Partial Stomatal Closure Research Articles

Involvement of malate and mannitol in the diurnal regulation of the water status in members of Oleaceae

This study examines water status regulation in plants of the Oleaceae family and in some other co-occurring species that are exposed to high solar radiation, in the same habitat. Fraxinus excelsior L., one of the most studied Oleaceae in this field exhibited, during the growing season, a close relationship between diurnal variations in leaf water potential and changes in malate, mannitol and K+ levels, depending on the weather conditions. On sunny days, similar variations can be observed in leaves of the other Oleaceae, with a concomitant decrease in the osmotic potential between predawn and solar noon. Malate, mannitol and the well-known osmoticum K+, contribute greatly to the osmotic potential decrease. This mechanism, which can be related to the osmotic adjustment described for both drought and salt-affected plants, appears as a general response in plants of the Oleaceae family. Among the other co-occurring species investigated, only Quercus robur L. displayed a similar mechanism under the same environmental conditions, but two other organic compounds, quinic and shikimic acids, are presumably involved. Alnus glutinosa (L.) Gaertn. and Robinia pseudacacia L. responded to a vapor deficit by partial stomatal closure, as transpiration progressed through the morning.

Elevated CO2reduces O3flux and O3-induced yield losses in soybeans: possible implications for elevated CO2studies

Soybeans were grown for three seasons in open-top field chambers to determine (1) whether elevated CO2 (360 versus 700 //mol mol1) alleviates some of the yield loss due to pollutant 0 3, (2) whether the partial stomatal closure resulting from chronic 03 exposure (charcoal-filtered air versus 1.5 x ambient concentrations) is a cause or result of decreased photosynthesis, and (3) possible implications of C0 2 /0 3 interactions to climate change studies using elevated CO2. Leaf conductance was reduced by elevated C02, regardless of O3 level, or by exposure to 03 alone. As. a result of these effects on conductance, high C0 2 reduced estimated midday 03 flux into the leaf by an average of 50% in charcoal-filtered air and 35% in the high 03 treatment. However, while exposure to O 3 reduced seed yields by 41% at ambient CO2 levels, the yield reduction was completely ameliorated by elevated CO2. The threshold midday 03 flux for yield loss appears to be 20-30 nmol m~2 s 1 in this study. Although elevated CO2 increased total biomass production, it did not increase seed yields. A/C, curves show a large reduction in the stomatal limitation to photosynthesis due to elevated CO2, but no effect of O3. These data demonstrate that (1) reduced conductance due to O3 is the result, and not the cause, of reduced photosynthesis, (2) 700 //mol mol' CO2 can completely ameliorate yield losses due to 03 within the limits of these experiments, and (3) some reports of increased yields under elevated CO2 treatments may, at least in part, reflect the amelioration of unrecognized suppression of yield by 03 or other stresses.

Performance of interior spruce seedlings treated with abscisic acid analogs

This research examined the performance of interior spruce (Piceaglauca (Moench) Voss × Piceaengelmannii Parry ex Engelm.) seedlings, each group treated with one of nine abscisic acid (ABA) analogs, during the initial stages of seedling establishment under a range of environmental conditions. Interior spruce seedlings were removed from frozen storage, ABA analog treatments were immediately applied, and seedlings were tested under low root temperature or moderate drought cycle conditions. Alternatively, seedlings were removed from frozen storage and held until bud break had occurred before ABA analog treatments were applied. These seedlings were then tested under severe drought or optimum environmental conditions. ABA analog 1, followed by ABA analog 2, had the most consistent performance of the nine tested ABA analogs under all combinations of environmental test conditions. These ABA analogs reduced needle conductance for 7–9 days when seedlings were tested under low root temperature conditions with only a reduction in net photosynthesis on the first day of testing. During three successive moderate drought cycles, seedlings treated with ABA analogs 1 and 2 had partial stomatal closure, thereby increasing mean shoot water potential by around 50%. During a severe drought, ABA analog 1 caused partial stomatal closure, which allowed seedlings to maintain a mean shoot water potential of greater than −3.0 MPa and a positive net photosynthesis up to 8 days longer than control seedlings. Under optimum environmental conditions, ABA analogs 1 and 2 reduced needle conductance for up to 7 days, with net photosynthesis reduced for 1 day. Root growth was not adversely affected in seedlings treated with any of the ABA analogs prior to bud break. However, when seedlings were treated after bud break, all ABA analogs reduced growth of long roots (>4.0 cm) by approximately 60%. ABA analogs 1 and 2 delayed bud break by 4 days, when compared with control seedlings. Results are discussed in reference to the establishment process of spruce seedlings on reforestation sites.

Responses to water stress in an ABA ‐ unresponsive hybrid poplar (Populus koreana×trichocarpa cv. Peace) I. Stomatal function

SUMMARYThe poplar cultivar ‘Peace’ (Populus trichocarpa×koreana) displays abnormal stomatal behaviour with a lack of sensitivity to exogenous abscisic acid (ABA) in mature leaves. We report details of the leaf‐age dependency of this feature, and present the responses of stomata to diverse closing stimuli in addition to ABA. Soil water depletion induced complete stomatal closure of the youngest leaves, whereas the oldest exhibited almost no closure. By contrast, no such age effect was observed in stomatal sensitivity to ABA: a complete lack of closure was observed on all leaves, even with 10‐3 M ABA. Moreover, a preconditioning by drought did not restore the ability of stomata to close in response to ABA. Supplying detached leaves with Ca2+ (5 × 10‐2 M) was effective in inducing stomatal closure in the youngest leaves, and reproduced the age‐dependent drought response. These results support the hypothesis that drought control of stomatal conductance in ‘Peace’ is ABA‐independent, and could involve calcium ions. A loss of stomatal sensitivity to calcium, with leaf maturation, could explain the dependency of stomatal closure on leafage. Xylem sap concentrations of calcium were lower than those required to close stomata, and no drought‐induced increase was recorded. The mature leaves were also insensitive to increased ambient CO2, but darkness promoted partial stomatal closure.

Effects of water and nitrogen interaction on net photosynthesis, stomatal conductance, and water‐use efficiency in two hybrid poplar clones

We examined the interactions of water and nitrogen availability by subjecting two Populus clones. Tristis and Eugenei, to five soil moisture and three soil nitrogen levels. Nitrogen application significantly increased net photosynthesis and stomatal conductance of flooded Eugenei and Tristis. The onset of flooding caused partial stomatal closure. Net photosynthesis significantly declined after a longer flooding period. Emergence of adventitious roots on the submerged portions of stems in both clones seemingly helped net photosynthesis fully recover in Eugenei and partially recover in Tristis. Under the progressive drought conditions, stomatal conductance was more sensitive to drought than net photosynthesis in both clones. Addition of nitrogen to progressively drying soil induced more stomatal closure in both clones. The highest water‐use efficiency was found on the high‐N/severe drought zone for Eugenei, whereas it was found on the high‐N/mild to moderate drought zone for Tristis.

Photosynthetic acclimation to light in juveniles of two cloud forest tree species

The photosynthetic response of juveniles of Decussocarpus rospigliosii, an emergent primary forest species and shade tolerant in its juvenile stages and Alchornea triplinervia, a gap-colonizing species of tropical cloud forest in Venezuela was studied. Daily courses of microenvironmental variables and gas exchange under contrasting light conditions (gap and understory) were carried out in their natural environment and transplanted to different light regimes (shade and sun) in the field. The photosynthetic response and some anatomical characteristics of plants from different treatments were analyzed in the laboratory. Photosynthetic rates were low for both species, and were negative during some diurnal periods, related to the low photosynthetically active radiation levels obtained at both gap (6% of total radiation) and understory (2%). A. triplinervia shows higher rates (1.5–3.0 μmolm-2-1) than D. rospigliosii (0.7–1.1 μmolm-2s-1). Both species showed increased photosynthetic rates when grown in gaps. A. triplinervia did not adjust its maximum photosynthetic rates to the prevailing light conditions. In contrast, D. rospigliosii responded to increased light levels. Both species showed low light compensation points when grown under total shade. There was a partial stomatal closure generally during midday in D. rospigliosii. A. triplinervia presented lower leaf conductances, transpiration rates and lesser stomatal control. Some leaf anatomical characteristics, in both species, were affected by variations in the light regime (i.e. increased leaf thickness, leaf specific weight and stomatal density). These results suggest that both species have the ability to respond to variations in their natural light environments, therefore maintaining a favorable carbon balance during the day.

Effects of elevated CO2 and/or O3 on growth, development and physiology of wheat (Triticum aestivum L.)

AbstractTwo cultivars of spring wheat (Triticum aestivum L. cvs. Alexandria and Hanno) and three cultivars of winter wheat (cvs. Riband, Mercia and Haven) were grown at two concentrations of CO2 [ambient (355 pmol mol−1) and elevated (708 μmol mol−1)] under two O3 regimes [clean air (< 5 nmol mol−1 O3) and polluted air (15 nmol mol−1 O3 at night rising to a midday maximum of 75 nmol mol−1)] in a phytotron at the University of Newcastle‐upon‐Tyne. Between the two‐leaf stage and anthesis, measurements of leaf gas‐exchange, non‐structural carbohydrate content, visible O3 damage, growth, dry matter partitioning, yield components and root development were made in order to examine responses to elevated CO2 and/or O3.Growth at elevated CO2 resulted in a sustained increase in the rate of CO2 assimilation, but after roughly 6 weeks' exposure there was evidence of a slight decline in the photosynthetic rate (c.‐15%) measured under growth conditions which was most pronounced in the winter cultivars. Enhanced rates of CO2 assimilation were accompanied by a decrease in stomatal conductance which improved the instantaneous water use efficiency of individual leaves. CO2 enrichment stimulated shoot and root growth to an equivalent extent, and increased tillering and yield components, however, non‐structural carbohydrates still accumulated in source leaves. In contrast, long‐term exposure to O3 resulted in a decreased CO2 assimilation rate (c. ‐13%), partial stomatal closure, and the accumulation of fructan and starch in leaves in the light. These effects were manifested in decreased rates of shoot and root growth, with root growth more severely affected than shoot growth.In the combined treatment growth of O3‐treated plants was enhanced by elevated CO2, but there was little evidence that CO2 enrichment afforded additional protection against O3 damage. The reduction in growth induced by O3 at elevated CO2 was similar to that induced by O3 at ambient CO2 despite additive effects of the individual gases on stomatal conductance that would be expected to reduce the O3 flux by 20%, and also CO2‐induced increases in the provision of substrates for detoxification and repair processes. These observations suggest that CO2 enrichment may render plants more susceptible to O3 damage at the cellular level. Possible mechanisms are discussed.

The effect of elevated atmospheric CO2and drought onstomatal frequency in groundnut (Arachis hypogaea(L.))

The effects of elevated atmospheric CO 2 , alone or in combination with water stress, on stomatal frequency in groundnut (Arachis hypogaea (L.) cv. Kadiri-3) were investigated. CO 2 exerted significant effects on stomatal frequency only in irrigated plants. The effects of drought on leaf development outweighed the smaller effects of CO 2 concentration, although reductions in stomatal frequency induced by elevated atmospheric CO 2 were still observed. When stands of groundnut were grown under irrigated conditions with unrestricted root systems, an increase in atmospheric CO 2 from 375 to 700 ppmv decreased stomatal frequency on both leaf surfaces by up to 16%; in droughted plants, stomatal frequency was reduced by 8% on the adaxial leaf surface only. Elevated atmospheric CO 2 promoted larger reductions in leaf conductance than the changes in stomatal frequency, indicating partial stomatal closure. As a result, the groundnut stands grown at elevated CO 2 utilized the available soil moisture more slowly than those grown under ambient CO 2 , thereby extending the growing period. Despite the large variations in cell frequencies induced by drought, there was no treatment effect on either stomatal index or the adaxial/abaxial stomatal frequency ratio. The data suggest that the effects of future increases in atmospheric CO 2 concentration on stomatal frequency in groundnut are likely to be small, especially under conditions of water stress, but that the combination of associated reductions in leaf conductance and enhanced assimilation at elevated CO 2 will be important in semi-arid regions.

Responses of the stomata ofAster tripoliumto calcium and sodium ions in relation to salinity tolerance

In previous work, the stomata of the maritime halophyte Aster tripolium L. were shown to close when NaCl concentrations rise in the vicinity of the guard cells. Further studies have now revealed important effects of calcium on the ionic responses of the stomata. When the guard cells were presented with KCl, Ca2+ suppressed opening in a manner similar to that which has become familiar in other species such as Commelina communis L. However, in the presence of NaCl, Ca2+ had the opposite effect, reducing the closing response to NaCl. This pattern of behaviour is discussed in relation to known salt effects on membranes, but the underlying physiological basis remains obscure. A previous study led to the hypothesis that the closing response of the stomata to Na+ ions may make an important contribution to the salinity tolerance of this species. Here we report that increasing supplies of Ca2+ ions reduce the effect of salinity on stomatal conductance in the whole plant as well as in the isolated epidermis. This finding is consistent with the well established role of calcium in increasing resistance to salinity: in the presence of high calcium the plant can tolerate a greater salt intake, and hence there is a reduced need for transpiration to be restricted by partial stomatal closure.

Stomatal responses to sodium ions inAster tripolium: a new hypothesis to explain salinity regulation in above‐ground tissues

ABSTRACTA study has been made of the ionic relations of stomata ofAster tripoliumL., a maritime halophyte which colonizes coastal saltmarshes. The results obtained allow us to add this species to the growing list for which an involvement of K+transport in stomatal movements has been demonstrated. However, an additional and ecologically important characteristic was found: there was a suppression of stomatal opening by increasing NaCl concentrations. A new hypothesis is offered of the mechanism for controlling salt and water relations inA. tripolium, a species which does not possess glands or other means of excreting salt. It is suggested that when the capacity of the tissues to accumulate salt in cell vacuoles is exceeded, the concentration of Na+ions in the apoplast around the guard cells begins to rise. This causes partial stomatal closure, reduces transpiration and increases water‐use‐efficiency. Therefore, the flow of salt into the leaves is reduced but growth (and the manufacture of the new photosynthates required to support it) can continue.Aster tripoliumcan be added to the small list of known species which readily yield isolated epidermis suitable for detailed stomatal studies. Throughout this study, we have compared its stomatal physiology withC. communis, which has been thoroughly investigated in the past.

LEAF GAS EXCHANGE IN QUERCUS MACROCARPA (FAGACEAE): RAPID STOMATAL RESPONSES TO VARIABILITY IN SUNLIGHT IN A TREE GROWTH FORM

Responses in net photosynthesis (A), stomatal conductance to water vapor (g), and leaf xylem pressure potential (ψ) were measured in the deciduous tree Quercus macrocarpa during alternating periods of sun (photosynthetic photon flux, PPF > 1,500 μmol m‐2 sec‐1) and shade (ca. 350 μmol m‐2 sec‐1 simulating cloud cover). Measurements were made on trees growing at the gallery forest‐prairie edge on the Konza Prairie Research Natural Area in northeast Kansas. The region is near the westernmost extension of the range of Q. macrocarpa where this species experiences significant seasonal water stress (minimum ψ < ‐2.9 MPa). Quercus macrocarpa was chosen for study because it has relatively high A (15 μmol m‐2 sec‐1) and g (300 mmol m‐2 sec‐1) in contrast to the deciduous and evergreen subalpine trees previously studied. Both trees and large saplings of Q. macrocarpa responded to alternating several minute periods of sun and shade with relatively rapid changes in A and g. Reductions in g (110 mmol m‐2 sec‐1) during shade periods lowered transpirational water losses (E) by 13% (and reduced A by 5%) relative to estimates of A and E made assuming g remained constant. Partial stomatal closure during shade was correlated with moderate enhancement in ψ (0.31 MPa) in Q. macrocarpa. However, greater increases in ψ were measured in adjacent prairie grasses exposed to similar periods of shade (0.72 MPa in Andropogon gerardii, 0.61 MPa in Sorghastrum nutans). Reduced variability in ψ in tree growth forms may reflect greater buffering of water relations associated with the large size of trees, the amount of tissue devoted to water storage, and differences in hydraulic resistance relative to herbs. Nonetheless, the gas exchange and water relations responses in Q. macrocarpa were much more similar to those previously measured in herbaceous subalpine and grassland species than to those documented for subalpine trees. Thus, rapid gas exchange responses to variable PPF may also occur in tree growth forms.

Growth of cotton under continuous salinity stress: influence on allocation pattern, stomatal and non-stomatal components of photosynthesis and dissipation of excess light energy

Cotton (Gossypium hirsutum L.) plants were grown in flowing-culture solutions containing 0%, 26% and 55% natural seawater under controlled and otherwise identical conditions. Leaf Na(+) content rose to 360 mM in 55% seawater, yet the K(+) content was maintained above 100 mM. The K(+)/Na(+) selectivity ratio was much greater in the saline plants than in the control plants. All plants were healthy and able to complete the life cycle but relative growth rate fell by 46% in 26% seawater and by 83% in 55% seawater. Much of this reduction in growth was caused by a decreased allocation of carbon to leaf growth versus root growth. The ratio of leaf area/plant dry weight fell by 32% in 26% seawater and by 50% in 55 % seawater while the rate of carbon gain per unit leaf area fell by only 20% in 26% seawater and by as much as 66% in 55% seawater. Partial stomatal closure accounted for nearly all of the fall in the photosynthesis rate in 26% seawater but in 55% seawater much of the fall also can be attributed to non-stomatal factors. As a result of the greater effect of salinity on stomatal conductance than on CO2-uptake rate, photosynthetic water-use efficiency was markedly improved by salinity. This was also confirmed by stablecarbon-isotope analyses of leaf sugar and of leaf cellulose and starch. - Although non-stomatal photosynthetic capacity at the growth light was reduced by as much as 42% in 55% seawater, no effects were detected on the intrinsic photon yield of photosynthesis nor on the efficiency of photosystem II photochemistry, chlorophyll a/b ratio, carotenoid composition or the operation of the xanthophyll cycle. Whereas salinity caused in increase in mesophyll thickness and content of chloroplast pigments it caused a decrease in total leaf nitrogen content. The results indicate that the salinity-induced reduction in non-stomatal photosynthetic capacity was not caused by any detrimental effect on the photosynthetic apparatus but reflects a decreased allocation to enzymes of carbon fixation. - Rates of energy dissipation via CO2 fixation and photorespiration, calculated from gas-exchange measurements, were insufficient to balance the rate of light-energy absorption at the growth light. Salinity therefore would have been expected to cause the excess excitation energy to rise, leading to an increased nonradiative dissipation in the pigment bed and resulting increases in non-photochemical fluorescence quenching and zeaxanthin formation. However, no such changes could be detected, implying that salinity may have increased energy dissipation via a yet unidentified energy-consuming process. This lack of a response to salinity stress is in contrast to the responses elicited by short-term water stress which caused strong non-photochemical quenching and massive zeaxanthin formation.

Canopy photosynthesis and respiration of kiwifruit (Actinidia deliciosa var. deliciosa) vines growing in the field.

Net CO(2) assimilation (A) for canopies of kiwifruit (Actinidia deliciosa var. deliciosa) vines enclosed in a whole-canopy cuvette was measured continuously for three periods of 15-20 days during late summer, near Hamilton, New Zealand (latitude 38.2 degrees S). Canopy A showed an asymptotic response to incident radiation (PAR), saturating at about 1300 micromol m(-2) s(-1) for one vine and about 800 micromol m(-2) s(-1) for two other vines. Radiation interception at low solar angles and low leaf area apparently limited the response of A to PAR. Radiation saturated rates of A were 25-30 micromol CO(2) m(-2) s(-1) for one vine, and 12-18 micromol CO(2) m(-2) s(-1) for two other vines. At any PAR, canopy A was often lower in the afternoon than in the morning. Canopy respiration averaged 8.9 micromol CO(2) m(-2) s(-1) at 12 degrees C, but increased only 24-34% over the range 7-17 degrees C. Net daily C gains for the whole canopy, calculated as the temporal integral of A, ranged from -0.8 g C m(-2) for a cloudy day (PAR </= 450 micromol m(-2) s(-1)) to 6.2 g C m(-2) for a day with intermittent cloud. Measurements of gas exchange for single leaves in a leaf cuvette were used to examine the basis of the whole-canopy responses to environmental variables. Partial stomatal closure due to water stress apparently contributed to the afternoon depressions in A, although non-stomatal limitations may also have been involved.

Stomatal and Nonstomatal Components to Inhibition of Photosynthesis in Leaves of Capsicum annuum during Progressive Exposure to NaCl Salinity

Young bell pepper (Capsicum annuum L.) plants grown in nutrient solution were gradually acclimated to 50, 100, or 150 moles per cubic meter NaCl, and photosynthetic rates of individual attached leaves were measured on several occasions during the salinization period at external CO(2) concentrations ranging from approximately 70 to 1900 micromoles per mole air. Net CO(2) assimilation (A) was plotted against computed leaf internal CO(2) concentration (C(i)), and the initial slope of this A-C(i) curve was used as a measure of photosynthetic ability. During the 10 to 14 days after salinization began, leaves from plants exposed to 50 moles per cubic meter NaCl showed little change in photosynthetic ability, whereas those treated to 100 or 150 moles per cubic meter NaCl had up to 85% inhibition, with increase in CO(2) compensation point. Leaves appeared healthy, and leaf chlorophyll content showed only a 14% reduction at the highest salinity levels. Partial stomatal closure occurred with salinization, but reductions in photosynthesis were primarily nonstomatal in origin. Photosynthetic ability was inversely related to the concentration of either Na(+) or Cl(-) in the leaf laminas sampled at the end of the experimental period. However, the concentration of Cl(-) expressed on a tissue water basis was greater, exceeding 300 moles per cubic meter, and Cl(-) was more closely associated (R(2) = 0.926) with the inhibition of photosynthetic ability. Leaf turgor was not reduced by salinization and leaf osmotic potential decreased to a slightly greater extent than the osmotic potential decreases of the nutrient solutions. Concentration of accumulated Na(+) and Cl(-) (on a tissue water basis) accounted quantitatively for maintenance of leaf osmotic balance, assuming that these ions were sequestered in the vacuoles.

Growth and Water Status of Pruned and Unpruned Woody Landscape Plants Treated with Sumagic (Uniconazole), Cutless (Flurprimidol), or Atrimmec (Dikegulac)

Abstract Efficacy of Cutless (flurprimidol), Sumagic (uniconazole), and Atrimmec (dikegulac) as affected by application date after pruning was investigated with Pyracantha × ‘Teton’, Ligustrum × vicaryi, and Euonymus fortunei var. coloratus. Cutless (flurprimidol) applied to the medium surface, or foliar sprays of Sumagic (uniconazole) or Atrimmec (dikegulac) were applied 1, 2, 3 or 7 days after pruning (DAP) and to unpruned plants in spring 1989. Height and width were recorded weekly into November, 1989. The effect of these plant growth regulators (PGRs) on short-term plant water relations was measured at 2, 3, 18, 22, or 24 days on unpruned plants and plants treated 2 DAP. Growth rate of Pyracantha was reduced for 6 to 7 weeks if Sumagic (uniconazole) was applied 2 DAP or less, or to unpruned plants; otherwise growth rate was reduced 4 to 5 weeks. Sumagic (uniconazole) reduced Pyracantha growth rate more than Atrimmec (dikegulac); however, Atrimmec-treated plants had a more normal growth habit by the end of the experiment. Atrimmec (dikegulac) also suppressed Ligustrum growth without detrimentally affecting plant appearance. Cutless (flurprimidol) was the only PGR that effectively inhibited growth of Euonymus; however, this treatment excessively dwarfed Pyracantha and Ligustrum. The influence of Atrimmec (dikegulac) and Cutless (flurprimidol) on leaf area varied with the species, and pruning had no effect. Short-ternl effects of Sumagic (uniconazole), Cutless (flurprimidol), and Atrimmec (dikegulac) on transpiration and stomatal conductance to water vapor were species dependent, with some species exhibiting partial stomatal closure. Improvement of plant water status (stem water potential and relative leaf water content) occurred in some cases.

Physiological Studies on the Actions of Triadimefon in Desert Plants II. Role on Transpiration in Groundnut

Triadimefon — a triazole fungicide decreased transpiration rate in groundnut. The soil application of the chemical was much more effective than foliar treatment. Further, an analysis of transpiration decline curves reveals that triadimefon reduced the transpiration rate by decreasing stomatal transpiration through partial stomatal closure.

Contributions of stomatal infiltration and cuticular penetration to enhancements of foliar uptake by surfactants

AbstractRadiolabelled deoxyglucose (DOG) and glyphosate were used to investigate the effects of certain non‐ionic surfactants on the kinetics of foliar uptake in three species. ‘Silwet L‐77’ (5 g litre−1), an organosilicone surfactant, enabled spray solutions to infiltrate stomata, providing uptake of DOG into Vicia bean (50%), oat (35%) and wheat (20%) within 10 min of application. ‘Silwet Y‐12301’, another organosilicone, also induced stomatal infiltration but to a lesser extent; unlike L‐77, this was attenuated by partial stomatal closure. A third organosilicone, ‘Silwet L‐7607’, and two conventional surfactants, ‘Triton X‐45’ (OP5) and ‘Agral 90’ (NP9), did not induce stomatal infiltration. The effective minimum concentration of L‐77 required to enable infiltration of stomata was 2 g litre−1. The uptake of glyphosate into bean did not differ from that of DOG but the ‘Roundup’ formulation of glyphosate partially antagonised the infiltration provided by L‐77. Addition of surfactants did not increase the rate of cuticular penetration of DOG into bean but total uptake was increased, except by NP9, either via infiltration (L‐77 and Y‐12301) or by extending the period during which penetration occurred (L‐7607 and OP5). The surfactants had a variable effect on rates of penetration of DOG into wheat and oat. In general, foliar uptake followed an exponential timecourse which was largely complete within 6 h and only rarely approached 100% of the applied chemical. The stomatal infiltration provided by L‐77 caused an increase in translocation of DOG in bean.

A vapor pressure deficit effect on crop canopy photosynthesis

Canopy CO2-exchange rates (CER), air temperatures, and dew points were measured throughout ten days during the 1987 growing season for cotton (Gossypium hirsutum L.), grain sorghum [Sorghum bicolor (L) Moench], and five soybean [Glycine max (L) Merr.] cultivars, and throughout seven days in 1988, on maize (Zea maize L.). The objective was to determine if the decline in CER per unit light during the afternoon is associated with a vapor pressure deficit (VPD) increase. Some of the soybean and maize plots were kept as dry as possible. A VPD term significantly contributed (P≤0.05) to a canopy CER regression model in 54 of 80 data sets in 1987. Grain sorghum was less sensitive than the well-watered soybean genotypes to an increasing VPD (P≤0.05) on three of the ten measurement days and less sensitive than cotton (P≤0.05) on only one day. Cotton demonstrated less VPD sensitivity than soybean (P≤0.05) on one day. The moisture stressed soybean plots showed a greater CER sensitivity to VPD (P≤0.05) than the well-watered soybean plots. In 1988, the frequently irrigated maize plots were less sensitive to VPD (P≤0.05) than the rain-fed plots early in the season, before the rain-fed plots were excessively damaged by moisture stress. These results indicate that the afternoon declines in canopy CER found in a number of different species are associated with increases in the VPD; recent work of others suggests that this may be due to partial stomatal closure.

The influence of soil aeration on growth and elemental absorption of greenhouse‐grown seedling pecan trees

Abstract ‘Dodd’ pecan seedlings were exposed to 3 levels of soil aeration for 30 days; 100%, 5%, and 0% of the container surface exposed to the atmosphere. These treatments resulted in about 21%, 13.5%, and 3% soil O2and 0.3%, 5%, and 13% soil CO2for 100%, 5%, and 0% of the container surface exposed, respectively. Restricting soil aeration induced partial stomatal closure, and decreased leaf number, leaf area, and leaf, trunk and root dry weights. The decrease in root dry weight associated with reduced soil aeration exceeded the decrease in top dry weight by about 50%. Translocation of N and P to the leaves was reduced when soil aeration was restricted, but root N and P concentrations were increased compared to trees grown in well aerated soil. Leaf elemental concentrations of Ca, Mg, and Mn were lower when trees were exposed to reduced soil aeration. Zinc and Fe concentrations were greater in the roots of trees with low aeration, but leaf and trunk concentrations of Zn and Fe were not affected

Net photosynthesis, water use efficiency, leaf water potential and leaf rolling as affected by water deficit in tropical upland rice

Growth and production of tropical upland rice is often impeded by drought. Little is known on varietal response of CO2 assimilation to water deficit under tropical field conditions. A drought-susceptible semidwarf (IR20) and a drought-resistant traditional (Azucena) rice were grown in a dryland field experiment with sprinkler irrigation during the dry season in the Philippines. Differential irrigation was imposed for 11 days during vegetative growth using a line source sprinkler. Net photosynthesis, leaf conductance, transpiration, leaf rolling and leaf water potential were determined during the stress cycle at pre-noon and afternoon, with all measurements on the same leaf. No varietal differences in maximum photosynthetic rate and in the relationship between photosynthesis and leaf conductance were observed. In both rices, partial stomatal closure and nonstomatal inhibition reduced assimilation rates in the afternoon. Leaf water deficits restricted gas exchange through at least three apparently independent mechanisms: leaf rolling, reduced stomatal conductance and non-stomata1 inhibition which became evident only at severe degrees of stress. Stomata1 closure and leaf rolling were more sensitive to water deficit in Azucena which maintained higher leaf water potential throughout the stress cycle. Both stomatal closure and leaf rolling improved water use efficiency at moderate stress while nonstomatal inhibition of photosynthesis reduced water use efficiency.