Foliar abscisic acid content underlies genotypic variation in stomatal responsiveness after growth at high relative air humidity

Background and Aims

Stomata formed at high relative air humidity (RH) respond less to abscisic acid (ABA), an effect that varies widely between cultivars. This study tested the hypotheses that this genotypic variation in stomatal responsiveness originates from differential impairment in intermediates of the ABA signalling pathway during closure and differences in leaf ABA concentration during growth.

Methods

Stomatal anatomical features and stomatal responsiveness to desiccation, feeding with ABA, three transduction elements of its signalling pathway (H₂O₂, NO, Ca²⁺) and elicitors of these elements were determined in four rose cultivars grown at moderate (60 %) and high (90 %) RH. Leaf ABA concentration was assessed throughout the photoperiod and following mild desiccation (10 % leaf weight loss).

Key Results

Stomatal responsiveness to desiccation and ABA feeding was little affected by high RH in two cultivars, whereas it was considerably attenuated in two other cultivars (thus termed sensitive). Leaf ABA concentration was lower in plants grown at high RH, an effect that was more pronounced in the sensitive cultivars. Mild desiccation triggered an increase in leaf ABA concentration and equalized differences between leaves grown at moderate and high RH. High RH impaired stomatal responses to all transduction elements, but cultivar differences were not observed.

Conclusions

High RH resulted in decreased leaf ABA concentration during growth as a result of lack of water deficit, since desiccation induced ABA accumulation. Sensitive cultivars underwent a larger decrease in leaf ABA concentration rather than having a higher ABA concentration threshold for inducing stomatal functioning. However, cultivar differences in stomatal closure following ABA feeding were not apparent in response to H₂O₂ and downstream elements, indicating that signalling events prior to H₂O₂ generation are involved in the observed genotypic variation.     

Variation in the carbon and oxygen isotope composition of plant biomass and its relationship to water‐use efficiency at the leaf‐ and ecosystem‐scales in a northern Great Plains grassland

Measurements of the carbon (δ¹³C_m) and oxygen (δ¹⁸O_m) isotope composition of C₃ plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ¹³C_m and δ¹⁸O_m values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ¹³C_m and biomass production during this time. By comparison, the lack of a significant relationship between δ¹³C_m and δ¹⁸O_m values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf c_i/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.     

Avoiding high relative air humidity during critical stages of leaf ontogeny is decisive for stomatal functioning

Plants of several species, if grown at high relative air humidity (RH ≥85%), develop stomata that fail to close fully in case of low leaf water potential. We studied the effect of a reciprocal change in RH, at different stages of leaf expansion of Rosa hybrida grown at moderate (60%) or high (95%) RH, on the stomatal closing ability. This was assessed by measuring the leaf transpiration rate in response to desiccation once the leaves had fully expanded. For leaves that started expanding at high RH but completed their expansion after transfer to moderate RH, the earlier this switch took place the better the stomatal functioning. Leaves initially expanding at moderate RH and transferred to high RH exhibited poor stomatal functioning, even when this transfer occurred very late during leaf expansion. Applying a daily abscisic acid (ABA) solution to the leaves of plants grown at continuous high RH was effective in inducing stomatal closure at low water potential, if done before full leaf expansion (FLE). After FLE, stomatal functioning was no longer affected either by the RH or ABA level. The results indicate that the degree of stomatal adaptation depends on both the timing and duration of exposure to high RH. It is concluded that stomatal functionality is strongly dependent on the humidity at which the leaf completed its expansion. The data also show that the effect of ambient RH and the alleviating role of ABA are restricted to the period of leaf expansion.     

Stomatal response characteristics of Tradescantia virginiana grown at high relative air humidity

Plants produced at high relative air humidity (RH) show poor control of water loss after transferring to low RH, a phenomenon which is thought to be due to their stomatal behaviour. The stomatal anatomy and responses of moderate (55%) and high (90%) RH grown Tradescantia virginiana plants to treatments that normally induce stomatal closure, i.e. desiccation, abscisic acid (ABA) application and exposure to darkness were studied using attached or detached young, fully expanded leaves. Compared with plants grown at moderate RH the transpiration rate, stomatal conductance and aperture of high RH grown plants measured at the same condition (40% RH) were, respectively, 112, 139 and 132% in light and 141, 188 and 370% in darkness. Besides the differences in stomatal size (guard cell length was 56.7 and 73.3 µm for moderate and high RH grown plants, respectively), there was a clear difference in stomatal behaviour. The stomata responded to desiccation, ABA and darkness in both moderate and high RH grown plants, but the high variability of stomatal closure in high RH grown plants was striking. Some stomata developed at high RH closed in response to darkness or to a decrease in relative water content to the same extent as did stomata from moderate RH grown plants, whereas others closed only partly or did not close at all. Evidently, some as yet unidentified physiological or anatomical changes during development disrupt the normal functioning of some stomata in leaves grown at high RH. The failure of some stomata to close fully in response to ABA suggests that ABA deficiency was not responsible for the lack of stomatal closure in response to desiccation.     

Effects of Salinity on Stomatal Conductance, Photosynthetic Capacity, and Carbon Isotope Discrimination of Salt-Tolerant (Gossypium hirsutum L.) and Salt-Sensitive (Phaseolus vulgaris L.) C(3) Non-Halophytes

The effects of salinity on growth, stomatal conductance, photosynthetic capacity, and carbon isotope discrimination (Δ) of Gossypium hirsutum L. and Phaseolus vulgaris L. were evaluated. Plants were grown at different NaCl concentrations from 10 days old until mature reproductive structures were formed. Plant growth and leaf area development were strongly reduced by salinity, in both cotton and bean. Stomatal conductance also was reduced by salinity. The Δ always declined with increasing external salinity concentration, indicating that stomatal limitation of photosynthesis was increased. In cotton plant dry matter, Δ correlated with the ratio of intercellular to atmospheric CO₂ partial pressures (p_l/p_a) calculated by gas exchange. This correlation was not clear in bean plants, although Δ showed a more pronounced salt induced decline in bean than in cotton. Possible effects of heterogeneity of stomatal aperture and consequent overestimation of p_l as determined from gas exchange could explain these results. Significant differences of Δ between leaf and seed material were observed in cotton and bean. This suggests different patterns of carbon allocation between leaves and seeds. The photon yield of O₂ evolution determined at rate-limiting photosynthetic photon flux density was insensitive to salinity in both species analyzed. The light- and CO₂-saturated rate of CO₂ uptake and O₂ evolution showed a salt induced decline in both species. Possible explanations of this observation are discussed. O₂ hypersensitivity was observed in salt stressed cotton plants. These results clearly demonstrate that the effect of salinity on assimilation rate was mostly due to the reduction of stomatal conductance, and that calculation of p_l may be overestimated in salt stressed plants, because of heterogeneity of stomatal aperture over the leaf surface.     

The role of stomata in sensitivity of Betula papyrifera seedlings to SO2 at different humidities

Summary Stomata of paper birch (Betula papyrifera Marsh.) seedlings were more open at high humidity than at low humidity and responded rapidly to changes in vapor pressure deficit. SO₂ at 0.2 or 0.8 l l^-1 caused partial stomatal closure. Seedlings fumigated with SO₂ at 0.2 or 0.5 l l^-1 for 30 h or 0.2 l l^-1 for 75 h took up more SO₂ at high than at low humidity. Differences in pollutant uptake could be explained by stomatal conductance with no need to invoke changes in mesophyll conductance. Betula seedlings were more sensitive to SO₂ when fumigated at high humidity, as manifested in more leaf necrosis, increased leaf abscission, and greater growth inhibition compared to seedlings fumigated at low humidity. Amount of injury to leaves increased with rate of SO₂ uptake, and inhibition of root growth increased with total SO₂ uptake.Abbreviations RH relative humidity - VPD vapor pressure deficit - RGR mean relative growth rate - PPFD photosynthetic photon flux density (400–700 nm) - LDC leaf diffusive conductance - water potential Research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison     

The role of abscisic acid in disturbed stomatal response characteristics of Tradescantia virginiana during growth at high relative air humidity

In this study, the role of abscisic acid (ABA) in altered stomatal responses of Tradescantia virginiana leaves grown at high relative air humidity (RH) was investigated. A lower ABA concentration was found in leaves grown at high RH compared with leaves grown at moderate RH. As a result of a daily application of 20 microM ABA to leaves for 3 weeks during growth at high RH, the stomata of ABA-treated leaves grown at high RH showed the same behaviour as did the stomata of leaves grown at moderate RH. For example, they closed rapidly when exposed to desiccation. Providing a high RH around a single leaf of a plant during growth at moderate RH changed the stomatal responses of this leaf. The stomata in this leaf grown at high RH did not close completely in response to desiccation in contrast to the stomata of the other leaves from the same plant. The ABA concentration on a fresh weight basis, though not on a dry weight basis, of this leaf was significantly lower than that of the others. Moreover, less closure of stomata was found in the older leaves of plants grown at high RH in response to desiccation compared with younger leaves. This was correlated with a lower ABA concentration in these leaves on a fresh weight basis, though not on a dry weight basis. Stomata of leaves grown at moderate RH closed in response to short-term application of ABA or sodium nitroprusside (SNP), while for leaves grown at high RH there was a clear difference in stomatal responses between the leaf margins and main-vein areas. The stomatal aperture in response to short-term application of ABA or SNP at the leaf margins of leaves grown at high RH remained significantly wider than in the main-vein areas. It was concluded that: (i) a long-term low ABA concentration in well-watered plants during growth at high RH could be a reason for less or no stomatal closure under conditions of drought stress; and (ii) the long-term ABA concentration on a fresh weight basis rather than on a dry weight basis is likely to be responsible for structural or physiological changes in stomata during leaf growth.     

Vapour pressure deficit during growth has little impact on genotypic differences of transpiration efficiency at leaf and whole‐plant level: an example from Populus nigra L.

Poplar genotypes differ in transpiration efficiency (TE) at leaf and whole‐plant level under similar conditions. We tested whether atmospheric vapour pressure deficit (VPD) affected TE to the same extent across genotypes. Six Populus nigra genotypes were grown under two VPD. We recorded (1) ¹³C content in soluble sugars; (2) ¹⁸O enrichment in leaf water; (3) leaf‐level gas exchange; and (4) whole‐plant biomass accumulation and water use. Whole‐plant and intrinsic leaf TE and ¹³C content in soluble sugars differed significantly among genotypes. Stomatal conductance contributed more to these differences than net CO₂ assimilation rate. VPD increased water use and reduced whole‐plant TE. It increased intrinsic leaf‐level TE due to a decline in stomatal conductance. It also promoted higher ¹⁸O enrichment in leaf water. VPD had no genotype‐specific effect. We detected a deviation in the relationship between ¹³C in leaf sugars and ¹³C predicted from gas exchange and the standard discrimination model. This may be partly due to genotypic differences in mesophyll conductance, and to its lack of sensitivity to VPD. Leaf‐level ¹³C discrimination was a powerful predictor of the genetic variability of whole‐plant TE irrespective of VPD during growth.     

The effects of salinity,crassulacean acid metabolism and plant age on the carbon isotope composition of <Emphasis Type="Italic">Mesembryanthemum crystallinum</Emphasis> L., a halophytic C<Subscript>3</Subscript>-CAM species

The carbon isotope composition of the halophyte Mesembryanthemum crystallinum L. (Aizoaceae) changes when plants are exposed to environmental stress and when they shift from C₃ to crassulacean acid metabolism (CAM). We examined the coupling between carbon isotope composition and photosynthetic pathway by subjecting plants of different ages to salinity and humidity treatments. Whole shoot ¹³C values became less negative in plants that were exposed to 400 mM NaCl in the hydroponic solution. The isotopic change had two components: a direct NaCl effect that was greatest in plants still operating in the C₃ mode and decreased proportionally with increasing levels of dark fixation, and a second component related to the degree of CAM expression. Ignoring the presumably diffusion-related NaCl effect on carbon isotope ratios results in an overestimation of nocturnal CO₂ gain in comparison to an isotope versus nocturnal CO₂ gain calibration established previously for C₃ and CAM species grown under well-watered conditions. It is widely taken for granted that the shift to CAM in M. crystallinum is partially under developmental control and that CAM is inevitably expressed in mature plants. Plants, cultivated under non-saline conditions and high relative humidity (RH) for up to 63 days, maintained diel CO₂ gas-exchange patterns and ¹³C values typical of C₃ plants. However, a weak CAM gas-exchange pattern and an increase in ¹³C value were observed in non-salt-treated plants grown at reduced RH. These observations are consistent with environmental control rather than developmental control of the induction of CAM in mature M. crystallinum under non-saline conditions.     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Stomatal conductance in relation to xylem sap abscisic acid concentrations in two tropical trees, Acacia confusa and Litsea glutinosa

Two tropical trees, Acacia confusa and Litsea glutinosa, were grown under controlled conditions with their roots subjected to soil drying and soil compaction treatments. In both species, a decline in stomatal conductance resulting from soil drying took place much earlier than the decline of leaf water potential. Soil compaction treatment also resulted in a substantial decrease in stomatal conductance but had little effect on leaf water potential. A rapid and substantial increase in xylem abscisic acid (ABA) concenation ([ABA]), rather than hulk leaf ABA, was closely related to soil drying and soil compaction. A significant relationship between stomatal conductance (g_s) and xylem [ABA] was observed in both species. Artificially feeding ABA solutions to excised leaves of both species showed that the relationship bet ween g_s and [ABA] was very similar to that obtained from the whole plant, i.e. the relationship between g_s and xylem [ABA]. These results suggest that xylem ABA may act as a stress signal in the control of stomatal conductance.     

Does root-sourced ABA have a role in mediating growth and stomatal responses to soil compaction in tomato (Lycopersicon esculentum)?

Isogenic wild-type (Ailsa Craig) and abscisic acid (ABA)-deficient mutant (flacca) genotypes of tomato were used to examine the role of root-sourced ABA in mediating growth and stomatal responses to compaction. Plants were grown in uniform soil columns providing low to moderate bulk densities (1.1–1.5 g cm^?3), or in a split-pot system, which allowed the roots to divide between soils of the same or differing bulk density (1.1/1.5 g cm^?3). Root and shoot growth and leaf expansion were reduced when plants were grown in compacted soil (1.5 g cm^?3) but leaf water status was not altered. However, stomatal conductance was affected, suggesting that non-hydraulic signal(s) transported in the transpiration stream were responsible for the observed effects. Xylem sap and foliar ABA concentrations increased with bulk density for 10 and 15 days after emergence (DAE), respectively, but were thereafter poorly correlated with the observed growth responses. Growth was reduced to a similar extent in both genotypes in compacted soil (1.5 g cm^?3), suggesting that ABA is not centrally involved in mediating growth in this severely limiting ‘critical’ compaction stress treatment. Growth performance in the 1.1/1.5 g cm^?3 split-pot treatment of Ailsa Craig was intermediate between the uniform 1.1 and 1.5 g cm^?3 treatments, whereas stomatal conductance was comparable to the compacted 1.5 g cm^?3 treatment. In contrast, shoot dry weight and leaf area in the split-pot treatment of flacca were similar to the 1.5 g cm^?3 treatment, but stomatal conductance was comparable to uncompacted control plants. These results suggest a role for root-sourced ABA in regulating growth and stomatal conductance during ‘sub-critical’ compaction stress, when genotypic differences in response are apparent. The observed genotypic differences are comparable to those previously reported for barley, but occurred at a much lower bulk density, reflecting the greater sensitivity of tomato to compaction. By alleviating the severe growth reductions induced when the entire root system encounters compacted soil, the split-pot approach has important applications for studies of the role of root-sourced signals in compaction-sensitive species such as tomato.     

Nonstomatal Inhibition of Net CO(2) Uptake by (+/-) Abscisic Acid in Pharbitis nil

(±) Abscisic acid (ABA) injected into petioles of attached transpiring leaves of Pharbitis nil Chois. cv violet reduced the photosynthetic capacity of the mesophyll of these leaves as well as the stomatal conductance to CO₂ diffusion. Greater than 75% of the injected ABA was recovered as ABA, suggesting that ABA rather than some metabolite thereof was the active compound. The nonstomatal effect of ABA increased from 30% reduction in photosynthesis at 0.25 micromolar ABA in the leaf blade to 90% reduction at 18 micromolar. Despite the effect of ABA on the nonstomatal component of leaf net CO₂ uptake, it was calculated that a substantial part of the reduction in leaf net CO₂ uptake (50-80%) could be accounted for by the effect of ABA on stomatal conductance.     

Stomatal sensitivities to changes in leaf water potential, air humidity, CO2 concentration and light intensity, and the effect of abscisic acid on the sensitivities in six temperate deciduous tree species

To characterise the stomata of six temperate deciduous tree species, sets of stomatal sensitivities to all the most important environmental factors were measured. To compare the importance of abscisic acid (ABA) in the different stomatal responses, the effect of exogenous ABA on all the stomatal sensitivities was determined.Almost all the stomatal sensitivities: the sensitivity to a decrease in leaf water potential, air humidity, CO₂ concentration ([CO₂]) and light intensity, and to an increase in [CO₂] and light intensity were the highest in the slow-growing species, and the lowest in the fast-growing species. Drought increased the sensitivity to the environmental changes that induce a decrease in the stomatal conductance, and decreased the sensitivity to the changes that induce an increase in this conductance. The sensitivities of the slow-growers were most strongly affected by drought and ABA. Therefore the success of the slow-growers in their ecological niches can be based on the highly sensitive and strictly regulated responses of their stomata. The fast-growers had the highest sensitivity to an increase in leaf water potential and this sensitivity was sharply reduced by drought and ABA. Thus, the dominance of the trees in riparian areas can be based on the ability of their stomata to quickly reach high conductance in well-watered conditions and to efficiently decrease this rate during drought.Stomatal sensitivities to the hydraulic environmental factors (water potentials in plant and air) had higher values in well-watered trees and a more pronounced response to drought than the sensitivities to the photosynthetic environmental factors ([CO₂] and light intensity). Thus, the hydraulic factors most likely prevail over the photosynthetic factors in determining stomatal conductance in these species.In response to exogenous ABA, the rates of stomatal closure, following a decrease in air humidity and light intensity, and an increase in [CO₂], were accelerated. Stomatal opening following an increase in air humidity and light intensity and a decrease in [CO₂] was replaced by slow closing. The rate of stomatal opening following an increase in leaf water potential was reduced. As the sensitivities to changes in light were modified less by the ABA than the other stomatal sensitivities, the prediction of stomatal responses on the basis of the sensitivity to light alone should be excluded in stomatal models.     

An oxygen isotope chronometer for cellulose deposition: the successive leaves formed by tillers of a C4 perennial grass

Multiannual time series of (palaeo)hydrological information can be reconstructed from the oxygen isotope composition of cellulose (δ¹⁸O_Cel) in biological archives, for example, tree rings, but our ability to temporally resolve information at subannual scale is limited. We capitalized on the short and predictable leaf appearance interval (2.4 d) of a perennial C₄ grass (Cleistogenes squarrosa), to assess its potential for providing highly time‐resolved δ¹⁸O_Cel records of vapour pressure deficit (VPD). Plants grown at low (0.63 kPa) or high (1.58 kPa) VPD were swapped between VPD environments and exposed to the new environment for 7 d with simultaneous ¹³CO₂ labelling. Then, leaves were sampled by age/position along individual tillers. Five leaves at different developmental stages were growing simultaneously. The period of most‐active leaf elongation, from 10 to 90% of final length, lasted 6.6 d, and ~80% of all carbon and oxygen incorporation in whole‐leaf cellulose occurred within 7 d. Cellulose deposition stopped at (or shortly after) full leaf expansion. The direction of change, low‐to‐high or high‐to‐low VPD, had no differential effect on new oxygen and carbon incorporation in cellulose. Successive leaves produced by tillers of C. squarrosa provide a δ¹⁸O_Cel record useful for reconstructions of short‐term hydrological dynamics.     

Carbon stable isotopic composition of soluble sugars in Tillandsia epiphytes varies in response to shifts in habitat

We studied C stable isotopic composition (δ¹³C) of bulk leaf tissue and extracted sugars of four epiphytic Tillandsia species to investigate flexibility in the use of crassulacean acid metabolism (CAM) and C₃ photosynthetic pathways. Plants growing in two seasonally dry tropical forest reserves in Mexico that differ in annual precipitation were measured during wet and dry seasons, and among secondary, mature, and wetland forest types within each site. Dry season sugars were more enriched in ¹³C than wet season sugars, but there was no seasonal difference in bulk tissues. Bulk tissue δ¹³C differed by species and by forest type, with values from open-canopied wetlands more enriched in ¹³C than mature or secondary forest types. The shifts within forest habitat were related to temporal and spatial changes in vapor pressure deficits (VPD). Modeling results estimate a possible 4% increase in the proportional contribution of the C₃ pathway during the wet season, emphasizing that any seasonal or habitat-mediated variation in photosynthetic pathway appears to be quite moderate and within the range of isotopic effects caused by variation in stomatal conductance during assimilation through the C₃ pathway and environmental variation in VPD. C isotopic analysis of sugars together with bulk leaf tissue offers a useful approach for incorporating short- and long-term measurements of C isotope discrimination during photosynthesis.     

Carbon isotope discrimination in irrigated and droughted potato (Solanum tuberosum L.)

Carbon isotope discrimination (Δ) was measured in irrigated and droughted potato. Under irrigation, Δ in leaflets at given nodes increased (P < 0.001) between 21 and 63 d after emergence (DAE), which was attributed to increasing stomatal conductance (g_s) during leaf expansion. The effect of leaf position on Δ was non-significant in mature leaves. Under drought, Δ decreased (P < 0.001) in successive leaves up the stem, reflecting changes in g_s and water stress. At each node Δ remained constant or decreased, suggesting that effects of water stress were greater than changes with leaf expansion. There were significant differences in Δ between cultivars in both treatments, and in the progressive decrease in Δ up the stem under drought. Differences in Δ between cultivars were consistent with differences in stomatal control of leaf water status following water stress. Values for Δ in tubers were consistently lower than in stem and leaf, and decreased more rapidly. Differences in Δ between cultivars did not reflect dry matter production in either treatment, and differences in water use were non-significant between cultivars under drought. So, plants can achieve similar dry matter production through different growth strategies when irrigated or droughted, and Δ does not provide a simple, indirect method of selecting for dry matter production under water stress.     

Oxygen isotope ratio stratification in a tropical moist forest

Summary Oxygen isotope ratios were determined in leaf cellulose from two plant species at Barro Colorado (Republic of Panama) in 4 different plots, two of which were undergoing an irrigation treatment during the dry season. There is a gradient in ¹⁸O values of leaf cellulose from the understory to canopy leaves, reflecting the differences in relative humidity between these two levels of the forest. This gradient is most pronounced in irrigated plots. For irrigated plots there was a highly significant correlation between ¹⁸O and ¹³C values, which was not observed in control plots. This relationship can be explained by humidity controlling stomatal conductance. Low humidity affects ¹⁸O values of leaf water during photosynthesis, which isotopically labels cellulose during its synthesis. Low humidity also decreases stomatal conductance, which affects discrimination against carbon-13 by photosynthetic reactions, thus affecting the ¹³C values of photosynthates. WUE values calculated by using plant carbon and oxygen isotope ratios were similar to those observed with gas exchange measurements in other tropical and temperate area. Thus the concurrent analysis of carbon and oxygen isotope ratios of leaf material can potentially be useful for long term estimation of assimilation and evapotranspiration regimes of plants.     

Response of photosynthesis to high light and drought for Arabidopsis thaliana grown under a UV-B enhanced light regime

Arabidopsis thaliana grown in a light regime that included ultraviolet-B (UV-B) radiation (6 kJ m⁻² d⁻¹) had similar light-saturated photosynthetic rates but up to 50% lower stomatal conductance rates, as compared to plants grown without UV-B radiation. Growth responses of Arabidopsis to UV-B radiation included lower leaf area (25%) and biomass (10%) and higher UV-B absorbing compounds (30%) and chlorophyll content (52%). Lower stomatal conductance rates for plants grown with UV-B radiation were, in part, due to lower stomatal density on the adaxial surface. Plants grown with UV-B radiation had more capacity to down regulate photochemical efficiency of photosystem II (PSII) as shown by up to 25% lower φ_PSII and 30% higher non-photochemical quenching of chlorophyll fluorescence under saturating light. These contributed to a smaller reduction in the maximum photochemical efficiency of PSII (F _v/F _m), greater dark-recovery of F _v/F _m, and higher light-saturated carbon assimilation and stomatal conductance and transpiration rates after a four-hour high light treatment for plants grown with UV-B radiation. Plants grown with UV-B were more tolerant to a 12 day drought treatment than plants grown without UV-B as indicated by two times higher photosynthetic rates and 12% higher relative water content. UV-B-grown plants also had three times higher proline content. Higher tolerance to drought stress for Arabidopsis plants grown under UV-B radiation may be attributed to both increased proline content and decreased stomatal conductance. Growth of Arabidopsis in a UV-B-enhanced light regime increased tolerance to high light exposure and drought stress.     

Mapping Intercellular CO2 Mole Fraction (Ci) in Rosa rubiginosa Leaves Fed with Abscisic Acid by Using Chlorophyll Fluorescence Imaging : Significance of Ci Estimated from Leaf Gas Exchange

Imaging of photochemical yield of photosystem II (PSII) computed from leaf chlorophyll fluorescence images and gas-exchange measurements were performed on Rosa rubiginosa leaflets during abscisic acid (ABA) addition. In air ABA induced a decrease of both the net CO₂ assimilation (A_n) and the stomatal water vapor conductance (g_s). After ABA treatment, imaging in transient nonphotorespiratory conditions (0.1% O₂) revealed a heterogeneous decrease of PSII photochemical yield. This decline was fully reversed by a transient high CO₂ concentration (7400 μmol mol⁻¹) in the leaf atmosphere. It was concluded that ABA primarily affected A_n by decreasing the CO₂ supply at ribulose-1,5-bisphosphate carboxylase/oxygenase. Therefore, the A_n versus intercellular mole fraction (C_i) relationship was assumed not to be affected by ABA, and images of C_i and g_s were constructed from images of PSII photochemical yield under nonphotorespiratory conditions. The distribution of g_s remained unimodal following ABA treatment. A comparison of calculations of C_i from images and gas exchange in ABA-treated leaves showed that the overestimation of C_i estimated from gas exchange was only partly due to heterogeneity. This overestimation was also attributed to the cuticular transpiration, which largely affects the calculation of the leaf conductance to CO₂, when leaf conductance to water is low.