Cellular Processes Limiting Leaf Growth in Plants under Hypoxic Root Stress

The effects of root hypoxia on leaf growth of a Populus trichocarpa? deltoides hybrid have been assessed. Clonal plants were subjectedto hypoxic root conditions in pot culture by flooding and insolution culture by gassing with nitrogen. The rate of leafexpansion declined within 8 h and was suppressed for the durationof the treatment. Final leaf size was reduced by 35% to 60%compared to aerated plants. Final epidermal cell size and numberdepended both on the developmental stage of the leaf at theonset of stress and on the duration of the treatment. No differencesin bulk leaf water potential were measured between the hypoxicand aerated plants. Cell wall extensibility was lower, leafsolute potential was more negative and turgor potential washigher in leaves of hypoxia-treated plants than of aerated plants.These data suggest that leaf growth of hypoxia-stressed plantsis limited by cell wall extensibility. The mechanism by whichthe root stress induces changes in leaf cell wall characteristicsis not known. Key words: Populus, flooding     

Field Estimates of Root and Xylem Resistances in Pinus contorta using Root Excision

A root excision technique was used to estimate the proportionof total resistance to water flux residing in the soil, theroot, and the xylem of lodgepole pine (Pinus contorta Douglex. Loud.) trees in the field. Root excision at mid-day alwaysresulted in rapid recovery of leaf water potential when waterwas supplied to the cut stem, suggesting a high soil-root resistance.Transpiration was unaffected if leaf water potential beforecutting was not limiting leaf conductance. By mid-June wateruptake by the excised stem always exceeded calculated crowntranspiration indicating recharge of internal sapwood storage.Predawn leaf water potential before root excision was highlycorrelated with total soil-plant resistance (r² = 0·89)and calculated root water uptake (r² = 0·92).     

Does ABA in the Xylem Control the Rate of Leaf Growth in Soil-Dried Maize and Sunflower Plants?

Maize (Zea mays L.) and sunflower (Helianthus annuus L.) plantswere grown in large volumes of soil and leaf growth rate wasmonitored on a daily basis. Half the plants were given a soildrying treatment and when they showed a significant restrictionof growth rate (compared to both their daily growth rate beforedrying and the average growth rate of well-watered plants onthe same day), leaf water relations were measured and xylemsap was extracted using several techniques. There was a significant negative log-linear relationship betweenthe rate of leaf growth and the concentration of ABA in thexylem for both species. There was no clear relationship betweenleaf growth rate and leaf water potential or turgor for eitherspecies. Assessment of different methods for sampling xylemsap suggests that exudates collected from stem stumps or samplescollected by pressurizing the whole root system are suitablefor estimating ABA concentration in xylem, at least with largeplants of maize or sunflower, provided the first few hundredcubic millimetres of collected sap are used for the assay. Centrifugationof sections of stems resulted in dilution of ABA in the xylemsap with sap squeezed from parenchyma tissue. This is because,at least in plants subjected to mild soil drying, the concentrationof the ABA in the xylem is far higher than that in the cellsap of stem tissue. Results support the proposal that ABA plays a major role asa chemical signal involved in the root-to-shoot communicationof the effects of soil drying. The non-hydraulic restrictionof leaf growth by a chemical signal can be explained by theextra root-sourced ABA in the xylem and may be an importantcomponent of the modification of growth and development whichresults from prolonged soil drought. Key words: Soil drying, ABA, leaf growth, Zea mays L., Helianthus annuus L.     

Leaf elongation and water relations of mycorrhizal sorghum in response to partial soil drying: two Glomus species at varying phosphorus fertilization

Arbuscular mycorrhizal symbiosis has previously been shown toalter the response of sorghum leaves to probable non-hydraulicsignals of soil drying. Our objectives here were to determine:(1) how changes in phosphorus nutrition affect this root-to-shootsignalling in sorghum, (2) if mycorrhizal symbiosis can affectthe signalling process independently of effects on host P nutrition,and (3) how two Glomus species compare in their influence onsignalling. Sorghum bicolor (L.) Moench ‘G1990A’ plants weregrown with root systems split between two pots. The 332 experimentaldesign included three levels of mycorrhizae (Glomus intraradicesSchenck & Smith, Glomus etunicatum Becker & Gerd., non-mycorrhizal),three levels of phosphorus fertilization and two levels of water(fully watered, half-dried). Declines in leaf elongation with soil drying were more consistentin non-mycorrhizal than mycorrhizal plants. Relative growthrate (RGR) of both mycorrhizal and non-mycorrhizal plants initiallydeclined when water was withheld from about half of the rootsystem. With further soil drying, RGR of mycorrhizal plantseventually returned to control levels, while RGR of non-mycorrhizalplants remained depressed throughout the drying episode. Bythe end of the drying episode, mycorrhizal symbiosis had eliminateddrying-induced declines in total plant leaf length. Shoot androot dry weight declines of half-dried plants were not affectedby mycorrhizae. Declines in stomatal conductance with soil dryingwere larger and more frequent in non-mycorrhizal than mycorrhizalplants. Leaf osmotic potential and relative water content remainedsimilar in control and half-dried plants during drying, suggestingthat altered leaf behaviour of half-dried plants was due tosome non-hydraulic factor. The two fungi did not differ substantiallyin their influence on leaf behaviour. The applied phosphorustreatments did not affect either growth or stomatal responseof halfdried plants to the root-to-shoot signal, but lengthdeclines were related to actual leaf phosphorus concentrations.Rate of soil drying did not appear to influence ultimate growthreductions. We conclude that mycorrhizal fungi can modify leaf growth responseto the root-to-shoot signal of soil drying, and that this mycorrhizaleffect can occur independently of mycorrhizal effects on plantsize or phosphorus nutrition. However, plant size and nutrition,which are commonly affected by mycorrhizal symbiosis, can alsomodify the signalling process. Key words: Drought, nutrition, root signal, Sorghum bicolor, vesicular-arbuscular     

Wheat Cultivars Respond Differently to a Drying Top Soil and a Possible Non-Hydraulic Root Signal

Research has shown that when plant roots are exposed to a dryingsoil a non-hydraulic (chemical) signal is produced in the rootand transported to the shoot, causing stomatal closure and growthretardation. This study was designed to reveal genetic diversityin wheat response to soil conditions which elicit a root signal,as the first step in the investigation of the genetic controlof the production of and the response to the root signal. Five spring wheat (Triticum aestivum L.) cultivars were establishedin the growth chamber in soil-filled polyvinyl chloride tubes,120 cm long and of an internal diameter of 10·2 cm. Soilwas well fertilized and wet to field capacity at emergence whentwo treatments were imposed: (1) tubes were watered from thetop as needed to eliminate stress (control); and (2) tubes hada constant water table at a soil depth of 100 to 120 cm, withno applied water. Measurements were performed on five dateson leaf water status and stomatal diffusive resistance. Above-groundbiomass and grain yield per plant were determined at maturity. The water table treatment resulted in dry and hard top soilconditions which were previously indicated to elicit a possibleroot signal. Under these experimental conditions, cultivarsdiffered in their leaf water status, stomatal diffusive resistance(R_s) and plant production. In the control treatment, R_s of cultivarsincreased with reductions in their relative water content (RWC)and leaf water potential (LWP), indicating the expected controlof R_s by leaf water status. Under conditions of a drying topsoil, relative water content (RWC) and leaf water potential(LWP) increased in cultivars that had a higher R_s, indicatingthat stomatal activity was controlling leaf water status. Itwas therefore suggested that the drying top soil elicited aroot signal which caused stomatal closure and reduced plantproduction. Under such conditions, two cultivars (Bethlehemand V748) consistently maintained relatively low R_s and highplant production, despite their relatively lower RWC and LWP,as compared with cvs C97, V747 and V652. Limited observationssuggest that in these two cultivars relatively fewer roots mayhave been exposed to the drying top soil, as compared with theother three cultivars. Key words: Triticum aestivum, cultivars, soil moistrue, drought stress, root, root signal, stomata, relative water content, leaf water potential, biomass, yield     

Comparison of Osmotic Adjustment Responses to Water and Temperature Stresses in Spring Wheat and Sudangrass

This study explores the mechanisms of osmotic adjustment bycomparing the growth of spring wheat and sudangrass, which exhibitdifferent degrees of osmotic adjustment, under soil water andtemperature stresses. Leaf water potential ( ₁), osmotic potential(), and rate of leaf area growth of spring wheat and sudangrassseedlings were measured at combinations of five soil water potentials,from -0·03 to -0·25 MPa, and six root temperatures,from 14 to 36°C. Spring wheat exhibit little osmotic adjustment.The leaf osmotic potential was not affected by either soil wateror root temperature stress. Osmotic potential of sudangrassdecreased in parallel with the decreasing leaf water potentialas a result of osmotic adjustment. As soil water potential decreasedfrom -0·03 to -0·25 MPa, the rates of growth andphotosynthesis of spring wheat both decreased by about 30%.For sudangrass with the same range of soil water potential,the photosynthesis rate decreased by only 10% while the leafarea growth rate decreased by 49%. We introduce a dimensionlessindex (R) to quantify the degree to which environmental stressesalter the balance between production of photosynthates and theiruse for growth. The index, R, is equal to 1 when stress reducesgrowth and photosynthesis by the same degree, i.e. the balancebetween production and consumption of photosynthate is not disturbed.R is smaller than 1 when growth is reduced more than photosynthesis.R was equal to 1 for spring wheat where there was no osmoticadjustment. For sudangrass, R decreased from 1 to 0·25as osmotic potential decreased from -1·10 to -1·63MPa. These findings lead to the hypothesis that osmotic adjustmentcould result from an imbalance between production, consumptionand translocation of photosynthates under stressed conditions.Copyright1993, 1999 Academic Press Osmotic adjustment, water stress, root temperature     

Effects of root pruning on the growth and water use efficiency of winter wheat

A pot experiment was conducted to study the effects of root pruning at the stem elongation stage on the growth and water use efficiency (WUE) of winter wheat (Triticum aestivum). The results showed that stomatal conductance (g) and transpiration (E) of wheat were very sensitive to root pruning. After root pruning, they declined rapidly and but returned to pre-pruning values 15 days after treatment. Under well-watered conditions, there was no significant difference in leaf water potential (ψ_leaf) between root pruned and control plants after root pruning. Under moderate drought stress, ψ_leaf of root pruned plants declined significantly compared to the control 3 days after root pruning. After 15 days, ψ_leaf of root pruned plants was similar to the controls. Under different soil moisture levels, net assimilation rate (A) of root pruned plants was lower than controls 3–7 days after root pruning, but was similar to the controls 15 days after pruning. At anthesis (50 days after root pruning), root pruned plants showed significantly higher A compared with the control. Leaf area per tiller and tiller number of root pruning plants was significant lower than the control at booting stage, which showed that root pruning restrained the growth of plants in the early growing stage, but leaf area per stem, of root pruned plants, was similar to the control at anthesis. Under both soil moisture levels, there was no significant difference in grain yield between root pruned and the control plants in the monoculture. In mixture with the control plants, the root pruned plants was less productive and had a lower relative yield (0.92 and 0.78, respectively) compared with the control (1.13 and 1.19, respectively), which suggested that the pruned plants lost some of its competing ability and showed a lower ability to acquire and use the same resources in the mixture compared with the control plant. Over the whole growing cycle, root pruning reduced water consumption (by 10% under well-watered conditions and 16% under moderate drought stress) of wheat significantly compared to the control (P < 0.05), and but there was no significant difference in grain yield between root pruned and control plants. Therefore root pruned wheat had a higher WUE with respect to grain yield compared with the controls. In conclusion, lowering water consumption by root pruning in the early growing stage is an effective way to improve water use efficiency in arid and semi arid areas.     

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.     

The Effect of Grain Number per Ear (Sink Size) on Source Activity and its Water-Relations in Wheat

Blum, A., Mayer, J. and Golan, G. 1988. The effect of grainnumber per ear (sink size) on source activity and its water-relationsin wheat.–J. exp. Bot. 39: 106–114. Work was done to evaluate the nature of sink-source relationshipsin wheat (Triticum aestivum L.), when the strength of the sinkwas modified by the removal of half of the grain from the earat about anthesis. The main hypothesis was that sink-sourcerelationship would be modified by water stress and that a weakersink would improve the drought resistance of the source. Two experiments were performed. The first experiment evaluatedthe effect of de-graining in two wheat varieties grown in thefield. The second experiment (in the greenhouse) evaluated theeffect of de-graining in plants subjected to water stress afteranthesis by immersing the root system in a solution of polyethyleneglycol (6000), as compared with non-stressed controls. In bothexperiments measurements were performed after de-graining toprovide data on leaf gas exchange, leaf water potential, osmoticadjustment of leaves and ears (greenhouse), the percent of stemweight loss as an index of stem reserve mobilization, finalroot weight (greenhouse) and ear weight components. De-graining caused a decrease in flag leaf stomatal conductance,carbon exchange rate (CER) and transpiration and an increasein flag leaf water potential. These effects were stronger withwater stress. De-graining did not affect osmotic adjustmentin the flag leaf but induced better adjustment in glumes andawns. De-graining decreased the percent of stem weight lossand increased final root weight, especially under drought stress. A weaker sink was, therefore, considered to improve plant droughtresistance in terms of the maintenance of higher leaf waterpotential, a larger root, a better osmotic adjustment in theear and, possibly, increased flag leaf longevity. The ‘cost’of this improved drought resistance was in reduced flag leafCER and reduced stem (and root?) reserve mobilization. Key words: Drought resistance, carbon exchange rate, stomata, transpiration, osmotic adjustment, leaf water potential, root, awns, yield     

Influence of Saturation Deficit on Leaf Production and Expafision in Stands of Groundnut-{Arachis hypogaea L.) Grown Without Irrigation

The response of leaf area expansion to atmospheric saturationdeficit (SD) and soil moisture deficit was examined in termsof leaf water potential (₁) and turgor potential (_p), as partof a wider study of the effects of SD on groundnut growth. Standsof plants were grown at four levels of SD and without irrigationin controlled environment glasshouses. A fifth stand was grownat low SD on soil kept irrigated to field capacity. Large saturation deficits accelerated the depletion of soilmoisture reserves in the unirrigated stands and greatly reducedleaf area index, particularly in the driest treatment. Leafnumber per plant and leaf size both decreased as SD increased,but the effect on leaf size was greater than on number. SD hadless effect than soil water deficit on leaf production. Turgorpotential and leaf extension rate (R) were both reduced at highsaturation deficits and R was linearly related to _p between0900 and 1600 h. However, leaf extension rate and turgor potentialwere poorly correlated between 0400 and 0700 h in the driesttreatment. Arachis hypogaea L., groundnut, saturation deficit, leaf growth, canopy development     

Acclimation to Drought in Acer pseudoplatanus L. (Sycamore) Seedlings

A glasshouse experiment was conducted with well-watered andwater-stressed seedlings of sycamore (Acer pseudoplatanus L.)grown in soil columns. Water was withheld when the seedlingswere 82-d-old. Effects of soil drying on stomatal behaviour,water relations, xylem cavitation, and growth of leaves androots were evaluated. Stomatal conductance declined well before any observable changein bulk leaf water potentials, and was correlated with soilwater status. At seven weeks, osmotic potential had declinedby 0·51 MPa and 0·44 MPa at full and zero turgor,respectively. Drought significantly increased both bulk elasticmodulus and leaf dry weight to turgid weight ratio of water-stressedplants. Drought had no effect on relative water content at zeroturgor. Water cavitation in the xylem was detected as ultrasonic acousticemissions (AE). Water-stressed plants displayed significantlyhigher rates of AE than well-watered plants. Maximum rate ofAE coincided with the minimum level of stomatal conductanceand apparent rehydration of the leaves. Drought caused changes in the root distribution profile andit increased the root weight. The increase in root weight wasmainly due to a substantial shift in assimilates allocated infavour of roots with total biomass being unaffected. Leaf growthwas maintained for six weeks without any significant declinein expansion rate. However, the development of severe waterstress reduced both leaf production and expansion.     

Modulation of Competition between Fruits and Leaves by Flower Pruning and Water Fogging, and Consequences on Tomato Leaf and Fruit Growth

The effects of water fogging and reducing plant fruit load werestudied in a tomato crop grown in a glasshouse under Mediterraneansummer conditions. The objective of these treatments was toreduce competition between leaves and fruits for carbohydratesand water. Flower pruning increased plant leaf area and increasedfruit, stem, lamina and petiole dry mass (DM). This indicatesthat leaf area growth was limited during the summer due to competitionbetween fruits and leaves for assimilates. In contrast, reducingthe air vapour pressure deficit (VPD) by water fogging had noeffect on plant leaf area or aerial plant DM. Interestingly,there was a significant interaction between plant fruit loadand VPD: the higher the leaf[ratio]fruit ratio the greater theresponses to a reduction in VPD (increase in fruit DM, fruitdiameter, fruit and leaf expansion rate). The data suggest thatunder high fruit loads, water and carbohydrates limit growthunder Mediterranean summer conditions. However, reducing VPDwas not always sufficient to enhance fruit and leaf growth.This might be due to the lower leaf area under high fruit load.In contrast, reducing VPD under low fruit load triggered higherrates of leaf and fruit expansion; this is probably linked toa greater availability of water and carbohydrates. Copyright2001 Annals of Botany Company Assimilate competition, assimilate supply, flower pruning, fruit load, fruit growth, generative/vegetative growth, leaf growth, Lycopersicon esculentum, specific leaf weight, tomato, vapour pressure deficit, water stress     

Interactive Effects of Soil Nutrients, Moisture and Sand Burial on the Development, Physiology, Biomass and Fitness of Cakile edentula

The relative importance and interactive effects of nutrientsupply, soil moisture content and sand burial on the development,physiology, biomass allocation and fitness ofCakile edentulawere examined under controlled greenhouse conditions. Planttraits were more frequently affected by nutrient supply thanby soil moisture content or sand burial. Measurements on mostplant traits also varied depending on the two or three way interactionsamong the three environmental factors. Plants partially buriedby sand had higher leaf chlorophyll concentration than thoseunburied at the early stages of development, especially underlow soil moisture content. High nutrient supply tended to lowerthe leaf chlorophyll concentration of mature plants, and thiseffect was more pronounced under high as compared to low soilmoisture content. High nutrient supply enhanced the photosyntheticcapacity of plants when they were water stressed. With adequatesoil moisture, high nutrient supply increased/decreased thephotosynthetic capacity of plants with/without previous experienceof water stress. High nutrient supply increased the biomassallocation to the root system of plants, especially at low soilmoisture content. Partial sand burial also promoted biomassallocation to the root system of plants grown at low soil moisturecontent. Soil nutrition; water supply; sand accretion; multiple stresses; biomass allocation; Cakile edentula     

Effects of periodic flooding and root pruning on Quercus nuttallii seedlings

In the southern United States, much of the emphasis in bottomland restoration is placed on establishing an oak-dominated forest. Artificial regeneration is an alternative for restoration on cleared lands and where a desirable seed source is not present. Currently the standard procedure for seedling preparation is to prune the roots prior to transplanting in the field. It is not fully known what effect(s) root pruning has on transplanted seedlings. In addition, bottomland restoration efforts inherently take place on floodplains. The potential interaction between root pruning and flooding on seedling performance is not known. This study consisted of two separate but related laboratory experiments. The purpose of the first experiment was to quantify the effects of various percentages of root removal and varying soil moisture regimes on transplanted Nuttall oak seedlings (Quercus nuttallii Palmer). Root pruning treatments consisted of removal of roots at 0%, 25% and 75% while soil moisture regime was maintained at non-flooded or periodically flooded conditions. Plant gas exchange, growth, and survival were measured. Root pruning alone had adverse effects on height growth during the first 72 days following transplanting. Periodic flooding also produced adverse effects on stomatal conductance (p = 0.0002), height growth (p = 0.005), and survival (p = 0.02). Photosynthetic data indicated that as pruning intensified in the periodically flooded seedlings, photosynthetic rates decreased. In contrast, as pruning intensified in the non-flooded seedlings, photosynthesis increased. This demonstrated that pruning rate had a varying effect on photosynthesis dependent upon soil moisture condition. Experiment 2 focused on the effects of varying degrees of root pruning on new root formation. The seedlings were grown under laboratory conditions, harvested at 0, 10, 20, and 30 days after treatment initiation, and analyzed for new root formation. Results of Experiment 2 indicated no difference in new root formation, root length, or root biomass due to the pruning treatment. Overall, our results from both experiments indicated that root pruning had no detectable long-term adverse effects on growth and survival of seedlings under drained soil conditions; however, as results from Experiment 1 demonstrated, if seedlings were planted in periodically flooded conditions, root pruning produced adverse effects. Thus, in restoration efforts utilizing Nuttall oak seedlings, the planting strategy and pruning rate should be carefully evaluated based on the knowledge of sites' hydrology. Alternatively, on sites with unpredictable flooding both pruned and unpruned seedlings may be utilized to ensure survival.     

Water relations, gas exchange and abscisic acid content of Lupinus cosentinii leaves in response to drying different proportions of the root system

Soil columns in which the root system was divided into threeequal layers, each 24 cm in diameter and 33 cm high were usedto examine the influence of drying different proportions ofthe root system on the water relations, gas exchange and abscisicacid (ABA) concentration of lupin (Lupinus cosentinii Guss.cv. Eregulla) leaves. The treatments imposed were (i) all threelayers adequately watered (control), (ii) the upper layer unwateredwith the remaining layers kept adequately watered, (iii) thetwo upper layers unwatered with the basal layer kept adequatelywatered, (iv) all three layers unwatered. The treatments wereapplied at 56 d after sowing (DAS), and continued for 21 d inthe treatment in which the three layers were dried and for 36d in the other three treatments. After 21 d, the soil matricpotential in the layers that were unwatered had decreased toemdash 1.3MPa, compared to - 0.03 MPa in the adequately-wateredlayers. Within 8 d of cessation of watering, plants with the entireroot system in drying soil had significantly lower stomatalconductances, lower rates of net photosynthesis, and higherleaf ABA contents than did adequately-watered plants. Whilethe leaf osmotic potential decreased within 8 d of cessationof watering, the leaf water potential did not change for thefirst 15 d after water was withheld. After withholding waterfrom all layers, the shoot dry matter was 63% lower than thatin the adequately-watered plants. In the two partially-droughtedtreatments, 17% and 48% of the root length was subjected todrying. Compared to the adequately-watered plants, drying upto 50% of the root system for 36 d, in the two partially-droughtedtreatments, did not reduce stomatal conductance, net photosynthesis,or plant growth. Similarly, there was no significant effecton leaf water potential or osmotic potential. When either theupper or upper and middle layers of soil were dried, the ABAcontent of the leaves for most of the drying period was slightly,but not significantly, higher than in leaves of the adequately-wateredplants. The results suggest that lupins with a well-established rootsystem can utilize localized supplies of available soil waterto maintain leaf gas exchange despite appreciable portions ofthe root system being in dry soil. In contrast to other studies,the results also suggest that when only a portion of the soilvolume is dry and adequate water is available in the wet zone,root signals do not influence stomatal conductance and leafgas exchange of lupin. Key words: Abscisic acid, gas exchange, lupins, split-roots, water deficit     

The Effect of Wind and a Reduced Supply of Water on the Growth and Water Relations of Festuca arundinacea Schreb

Festuca arundinacea was grown at high and low wind-speeds attwo levels of soil water. Transpiration was increased at highwind-speed and accompanied by leaf water stress. Growth of leafarea was progressively reduced according to the severity ofthe experimental treatments in the sequence: wet soil and lowwind; dry soil and low wind; wet soil and high wind; dry soiland high wind. The leaf water potential was also reduced inthis sequence. Festuca arundinacea Schreb., transpiration, water stress, wind, water potential     

The gradient between pre-dawn rhizoplane and bulk soil matric potentials, and its relation to the pre-dawn root and leaf water potentials of four species

Uptake of soil water by plants may result in significant gradients between bulk soil and soil in the vicinity of roots. Few experimental studies of water potential gradients in close proximity to roots, and no studies on the relationship of water potential gradients to the root and leaf water potentials, have been conducted. The occurrence and importance of pre-dawn gradients in the soil and their relation to the pre-dawn root and leaf water potentials were investigated with seedlings of four species. Pre-germinated seeds were grown without watering for 7 and lid in a silt loam soil with initial soil matric potentials of -0.02, -0.1 and -0.22 MPa. Significant gradients, independent of the species, were observed only at pre-dawn soil matric potentials lower than -0.25 MPa; the initial soil matric potentials were -0.1 MPa. At an initial bulk soil matric potential of -0.22 MPa, a steep gradient between bulk and rhizoplane soil was observed after 7 d for maize (Zea mays L. cv. Issa) and sunflower (Helianthus annuus L. cv. Nanus), in contrast to barley (Hordeum vulgare L. cv. Athos) and wheat (Triticum aestivum L. cv. Kolibri). Pre-dawn root water potentials were usually about the same as the bulk soil matric potential and were higher than the rhizoplane soil matric potential. Pre-dawn root and leaf water potentials tended to be much higher than rhizoplane soil matric potentials when the latter were lower than -0.5 MPa. It is concluded that plants tend to become equilibrated overnight with the wetter bulk soil or with wetter zones in the bulk soil. Plants can thus circumvent negative effects of localized steep pre-dawn soil matric potential gradients. This may be of considerable importance for water uptake and growth in drying soil.     

Effects of Water and Nutrient Availability on Water Relations, Gas Exchange and Growth Rate of Mature Plants and Resprouts of Arbutus unedo L.

This study examines the effects of water supply and nutritionon the water status, gas exchange and growth of mature plantsand resprouts of Arbutus unedo, a Mediterranean evergreen shrubadapted to drought and poor nutrition. Mature plants of A. unedorespond to irrigation with increased leaf water potential duringsummer drought, but they show a very conservative use of waterand they do not increase leaf conductance. There is also a verysmall increase in net photosynthesis and growth, which doesnot significantly increase productivity. Resprouts of A. unedo increase water potential, leaf conductance,transpiration rate, net photosynthesis and growth rate in responseto watering, showing a less conservative use of water than matureplants. Increased growth rates, both in mature plants and resprouts,are likely to be due to the higher cell turgor caused by improvedleaf water potential, rather than to increased photosynthesis. The only effect of nutrient addition on mature plants is anincrease in leaf nutrient content, and other aspects of thephysiology and growth of resprouts were unaffected. We thereforeconclude that water is a more limiting factor than nutrientsfor mature plants and resprouts of A. unedo growing in the studyarea. These results support previous data which indicate thathigher growth rates in resprouts than in mature plants of A.unedo are mainly the result of a higher water availability.Copyright1994, 1999 Academic Press Arbutus unedo L., strawberry tree, resprouts, water stress, nutrient availability, water relations, gas exchange, growth rate, regeneration     

Influence of temperature and water potential on root growth of white oak

Root growth of white oak ( Quercus alba L.) was observed under field conditions using a rhizotron. The effects of temperature, soil water potential, and leaf water potential were evaluated on three measures of root growth and development: root elongation rate, number of growing roots, and root growth intensity (sum of projected root area compared to the total root viewing area). Root elongation rate was linearly related to changes in soil temperature and soil water potential. At soil temperatures less than 17deg;C, temperature was the dominant factor affecting rate of growth, bat at temperatures greater than 17°C soil water potential became the important factor. Unlike root elongation rate, the number of growing roots and root growth intensity increased at cold soil temperatures (8°C) and at soil water potentials of-0.3 to -0.8 MPa. At high soil water potentials (-0.1 MPa) root elongation rate reached a maximum while the number of growing roots and root growth intensity were low. These differences showed that root growth and development were not exclusively affected by the soil environment. In addition, the relationship between root growth and predawn leaf water potential suggested that root growth was a contributing factor to the drought resistance of white oak.     

不同土壤水分条件下核桃的生理生态特性研究

采用完全随机设计试验方法对不同土壤水分条件下核桃生理生态特性进行研究.结果表明,土壤贮水量、耗水量、水分盈亏值均与灌水量成正比.轻剪、中剪和重剪可分别提高土壤贮水量0.2％、0.5％和0.9％,耗水量-0.5％、-2.0％和-2.5％以及水分盈亏值的1.5、1.9和2.1倍.灌水、覆草、覆膜、修剪处理下的土壤贮水量分别提高了4.4％、1.2％、1.6％和0.5％.灌水(225 kg,4月1日)、中剪、覆草、覆膜处理对增大小叶夹角、提高叶水势效果明显,重剪处理使叶绿素含量增加幅度最大（0.27 mg·dm^-2）.小叶夹角与叶水势呈正相关关系.以小叶夹角作为评估核桃树体水分状况及其立地土壤水分含量状况指标,具有可靠、直观、及时和简单等特点,并具有较高的应用价值.