Leaf and root growth dynamics in Eucalyptus globulus seedlings grown in drying soil

Summary Seedlings of Eucalyptus globulus growing in soil columns were subjected to a 24 day soil drying treatment. Water and solute potentials of both young expanding and fully expanded leaves declined under reduced soil water availability, while slightly higher turgor was sustained by the fully expanded leaves. Although leaf area of unwatered seedlings was smaller, the corresponding leaf dry weight was quite similar to that of well-watered seedlings. Soon after rewatering, leaf area of plants experiencing water shortage was comparable to that of well-watered plants. It seems that a difference in wall properties between juvenile and mature leaves allows for an effective pattern of water use by eucalypt plants growing in drying soil. Some stomatal opening is sustained and therefore, presumably, some carbon may be fixed, keeping the carbon balance of the whole plant positive, and allowing a continuous cell division despite the limited water supply. The highest root density of both well-watered and unwatered plants was found in the upper soil layers. However, root growth of unwatered seedlings was gradually increased in the deeper soil layers, where thicker root apices and higher soil water depletion rates per unit root length were recorded. As a consequence, root absorbing surface area was as large in unwatered plants as in well-watered plants.     

Solute regulation and growth by roots and shoots of water-stressed maize plants

Potted maize seedlings were subjected to a single period of water stress. As the severity of water stress increased, measurements were made of leaf and root solute and water potentials, leaf diffusive conductance and leaf and root growth. After day four of the drying cycle, the rate of leaf extension and the development of leaf area were reduced. This reduction correlated well with a reduction in leaf turgor which occurred at this time. A significant accumulation of solutes in the root tips of the unwatered plants resulted in the maintenance of root turgor for the duration of the water stress treatment. Root growth of the unwatered plants was also maintained as the severity of water stress increased. A mild degree of water stress resulted in a net increase in root growth compared to the situation in well-watered plants. The significance of solute regulation and continued root growth for plants growing in drying soil is discussed.Abbreviations PAR photosynthetically active radiation - MPa mega pascat     

Solute accumulation in leaves and roots of woody plants subjected to water stress

Summary Young seedlings of English Oak, Quercus robur L., and Silver Birch, Betula verrucosa Ehrl., were subjected to a number of consecutive periods during which water was withheld. During one 14-day period leaf-and soil-water potentials and leaf- and root-solute potentials of two groups of plants were sampled at noon of each day. One group of plants was watered every day while water was withheld from the other group. Solute accumulation in roots and leaves of oak seedlings subjected to water stress resulted in maintenance of turgor and high leaf conductance as the soil dried. In birch seedlings turgor was only maintained by stomatal closure at high soil water potential.Fourteen consecutive water stress cycles greatly reduced the growth of birch seedlings but had little effect on oak seedlings other than to alter root morphology. Water stress treatment resulted in the production of long thin roots in this plant. Stomatal behaviour in oak and birch seedlings during the 14-week stress period was consistent with observed changes in leaf water and solute potentials. Daily solute accumulation in oak leaves was presumably responsible for the maintenance of plant growth as water potentials fell.     

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.     

Abscisic acid produced in dehydrating roots may enable the plant to measure the water status of the soil

Abstract. Maize plants were grown in 1-m-long tubes of John Innes No. 2 potting compost. From the start of the experimental period, half of the plants were unwatered. Stomatal conductance of these plants was restricted 6 d after last watering and continued to decline thereafter. This was despite the fact that as a result of solute accumulation, unwatered plants showed consistently higher leaf turgors than well-watered plants. Leaf water potentials of unwatered plants were not significantly lower than those of plants that were watered well. Main seminal and nodal roots showed solute regulation in drying soil and continued to grow even in the driest soil, and plants growing in drying soil showed consistently higher root dry weights than did well-watered plants, water potentials and turgors of the tips of fine roots in the upper part of the column decreased as the soil dried. Soil drying below a water content of around 0–25 g cm⁻³ (a bulk soil water potential of between -0.2 and -0.3 MPa) resulted in a substantial increase in the ABA content of roots. As soil columns dried progressively from the top, ABA content increased in roots deeper and deeper in the soil. These responses suggest that ABA produced by dehydrating roots and which was subsequently transported to the shoots provided a sensitive indication of the degree of soil drying.     

Farnesol-like antitranspirant activity and stomatal behaviour in maize and Sorghum lines of differing drought tolerance

Abstract Potted seedlings of four lines of maize and Sorghum of differing drought tolerance were subjected to a single soil drying cycle and were only rewatered when the plants showed the first signs of wilting. Other plants remained well-watered throughout the experimental period. As plant water potentials decreased in the unwatered plants of three of the lines investigated (Sorghum Piper and M35-1, V-4146 and maize Farz 27), endogenous levels of farnesol-like antitranspirants increased. Closure of stomata correlated well with the increase in endogenous antitranspirant. In the fourth line (Sorghum M35-1, V-4184), stomata did not close as the level of plant water stress increased, although leaf diffusion resistance of even the well-watered plants of this line was quite high. In this line, there was no consistent relationship between plant water stress and antitranspirant level or between stomatal behaviour and antitranspirant level. The involvement of farnesol-like antitranspirants in the control of stomatal behaviour in water-stressed plants is discussed.     

Changes in the concentration of ABA in xylem sap as a function of changing soil water status can account for changes in leaf conductance and growth

Abstract. Maize seedlings ( Zea mays L. John Innes F1 hybrid) were grown in a greenhouse in l-m-long tubes of soil. When the plants were well established, water was withheld from half of the tubes. Control plants were watered every day during the 20-d experimental period. The soil drying treatment resulted in a substantial restriction of stomatal conductance and a limitation in shoot growth, even though there was no detectable difference in the water relations of watered and unwatered plants. From day 7 of the soil drying treatment, xylem ABA concentrations (measured using the sap exuded from detopped plants) were substantially increased in unwatered plants compared to values recorded with sap from plants watered every day. Measurements of water potential through the profile of unwatered soil suggest that xylem ABA concentrations reflects the extent of soil drying. Leaf ABA content was a much less sensitive indicator of the effect of soil drying and during the whole of experimental period there was no significant difference between ABA concentration in leaves of well watered and unwatered plants. In a second set of experiments, ABA was fed to part of the roots of potted maize plants to manipulate xylem ABA concentration. These manipulations suggested that the increases in ABA concentration in xylem sap, which resulted from soil drying, were adequate to explain the observed variation in stomatal conductance and might also explain the restriction in leaf growth rate. These results are discussed in the light of recent work which suggests that stomatal responses to soil drying are partly attributable to an as-yet unidentified inhibitor of stomatal opening.     

在旱化土壤条件下来自不同生境的20种植物幼苗根系的形态反应的比较(英文)

在浇水和未浇水的塑料管中栽培了 2 0个植物种 ,测量了其幼苗的根深、根重和茎重。这些种的原始生境含水状况差异较大 ,是从沼泽到沙漠的系列。植物种原始生境的水分状况用Ellenberg水分序数定量。幼苗首先在湿沙中生长 2 1d ,然后进入为期也是 2 1d的处理阶段 (浇水和不浇水 )。浇水植株的根深与Ellenberg水分序数无关。在旱化的沙层中 ,源于干旱生境的植物的根深趋向于增加 ,来自湿润生境的则减少。根深塑性 (即未浇水的根深 /浇水的根深 )与Ellenberg水分序数显著相关 (R2 =0 .5 6 ) ,茎 /根比值塑性也与Ellenberg水分序数相关 ,但不如根深塑性的关系明显。根深塑性表现最为明显的植物种具有在未浇水沙层中维持茎生长的最大能力。有迹象表明 ,在浇水处理时 ,来自很干旱生境的植物生长减弱。研究结果表明 :幼苗利用深层水分的能力是植物对干旱生境的主要适应。     

Root Growth and Water Uptake by Maize Plants in Drying Soil

Sharp, R. E and Da vies, W. J. 1985. Root growth and water uptakeby maize plants in drying soil.— J. exp. Bot. 36: 1441–1456. The influence of soil drying on maize (Zea mays L.) root distributionand use of soil water was examined using plants growing in thegreenhouse in soil columns. The roots of plants which were wateredwell throughout the 18 d experimental period penetrated thesoil profile to a depth of 60 cm while the greatest percentageof total root length was between 20–40 cm. High soil waterdepletion rates corresponded with these high root densities.Withholding water greatly restricted root proliferation in theupper part of the profile, but resulted in deeper penetrationand higher soil water depletion rates at depth, compared withthe well watered columns. The deep roots of the unwatered plantsexhibited very high soil water depletion rates per unit rootlength. Key words: Maize, roots, water deficit, soil water depletion     

The Response of Leaf Water Potential and Crassulacean Acid Metabolism to Prolonged Drought in Sedum rubrotinctum

Plants of Sedum rubrotinctum R. T. Clausen were studied in a green-house over a 2-year period without watering. Only the apical leaves survived and were turgid at the end of the experiment. The midday leaf water potential of these apical leaves was −1.20 megapascals, while the leaf water potential of comparable leaves on well-watered control plants was −0.20 megapascals. The unwatered plants appear to have maintained turgor by means of an osmotic adjustment. After 2 years without water the plants no longer exhibited a nocturnal accumulation of titratable acidity. However, the daytime levels of titratable acidity of the unwatered plants were more than 2-fold greater than the levels in well-watered control plants. Well-watered plants of S. rubrotinctum exhibited seasonal shifts in biomass stble carbon isotope ratios, indicating a greater proportion of day versus night CO₂ uptake in the winter than in the summer. The imposition of water stress prevented the expression of this seasonal rhythm and restricted the plants to dark CO₂ uptake.     

Non-hydraulic regulation of fruit growth in tomato plants (Lycopersicon esculentum cv. Solairo) growing in drying soil

Tomato (Lycopersicon esculentum cv. Solairo) fruit growth, fruit mesocarp and leaf epidermal cell turgor, and fruit and leaf sub-epidermal apoplastic pH were monitored as plants were allowed to dry the soil in which they were rooted. Soil drying regimes involved splitting the root system of plants between two halves of a single pot separated by a solid impervious membrane to form a split-root system. Plants were then allowed to dry the soil in both halves of the pot (a soil-drying (SD) treatment) or water was supplied to one-half of the pot (a partial root-drying (PRD) treatment), allowing only one-half of the root system to dry the soil. A well-watered control treatment watered the soil on both halves of the pot. The rate of fruit growth was highly correlated with the soil water content of both sides of the SD treatment and the dry side of the PRD treatment. Soil drying caused a significant restriction in fruit growth rate, which was independent of any changes in the turgor of expanding fruit mesocarp cells in the PRD treatment. By supplying water to half of the root system, the turgors of mesocarp cells were maintained at values above those recorded in well-watered controls. The turgor of leaf epidermal cells exhibited a similar response. The pH of the sub-epidermal apoplastic compartment in leaves and fruit increased with soil drying. The dynamics of this increase in leaves and fruit were identical, suggesting free transport of this signal from shoot to fruit. Fruit growth rate and sub-epidermal pH within the fruit showed a strong correlation. The similarity of fruit growth response in the SD and PRD treatment, suggests that tomato plants respond to a discrete measure of soil water status and do not integrate measures to determine total soil water availability. The results of this study are not consistent with Lockhartian models of growth regulation in expanding fruit of a higher plant. A non-hydraulic, chemical-based signalling control of fruit growth in plants growing in drying soil is proposed.     

Responses of cowpeas (Vigna unguiculata (L.) Walp.) to progressive soil drought

Summary Greenhouse-grown cowpeas, Vigna unguiculata (L.) Walp., were subjected either to well-watered or to progressive soil drought conditions between 10–40 days after emergence. Stomatal closure was found to correlate with the progressive drying of soil while leaf water potentials were not very different from the well-watered plants. Reduction in leaf turgor resulted in a reduced rate of leaf extension but increased that of root. Stomatal conductance and transpiration rates of soil-drought plants were similar to well-watered plants in the morning, but were greatly reduced in the afternoon till evening. It is suggested that the maintenance of transpiration rates per unit leaf area of soil-drought cowpeas in the morning is due to the reduction in the leaf area per plant and possibly the hydration of the plants in the night through enhanced root growth.     

Effect of water stress on leaf and root growth,and water uptake ofGmelina arborea ROXB. seedlings

Young seedlings ofGmelina arborea Roxb. were subjected to 2 weeks of drought. Despite the gradual reduction in stomatal conductance, leaf and root growth was not affected until the later part of the stress period. This was attributed to solute adjustment in the roots of the plants. As the severity of water stress increased, root growth was prolific in all the soil segments. As a result, water in the lowest soil segment was used to maintain plant turgor, which in turn sustains the leaf and root growth during the water-stress treatment. The influence of soil water content and soil water potential upon soil water uptake rate was also evaluated on soil profile basis. Rates of extraction began to decline in all soil segments as soon as soil water potential fell below -0.06 MPa, presumably as a result of vapour gaps between the root and soil (root: soil interface resistance). It is suggested that the growth of roots ofGmelina plants away from drying soil will minimize the resistance to water uptake.     

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.     

Water relations and root activities of Buchloe dactyloides and Zoysia japonica in response to localized soil drying

Effects of localized soil drought stress on water relations, root growth, and nutrient uptake were examined in drought tolerant ‘Prairie’ buffalograss [Buchloe dactyloides (Nutt.) Engelm.] and sensitive ‘Meyer’ zoysiagrass (Zoysia japonica Steud.). Grasses were grown in small rhizotrons in a greenhouse and subjected to three soil moisture regimes: (1) watering the entire 80-cm soil profile (well-watered control); (2) drying 0–40 cm soil and watering the lower 40 cm (partially dried); (3) and drying the entire soil profile (fully dried). Drying the 0–40 cm soil for 28 days had no effect on leaf water potential (Ψ _leaf ) in Prairie buffalograss compared to the well-watered control but reduced that in Meyer zoysiagrass. Root elongation rate was greater for Prairie buffalograss than Meyer zoysiagrass under well-watered or fully dried conditions. Rooting depth increased with surface soil drying; with Prairie buffalograss having a larger proportion of roots in the lower 40 cm than Meyer zoysiagrass. The higher rates of water uptake in the deeper soil profile in the partially dried compared to the well-watered treatment and by Prairie buffalograss compared to Meyer zoysiagrass could be due to differences in root distribution. Root ¹⁵N uptake for Prairie buffalograss was higher in 0–20 cm drying soil in the partially dried treatment than in the fully dried treatment. Diurnal fluctuations in soil water content in the upper 20 cm of soil when the lower 40 cm were well-watered indicated water efflux from the deeper roots to the drying surface soil. This could help sustain root growth, maintain nutrient uptake in the upper drying soil layer, and prolong turfgrass growth under localized drying conditions, especially for the deep-rooted Prairie buffalograss. This revised version was published online in June 2006 with corrections to the Cover Date.     

在旱化土壤条件下来自不同生境的20种植物幼苗 根系的形态反应的比较

Rooting depth and root and shoot biomass were measured for seedlings of 20 species in both watered and unwatered sand columns. The species were from habitats of widely varying moisture status, ranging from marsh to desert. Moisture status of the species' habitats was quantified as Ellenberg moisture number. Seedlings were allowed to grow in moist sand for 21 days and were then exposed to the treatments (watered and unwatered) for a further 21 days. Rooting depth of control plants was not correlated with Ellenberg number. Riit depth of plants from dry habitats tended to increase in drying sand, while roots of plants from wet habitats decreased in depth. Plasticity of rooting depth (depth in unwatered/depth in watered sand) was significantly correlated with Ellenberg number (r2=0.56). Plasticity of shoot/root ratio was also correlated with Ellenberg number, but the relationship was weaker than for rooting depth plasticity. Species that showed the greatest pasticity in rooting depth also showed the greatest ability to sustain shoot growth in unwatered sand. There was some evidence that growth of plants from very dry habitats was reduced in the watered treatment. Results of this study suggest that a major, although not the only, adaptation of plants of dry habitats is the ability of their seedlings to exploit deeply buried water resources.     

Genotypic variation in tissue water relations of leaves and roots of black walnut (Juglans nigra) seedlings

Genetic variation in the drought response of leaf and root tissue water relations of seedlings of eight sources of black walnut ( Juglans nigra L.) was investigated using the pressure-volume technique. Tissue water relations were characterized at three stages of a drying cycle during which well-watered plants were allowed to desiccate and then were reirrigated.
Sources varied both in the capacity for and degree of leaf and root osmotic adjustment, and in the mechanism by which it was achieved. A decrease in osmotic potential at the turgor loss point (ψ_πp) of 0.4 MPa was attributable to increased leaf tissue elasticity in seedlings of four sources, while seedlings of an Ontario source exhibited a 0.7–0.8 MPa decline in ψ_πp as a result of both increased solute content and increased leaf tissue elasticity. Seedlings of a New York source showed no detectable osmotic adjustment.
In roots, decreased ψ_πp (osmotic potential at full hydration) and ψ_πp were observed under drought. Sources that exhibited significant leaf osmotic adjustment also generally showed a similar response in roots. Tissue elasticity and ψ_πp of roots were higher than those of shoots, whereas ψ_πp of the two organs was similar for most sources. Because of greater elasticity, roots exhibited a more gradual decline in turgor and total water potential than did leaves as tissue relative water content decreased.     

Effects of soil water level on the development of adult plant resistance to powdery mildew in barley

Barley grown in dry soil developed greater adult plant resistance (APR) to powdery mildew (Erysiphe graminis DC. f. sp. hordei Mérat) than barley grown in wet soil. Conidial germination and appressorium formation were less, and fungal development between formation of appressoria and elongating secondary hyphae on upper leaves was inhibited, when adult plants were grown in dry soil. Mildew colonies expanded more slowly on leaves of adult plants than on leaves of seedlings, especially if adult plants had grown in dry soil. APR was reduced if plants, previously grown in dry soil, were well watered more than 32 h before inoculation. Conidia originating from plants grown in dry soil had a lower solute potential and greater ability to infect plants grown in dry but not wet soil than conidia originating from plants grown in wet soil. APR could not be attributed simply to increased cell wall or cuticle thickness, nor to lowered leaf solute potentials, as has sometimes been suggested for powdery mildew diseases. Increasing plant age and water stress induced increases in cell wall and cuticle thickness, but these changes did not always coincide with changes in disease resistance. Increasing plant age and water stress also lowered leaf solute potentials but fungal solute potentials were lower than leaf solute potentials and, more importantly, were lower than leaf water potentials. Thus, fungal growth was not limited by the availability of water from the host during penetration and hyphal establishment. It is suggested that resistance levels may be determined not by the thickness of epidermal structures, nor by lowering of solute potential per se, but by specific substances harmful to the fungus which accumulate in either cell wall, cuticle or sap, and whose concentration is dependent on the age and water stress of leaves.     

Defense characteristics of seral deciduous broad-leaved tree seedlings grown under differing levels of CO2 and nitrogen

Mixed broad-leaved forests are abundantly populated by several kinds of herbivore species in northern Japan. The life of herbivores depends strongly on the quality of food leaves. Leaf quality is changing with increasing atmospheric CO₂ and nitrogen deposition. Four seral species of deciduous broad-leaved tree seedlings (alder, birch, oak and maple) were raised in all four combinations of two levels of CO₂ and two levels of nutrient, to examine foliar defense traits. To evaluate the relative defense capacity of the four tree species, we used wild silkworms (Erisan; a generalist herbivore) as a bioassay material. Except with alder, the survival rate and longevity (ML₅₀) of the silkworms were least when they were fed with leaves of seedlings raised under high CO₂ and infertile soil conditions, and longest on a diet of leaves grown in ambient CO₂ and fertile soil, especially in birch and maple. The longevity of Erisan decreased in the order birch, oak, and maple. The longevity of Erisan fed with alder leaves was independent of CO₂ levels and was longer on alder seedlings grown in infertile soil conditions. Alder is an actinorhizal plant that can fix atmospheric nitrogen in root nodules formed by the actinomycetes Frankia sp. The activity of symbiotic microbes would have been enhanced by the greater amount of photosynthates received from the host plants at high CO₂, improving the food quality for the silkworms. It was concluded that in all but alder, leaf chemical traits, especially C/N ratio, affect the ML₅₀ of Erisan larvae.     

Effects of zinc and soil moisture on photosynthetic rate and chlorophyll fluorescence parameters of maize

Effects of zinc [0 and 5.0 mg Zn kg⁻¹ (soil)] on photosynthetic rate (P_N), and chlorophyll fluorescence in leaves of maize (Zea mays L.) cv. Zhongdan 9409 seedlings grown under different soil moisture regimes (40–45 % and 70–75 % of soil saturated water content) were studied. Zn application did not enhance maize plant adaptation to drought stress. The relative water content and the water potential of leaves were not affected by Zn treatment. Moreover, The P_N of drought-stressed plants was not improved by Zn supply. The increases of plant biomass, stomatal conductance and quantum yield of photosystem 2 due to Zn addition were notable in well-watered plants.