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     

Root growth and water relations of oak and birch seedlings

Summary First year seedlings of English oak (Quercus Cobur) and silver birch (Betula pendula) were subjected to pressure-volume analysis to investigate the water potential components and cell wall properties of single leaves. It was hoped that this rapid-drying technique would differentiate between reductions in plant solute potential resulting from dehydration and the effects of solute accumulation.Comparison of results from these experiments with those of slow drying treatments (over a number of days) with plants growing in tubes of soil, indicated that some solute accumulation may have occurred in drying oak leaves. High leaf turgor and leaf conductance were maintained for a significant period of the drying cycle. Roots of well-watered oak plants extended deep into the soil profile, and possibly as a result of solute regulation and therefore turgor maintenance, root growth of unwatered plants was greater than that of their well-watered counterparts. This was particularly the case deep in the profile. As a result of deep root penetration, water deep in the soil core was used by oak plants to maintain plant turgor, and quite low soil water potentials were recorded in the lower soil segments.Root growth of well-watered birch seedlings was prolific but roots of both well-watered and unwatered plants were restricted to the upper part of the profile. Root growth of unwatered plants was reduced despite the existence of high soil water potentials deep in the profile. Shallow rooting birch seedlings were unable to use this water.Pressure-volume analysis indicated that significant reductions of water potential, which are required for water uptake from drying soil, would occur in oak with only a small reduction in plant water content compared to the situation in birch. This was a result of the low solute potential in oak leaves combined with a high modulus of elasticity of cell walls. Deep rooting of oak seedlings, combined with these characteristics, which will be particularly important when soil deep in the profile begins to dry, mean that this species may be comparatively successful when growing on dry sites.     

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.     

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     

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.     

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.     

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.     

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.     

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.     

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.     

Sequential response of whole plant water relations to prolonged soil drying and the involvement of xylem sap ABA in the regulation of stomatal behaviour of sunflower plants

Sunflower plants ( Helianihus animus cv. Tall Single Yellow} were grown in the greenhouse in drain pipes (100 mm inside diameter and 1 m long) rilled with John Innes No. 2 compost. When the fifth leaf had emerged, half of the plants were left unwatered for 6 days, rewatered for 2 days and then not watered for another 12 days. Measurements of water relations and abaxial stomatal conductance were made at each leaf position at regular intervals during the experimental period. Estimates were also made of soil water potentials along the soil profile and of ABA concentrations in xylem sap and leaves.
Soil drying led to some reduction in stomatal conductance alter only 3 days but leaf turgors were not reduced until day 13 (6 days after rewatering). When the water relations of leaves did change, older leases became substantially dehydrated while high turgors were recorded in younger leaves. Leaf ABA content measured on the third youngest leaf hardly changed over the first 13 days of the experiment, despite substantial soil drying, while xylem ABA concentrations changed very significantly and dynamically as soil water status varied, even when there was no effect of soil drying on leaf water relations. We argue that the highest ABA concentrations in the xylem, found as a result of substantial soil drying, arise from synthesis in both the roots and the older leaves, and act to delay the development of water deficit in younger leases.
In other experiments ABA solutions were watered on to the root systems of sunflower plants to increase ABA concentrations in xylem sap. The stomatal response to applied ABA was quantitatively very similar to that to ABA generated as a result of soil drying. There was a log-linear relationship between the reduction of leaf conductance and the increase of ABA concentration m xylem sap.     

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     

土壤结构与光照水平对秋茄某些生长和生理参数的影响

在香港的３个红树森样地即黄竹湾（沙土）、西径（沙壤土）和米埔（粘壤土）进行了土壤结构对秋茄（Kandelia candel(L.)Durce)生长和生理影响的研究,并在米埔比较了林内和林外秋茄幼苗的生长和生理参数以观察光照水平的效应。在沙土和沙壤土生长的１．５年秋茄幼苗比粘壤土具有较粗的基径的较高的总生物量,说明秋茄幼苗在沙土和沙壤土中比在粘壤土中生长更好。沙土１．５ 茄幼苗的叶片厚度分别为沙壤土和粘壤土的１．７５和２．０５倍,表明沙土中的秋茄幼苗具有旱生结构以维持体内水分。然而,沙土和沙壤土４．５年秋茄幼树的叶片厚度无显著差异,沙土和沙壤土中１．５年秋茄幼苗分配于根系的生物量比例约为５０％,高于粘壤土的值（约４０％）。沙土和沙壤土中１．５年的秋茄比粘壤土具有较低的叶绿素含量、根系活力、硝酸盐还原酶活性、过氧化物酶（ＰＯＸ）活性、超氧化物歧化酶（ＳＯＤ）活性及较高的丙二醛（ＭＤＡ）含量。米埔１．５年秋茄幼苗在红树林外比林内有更好的长势,具有更大的叶面积、特殊叶面积、叶片数量及生物量。林内幼苗具有较高叶绿素含量,较低叶绿素a/b比值,较高硝酸盐还原酶活性和较强的根系活力,林外幼苗的叶片ＰＯＸ和ＳＯＤ活性比林内的值稍高,ＭＤＡ含量比林内显著要高。     

Photosynthesis,respiration, and carbon allocation of two cool-season perennial grasses in response to surface soil drying

The study was conducted to investigate carbon metabolic responses to surface soil drying for cool-season grasses. Kentucky bluegrass (Poa pratensis L.) and tall fescue (Festuca arundinaceae Schreb.) were grown in a greenhouse in split tubes consisting of two sections. Plants were subjected to three soil moisture regimes: (1) well-watered control; (2) drying of upper 20-cm soil (upper drying); and (3) drying of whole 40-cm soil profile (full drying). Upper drying for 30 d had no dramatic effects on leaf water potential (Ψ_leaf) and canopy photosynthetic rate (P_n) in either grass species compared to the well-watered control, but it reduced canopy respiration rate (R_canopy) and root respiration rate in the top 20 cm of soil (R_top). For both species in the lower 20 cm of wet soil, root respiration rates (R_bottom) were similar to the control levels, and carbon allocation to roots increased with the upper soil drying, particularly for tall fescue. The proportion of roots decreased in the 0-20 cm drying soil, but increased in the lower 20 cm wet soil for both grass species; the increase was greater for tall fescue. The Ψ_leaf, P_n, R_canopy, R_top, R_bottom, and carbon allocation to roots in both soil layers were all significantly higher for upper dried plants than for fully dried plants of both grass species. The reductions in R_canopy and R_top in surface drying soil and increases in root respiration and carbon allocation to roots in lower wet soil could help these grasses cope with surface-soil drought stress. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cytokinin producing bacteria enhance plant growth in drying soil

Cytokinins can promote stomatal opening, stimulate shoot growth and decrease root growth. When soil is drying, natural cytokinin concentrations decrease in association with stomatal closure and a redirection of growth away from the shoots to the roots. We asked if decreased cytokinin concentrations mediate these adaptive responses by lessening water loss and promoting root growth thereby favouring exploration for soil water. Our approach was to follow the consequences for 12-d-old lettuce seedlings of inoculating the growing medium with cytokinin-producing bacteria under conditions of water sufficiency and deficit. Inoculation increased shoot cytokinins as assessed by immunoassay and mass spectrometry. Inoculation also promoted the accumulation of shoot mass and shortened roots while having a smaller effect on root mass. Inoculation did not raise stomatal conductance. The possible promoting effect of these cytokinins on stomatal conductance was seemingly hampered by increases in shoot ABA that inoculation also induced. Inoculation lowered root/shoot ratios by stimulating shoot growth. The effect was greater in non-droughted plants but remained sufficiently strong for shoot mass of inoculated droughted plants to exceed that of well-watered non-inoculated plants. We conclude that compensating for the loss of natural cytokinins in droughted plants interferes with the suppression of shoot growth and the enhancement of root elongation normally seen in droughted plants.     

Abscisic Acid is not the only stomatal inhibitor in the transpiration stream of wheat plants

Xylem sap was collected from the transpiration stream of wheat (Triticum aestivum L.) plants and assayed for the presence of an inhibitor of transpiration using leaves detached from well-watered plants. Transpiration of detached leaves was reduced by nearly 60% by sap collected from plants in drying soil, and to a lesser extent (about 25%) by sap from plants in well-watered soil. As the soil dried the abscisic acid (ABA) concentration in the sap increased by about 50 times to 5 × 10⁻⁸ molar. However, the ABA in the sap did not cause its inhibitory activity. Synthetic ABA of one hundred times this concentration was needed to reduce transpiration rates of detached leaves to the same extent. Furthermore, inhibitory activity of the sap was retained after its passage through an immunoaffinity column to remove ABA. Xylem sap was also collected by applying pressure to the roots of plants whose leaf water status was kept high as the soil dried. Sap collected from these plants reduced transpiration to a lesser extent than sap from nonpressurised plants. This suggests that the inhibitory activity was triggered partly by leaf water deficit and partly by root water deficit.     

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     

根系分区灌溉和水分利用效率

根系分区灌溉是指仅仅部分根系受到正常的灌溉,其余根系则受到人为的干旱,两项理论根据指出这种措施可减少植物的水分肖耗,并保持一定的生物产量,其一是植物蒸腾失水与气孔导性是线性关系,而光合作用与气孔导性则是一种渐趋饱和的关系,如果气孔导性从最大值适应调低,可显著降低蒸腾,但对光合影响应小得多,其二是处于干燥土壤中的根系可感觉干旱,产生干旱信号来调节地上部分的气孔开度,显然,这项措施在田间有多大效用值得深入研究。 先是大田作物的蒸腾失水仅部分地受气孔控制,界面层的扩散阻力起很大作用。因此该措施可能对界面层阻力较小的,如果树等作用大些,另外,根系干旱信号可否“长期”地产生和调控气孔仍需试验证明。     

Sequence of drought response of maize seedlings in drying soil

Leaf elongation in monocotyledonous plants is sensitive to drought. To better understand the sequence of events in plants subjected to soil drying, leaf elongation and transpiration of maize seedlings ( Zea mays L.) of 4 cultivars were monitored continuously and the diurnal courses of the root and leaf water relations were determined. Results from this study indicate the following sequence of drought response: Leaf elongation decreased before changes in the leaf water relations of non‐growing zones of leaf blades were detected and before transpiration decreased. Reductions in leaf elongation preceded changes in the root water potential (ψ_w). Root ψ_w was not a very sensitive indicator of soil dryness, whereas the root osmotic potential (ψ_s) and root turgor (ψ_p) were more sensitive indicators. The earliest events observed in drying soil were a significant increase in the largest root diameter class (1 720 to 1 960 gm) and a decrease in leaf elongation ( P = 0.08) 2 days after withholding water. Significant increases in root length were observed 2 days later. Soil drying increased the number of fine roots with diameters of <240 µm. Slight increases in soil strength did not affect leaf elongation in the drying soil.