Stomatal behavior and water relations of waterlogged tomato plants

The effects of waterlogging the soil on leaf water potential, leaf epidermal conductance, transpiration, root conductance to water flow, and petiole epinasty have been examined in the tomato (Lycopersicon esculentum Mill.). Stomatal conductance and transpiration are reduced by 30% to 40% after approximately 24 hours of soil flooding. This is not due to a transient water deficit, as leaf water potential is unchanged, even though root conductance is decreased by the stress. The stomatal response apparently prevents any reduction in leaf water potential. Experiments with varied time of flooding, root excision, and stem girdling provide indirect evidence for an influence of roots in maintaining stomatal opening potential. This root-effect cannot be entirely accounted for by alterations in source-sink relationships. Although 1-aminocyclopropane-1-carboxylic acid, the immediate precursor of ethylene, is transported from the roots to the shoots of waterlogged tomato plants, it has no direct effect on stomatal conductance. Ethylene-induced petiole epinasty develops coincident with partial stomatal closure in waterlogged plants. Leaf epinasty may have beneficial effects on plant water balance by reducing light interception.     

A coupled model of stomatal conductance, photosynthesis and transpiration

A model that couples stomatal conductance, photosynthesis, leaf energy balance and transport of water through the soil–plant–atmosphere continuum is presented. Stomatal conductance in the model depends on light, temperature and intercellular CO₂ concentration via photosynthesis and on leaf water potential, which in turn is a function of soil water potential, the rate of water flow through the soil and plant, and on xylem hydraulic resistance. Water transport from soil to roots is simulated through solution of Richards’ equation. The model captures the observed hysteresis in diurnal variations in stomatal conductance, assimilation rate and transpiration for plant canopies. Hysteresis arises because atmospheric demand for water from the leaves typically peaks in mid‐afternoon and because of uneven distribution of soil matric potentials with distance from the roots. Potentials at the root surfaces are lower than in the bulk soil, and once soil water supply starts to limit transpiration, root potentials are substantially less negative in the morning than in the afternoon. This leads to higher stomatal conductances, CO₂ assimilation and transpiration in the morning compared to later in the day. Stomatal conductance is sensitive to soil and plant hydraulic properties and to root length density only after approximately 10 d of soil drying, when supply of water by the soil to the roots becomes limiting. High atmospheric demand causes transpiration rates, LE, to decline at a slightly higher soil water content, θ_s, than at low atmospheric demand, but all curves of LE versus θ_s fall on the same line when soil water supply limits transpiration. Stomatal conductance cannot be modelled in isolation, but must be fully coupled with models of photosynthesis/respiration and the transport of water from soil, through roots, stems and leaves to the atmosphere.     

Stomatal sensitivity of six temperate, deciduous tree species to non-hydraulic root-to-shoot signalling of partial soil drying

The objective of this study were to (1) characterize stomatal response of six deciduous tree species to non-hydraulic, root-sourced signals of soil drying, and (2) test whether species sensitivity to non-hydraulic signalling is allied with their drought avoidance and tolerance profiles. Saplings were grown with roots divided between two pots. Three treatments were compared: one half of the root system watered and half droughted (WD), one half of the root system watered and half severed (WS), both halves watered (WW). Drying about half of the root system caused non-hydraulic declines in stomatal conductance (gs) in all species, with gs of WD plants reduced to from 40% to 60% of WS controls. Declines in stomatal conductance were closely related to declining soil matric potential (m) between -0.01 and -0.10 MPa. Soil m required to cause declines in gs of WD plants to 80% of WS controls varied from a high of -0.013 to a low of -0.044 MPa. Stomatal inhibition varied somewhat with leaf age in half of the species. Leaf osmotic potentials during soil drying were mostly similar among treatments. Although stomatal sensitivity to the non-hydraulic, root-sourced signal (characterized as decline in gs per unit decline in soil ) was not closely correlated with previously identified lethal leaf water potentials or capacity for osmotic adjustment, species having the highest stomatal sensitivity also had the least hydration tolerance. This suggests that stomatal sensitivity to non-hydraulic root signals may be mechanistically linked to a limited extent with other characteristics defining relative species drought tolerance.     

Non-hydraulic signals from maize roots in drying soil: inhibition of leaf elongation but not stomatal conductance

Conditions of soil drying and plant growth that lead to non-hydraulic inhibition of leaf elongation and stomatal conductance in maize (Zea mays L.) were investigated using plants grown with their root systems divided between two containers. The soil in one container was allowed to dry while the other container was kept well-watered. Soil drying resulted in a maximum 35% inhibition of leaf elongation rate which occurred during the light hours, with no measurable decline in leaf water potential (_w). Leaf area was 15% less than in control plants after 18 d of soil drying. The inhibition of elongation was observed only when the soil _w declined to below that of the leaves and, thus, the drying soil no longer contributed to transpiration. However, midday root _w in the dry container (-0.29 MPa) remained much higher than that of the surrounding soil (-1.0 MPa) after 15 d of drying, indicating that the roots in drying soil were rehydrated in the dark.To prove that the inhibition of leaf elongation was not caused by undetectable changes in leaf water status as a result of loss of half the watergathering capacity, one-half of the root system of control plants was excised. This treatment had no effect on leaf elongation or stomatal conductance. The inhibition of leaf elongation was also not explained by reductions in nutrient supply.Soil drying had no effect on stomatal conductance despite variations in the rate or extent of soild drying, light, humidity or nutrition. The results indicate that non-hydraulic inhibition of leaf elongation may act to conserve water as the soil dries before the occurrence of shoot water deficits.Symbol _w water potential Contribution from the Missouri Agricultural Experiment Station, Journal Series No. 10881     

The Influence of Leaf Water Status and ABA on Leaf Growth and Stomata of Phaseolus Seedlings with Hypoxic Roots

The objective of this study was to assess the relative rolesof leaf water status and root-sourced signals in mediating beanleaf responses to root hypoxia. To do so, the roots of beanplants under varied VPD (0.95 kPa to 0.25 KPa) were made hypoxic.Under all conditions, leaf growth rates and stomatal conductanceswere reduced. There was a transitory decline in leaf water potentialat high VPD which accounted for the initial reduction in leafgrowth rates and stomatal conductance. At low VPD, no waterdeficits were detected. Leaf growth inhibition and reduced stomatalconductance under low VPD treatments were unrelated to leafwater status and must be induced by some other factor. In vitrogrowth of leaf discs was reduced by xylem sap collected fromhypoxic roots. Exogenously applied ABA, at high concentrationsin KCl and sucrose, or at low concentrations diluted in xylemsap from aerated plants, inhibited in vitro growth of leaf discs.Applications of ABA in the transpiration stream reduced stomatalconductance.     

Shoot and root physiological responses to localized zones of soil moisture in cultivated and wild lettuce (Lactuca spp.)

Cultivated crisphead lettuce (Lactuca sativa L.) has a shallower root system than its wild relative, Lactuca serriola L. The effects of localized soil water, at depth, on plant water relations, gas exchange and root distribution were examined in the two species using soil columns with the soil hydraulic-ally separated into two layers, at (0–20 cm and 20–81) cm, but permitting root growth between the layers. Three treatments were imposed on 7-week-old plants, and maintained for 4 weeks: (i) watering, both layers to field capacity; (ii) drying the upper layer while watering the lower layer to field capacity, and (iii) drying both layers. Drying only 0–20 cm of soil had no effect on leaf water status, net photosynthesis, stomatal conductance or biomass production in L. serriola compared to a well-watered control, but caused a short-term reduction (10 d) in leaf water status and photosynthesis in L. sativa that reduced final shoot production. The different responses may be explained by differences in root distribution. Just before the treatments commenced, L. serriola had 50% of total root length at 20–80 cm compared to 35% in L. sativa. Allocation of total biomass to roots in L. serriola was approximately double that in L. sativa. The wild species could provide germplasm for cultivated lettuces to extract more soil water from depth, which may improve irrigation efficiency.     

Soil temperature and flooding effects on two species of citrus

Summary Rough lemon (Citrus jambhiri Lush.) and sour orange (C. aurantium L.) seedlings were grown at constant soil temperatures of 16, 24, and 33 C for 3 months. Shoot and root growth of rough lemon was greatest at 33 C while growth of sour orange was greatest at 24 C. There were no significant effects of soil temperature on shoot: root ratio, leaf water potential or stomatal conductance. The hydraulic conductivity of intact root systems of both species was highest when seedlings were grown at 16 C. Thus, acclimation through greater root conductivity at low soil temperature may have compensated for decreased root growth at 16 C and negated effects of soil temperature on plant water relations. Half the plants growing at each soil temperature were subsequently flooded. Within 1 week, the soil redox potential (Eh) dropped below zero mV, reaching a minimum Eh of –250mV after 3 weeks of flooded conditions. Flooded plants exhibited lower root conductivity, a cessation of shoot growth, lower leaf water potentials, lower stomatal conductances, and visual sloughing of fibrous roots. Decreases in root conductivity in response to flooding were large enough to account for the observed decreases in stomatal conductance.Florida Agricultural Experiment Stations Journal Series No. 4080.     

Water status of soil and vegetation in a shortgrass steppe

Summary In an attempt to describe some major relationships between soil and plant compartments in a shortgrass steppe, the process of water loss from the system and plant water relations throughout a drying cycle were studied. The water supply was manipulated and some soil and plant variables monitored throughout a drying cycle. Leaf conductance and leaf water potential of blue grama (Bouteloua gracilis) were measured periodically at predawn and noon. Soil water content and water potential of different layers were also monitored.Three different periods were distinguished in the water loss process throughout a drying cycle. These distinctions were made taking into account the relative contribution of different soil layers. Leaf conductance and water potential at noon slowly declined throughout the first 50 days of plant growth. After that, they rapidly decreased, reaching values of 0.29 mm s^-1 and-5.0 MPa, respectively. The predawn leaf water potential remained unchanged around-0.5 MPa during the first 45 days, then rapidly decreased. This occurred when soil water of the wettest soil layer was near depletion.Predawn leaf water potentials were highly correlated with water potentials of the wettest layer. Leaf conductance and water potential at noon were correlated with effective soil water potential (soil water potential weighted by the root distribution in the profile). We concluded that root surface area limited the water flow through an important part of the day in this semiarid ecosystem. Axial root resistance did not appear important in determining the equilibrium status between leaves and the wettest soil layer.     

Hydraulic control of stomatal conductance in Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] and alder [Alnus rubra (Bong)] seedlings

Experiments were conducted on 1-year-old Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] and 2- to 3-month-old alder [Alnus rubra (Bong)] seedlings growing in drying soils to determine the relative influence of root and leaf water status on stomatal conductance (g_c). The water status of shoots was manipulated independently of that of the roots using a pressure chamber that enclosed the root system. Pressurizing the chamber increases the turgor of cells in the shoot but not in the roots. Seedling shoots were enclosed in a whole-plant cuvette and transpiration and net photosynthesis rates measured continuously. In both species, stomatal closure in response to soil drying was progressively reversed with increasing pressurization. Responses occurred within minutes of pressurization and measurements almost immediately returned to pre-pressurization levels when the pressure was released. Even in wet soils there was a significant increase in g_c with pressurization. In Douglas fir, the stomatal response to pressurization was the same for seedlings grown in dry soils for up to 120 d as for those subjected to drought stress over 40 to 60 d. The stomatal conductance of both Douglas fir and alder seedlings was less sensitive to root chamber pressure at higher vapour pressure deficits (D), and stomatal closure in response to increasing D from 1.04 to 2.06 kPa was only partially reversed by pressurization. Our results are in contrast to those of other studies on herbaceous species, even though we followed the same experimental approach. They suggest that it is not always appropriate to invoke a ‘feedforward’ model of short-term stomatal response to soil drying, whereby chemical messengers from the roots bring about stomatal closure.     

Influence of soil drying on root development,water relations and leaf growth of Ceratonia siliqua L.

Summary Seedlings of Ceratonia siliqua L., an evergreen sclerophyll species native to the Mediterranean region, were grown in 30-cm deep tubes of John Innes II potting compost in a growth cabinet maintained at 15° C during a 12-h day where PAR was 400 mol m^–2 s^–1. After a period of acclimatisation to the conditions in the cabinet during which plants were watered every day, water was withheld from the soil in some tubes for 24 days. These conditions may be regarded as a simulation of the natural situation. Estimates of leaf and root water potential and solute potential, leaf growth and root development were made at intervals during the soil drying cycle on both watered and unwatered plants. Water potential and solute potential measurements were made both on young expanding and on fully expanded leaves. During the experimental period, root growth of C. siliqua was not much affected by soil drying, and roots in both the watered and the unwatered columns penetrated to the bottom of the soil tubes by the end of the drying treatment. Expanded leaves showed significant limitation in stomatal conductance as soil drying progressed. Leaf water potential of fully expanded leaves of unwatered plants declined substantially. In contrast, water potential of young expanding leaves on unwatered plants declined to only a limited extent and turgor was sustained. As the soil dried, stomatal conductance of young leaves was always higher than that of mature leaves; also, placticity and elasticity of young leaves slowly decreased whereas mature leaves became stiff. Changing leaf cell wall properties may determine different patterns of water use as the leaves age. A mechanism of continuous diffusion of water through the soil towards the tip and pumping towards the young leaves is proposed.     

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     

Leaf and root control of stomatal closure during drying in soybean

The stomatal conductance of an illuminated 2.5 cm² area of an intact soybean leaflet was the same whether the rest of the shoot was in light or darkness. This was true throughout soil drying cycles. Water potential of tissue immediately outside the illuminated area consistently decreased about 0.3 MPa upon illumination of the shoot. This erroneously suggested that stomatal conductance during soil drying did not respond to diurnal reductions in leaf water potential, but was controlled by root or soil water status. Tests showed that the water potential of tissue in the illuminated area did not change in the steady-state upon illumination of the rest of the shoot. Water potentials of shaded sections of leaves were not different from predawn water potentials, and were higher than leaf xylem pressure potentials as determined with a pressure chamber. These steep local gradients of leaf water potential suggest that there is minimal interchange of water among xylem elements leading from roots to different sections of leaves. The relationship between stomatal conductance and leaf water potential was the same whether leaf water potential was reduced by soil drying, application of polyethylene glycol (PEG) to the root system, lowering root temperature, or leaf excision. In the root cooling experiment, there was no soil drying, and with leaf excision, there was no root drying. The similarity of stomatal responses to leaf water potential in all cases strongly suggests control of conductance by a signal produced by local leaf water potential rather than root or soil water status in these experiments.     

闽西南红壤侵蚀区黑莎草功能性状特征及其与土壤因子的关系

为了解黑莎草(Gahnia tristis)在南方红壤侵蚀区的适应状况,测定了长汀县红壤侵蚀区的黑莎草叶片、根系的功能性状及土壤理化性质,并应用数理统计方法分析了黑莎草叶片与根系功能性状之间的相关性,及其对土壤因子的响应。结果表明,黑莎草叶片表型性状在冬夏季间存在显著差异,叶长、叶宽、叶组织密度和叶绿素表现为夏季显著大于冬季,根系表型性状则更具稳定性,冬季的根系养分含量均高于夏季,养分的分配上叶片养分高于根系养分。叶组织密度与叶绿素含量呈显著正相关,与比叶面积呈显著负相关;根组织密度与比根长和比根面积均呈显著负相关,叶片和根系养分间均呈显著正相关,土壤碳、氮、磷含量是影响黑莎草功能性状主要因子。因此,黑莎草可通过调节功能性状以适应环境变化,可作为地带性植物应用于南方红壤侵蚀区的植被恢复和水土流失治理。     

冬小麦叶片气孔导度模型水分响应函数的参数化

植物气孔导度模型的水分响应函数用来模拟水分胁迫对气孔导度的影响过程, 是模拟缺水环境下植物与大气间水、碳交换过程的关键算法。水分响应函数包括空气湿度响应函数和土壤湿度(或植物水势)响应函数, 该研究基于田间实验观测, 分析了冬小麦(Triticum aestivum)叶片气孔导度对不同空气饱和差和不同土壤体积含水量或叶水势的响应规律。一个土壤水分梯度的田间处理在中国科学院禹城综合试验站实施, 不同水分胁迫下的冬小麦叶片气体交换过程和气孔导度以及其他的温湿度数据被观测, 同时观测了土壤含水量和叶水势。实验数据表明, 冬小麦叶片气孔导度对空气饱和差的响应呈现双曲线规律, 变化趋势显示大约1 kPa空气饱和差是一个有用的阈值, 在小于1 kPa时, 冬小麦气孔导度对空气饱和差变化反应敏感, 而大于1 kPa后则反应缓慢; 分析土壤体积含水量与中午叶片气孔导度的关系发现, 中午叶片气孔导度随土壤含水量增加大致呈现线性增加趋势, 但在平均土壤体积含水量大于大约25%以后, 气孔导度不再明显增加, 而是维持在较高导度值上下波动; 冬小麦中午叶片水势与相应的气孔导度之间, 随着叶水势的增加, 气孔导度呈现增加趋势。根据冬小麦气孔导度对空气湿度、土壤湿度和叶水势的响应规律, 研究分别采用双曲线和幂指数形式拟合了水汽响应函数, 用三段线性方程拟合了土壤湿度响应函数和植物水势响应函数, 得到的参数可以为模型模拟冬小麦的各类水、热、碳交换过程采用。     

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

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

Stomatal action directly feeds back on leaf turgor: new insights into the regulation of the plant water status from non‐invasive pressure probe measurements

Uptake of CO₂ by the leaf is associated with loss of water. Control of stomatal aperture by volume changes of guard cell pairs optimizes the efficiency of water use. Under water stress, the protein kinase OPEN STOMATA 1 (OST1) activates the guard‐cell anion release channel SLOW ANION CHANNEL‐ASSOCIATED 1 (SLAC1), and thereby triggers stomatal closure. Plants with mutated OST1 and SLAC1 are defective in guard‐cell turgor regulation. To study the effect of stomatal movement on leaf turgor using intact leaves of Arabidopsis, we used a new pressure probe to monitor transpiration and turgor pressure simultaneously and non‐invasively. This probe permits routine easy access to parameters related to water status and stomatal conductance under physiological conditions using the model plant Arabidopsis thaliana. Long‐term leaf turgor pressure recordings over several weeks showed a drop in turgor during the day and recovery at night. Thus pressure changes directly correlated with the degree of plant transpiration. Leaf turgor of wild‐type plants responded to CO₂, light, humidity, ozone and abscisic acid (ABA) in a guard cell‐specific manner. Pressure probe measurements of mutants lacking OST1 and SLAC1 function indicated impairment in stomatal responses to light and humidity. In contrast to wild‐type plants, leaves from well‐watered ost1 plants exposed to a dry atmosphere wilted after light‐induced stomatal opening. Experiments with open stomata mutants indicated that the hydraulic conductance of leaf stomata is higher than that of the root–shoot continuum. Thus leaf turgor appears to rely to a large extent on the anion channel activity of autonomously regulated stomatal guard cells.     

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.     

Stomatal response to environment and a possible interrelation between stomatal effects on transpiration and CO2 assimilation

Abstract It had been hypothesized that if daily CO₂ assimilation is to be maximized at a given level of daily transpiration, stomatal apertures should change during the day so that the gain ratio (?A/?g)/(?E/?g) remains constant. These partial differentials describe the sensitivity of assimilation rate (A) and transpiration rate (E) to changes in stomatal conductance (g). Experiments were conducted to determine whether stomata respond to environment in a manner which results in constant gain ratios. Gas–exchange measurements were made of the stomatal and photosynthetic responses of Vigna unguiculata L. Walp. in controlled environments. Leaf conductance to water vapour responded to step changes in temperature and humidity so that for different steady-state conditions the gain ratio remained constant on all but one day. Depletion of water in the root zone resulted in day-to-day increases in gain ratio which were correlated with decreases in maximum leaf conductance to water vapour. The significance of the results for plant adaptation and stomatal mechanisms, and methods for measuring the gain ratio, are discussed.     

Nonhydraulic signalling of soil drying in mycorrhizal maize

Our objectives were to (1) verify that nonhydraulic signalling of soil drying can reduce leaf growth of maize, (2) determine if a mycorrhizal influence on such signalling can occur independently of a mycorrhizal effect on leaf phosphorus concentration, plant size or soil drying rate, and (3) determine if leaf phosphorus concentration can affect response to the signalling process. Maize (Zea mays L. Pioneer 3147) seedlings were grown in a glasshouse with root systems split between two pots. The 2 x 3 x 2 experimental design included two levels of mycorrhizal colonization (presence or absence of Glomus intraradices Schenck & Smith), three levels of phosphorus fertilization within each mycorrhizal treatment and two levels of water (both pots watered or one pot watered, one pot allowed to dry). Fully watered mycorrhizal and nonmycorrhizal control plants had similar total leaf lengths throughout the experiment, and similar final shoot dry weights, root dry weights and leaf length/root dry weight ratios. Leaf growth of mycorrhizal plants was not affected by partial soil drying, but final plant leaf length and shoot dry weight were reduced in half-dried nonmycorrhizal plants. At low P fertilization, effects of nonhydraulic signalling were not evident. At medium and high P fertilization, final total plant leaf length of nonmycorrhizal plants was reduced by 9% and 10%, respectively. These growth reductions preceded restriction of stomatal conductance by 7 d. This and the fact that leaf water potentials were unaffected by partial soil drying suggested that leaf growth reductions were nonhydraulically induced. Stomatal conductance of plants given low phosphorus was less influenced by nonhydraulic signalling of soil drying than plants given higher phosphorus. Soil drying was not affected by mycorrhizal colonization, and reductions in leaf growth were not related to soil drying rate (characterized by time required for soil matric potential to drop below control levels and by time roots were exposed to soil matric potential below typical leaf water potential). We conclude that mycorrhizal symbiosis acted independently of phosphorus nutrition, plant size or soil drying rate in eliminating leaf growth response to nonhydraulic root-to-shoot communication of soil drying.Abbreviations and Symbols ANOVA analysis of variance - Cs stomatal conductance(s) - med medium - P probability - matric potential(s) - water potential(s) This work was supported by the U.S. Department of Agriculture grant No. 91-37100-6723 and a University of Tennessee Professional Development Research Award to R.M.A. We thank Angela Berry for the graphics.