatal response to VPD showed a higher sensitivity than its congeners, M. cerifera, and Laurus azorica, tree species co-occurring in the Canarian laurel forest. In general, all these gas exchange characteristics lead us to consider this species more similar to subtropical plants of humid regions than to species of the Mediterranean region.     

Ethanol in the stems of trees

Acetaldehyde and ethanol are usually thought to be produced in plant tissues as a mechanism to tolerate hypoxic conditions. We have found acetaldehyde and ethanol to be common in the vascular cambium and in the transpiration stream of trees. In nonflooded trees, acetaldehyde and ethanol concentrations averaged 130 and 40 μ M in the cambium and 130 and 50 μ M in the xylem sap, respectively. Ethanol concentrations in the transpiration stream and the cambium increased to as much as 5 m M upon flooding. Ethanol concentrations in the vascular cambium of Populus deltoides could not be eliminated by placing logs from nonflooded trees in a pure oxygen environment for as long as 96 h, but increased by almost 3 orders of magnitude when logs were exposed to low external partial pressures of O₂. These results suggest that the vascular cambium was not hypoxic, despite the presence of acetaldehyde, ethanol and the enzymes for their synthesis.