Leaf Gas Exchange Responses Research Articles

Stomatal behaviour, photosynthesis and transpiration under rising CO2

Definitions of the variables used and the units are given in Table 1 The literature reports enormous variation between species in the extent of stomatal responses to rising CO2. This paper attempts to provide a framework within which some of this diversity can be explained. We describe the role of stomata in the short‐term response of leaf gas exchange to increases in ambient CO2 concentration by developing the recently proposed stomatal model of Jarvis & Davies (1998). In this model stomatal conductance is correlated with the functioning of the photosynthetic system so that the effects of increases in CO2 on stomata are experienced through changes in the rate of photosynthesis in a simple and mechanistically transparent way. This model also allows us to consider the effects of evaporative demand and soil moisture availability on stomatal responses to photosynthesis and therefore provides a means of considering these additional sources of variation. We emphasize that the relationship between the rate of photosynthesis and the internal CO2 concentration and also drought will have important effects on the relative gains to be achieved under rising CO2. . Abbreviations image

Effects of elevated CO2 and soil quality on leaf gas exchange and above-ground growth in beech-spruce model ecosystems.

Responses of leaf gas exchange and above-ground growth of beech (Fagus sylvatica L.) and Norway spruce (Picea abies Karst.) to atmospheric CO2 enrichment (374 μl l-1 vs. 590 μl l-1 ) and increased wet deposition of N (5 vs. 50 kg N ha-1 a-1 ) in combination with two natural forest soil types ('acidic' and 'calcareous') were studied in large open-top chambers. Eight juvenile beech and spruce trees from different provenances, together with a ground cover composed of five understorey species, were established in each of 32 model ecosystems. Both beech and spruce showed sustained enhancement of photosynthesis in response to atmospheric CO2 enrichment during the first 2 yr of treatment. Nevertheless, switching measurement CO2 concentrations revealed partial downward adjustment of photosynthesis in trees grown in elevated CO2 , beech generally showing more pronounced downward adjustment than spruce. The responsiveness of photosynthesis to CO2 enrichment did not vary significantly among trees from different provenances. Stomatal conductance was reduced under elevated CO2 in both tree species. In spruce, the radial growth of the main stem and the annual production of wood (shoot-wood dry mass of current-year lateral shoots), needle dry mass, and assimilation area per tree were stimulated both by CO2 enrichment and increased N deposition, but were not significantly affected by soil type by year 2. In contrast, in beech, the radial growth of the stem and the total leaf number, foliage dry mass, and assimilation area per tree were all not significantly affected by elevated CO2 and increased N deposition when responses of the two soil types were pooled, but were greater on calcareous than on acidic soil by year 2. However, CO2 interacted with soil type in beech: irrespective of the N deposition rate, saplings showed growth stimulation on the calcareous soil but responded negatively to CO2 enrichment on the acidic soil (where growth was slower). Our results suggest that complex interactions between CO2 , species and soil quality need to be accounted for when attempting to predict forest development in a future CO2 -rich world.

Independent effects of the environment on the leaf gas exchange of three banana ( Musa sp.) cultivars of different genomic constitution

Circumstantial evidence suggests that the Musa balbisiana (B) genome confers greater drought tolerance to bananas and plantains than the Musa acuminata (A) genome. Hence the genetic makeup of bananas and plantains may affect the response of leaf gas exchange to the environment. Field data cannot be readily used to study the independent effects of environment but laboratory studies allow independent control of environmental parameters. We examined the independent effects of photosynthetic photon flux density, I, from 0 to 1450 μmol quanta m −2 s −1, leaf temperature, T l, from 21°C to 43°C, and leaf–air vapour pressure difference, Δ e, from 1.5 to 5.7 kPa on the stomatal conductance, g s, transpiration, E t, net photosynthesis, P n, internal CO 2 concentration, C i, and instantaneous water use efficiency, E w, of three Musa cultivars: cv Williams (AAA), cv Lady Finger (AAB), and cv Bluggoe (ABB). M. balbisiana genomes reduced the sensitivity of g s and P n to Δ e more than M. acuminata genomes. Genomic composition did not affect the responses to T l. As Δ e increased, g s and P n declined linearly at the rate of approximately 10% of predicted maximum g s and P n per 1 kPa increase in Δ e. This reduced stomatal aperture reduced C i, which declined exponentially, thereby limiting P n. Optimum temperatures for g s were 35°C and 39°C when Δ e was 1.5 and 3.0 kPa respectively. Optimum temperatures for P n were about 29°C when Δ e was 1.5 kPa and 33°C when Δ e was 3.0 kPa. The predicted maximum temperature where P n=0.0 would occur was 43°C to 44°C for all responses regardless of Δ e. The Williams cultivar was least sensitive to I showing less than 70% of predicted maximum photosynthesis and less than 50% of predicted maximum stomatal conductance at 1250 μmol quanta m −2 s −1. We conclude that there are genetic differences in the response of leaf gas exchange to changing environment within banana and plantains. The mechanism underlying the response of leaf gas exchange is through an effect of Δ e and I on the stomata, rather than an effect of T l on photosynthetic activity. Increasing proportions of B genomes decrease the sensitivity of stomata to Δ e but increases the sensitivity to I, especially at low photosynthetic photon flux densities. They also increase water use efficiency at the leaf level of organisation. The lower sensitivity of g s and P n to Δ e of cultivars containing more B genomes is consistent with the view that the B genome contributes to drought tolerance in Musa sp.

Leaf Gas Exchange and Growth Responses of Young, Container-grown Annona Trees to Flooding

Commercial Annona species such as sugar apple and atemoya, are sensitive to flooding. Annona glabra (pond apple) is a non-commercial species that grows well in flooded soils and may have potential as a flood-tolerant rootstock for commercial Annonas. The effects of flooding on net CO2 assimilation (A), stomatal conductance (gs), and vegetative growth of 2-year-old, container-grown Annona trees were studied under glasshouse conditions. Seedlings of A. glabra, A. muricata, and A. squamosa, and plants of `49-11' [a cross between atemoya (Annona. spp. hybrid cv. Gefner) × A. reticulata] grafted onto A. glabra or A. reticulata rootstock (`49-11'/A. glabra; `49-11'/A. reticulata) were exposed to 50 days of continuous flooding. Non-flooded (control) plants were irrigated daily. Flooded soil became anaerobic from day 3 of initiating the flooding treatment. Seedlings of A. glabra and A. muricata, and trees of `49-11'/A. glabra, produced more vegetative growth during flooding than A. squamosa and `49-11'/A. reticulata. Flooding decreased A and gs of A. squamosa and `49-11'/A. reticulata, and caused tree mortality (80%) of A. squamosa. Morphological adaptations of A. glabra (seedlings and rootstock) to flooding included adventitious root formation into flood water, development of hypertrophied (swollen) lenticels on trunk and adventitious roots, and development of trunk aerenchyma. All seedling trees of A. glabra and A. muricata, and all grafted trees of `49-11'/A. glabra survived 120 days of continuous flooding. A. glabra rootstock imparted flood tolerance to the `49-11' scion and may therefore impart flood tolerance to other Annona species. A. muricata seedlings may withstand extended periods of waterlogging under field conditions.

Response of leaf surfaces and gas exchange to wind stress and acid mist in birch (

Response of leaf surfaces and gas exchange to wind stress and acid mist in birch (

Salinity Influences Photosynthetic Characteristics, Water Relations, and Foliar Mineral Composition of Annona squamosa L.

Leaf gas exchange, chlorophyll fluorescence, water relations, and mineral nutrient relations responses of Annona squamosa seedlings to mild salinity were studied in sand culture in five experiments during 1990, 1991, and 1993. Trees were irrigated with a complete nutrient solution (control) or with this solution amended to 3 or 6 dS·m-1 with sea salt. Inhibition of net CO2 assimilation, stomatal conductance of CO2, and transpiration was apparent within 2 weeks of initiating salinity treatments, and gas exchange continued to decline until day 30 to 35. The diurnal pattern of leaf gas exchange was not altered by increased salinity. Salinity reduced CO2, light energy, and water-use efficiencies. Salinity sometimes reduced the ratio of variable to maximum fluorescence below that of the control, and this response was highly dependent on the ambient light conditions that preceded the measurements. Dark respiration was unaffected by salinity stress. Root zone salinity of 3 dS·m-1 administered for 52 days did not influence foliar sodium concentration or the ratio of sodium to potassium, but increased chloride concentration and decreased nitrogen concentration. The sodium response indicated that some form of exclusion or compartmentation occurred. Salinity reduced osmotic potential of root tissue but did not influence foliar osmotic or predawn xylem potential. These results indicate that A. squamosa is sensitive to salinity stress, and that the responses to salinity are consistent with other salt-sensitive woody perennial species.

Leaf gas exchange and carbon isotope composition responses to drought in a drought‐avoiding (Pinus pinaster) and a drought‐tolerant (Quercus petraea) species under present and elevated atmospheric CO2 concentrations

ABSTRACTThe responses of predawn leaf water potential (φwp), leaf conductance to water vapour diffusion (g), CO2 assimilation rate (A) and carbon isotope competition (δ13C) to a soil drying cycle were assessed in Pinus pinaster, a drought‐avoiding species with high stomatal sensitivity to drought, and Quercus petraea, a drought‐tolerant species with lower stomatal sensitivity to drought, under present (350 μmol−1) and elevated (700 μmol−1) atmospheric CO2 concentrations ([CO2]). In P. pinaster, decreasing A in response to drought was associated with increasing plant intrinsic water use efficiency (A/g) and with decreasing calculated intercellular [CO2] (C1), suggesting a stomatal limitation of A. In contrast, in Q. petraea, A/g declined and C1 increased during the drying cycle, which suggests a non‐stomatal origin for the decrease in A. In P. pinaster, a negative relationship was observed between the gas exchange‐derived values of Ci/Ca and δ13C, which conforms to the classical two‐step carbon isotope discrimination model. In Q. petraea, the relationship between C1/Ca and δ13C was positive. Possible causes of this discrepancy are discussed. Lower g values were observed under elevated [CO2] than under present [CO2] in Q. petraea, whereas g was unaffected in P. pinaster. A stimulation of A by elevated [CO2] was found in P. pinaster but not in Q. petraea. In both species, A/g was markedly higher under elevated than under present [CO2]. Whether the differences in the g response to elevated [CO2] found here can be generalized to other drought‐avoiding and non‐avoiding species remains to be assessed.

Physiological responses of banana (MusaAAA; Cavendish sub-group) in the subtropics. VI. Seasonal responses of leaf gas exchange to short-term water stress

SummaryA banana tissue culture block, cv. Williams, was divided in eight sub-plots each with independent irrigation control, to investigate effects of short-term water stress on the assimilation potential of vigorous plants never previously stressed. Supporting studies included recovery from short term water stress, validation of current crop factors of 0.8 × E0 (Autumn/Summer) and 0.5 × E0 (Winter), comparison of USDA class A Pan with tensiometer scheduling, and investigation of specific water demands of tissue culture plantings. Irrigation treatments during an autumn and winter study included a control (“daily” irrigation using A-Pan), a “water stress” treatment (water shortage until severe reductions in assimilation potential occurred) and a “normal” irrigation treatment using currently recommended crop factors. During a summer study, “normal” irrigation was scheduled physiologically, i.e. when plants showed a significant reduction in photosynthesis rate compared with the control. Soil moisture was monitored by tensiometer and neutron probe. The physiological responses of banana plants to different irrigation treatments were monitored by measurements of leaf gas exchange (photosynthesis rate, transpiration rate, stomatal conductance and internal CO2 concentration). During autumn, the first signs of water stress appeared after 4 d of water shortage. Twelve days of water shortage resulted in 79% reduction in photosynthesis, caused by a combination of stomatal limitations (84% reduction in stomatal conductance) and disturbed photosynthesis reaction (13% higher internal CO2 concentration). “Normal” irrigation at a crop factor of 0.8 showed no restricted leaf gas exchange indicating that soil water potential should be kept at – 15 to –20 kPa during warm seasons (summer to autumn) and that irrigation intervals should not be longer than three days. During winter, reductions in photosynthesis were only measured 8 d after initiation of water stress. A maximum reduction in photosynthesis of 32% occurred after 32 d of water shortage at a soil water potential of –69 kPa showing that water shortages during winter were less severe on assimilation than during warmer seasons. Irrigation at soil water potential of –30 kPa did not cause severe reductions in assimilation potential and a crop factor of 0.5 did not show long-term difference in gas exchange compared with “daily”. During summer, cloudy weather and reduced evaporative demand minimized symptoms of water stress until 7 d of water shortage at a soil water potential of –21.8 kPa. Physiological recovery of the plant from short term water stress took longer than 3 d in all three irrigation studies. In all measurements, irrigation scheduling with A-pan did not correlate with available soil water. Irrigation scheduling using soil water content (tensiometer or neutron probe) is a more effective indicator of actual water demand for bananas in the subtropics.

Physiological and production responses of plant growth forms to increases in limiting resources in alpine tundra: implications for differential community response to environmental change.

Physiological and growth measurements were made on forbs and graminoids following additions of water and N+water in a graminoid-dominated dry meadow and a forb-dominated moist meadow, to determine if the community-level response was related to differential responses between the growth forms. Graminoids had higher photosynthetic rates and lower transpiration rates and foliar N concentrations than forbs, and consequently maintained higher photosynthetic N- and water-use efficiencies. Photosynthetic rates, stomatal conductance, and transpiration rates increased significantly only in response to N fertilization and only in moist meadow species. The increase in photosynthetic rates was unrelated to variation in foliar N concentration, but instead correlated with variation in stomatal conductance. Growth based N-use efficiency was higher in moist meadow graminoids than in moist meadow forbs, but did not differ between the growth forms in the dry meadow. The moist meadow community had higher biomass and N standing crops, but the relative increase in these factors in response to N fertilization was greater in the dry meadow. Graminoids had a greater relative increase in biomass and N accumulation than forbs following N fertilization, but moist meadow graminoids exhibited a greater response than dry meadow graminoids. The difference in the growth response between the dry meadow and moist meadow graminoids to N fertilization was correlated with more conservative leaf gas exchange responses in dry meadow species, presumably related to a higher frequency of soil water deficits in this community. Community-level response to the resource additions was therefore mediated by the plant growth form response, corresponding with differences between the growth forms in physiological factors related to resource acquisition and use.

558 PB 377 SALINITY AND LEAF GAS-EXCHANGE OF ANNONA SOUAMOSA

Leaf gas-exchange responses of A. souamosa seedlings to salinity were studied in sand culture in a series of glasshouse experiments. Trees were irrigated with a complete nutrient solution as the control, or with this solution amended to 3 or 6 dS/m with sea salt. Inhibition of net CO2 assimilation, stomatal conductance of CO2, and transpiration was apparent 14 days after treatments were imposed, and continued to decline until day 30 to 35. The diurnal pattern of leaf gas exchange was not altered by salinity. Salinity reduced CO2, light energy, and water use efficiencies. Dark respiration and internal partial pressure of CO2 were unaffected by salinity stress. Results indicated that substrate salinity inhibited photosynthesis of A. souamosa via limitations on mesophyll capacity for CO2 assimilation and had little effect on gas phase limitations.

Leaf gas exchange of beech (Fagus sylvatica L.) seedlings in lightflecks: effects of fleck length and leaf temperature in leaves grown in deep and partial shade

Responses of leaf gas exchange in shade and half-shade grown seedlings of the European beech, Fagus sylvatica L., to constant light conditions indicate different phases of photosynthetic induction: an immediate, a fast and a subsequent slow phase. The slow phase has both biochemical and stomatal components. The higher the induction, the higher the lightfleck utilization efficiency (LUE) attributable to a lightfleck. LUE can be higher than 100% compared to a theoretical instantaneous response. Lightfleck quantum yield (total carbon gain attributable to a lightfleck per incident quantum density in the fleck) is highest in short pulses of light. Post-illumination carbon gain initially increases with fleck length, levelling off above 20 s. The lower the induction, the longer carbon is fixed post-illuminatively (up to 84 s) but the less carbon is gained. Shade leaves are induced much faster than partial shade leaves. They utilize series of lightflecks to become fully induced, while half-shade (and sun) leaves depend on continuous high light. Half-shade leaves lose induction faster in low light between lightflecks. High as well as low temperatures strongly delay induction in half-shade but not in shade leaves. In general, shade leaves are much better adapted to the dynamic light environment of the forest understorey; however, their water-use efficiency during induction is lower.

Leaf gas exchange of beech (Fagus sylvatica L.) seedlings in lightflecks: A system for measuring rapid changes in CO2 partial pressures

A system is described for the measurement of fast transient responses of leaf gas exchange to lightflecks. Any chamber adulterates a true signal unless in the steady-state; therefore, a procedure for estimating an 'effective chamber volume' is presented which is an essential parameter for correcting a measured transient signal. A common correction is critically commented on. With the system, responses of net photosynthesis to lightflecks as short as 1 s have been observed in leaves of seedlings of Fagus sylvatica L.

An investigation of leaf-blade anatomy and photosynthetic characteristics of four Cyperaceae species from the Albany and Bathurst districts in the eastern Cape

The anatomy of the leaf-blades of four field-grown Cyperaceae, namely Cyperus albostriatus Schrad., Cyperus fastigiatus Rottb., Cyperus pulcher Thunb. and Mariscus congestus (Vahl.) C.B.CI., was investigated with light and scanning electron microscopes, to determine stomatal type and frequencies. Photosynthetic characteristics, including light and temperature response as well as post-illumination CO2 burst effect, were investigated using an ADC 225 MKIII infra-red gas analyser in open circuit. The leaf anatomy and gas exchange response suggest that C. pulcher is a C3 species, whilst C. albostriatus, C. fastigiatus and M. congestus are C4. The post-illumination CO2 burst studies suggest that C. albostriatus and M. congestus are C4-NADP-Me species type and that C. fastigiatus is either NAD-Me or PCK.

Seasonal responses of leaf gas exchange to elevated carbon dioxide in Populusgrandidentata

Rising atmospheric carbon dioxide concentrations may have important consequences for forest ecosystems. We studied above- and below-ground growth and leaf gas exchange responses of Populusgrandidentata Michx. to elevated CO2 under natural forest conditions over the course of a growing season. Recently emerged P. grandidentata seedlings were grown in native, nutrient-poor soils at ambient and twice ambient (707 μbar (1 bar = 100 kPa)) CO2 partial pressure for 70 days in open-top chambers in northern lower Michigan. Total leaf area and shoot and root dry weight all increased in high CO2 grown plants. Photosynthetic light and CO2 response characteristics were measured 28, 45, and 68 days after exposure to elevated CO2. In ambient grown plants, light saturated assimilation rates increased from day 28 to day 45 and then declined at day 68 (15 September). This late-season decline, typical of senescing Populus leaves, was due both to a decrease in the initial slope of the net CO2 assimilation versus intercellular CO2 partial pressure relationship and to decreased CO2 saturated assimilation rates. Specific leaf nitrogen (mg N•(cm2 leaf area)−1) did not change during this period, although leaf carbon content and leaf weight (mg•cm−2) both increased. In ambient grown plants stomatal conductance also declined at day 68. In contrast, plants grown at elevated CO2 showed no late-season decline in photosynthetic capacity or changes in leaf weight, suggesting a delay in senescence with long-term exposure to high CO2. High CO2 grown plants also maintained photosynthetic sensitivity to increasing Ci throughout the exposure period, while ambient CO2 grown plants were insensitive to Ci above 400 μbar on day 68. These results indicate the potential for direct CO2 fertilization of P. grandidentata in the field and provide evidence for a new mechanism by which elevated atmospheric CO2 could influence seasonal carbon gain.

Flooding Affects Snap Bean Yield and Genotypic Variation in Leaf Gas Exchange and Root Growth Response

Greenhouse experiments were conducted in 1987 and 1988 to evaluate the effect of timing of a 4-day flooding stress on growth and yield of snap bean (Phaseolus vulgaris L. cv. Bush Blue Lake 274, BBL). Plant survival was reduced when flooding was imposed at postflowering growth stages, but most plants survived when flooded before flowering or when reproductive development was prevented by deflowering. Early yields of surviving plants were very low in all flooded treatments, regardless of timing, in both years. Total yield response to timing of flooding was linear in 1987, with lowest yields when flooding was imposed at later growth stages. The trend was not linear in 1988, but in both years the latest flooding treatment (36 days after planting) had few surviving plants and no measurable pod yield. Additional greenhouse experiments revealed that leaf conductance of BBL and another bean cultivar, Luna (LN), declined within the first day of flooding. This decline was concomitant with one in leaf water potential and photosynthesis (Pn) in BBL, but decline of these responses occurred 1 to 2 days later for LN. After 4 days of flooding, Pn fell to near 0 for BBL, and to 15% of the prestress value for LN. Pn of both cultivars had recovered to 18.5 μmol·m-2·s-1 10 days after termination of flooding. LN had a larger adventitious root biomass, higher percentage of adventitious roots, and a consistently lower leaf: root ratio than BBL during recovery.

Carambola Growth and Leaf Gas-exchange Responses to Seismic or Wind Stress

Growth and leaf gas-exchange responses of carambola (Averrhoa carambola L.) seedlings to wind or seismic stress were studied under glasshouse conditions. Forty days of twice daily seismic stress applied for 10 seconds consistently reduced carambola height, leaf area, dry weight, relative growth rate, and leaf-area ratio, but increased trunk cross-sectional area compared with plants receiving no seismic stress. Fifty-one days of wind load reduced plant height, leaf area, dry weight, trunk cross-sectional area, net assimilation rate, relative growth rate, leaf-area ratio, and stomatal conductance compared with plants receiving no wind stress. Morphological appearance was similar for plants receiving wind or seismic stress. Net CO2 assimilation of carambola leaflets was reduced by 30 minutes of wind load for up to 6 hours following the stress. Results suggest that wind may reduce carambola growth at least partially by influencing leaf gas exchange or by the mechanical stress associated with wind.

Comparative drought resistance among six species of birch (Betula): influence of mild water stress on water relations and leaf gas exchange

Responses of plant water relations and leaf gas exchange to mild water stress were monitored and compared among six species of birch; monarch birch (Bet& manimowicziana Reg.), river birch (B. nigra L.), paper birch (I?. pupyrifera Marsh.), European white birch (B, pendula Roth.), ‘Whitespire’ Japanese birch (B. platyphylla var. japonica Hara. ‘Whitespire’), and gray birch (B. populifolia Marsh.). Before imposition of water stress, ‘Whitespire’ Japanese birch and river birch maintained the highest stomatal conductances and net rates of photosynthesis of the species examined. After irrigation was withheld, stomatal conductance and rate of net photosynthesis gradually declined for most species. After 28 days without irrigation, ‘Whitespire’ Japanese birch maintained significantly higher stomatal conductance and rate of net photosynthesis than did the other species despite having one of the lowest midday water potentials. There was no evidence of osmotic adjustment by any of the species in response to the imposed drought. However, there was substantial variation in the water potential at the turgor loss point among the species, from a high of -1.34 MPa for river birch to a low of -1.78 MPa for ‘Whitespire’ Japanese birch. Stomata1 conductance and net photosynthesis under mild water stress (average predawn leaf water potential of -0.6 1 MPa) were negatively correlated with leaf osmotic potential at full turgor and leaf water potential at the turgor loss point. Thus, the greater net photosynthesis of ‘Whitespire’ Japanese birch under water stress compared with the other species appears to have resulted from a superior capacity to maintain turgor at low leaf water potentials, which in turn provided for greater stomatal conductance and CO2 uptake. These results indicate that ‘Whitespire’ Japanese birch is better adapted to dry sites than the other species.

Variation among wheat cultivars in the response of leaf gas exchange to light

SUMMARYThis research was done in order to explore genetic variation in carbon exchange rate (CER) of spring wheat (Triticum aestivum) leaves in response to variable photosynthetically active radiation (PAR) and to compare old and new Israeli cultivarsin this respect. Leaf gas exchange was measured in detached turgid leaves of 17 cultivars inan open system at 25 °C when PAR was reduced from c. 1200 to 200 μmol/m2 per s. Linear regressions of CER, stomatal conductance, transpirationand leaf internal CO2 concentration (Ci) on log PAR were fitted for each leaf(regression r2 was never <0·79) and the regressions were compared among cultivars by analysis of variance.Genotypes differed significantly for the slope (b) but not the intercept (a) of the regression of CER on log PAR, indicating that genotypic differences for CER increased with increasing PAR. Photosynthetic capacity, as expressed by the ratio of CER/Ci, differed significantly among cultivars only at high PAR. Stomatal conductanceand transpiration increased in a linear or a nonlinear fashion with log PAR and differences among cultivars for both were greatest at medium to low PAR. Photosynthetic water–use efficiency (WUE) and its variation among cultivars were greatest at the highest PAR. Genotypic variation in CER athigh PAR was confirmed by repeated results for 11 cultivars intwo independent experiments.The recently developed high-yielding cultivar V652 had a higher maximum CER, higher photosynthetic capacity and greater WUE at high PAR than older and lower-yielding cultivars. The results suggest an upward genetic shift in photosynthetic capacity andin CER at high PAR when selection for yield was performed under the high-irradiation conditions of Israel.

Gas exchange of Agropyron desertorum: diurnal patterns and responses to water vapor gradient and temperature.

The response of leaf gas exchange to environmental variables were measured at different levels of drought stress for Agropyron desertorum, a naturalized perennial bunchgrass of the semiarid shrub steppes of western North America. Leaf conductance (stomatal plus boundary layer) was more sensitive to changes in water vapor gradient than to changes in leaf temperature. Assimilation was sensitive to both temperature and vapor gradient, and also appeared to be affected by conductance and high transpiration rates. The magnitudes of both assimilation and conductance decreased with increased drought conditions. Diurnal patterns of gas exchange were measured during 3 growing seasons. For a typical spring day with moderate leaf temperature and vapor gradient, diurnal patterns were similar for plants at different levels of soil water availability. Assimilation was relatively constant during most of the day, but conductance decreased during the afternoon. Total daily carbon gain was decreased to a lesser extent than daily water loss as soil water was depleted. Consequently, the ratio of daily carbon gain to daily water loss, i.e. daily water use efficiency, increased with decreased soil water content for diurnals under spring conditions. Diurnal patterns of assimilation for a typical summer day with high leaf temperature and vapor gradient differend from those for a spring day. An afternoon decrease in assimilation was typical during a summer day. Daily carbon gain, water use, and water use efficiency for summer diurnals decreased only under severe drought conditions. Almost complete recovery of assimilation and conductance occurred if leaf microclimate was ameliorated during the afternoon of either spring or summer diurnals. Thus, conditions responsible for a midday depression in assimilation during a single day did not have persistent effects on leaf gas exchange. Daily carbon gain of a typical summer day was restricted by leaf microclimate during the afternoon, but daily water use efficiency was not relatively increased by the amelioration of leaf microclimate.

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content : III. In the sclerophyllous woody species Nerium oleander.

The responses of leaf conductance, leaf water potential and rates of transpiration and net photosynthesis at different vapour pressure deficits ranging from 10 to 30 Pa kPa-1 were followed in the sclerophyllous woody shrub Nerium oleander L. as the extractable soil water content decreased. When the vapour pressure deficit around a plant was kept constant at 25 Pa kPa-1 as the soil water content decreased, the leaf conductance and transpiration rate showed a marked closing response to leaf water potential at-1.1 to-1.2 MPa, whereas when the vapour pressure deficit around the plant was kept constant at 10 Pa kPa-1, leaf conductance decreased almost linearly from-0.4 to-1.1 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps, decreased leaf conductance at all exchangeable soil water contents. Changing the leaf water potential in a single leaf by exposing the remainder of the plant to a high rate of transpiration decreased the water potential of that leaf, but did not influence leaf conductance when the soil water content was high. As the soil water content was decreased, leaf conductances and photosynthetic rates were higher at equal levels of water potential when the decrease in potential was caused by short-term increases in transpiration than when the potential was decreased by soil drying.As the soil dried and the stomata closed, the rate of photosynthesis decreased with a decrease in the internal carbon dioxide partial pressure, but neither the net photosynthetic rate nor the internal CO2 partial pressure were affected by low water potentials resulting from short-term increases in the rate of transpiration. Leaf conductance, transpiration rate and net photosynthetic rate showed no unique relationship to leaf water potential, but in all experiments the leaf gas exchange decreased when about one half of the extractable soil water had been utilized. We conclude that soil water status rather than leaf water status controls leaf gas exchange in N. oleander.