Hybrid poplar stomata unresponsive to changes in environmental conditions

Summary The factors affecting stomatal conductance (g_s) of I-214 (Populus euramericana) and a hybrid poplar, Peace (P. koreana x P. trichocarpa), were examined in the field and under controlled environment conditions. Unusual opening of the stomata was observed with Peace leaves at all positions. Ontogenetic changes in g_s were similar between these two poplar species in the light. However, the dark/light ratio of g_s in Peace poplar varied from 0.58 to 1.23 with the insertion level while that of I-214 poplar was zero except for the third leaf from the top. The g_s of I-214 poplar changed with time of the day, varying from 0.74 mol m^-2s^-1 in the morning to zero at night, while the g_s of Peace poplar changed only from the minimum value of 0.23 mol m^-2s^-1 at night to the maximum of 0.48 mol m^-2s^-1 in the morning. Under severe water stress, below -1.5 MPa, which decreased the g_s of I-214 poplar to almost zero, the g_s of Peace poplar remained about onethird of that observed with well-watered leaves. Exposure to a relatively high concentration of O₃ caused the g_s of I-214 poplar to decrease nearly to zero but had no effect on the g_s of Peace. Stomata of Peace poplar were not affected by ABA and the g_s did not change even with 10^-4 M ABA, while the g_s of I-214 decreased to almost zero on the application of this concentration of ABA.     

Association genetics,geography and ecophysiology link stomatal patterning in Populus trichocarpa with carbon gain and disease resistance trade‐offs

Stomata are essential for diffusive entry of gases to support photosynthesis, but may also expose internal leaf tissues to pathogens. To uncover trade‐offs in range‐wide adaptation relating to stomata, we investigated the underlying genetics of stomatal traits and linked variability in these traits with geoclimate, ecophysiology, condensed foliar tannins and pathogen susceptibility in black cottonwood (Populus trichocarpa). Upper (adaxial) and lower (abaxial) leaf stomatal traits were measured from 454 accessions collected throughout much of the species range. We calculated broad‐sense heritability (H²) of stomatal traits and, using SNP data from a 34K Populus SNP array, performed a genome‐wide association studies (GWAS) to uncover genes underlying stomatal trait variation. H² values for stomatal traits were moderate (average H² = 0.33). GWAS identified genes associated primarily with adaxial stomata, including polarity genes (PHABULOSA), stomatal development genes (BRASSINOSTEROID‐INSENSITIVE 2) and disease/wound‐response genes (GLUTAMATE‐CYSTEINE LIGASE). Stomatal traits correlated with latitude, gas exchange, condensed tannins and leaf rust (Melampsora) infection. Latitudinal trends of greater adaxial stomata numbers and guard cell pore size corresponded with higher stomatal conductance (g_s) and photosynthesis (A_max), faster shoot elongation, lower foliar tannins and greater Melampsora susceptibility. This suggests an evolutionary trade‐off related to differing selection pressures across the species range. In northern environments, more adaxial stomata and larger pore sizes reflect selection for rapid carbon gain and growth. By contrast, southern genotypes have fewer adaxial stomata, smaller pore sizes and higher levels of condensed tannins, possibly linked to greater pressure from natural leaf pathogens, which are less significant in northern ecosystems.     

Genotype differences in 13C discrimination between atmosphere and leaf matter match differences in transpiration efficiency at leaf and whole‐plant levels in hybrid Populus deltoides ×nigra

¹³C discrimination between atmosphere and bulk leaf matter (Δ¹³C_lb) is frequently used as a proxy for transpiration efficiency (TE). Nevertheless, its relevance is challenged due to: (1) potential deviations from the theoretical discrimination model, and (2) complex time integration and upscaling from leaf to whole plant. Six hybrid genotypes of Populus deltoides×nigra genotypes were grown in climate chambers and tested for whole‐plant TE (i.e. accumulated biomass/water transpired). Net CO₂ assimilation rates (A) and stomatal conductance (g_s) were recorded in parallel to: (1) ¹³C in leaf bulk material (δ¹³C_lb) and in soluble sugars (δ¹³C_ss) and (2) ¹⁸O in leaf water and bulk leaf material. Genotypic means of δ¹³C_lb and δ¹³C_ss were tightly correlated. Discrimination between atmosphere and soluble sugars was correlated with daily intrinsic TE at leaf level (daily mean A/g_s), and with whole‐plant TE. Finally, g_s was positively correlated to ¹⁸O enrichment of bulk matter or water of leaves at individual level, but not at genotype level. We conclude that Δ¹³C_lb captures efficiently the genetic variability of whole‐plant TE in poplar. Nevertheless, scaling from leaf level to whole‐plant TE requires to take into account water losses and respiration independent of photosynthesis, which remain poorly documented.     

Leaf morphological plasticity and stomatal conductance in three Populus alba L. genotypes subjected to salt stress

The effect of salt stress on leaf morphology and functionality was studied in three Populus alba genotypes differing in tolerance to salinity: 6K3 (sensitive), 2AS11 (moderately tolerant), and 14P11 (tolerant). Plants were subjected to an intense and progressive salt stress from 50 to 250 mM NaCl by 50 mM steps at 10-day intervals. The micromorphological results highlighted phenotypic variation among the three genotypes already in control plants, with the genotype 14P11 having significantly smaller epidermal cells and higher stomatal density. Salt-treated plants modulated differently the expansion of stomata compared with epidermal cells. Regression analysis showed significant correlations between decrease of stomatal area and stomatal conductance (g_s) in genotypes 14P11 and 6K3. So, the common reduction of stomatal area could be an early mechanism to save water in this species. However, only genotype 14P11 showed further significant decrease of this trait under the highest salinity level, combined with a significant reduction in leaf length. In addition, this genotype showed the lowest leaf abscission rate at the end of salt stress period. The genotype 6K3 was severely affected by leaf necrosis and showed the highest leaf abscission rate in salt stress conditions. In the moderately tolerant genotype 2AS11, an intermediate plastic behaviour in both leaf morphology and physiology was observed during the experiment. The phenotypic variation among the three genotypes in terms of micromorphology and stomatal conductance is discussed in relation to plant functionality in salt stress conditions. Overall results suggest that leaf morphological habit contributes to salt tolerance in P. alba.     

Using the rapid A‐Ci response (RACiR) in the Li‐Cor 6400 to measure developmental gradients of photosynthetic capacity in poplar

The rapid A‐C_i response (RACiR) technique alleviates limitations of measuring photosynthetic capacity by reducing the time needed to determine the maximum carboxylation rate (V_cmax) and electron transport rate (J_max) in leaves. Photosynthetic capacity and its relationships with leaf development are important for understanding ecological and agricultural productivity; however, our current understanding is incomplete. Here, we show that RACiR can be used in previous generation gas exchange systems (i.e., the LI‐6400) and apply this method to rapidly investigate developmental gradients of photosynthetic capacity in poplar. We compared RACiR‐determined V_cmax and J_max as well as respiration and stomatal conductance (g_s) across four stages of leaf expansion in Populus deltoides and the poplar hybrid 717‐1B4 (Populus tremula × Populus alba). These physiological data were paired with leaf traits including nitrogen concentration, chlorophyll concentrations, and specific leaf area. Several traits displayed developmental trends that differed between the poplar species, demonstrating the utility of RACiR approaches to rapidly generate accurate measures of photosynthetic capacity. By using both new and old machines, we have shown how more investigators will be able to incorporate measurements of important photosynthetic traits in future studies and further our understanding of relationships between development and leaf‐level physiology.     

The boundary layer and the apparent responses of stomatal conductance to wind speed and to the mole fractions of CO2 and water vapour in the air

The experiments and simulations reported in this paper show that, for stomata sensitive to both CO₂ and water vapour concentrations, responses of stomatal conductance (g^w_s) to boundary layer thickness have two components, one resulting from changes in intercellular CO₂ concentration (χ^c_i) and another from changes in leaf surface water vapour saturation deficit (D^w_s). The experiments and simulations also show that the boundary layer conductance (g^w_b) can significantly alter the apparent response of g^w_s to ambient air CO₂ mole fraction (χ^c_a) and water vapour mole fraction (χ^w_a). Because of the feedback loop involved the responses of g^w_s for χ^c_a and χ^w_a each include responses to both χ^c_i and D^w_s. The boundary layer alters the state of the variables sensed by the guard cells—i.e. χ^c_i and D^w_s—and so it is a source of feedback. Thus, when scaling up from responses of stomata to the response of g^w_s for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of g^w_s to χ^c_i and D^w_s, the apparent responses of g^w_s to D^w_a and χ^c_a depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods.     

Early season cuticular conductance and gas exchange in two oaks near the western edge of their range

Seasonal changes in minimum leaf conductance to water vapor (g_min), an estimate of cuticular conductance, and photosynthetic gas exchange in two co-occurring oak species in north-east Kansas (USA) were examined to determine if leaf gas exchange characteristics correlated with differences in tree distribution. Bur oak (Quercus macrocarpa Michx.) is more abundant in mesic gallery forest sites, whereas chinquapin oak (Quercus muehlenbergii Englm.) is more abundant in xeric sites. Early, during leaf expansion, g_min was significantly lower in chinquapin oak than in bur oak, though midday water potentials were similar. After leaves had fully expanded, g_min decreased to seasonal minimum values of 4.57 (±0.274) mmol m^-2 s^-1 in bur oak, and 2.66 (±0.156) mmol m^-2 s^-1 in chinquapin oak. Water potentials at these times were significantly higher in chinquapin oak. As leaves were expanding, photosynthesis (A_net) was significantly higher in chinquapin oak than in bur oak. Later in the growing season, A_net and gleaf increased dramatically in both species, and were significantly higher in bur oak relative to chinquapin oak. We concluded that bur and chinquapin oak have a number of leaf gas exchange characteristics that minimize seasonal water loss. These characteristics are distinct from trees from more mesic sites, and are consistent with the distribution patterns of these trees in tall-grass prairie gallery forests.     

Impact of simulated herbivory on water relations of aspen (<Emphasis Type="Italic">Populus tremuloides</Emphasis>) seedlings: the role of new tissue in the hydraulic conductivity recovery cycle

Physiological mechanisms behind plant–herbivore interactions are commonly approached as input–output systems where the role of plant physiology is viewed as a black box. Studies evaluating impacts of defoliation on plant physiology have mostly focused on changes in photosynthesis while the overall impact on plant water relations is largely unknown. Stem hydraulic conductivity (k _h), stem specific conductivity (k _s), percent loss of hydraulic conductivity (PLC), CO₂ assimilation (A) and stomatal conductance (g _s) were measured on well-irrigated 1-month-old Populus tremuloides (Michx.) defoliated and control seedlings until complete refoliation. PLC values of defoliated seedlings gradually increased during the refoliation process despite them being kept well irrigated. k _s of defoliated seedlings gradually decreased during refoliation. PLC and k _s values of control seedlings remained constant during refoliation. k _s of new stems, leaf specific conductivity and A of leaves grown from new stems in defoliated and control seedlings were not significantly different, but g _s was higher in defoliated than in control seedlings. The gradual increase of PLC and decrease of k _s values in old stems after defoliation was unexpected under well-irrigated conditions, but appeared to have little impact on new stems formed after defoliation. The gradual loss of conductivity measured during the refoliation process under well-irrigated conditions suggests that young seedlings of P. tremuloides may be more susceptible to cavitation after herbivore damage under drought conditions.     

Most stomatal closure in woody species under moderate drought can be explained by stomatal responses to leaf turgor

Reduced stomatal conductance (g_s) during soil drought in angiosperms may result from effects of leaf turgor on stomata and/or factors that do not directly depend on leaf turgor, including root‐derived abscisic acid (ABA) signals. To quantify the roles of leaf turgor‐mediated and leaf turgor‐independent mechanisms in g_s decline during drought, we measured drought responses of g_s and water relations in three woody species (almond, grapevine and olive) under a range of conditions designed to generate independent variation in leaf and root turgor, including diurnal variation in evaporative demand and changes in plant hydraulic conductance and leaf osmotic pressure. We then applied these data to a process‐based g_s model and used a novel method to partition observed declines in g_s during drought into contributions from each parameter in the model. Soil drought reduced g_s by 63–84% across species, and the model reproduced these changes well (r² = 0.91, P < 0.0001, n = 44) despite having only a single fitted parameter. Our analysis concluded that responses mediated by leaf turgor could explain over 87% of the observed decline in g_s across species, adding to a growing body of evidence that challenges the root ABA‐centric model of stomatal responses to drought.     

Aquaporin expression in response to different water stress intensities and recovery in Richter-110 (<Emphasis Type="Italic">Vitis</Emphasis> sp.): relationship with ecophysiological status

Aquaporins seem essential for the regulation of plant water status and expenses. Richter-110 is a Vitis hybrid (Vitis berlandieri × rupestris) reputed to be strongly drought-tolerant. Three irrigation treatments were established in Richter-110 plants growing outdoors defined by the resulting maximum stomatal conductance (g _s), and ensuring water stress situations not severe enough as to stop photosynthesis and growth: well-watered plants (g _s about 250 mmol H₂O m⁻² s⁻¹), moderate water stress (g _s about 150 mmol H₂O m⁻² s⁻¹) and severe water stress (g _s about 50 mmol H₂O m⁻² s⁻¹). Plants under water stress were kept at constant water availability for 7 days to check for possible acclimation. Finally, plants were re-watered, and allowed to recover, for 3 days. Stomatal conductance, leaf water potential, xylem abscisic acid (ABA) content and root and stem hydraulic conductivity were determined. The relative amounts of expression of mRNA encoding seven putative aquaporins were determined in roots and leaves by RT-PCR. The decrease in stomatal conductance with moderate and severe water stress was associated with increasing ABA contents, but not with the leaf water potential and hydraulic conductivities, which remained unchanged during the entire experiment. Aquaporin gene expression varied depending on which aquaporin, water stress level and the plant organ. We suggest that aquaporin expression was responsive to water stress as part of the homeostasis, which resulted in constant leaf water potential and hydraulic conductivity.     

Environmental and physiological regulation of transpiration in tropical forest gap species: the influence of boundary layer and hydraulic properties

Environmental and physiological regulation of transpiration were examined in several gap-colonizing shrub and tree species during two consecutive dry seasons in a moist, lowland tropical forest on Barro Colorado Island, Panama. Whole plant transpiration, stomatal and total vapor phase (stomatal + boundary layer) conductance, plant water potential and environmental variables were measured concurrently. This allowed control of transpiration (E) to be partitioned quantitatively between stomatal (g _s) and boundary layer (g _b) conductance and permitted the impact of invividual environmental and physiological variables on stomatal behavior and E to be assessed. Wind speed in treefall gap sites was often below the 0.25 m s^–1 stalling speed of the anemometer used and was rarely above 0.5 m s^–1, resulting in uniformly low g _b (c. 200–300 mmol m^–2 s^–1) among all species studied regardless of leaf size. Stomatal conductance was typically equal to or somewhat greater than g _b. This strongly decoupled E from control by stomata, so that in Miconia argentea a 10% change in g _s when g _s was near its mean value was predicted to yield only a 2.5% change in E. Porometric estimates of E, obtained as the product of g _s and the leaf-bulk air vapor pressure difference (VPD) without taking g _b into account, were up to 300% higher than actual E determined from sap flow measurements. Porometry was thus inadequate as a means of assessing the physiological consequences of stomatal behavior in different gap colonizing species. Stomatal responses to humidity strongly limited the increase in E with increasing evaporative demand. Stomata of all species studied appeared to respond to increasing evaporative demand in the same manner when the leaf surface was selected as the reference point for determination of external vapor pressure and when simultaneous variation of light and leaf-air VPD was taken into account. This result suggests that contrasting stomatal responses to similar leaf-bulk air VPD may be governed as much by the external boundary layer as by intrinsic physiological differences among species. Both E and g _s initially increased sharply with increasing leaf area-specific total hydraulic conductance of the soil/root/leaf pathway (G _t), becoming asymptotic at higher values of G _t. For both E and g _s a unique relationship appeared to describe the response of all species to variations in G _t. The relatively weak correlation observed between g _s and midday leaf water potential suggested that stomatal adjustment to variations in water availability coordinated E with water transport efficiency rather than bulk leaf water status.     

Direct and acclimatory responses of stomatal conductance to elevated carbon dioxide in four herbaceous crop species in the field

In order to separate the net effect of growth at elevated [CO₂] on stomatal conductance (g_s) into direct and acclimatory responses, mid‐day values of g_s were measured for plants grown in field plots in open‐topped chambers at the current ambient [CO₂], which averaged 350 μmol mol^?1 in the daytime, and at ambient + 350 μmol mol^?1[CO₂] for winter wheat, winter barley, potato and sorghum. The acclimatory response was determined by comparing g_s measured at 700 μmol mol^?1[CO₂] for plants grown at the two [CO₂]. The direct effect of increasing [CO₂] from 350 to 700 μmol mol^?1 was determined for plants grown at the lower concentration. Photosynthetic rates were measured concurrently with g_s. For all species, growth at the higher [CO₂] significantly reduced g_s measured at 700 μmol mol^?1[CO₂]. The reduction in g_s caused by growth at the higher [CO₂] was larger for all species on days with low leaf to air water vapour pressure difference for a given temperature, which coincided with highest conductances and also the smallest direct effects of increased [CO₂] on conductance. For barley, there was no other evidence for stomatal acclimation, despite consistent down‐regulation of photosynthetic rate in plants grown at the higher [CO₂]. In wheat and potato, in addition to the vapour pressure difference interaction, the magnitude of stomatal acclimation varied directly in proportion to the magnitude of down‐regulation of photosynthetic rate through the season. In sorghum, g_s consistently exhibited acclimation, but there was no down‐regulation of photosynthetic rate. In none of the species except barley was the direct effect the larger component of the net reduction in g_s when averaged over measurement dates. The net effect of growth at elevated [CO₂] on mid‐day g_s resulted from unique combinations of direct and acclimatory responses in the various species.     

Water relations and gas exchange in Fagus sylvatica L. and Quercus petraea (Mattuschka) Liebl. in a mixed stand at their southern limit of distribution in Europe

Water status and gas exchange of beech (Fagus sylvatica L.) and sessile oak [Quercus petraea (Mattuschka) Liebl.] were studied in a mixed stand in the Montejo de la Sierra forest (central Iberian Peninsula), one of the southernmost locations of both species in Europe. Gas exchange and water potential were measured in leaves at different canopy levels over several days in two growing seasons. The daily variation pattern was established with the measurements of three selected dates per year, representative of the soil moisture content situations in early, mid- and late summer. A similar daily time course of leaf water potential was found for the two species. Nevertheless, beech showed a most noticeable decrease of water potential at midmorning and maintained lower leaf water potential than oak in the early afternoon. In 1994 the sessile oak saplings showed higher values of predawn water potential (Ψ_pd) than beech at the end of summer, when soil moisture content was lowest (20 cm depth). Beech showed a significantly lower net assimilation rate (A) than sessile oak for leaves under the same PPFD. Maximum net photosynthesis values (A _max) for beech and sessile oak on sunny leaves were 10.1±0.4 μmol m^–2 s^–1 and 17.8±1.7 μmol m^–2 s^–1 respectively, and those for water vapour stomatal conductance (g _wv) were 265±31 mmol m^–2 s^–1 and 438±74 mmol m^–2 s^–1. Differences in A and g _wv between the two species were maintained throughout the day on all measurement dates. No clear relationship was found between water status of saplings and stomata performance; there was only a negative correlation between Ψ_pd and g _wvmid in beech. Nevertheless, a significant response to the air vapour pressure gradient between leaf and air was translated into stomata closure on an hourly basis, more intensively in beech. Received: 4 March 1999 / Accepted: 21 December 1999     

Transpiration and whole-tree conductance in ponderosa pine trees of different heights

Changes in leaf physiology with tree age and size could alter forest growth, water yield, and carbon fluxes. We measured tree water flux (Q) for 14 ponderosa pine trees in two size classes (12 m tall and ∼40 years old, and 36 m tall and ∼ 290 years old) to determine if transpiration (E) and whole-tree conductance (g _t) differed between the two sizes of trees. For both size classes, E was approximately equal to Q measured 2 m above the ground: Q was most highly correlated with current, not lagged, water vapor pressure deficit, and night Q was <12% of total daily flux. E for days 165–195 and 240–260 averaged 0.97 mmol m^–2 (leaf area, projected) s^–1 for the 12-m trees and 0.57 mmol m^–2 (leaf area) s^–1 for the 36-m trees. When photosynthetically active radiation (I _P) exceeded the light saturation for photosynthesis in ponderosa pine (900 μmol m^–2 (ground) s^–1), differences in E were more pronounced: 2.4 mmol m^–2 (leaf area) s^–1 for the 12-m trees and 1.2 mmol m^–2 s^–1 for the 36-m trees, yielding g _t of 140 mmol m^–2 (leaf area) s^–1 for the 12-m trees and 72 mmol m^–2 s^–1 for the 36-m trees. Extrapolated to forests with leaf area index =1, the 36-m trees would transpire 117 mm between 1 June and 31 August compared to 170 mm for the 12-m trees, a difference of 15% of average annual precipitation. Lower g _t in the taller trees also likely lowers photosynthesis during the growing season. Received: 19 April 1999 / Accepted: 23 March 2000     

Seasonal relationships between leaf nitrogen content (photosynthetic capacity) and leaf canopy light exposure in peach (Prunus persica)

Abstract Field gas exchange measurements on intact peach (Prunus persica (L.) Batsch) leaves indicate that leaf nitrogen content (N_L) and leaf weight per unit leaf area (W_a) are highly correlated with CO₂ assimilation rate (A) and mesophyll conductance (g_m). Therefore, N_L and W_a were used to study seasonal relationships between leaf carboxylation capacity and natural light exposure in tree canopies. From mid-season onwards, N_L and W_a were linearly correlated with light exposure expressed as the amount of time during a clear day that a leaf was exposed to a photosynthetic photon flux density (Q) of ≥ 100 μmol m^?2 s^?1. The data support the hypothesis that whole-tree photosynthesis is optimized by partitioning of photosynthetic capacity among leaves in deciduous tree canopies with respect to natural light exposure.     

Expression levels of genes involved in metal homeostasis,physiological adaptation,and growth characteristics of rice (Oryza sativa L.) genotypes under Fe and/or Al toxicity

Acid sulphate soil contains high amounts of iron (Fe) and aluminum (Al), and their contamination has been reported as major problems, especially in rainfed and irrigated lowland paddy fields. Rice is sensitive to Fe and Al grown in acid soil (pH < 5.5), leading to growth inhibition and grain yield loss. The objective of this study was to evaluate Fe and/or Al uptake, translocation, physiological adaptation, metal toxicity, and growth inhibition in rice genotypes grown in acid soil. Fe and Al in the root tissues of all rice genotypes were enriched depending on the exogenous application of either Fe or Al in the soil solution, leading to root growth inhibition, especially in the KDML105 genotype. Expression level of OsYSL1 in KDML105 was increased in relation to metal uptake into root tissues, whereas OsVIT2 was downregulated, leading to Fe (50.3 mg g⁻¹ DW or 13.1 folds over the control) and Al (4.8 mg g⁻¹ DW or 2.2 folds over the control) translocation to leaf tissues. Consequently, leaf greenness (SPAD), net photosynthetic rate (P_n), stomatal conductance (g_s), and transpiration rate (E) in the leaf tissues of genotype KDML105 under Fe + Al toxicity significantly declined by 28.4%, 35.3%, 55.6%, and 51.6% over the control, respectively. In Azucena (AZU; Fe/Al tolerant), there was a rapid uptake of Fe and Al by OsYSL1 expression in the root tissues, but a limited secretion into vacuole organelles by OsVIT2, leading to a maintenance of low level of toxicity driven by an enhanced accumulation of glutathione together with downregulation of OsGR expression level. In addition, Fe and Al restrictions in the root tissues of genotype RD35 were evident; therefore, crop stress index (CSI) of Fe + Al–treated plants was the maximum, leading to an inhibition of g_s (53.6% over the control) and E (49.0% over the control). Consequently, free proline, total phenolic compounds, and ascorbic acid in the leaf tissues of rice under Fe + Al toxicity significantly increased by 3.2, 1.2, and 1.5 folds over the control, respectively, indicating their functions in non-enzymatic antioxidant defense. Moreover, physiological parameters including leaf temperature (T_leaf) increment, high level of CSI (>0.6), SPAD reduction, photon yield of PSII (Φ_PSII) diminution, P_n, g_s, and E inhibition in rice genotype IR64 (Fe/Al-sensitive) under Fe + Al treatment were clearly demonstrated as good indicators of metal-induced toxicity. Our results on Fe- and/or Al-tolerant screening to find out the candidate genotypes will contribute to present screening and breeding efforts, which in turn help increase rice production in the Fe/Al-contaminated acid soil under lowland conditions.

     

Leaf temperature effects on gas exchange in Quercus ilex L. growing under field conditions

ABSTRACT

Gas exchange temperature dependence in Quercus ilex shrubs growing in the Mediterranean maquis was analysed. The gas exchange trend was monitored during the year: photosynthetic activity (A _net) reached the highest average rates in early spring and autumn (12.5 µmol m^-2s^-1 was the absolute maximum A _net measured) and the lowest rates were monitored in the middle of June. There was a good correlation (r = 0.72) between A _net and g _s (A _net = 4.1246 ln g _s + 4316; P < 0.01), indicating that stomatal control of CO₂ diffusion plays an important role in controlling photosynthetic activity. Leaf temperature allowing the highest photosynthetic and stomatal conductance rates of Quercus ilex were in the range 17.5 – 29°C. A _net and g_s dropped below half its maximum value when leaf temperatures were below 11.5°C and above 35.7°C. Transpiration rates (E) were strongly related to leaf temperature; E increased as leaf temperature increased and the highest E rates were monitored in June, despite a 46% decrease in g _s. Leaf water loss from transpiration, during the drought period, could result in leaf water stress which would exacerbate heat effects on photosynthesis. During summer, the increase in leaf temperatures decreased g _s which in turn decreased A _net. Consequently, stomatal control in Quercus ilex may be considered as an adaptive strategy during drought.     

Variations in photosynthetic rate and associated parameters with age of oil palm leaves under irrigation

Net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) in an adult oil palm (Elaeis guineensis) canopy were highest in the 9^th leaf and progressively declined with leaf age. Larger leaf area (LA) and leaf dry mass (LDM) were recorded in middle leaves. P _N showed a significant positive correlation with g _s and a negative relationship with leaf mass per area (ALM). The oil palm leaf remains photosynthetically active for a longer time in the canopy which contributes significantly to larger dry matter production in general and greater fresh fruit bunch yields in particular.     

Control of transpiration from the upper canopy of a tropical forest: the role of stomatal, boundary layer and hydraulic architecture components

Concurrent, independent measurements of stomatal conductance (g_s), transpiration (E) and microenvironmental variables were used to characterize control of crown transpiration in four tree species growing in a moist, lowland tropical forest. Access to the upper forest canopy was provided by a construction crane equipped with a gondola. Estimates of boundary layer conductance (g_b) obtained with two independent methods permitted control of E to be partitioned quantitatively between g_s and g_b using a dimensionless decoupling coefficient (Ω) ranging from zero to 1. A combination of high g_s (c. 300–600 mmol m^?2 s^?1) and low wind speed, and therefore relatively low g_b (c. 100–800 mmol m^?2 s^?1), strongly decoupled E from control by stomata in all four species (Ω= 0.7–0.9). Photosynthetic water-use efficiency was predicted to increase rather than decrease with increasing g_s because g_b was relatively low and internal conductance to CO₂ transfer was relatively high. Responses of g_s to humidity were apparent only when the leaf surface, and not the bulk air, was used as the reference point for determination of external vapour pressure. However, independent measurements of crown conductance (g_c), a total vapour phase conductance that included stomatal and boundary layer components, revealed a clear decline in g_c with increasing leaf-to-bulk air vapour pressure difference (V_a because the external reference points for determination of g_c and V_a were compatible. The relationships between g_c and V_c and between g_s and V_s appeared to be distinct for each species. However, when g_s and g_c were normalized by the branch-specific ratio of leaf area to sapwood area (LA/SA), a morphological index of potential transpirational demand relative to water transport capacity, a common relationship between conductance and evaporative demand for all four species emerged. Taken together, these results implied that, at a given combination of LA/SA and evaporative demand scaled to the appropriate reference point, the vapour phase conductance and therefore transpiration rates on a leaf area basis were identical in all four contrasting species studied.     

Drought‐introduced variability of mesophyll conductance in Gossypium and its relationship with leaf anatomy

Mesophyll conductance (g_m) is one of the major determinants of photosynthetic rate, for which it has an impact on crop yield. However, the regulatory mechanisms behind the decline in g_m of cotton (Gossypium. spp) by drought are unclear. An upland cotton (Gossypium hirsutum) genotype and a pima cotton (Gossypium barbadense) genotype were used to determine the gas exchange parameters, leaf anatomical structure as well as aquaporin and carbonic anhydrase gene expression under well‐watered and drought treatment conditions. In this study, the decrease of net photosynthetic rate (A_N) under drought conditions was related to a decline in g_m and in stomatal conductance (g_s). g_m and g_s coordinate with each other to ensure optimum state of CO₂ diffusion and achieve the balance of water and CO₂ demand in the process of photosynthesis. Meanwhile, mesophyll limitations to photosynthesis are equally important to the stomatal limitations. Considering g_m, its decline in cotton leaves under drought was mostly regulated by the chloroplast surface area exposed to leaf intercellular air spaces per leaf area (S_c/S) and might also be regulated by the expression of leaf CARBONIC ANHYDRASE (CA1). Meanwhile, cotton leaves can minimize the decrease in g_m under drought by maintaining cell wall thickness (T_cw). Our results indicated that modification of chloroplasts might be a target trait in future attempts to improve cotton drought tolerance.