Turgor Differences and Water Stress in Maize and Sorghum Leaves During Drought and Recovery

The pressure potentials (turgor pressure) in leaves of maize(Zea mays L.) and grain sorghum (Sorghum vulgare Pers.) plantssubjected to water stress in a controlled environment were estimatedfrom measurements of water and osmotic potentials. Changes inturgor pressure were larger in sorghum than in maize duringthe development of water stress and after re-watering. It issuggested that this indicates a lower cell wall elasticity insorghum than in maize. This fact may affect some of the physiologicalactivities of sorghum     

Transient Responses of Cell Turgor and Growth of Maize Roots as Affected by Changes in Water Potential

Transient responses of cell turgor (P) and root elongation to changes in water potential were measured in maize (Zea mays L.) to evaluate mechanisms of adaptation to water stress. Changes of water potential were induced by exposing roots to solutions of KCl and mannitol (osmotic pressure about 0.3 MPa). Prior to a treatment, root elongation was about 1.2 mm h-1 and P was about 0.67 MPa across the cortex of the expansion zone (3-10 mm behind the root tip). Upon addition of an osmoticum, P decreased rapidly and growth stopped completely at pressure below approximately 0.6 MPa, which indicated that the yield threshold (Ytrans,1) was just below the initial turgor. Turgor recovered partly within the next 30 min and reached a new steady value at about 0.53 MPa. The root continued to elongate as soon as P rose above a new threshold (Ytrans,2) of about 0.45 MPa. The time between Ytrans,1 and Ytrans,2 was about 10 min. During this transition turgor gradients of as much as 0.15 MPa were measured across the cortex. They resulted from a faster rate of turgor recovery of cells deeper inside the tissue compared with cells near the root periphery. Presumably, the phloem was the source of the compounds for the osmotic adjustment. Turgor recovery was restricted to the expansion zone, as was confirmed by measurements of pressure kinetics in mature root tissue. Withdrawal of the osmoticum caused an enormous transient increase of elongation, which was related to only a small initial increase of P. Throughout the experiment, the relationship between root elongation rate and turgor was nonlinear. Consequently, when Y were calculated from steady-state conditions of P and root elongation before and after the osmotic treatment, Yss was only 0.21 MPa and significantly smaller compared with the values obtained from direct measurements (0.42-0.64 MPa). Thus, we strongly emphasize the need for measurements of short-term responses of elongation and turgor to determine cell wall mechanics appropriately. Our results indicate that the rate of solute flow into the growth zone could become rate-limiting for cell expansion under conditions of mild water stress.     

Diurnal and Seasonal Changes in Leaf Water Potential Components and Elastic Properties in Response to Water Stress in Apple Trees

Apple trees are very drought tolerant, having the capability to grow and carry on photosynthesis even at low water potentials. Much of the tolerance is due to the ability of apple leaves to maintain turgor potentials at levels conducive to growth and stomatal opening. Diurnally, leaf turgor is maintained through decreases in osmotic potentials (due to active solute accumulation), osmotic adjustment, or to concentration of solutes via tissue water loss. These two processes combined may decrease osmotic potentials by as much as 1.65 MPn during the day. Seasonally, osmotic potentials remain fairly constant, but leaf elasticity increases, allowing growth to continue and stomata to remain open us water and turgor potentials become progressively lower. Release of stored water from plant tissues to the transpiration stream is another means of preventing water potentials from reaching critical values for stomatal closure. A combination of a number of these physiological adaptations may account for much of the drought tolerance in apple trees.     

Behavior of Corn and Sorghum under Water Stress and during Recovery

Corn (Zea mays L.) and sorghum (Sorghum vulgare, Pers.) plants were grown in a vermiculite-gravel mixture in controlled environment chambers until they were 40 days old. Water was withheld until they were severely wilted, and they were then rewatered. During drying and after rewatering stomatal resistance was measured with a diffusion porometer each morning, and water saturation deficit and water potential were measured on leaf samples. The average resistance of the lower epidermis of well watered plants was lower for corn than for sorghum. When water stress developed, the stomata began to close at a higher water potential in corn than in sorghum. The stomata of both species began to reopen normally soon after the wilted plants were rewatered, and on the 2nd day the leaf resistances were nearly as low as those of the controls. The average leaf water potential of well watered corn was −4.5 bars; that of sorghum, −6.4 bars. The lowest leaf water potential in stressed corn was −12.8 bars at a water saturation deficit of 45%. The lowest leaf water potential in stressed sorghum was −15.7 bars, but the water saturation deficit was only 29%. At these values the leaves of both species were tightly rolled or folded and some injury was apparent. Thus, although the average leaf resistance of corn is little lower than that of sorghum, corn loses much more of its water before the stomata are fully closed than does sorghum. The smaller reduction in water content of sorghum for a given reduction in leaf water potential is characteristic of drought-resistant species.     

Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions: I. At High Soil Water Potential

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), stomatal resistance (r_s), leaf water potential (ψ), osmotic potential (π), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. var. Pa 602A), sorghum (Sorghum bicolor [L.] Moench var. RS610), and tobacco (Nicotiana tabacum L. var. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were high. In all three crops the r_s, measured with a ventilated diffusion porometer, the ψ, measured with the pressure chamber, the π, measured with a vapor pressure osmometer, and the calculated P, decreased from sunrise to reach minimum values near midday and then increased again in the afternoon. The diurnal range of all the variables was greater for leaves in the upper canopy than for those in the lower canopy. P was observed to decrease with decreasing ψ, but never became zero. Sorghum had a higher P at a ψ of, say −10 bars, than did maize, and maize had a higher P than tobacco at the same ψ. Moreover, at the same ψ the upper leaves in all canopies had a higher P than the lower leaves. When compared at high irradiances, r_s did not increase as ψ declined to −13, −15, and −10 bars or as P declined to 0.3, 3.5, and 1.2 bars in maize, sorghum, and tobacco, respectively. The relation between r_s and I in the upper, nonsenescent leaves of all three crops fits a hyperbolic curve, but the response varied with species and leaf senescence. The adaxial and abaxial epidermises had the same response of r_s to I in maize and sorghum, whereas in tobacco the adaxial epidermis had a higher r_s than the abaxial epidermis at all values of I. At equal values of I, tobacco had the lowest leaf resistance (r_l) and maize had the highest r_l. Senescent maize leaves had nonfunctional stomata, whereas the lowermost sorghum leaves had higher stomatal resistances on average than the other leaves.     

Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions: II. At Low Soil Water Potential

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), leaf resistance (r₁), leaf water potential (ψ), osmotic potential (π), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. cv. Pa602A), sorghum (Sorghum bicolor [L.] Moench cv. RS 610), and tobacco (Nicotiana tabacum L. cv. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were low. The r₁, measured with a ventilated diffusion porometer, of the leaves in the upper canopy decreased temporarily after sunrise [~0530 hours Eastern Standard Time] as I increased, but then r₁ increased again between 0700 and 0830 hr Eastern Standard Time as the ψ, measured with a pressure chamber, decreased rapidly from the values of −7, −4 and −6 bars at sunrise to minimal values of −18, −22 and −15 bars near midday in the maize, sorghum, and tobacco, respectively. The π, measured with a vapor pressure osmometer, also decreased after sunrise, but not to the same degree as the decrease in ψ, so that a P of zero was reached in some leaves between 0730 and 0800 hours. The lower (more negative) π of leaves in the upper canopy than those in the lower canopy gave the upper leaves a higher P at a given ψ than the lower leaves in all three species; leaves at intermediate heights had an intermediate P. This difference between leaves at the three heights in the canopy was maintained at all values of ψ. The r₁ remained unchanged over a wide range of P and then increased markedly at a P of 2 bars in maize, −1 bar in sorghum, and near zero P in tobacco: r₁ also remained constant until ψ decreased to −17, −20, and −13 bars in leaves at intermediate heights in maize, sorghum, and tobacco, respectively. In all three species r₁ of leaves in the upper canopy increased at more negative values of ψ than those at the base of the canopy, and in tobacco, leaves in the upper canopy wilted at more negative values of ψ than those in the lower canopy.     

A New Turgor/Membrane Potential Probe Simultaneously Measures Turgor and Electrical Membrane Potential

Abstract: A new combined turgor/membrane potential probe (T-EP probe) monitored cell turgor and membrane potential simultaneously in single giant cells. The new probe consisted of a silicone oil-filled micropipette (oil-microelectrode), which conducted electric current. Measurements of turgor and hydraulic conductivity were performed as with the conventional cell pressure probe besides the membrane potential. In internodal cells of Chara corallina, steady state turgor (0.5-0.7 MPa) and resting potentials (-200 to ?220 mV) in APW, and hydraulic conductivity (0.07 to 0.21 × 10～⁵ m s^?1 MPa^?1) were measured with the new probe, and cells exhibited healthy cytoplasmic streaming for at least 24 h during measurements. When internodal cells of Chara corallina were treated with 30, 20, 10, and 5 mM KCI, turgor responded immediately to all concentrations, and the osmotic changes in the medium were measured. Action potentials, which brought the membrane potential to a steady depolarization that measured the concentration difference of K⁺ in the medium, were induced in a concentration — dependent delay and occurred only 30, 20, and 10 mM of KCl. When the solution was changed back to APW, the repolarization of membrane potential consisted of a quick and a following slow phase. During the quick phase, which took place immediately and lasted 1 to 3 min, the plasma membrane remained activated. The membrane was gradually deactivated in the slow phase, and entirely deactivated when the membrane potential recovered to the resting potential in APW. Although the activated plasma membrane was permeable to K⁺, no major ion channels were activated on the tonoplast, and therefore, internodal cells of Chara corallina did not regulate turgor when osmotic potential changed in the surrounding medium.     

Growth, Turgor, Water Potential, and Young's Modulus in Pea Internodes

The relations between longitudinal growth, Young's modulus, turgor, water potential, and tissue tensions have been studied on growing internodes of etiolated pea seedlings in an attempt to apply some physical concepts to the growth of a well-known plant material. The modulus has been determined by the resonance frequency method and expressed as E_tissue It increases nearly proportional to the turgor pressure and is at water saturation more than 50 times higher than at plasmolysis. E_tissue is higher in the epidermis than in the ground parenchyma. Indoleacetic acid causes a decrease in Etissue Other properties have been studied on intact and split segments of internodes in solutions of graded mannitol additions. — The following tentative picture of the normal course of the growth has been obtained. Auxin induces growth both in the periphery (epidermis) and in the central core (parenchyma) under a decrease in E_tissue This is followed by an increase of E_tissue which is independent of auxin but depending upon the turgor pressure. It is assumed to involve internal structural changes of the cell walls of the type of creep. The rapid growth takes place in a dynamic system with a low water potential despite favourable water conditions. Epidermis and parenchyma grow equally rapid without tissue tensions. — Such can be produced artificially by splitting of segments and water uptake. The parenchyma thereby loses its sensitivity to auxin. This is the background of the split stem test for auxin. — E_tissue increases when growth is slowing down, probably owing to both synthesis of wall substance and structural changes within the wall. The cells attain a more static condition with E_tissue higher in epidermis than in parenchyma. This leads to the normal tissue tensions. — The result agrees with growth according to the multi-net-principle. The cause of the low water potential and low turgor is discussed with reference to the dynamic nature of both growth and water transport and a probably low matric potential of the streaming water. The decrease in E_tissue following auxin addition is small but is the net difference between an auxin-induced decrease and an increase through the assumed creep.     

外源芦丁预处理对水分胁迫下玉米幼苗的生理效应

以玉米(Zeamays L.)品种'郏单958'为材料.采用营养液水培法,研究了外源芦丁(Rutin)对聚乙二醇(PEG)胁迫下幼苗叶片质膜相对透性、脯氨酸、可溶性糖含量及保护酶活性的影响.结果显示:(1)在15%PEG-6000胁迫下,玉米叶片的MDA含量、质膜相对透性、脯氨酸和可溶性蛋白质含量均显著增加,保护酶SOD、CAT、POD活性显著升高.(2)一定浓度芦丁(>0.40 g/L)预处理可显著抑制水分胁迫下玉米幼苗叶片MDA含量的上升,降低叶片质膜相对透性,并诱导SOD、POD和CAT活性提高.降低脯氨酸和可溶性蛋白质含量.说明外源芦丁能够提高玉米幼苗的抗氧化作用,缓解水分胁迫引起的膜脂过氧化,保护细胞膜免受或减少损伤·达到提高植物抗旱性的目的.     

Plants under Climatic Stress: VI. Chilling and Light Effects on Photosynthetic Enzymes of Sorghum and Maize

The activity of several photosynthetic enzymes was unaltered by exposure of sorghum or maize to low temperatures (10 C) and light (170 w m⁻²). Two light-activated C₄-pathway enzymes, NADP-malate dehydrogenase and pyruvate Pi dikinase, were reduced in activity, and this was largely attributable to a loss of enzyme rather than to incomplete enzyme activation. Loss of NADP-malate dehydrogenase was more marked in sorghum than in maize, and in both species no loss occurred at 10 C when light levels were reduced from 170 to 50 w m⁻². A light-dependent, low temperature-induced loss of catalase activity was also observed in maize leaves.     

Effect of Water Stress on Turgor Differences and C-Assimilate Movement in Phloem of Ecballium elaterium

The effects of water stress on pressure differences and ¹⁴C-assimilate translocation in sieve tubes of squirting cucumber Ecballium elaterium A. Rich were studied. Water stress was induced by transfer of plants from culture solution to a polyethylene glycol 6,000 solution having an osmotic potential of −18.2 atm. Sieve tube turgor, turgor differences between source and sink, and translocation rate were decreased. After 260 minutes of translocation, only 19% of the total fixed ¹⁴CO₂ had moved out of the leaf, compared to the control value of 62% after the same period of time. The results suggest that water stress slows translocation by lowering sieve tube turgor differences, which are essential for the pressure flow mechanism of conduction.     

水分胁迫下荔枝叶片过氧化物酶和IAA氧化酶活性的变化

以适应山地栽培的抗旱性较强的东刘1号和适应河边栽培的抗旱性较弱的陈紫2年生荔枝（Litchi chinensis Sonn.)实生苗为试验材料,研究了水分胁迫下叶片细胞胞质,与（细胞）壁以离子键结合和壁以共价键结合的过氧化的酶（POD）和IAA氧化酶活性的变化。结果表明：在叶片中POD主要是以与壁以离子键结合的POD存在,占总活性的51．15％－52．15％,其次是细胞胞质POD,占44．20％－44．74％,与壁以共价键结合的POD活性最低,仅占3．44％－3．65％。与POD不同,IAA氧化酶绝大多数存在于细胞胞质中,占总活性的88．93％－89．29％,其次是少量的与壁以离子键结合的IAA氧化酶,占7．32％－7．63％,与壁以共价键结合的IAA氧化酶活性最低,仅占3．39％－3．44％;2个品种间差异不明显。水分胁迫下,叶片细胞胞质以及与壁以离子键和壁以共价键结合的POD和IAA氧化酶（比）活性均上升,抗旱笥较强的品种上升的幅度均大于抗旱性较弱的品种。     

Turgor Regulation in a Brackish Water Charophyte, Lamprothamnium succinctum III. Changes in Cytoplasmic Free Amino Acids and Sucrose Contents during Turgor Regulation

Cytoplasm and cell sap of Lamprothamnium succinctum were analyzedseparately for the contents of free amino acids and sucroseto find whether they contribute to turgor regulation. In thevacuole, both amino acids and sucrose were found to be minorcomponents contributing to the generation of osmotic pressure.Their amounts were almost insensitive to changes in externalosmotic pressure. In the cytoplasm, both amino acids and sucrosein the cytoplasm contributed about 20% to the osmotic pressure.Hypotonic treatment did not affect the contents of either, buthypertonic treatment, while not affecting the amino acid contents,caused a significant increase in sucrose content. The cytoplasmicsucrose content increased linearly with an increase in externalosmotic pressure, accounting for 40% of the increased osmoticpressure. ¹ Present address: Department of Biology, Osaka Medical College,Sawaragi-cho, Takatsuki, Osaka 569, Japan ² Present address: Department of Applied Physiology, NationalInstitute of Agrobiological Resources, Yatabe, Tsukuda, Ibaragi305, Japan (Received November 25, 1986; Accepted March 18, 1987)     

Growth Inhibition, Turgor Maintenance, and Changes in Yield Threshold after Cessation of Solute Import in Pea Epicotyls

The dependence of stem elongation on solute import was investigated in etiolated pea seedlings (Pisum sativum L. var Alaska) by excising the cotyledons. Stem elongation was inhibited by 60% within 5 hours of excision. Dry weight accumulation into the growing region stopped and osmotic pressure of the cell sap declined by 0.14 megapascal over 5 hours. Attempts to assay phloem transport via ethylenediaminetetraacetate-enhanced exudation from cut stems revealed no effect of cotyledon excision, indicating that the technique measured artifactual leakage from cells. Despite the drop in cell osmotic pressure, turgor pressure (measured directly via a pressure probe) did not decline. Turgor maintenance is postulated to occur via uptake of solutes from the free space, thereby maintaining the osmotic pressure difference across the cell membrane. Cell wall properties were measured by the pressure-block stress relaxation technique. Results indicate that growth inhibition after cotyledon excision was mediated primarily via an increase in the wall yield threshold.     

水分胁迫对春播玉米苗期生长及其生理生化特性的影响

采用盆栽试验,以‘蠡玉18'玉米单交种为供试材料,设置充分供水(CK)、轻度水分胁迫(LS)、中度水分胁迫(MS)和重度水分胁迫(SS)4个水分处理水平,研究了水分胁迫对春播玉米苗期保护酶活性和生长的影响,以探讨土壤水分胁迫对玉米苗期生长发育及其生理过程的影响机制.结果表明:(1)随着水分胁迫程度的加剧,玉米幼苗的生物量显著下降,根冠比、根系活力和脯氨酸含量增加,且水分胁迫对玉米幼苗地上部生物量的抑制作用更大;可溶性蛋白含量差异不明显,MDA含量波动变化.(2)随着水分胁迫时间的延长,根冠比、根系活力和植株脯氨酸含量先升高后降低,可溶性蛋白含量呈先下降后升高的趋势;玉米幼苗叶片和根系MDA积累波动变化,而叶片MDA含量始终高于根系.(3)在水分胁迫初期,玉米叶片中CAT活性较SOD、POD响应更敏感;玉米苗期根系在中度水分胁迫下主要依赖CAT来降低氧化危害,而在重度水分胁迫下前期主要依赖CAT、后期通过CAT和POD的共同作用来降低氧化伤害;水分胁迫条件下,叶片和根系POD同步降低氧化伤害,而SOD和CAT在叶片和根系间存在互补作用.研究表明,在不同程度的水分胁迫条件下,玉米幼苗的生长受到一定程度的抑制,但其能够通过调节自身的保护酶活性和渗透调节物质含量来减轻干旱伤害,维持植株的正常生理代谢功能.     

Physiological Changes in Cultured Sorghum Cells in Response to Induced Water Stress : II. Soluble Carbohydrates and Organic Acids

Eight cultivars Sorghum bicolor (L.) Moench were grown as callus cultures under induced, prolonged water stress (8 weeks), with polyethylene glycol in the medium. Concentrations of soluble carbohydrates and organic acids in callus were measured at the end of the growth period to determine differences in response to prolonged water stress. Sucrose, glucose, fructose, and malate were the predominant solutes detected in all callus at all water potentials. All cultivars had high levels of solutes in the absence of water stress and low levels in the presence of prolonged water stress. However, at low water potentials, low levels of solutes were observed in drought-tolerant cultivar callus and high solute levels were observed in drought-susceptible cultivar callus. Estimated sucrose concentrations were significantly higher in water-stressed, susceptible cultivar callus. Large solute concentrations in susceptible cultivar callus were attributed to osmotic adjustment and/or reduced growth during water stress.     

供氮形态与部位对局部根区水分胁迫下玉米水分吸收与利用的影响

试验以玉米品种'金海五号'幼苗为材料,在分根条件下采用聚乙二醇(PEG-6000)模拟局部根区水分胁迫,设置3种供氮形态(硝态氮、铵态氮、两者各占50%的混合氮)和2种供氮部位(水氮同区,氮加入到无PEG侧;水氮异区,氮加入到含PEG侧),研究局部根区水分胁迫下氮形态与供应部位对玉米水分吸收和利用的调节与作用机制,为局部根区灌溉水分高效利用提供理论依据.结果发现:(1)同一氮形态下水氮同区供应的植株蒸腾速率、耗水量、木质部汁液流速和生物量较高,加有硝态氮源处理无PEG侧根系的导管数目及单一氮形态处理无PEG侧根系的导管直径较高,但木质部汁液、叶片中脱落酸(ABA)浓度以及水分利用效率均较低.(2)同一供氮部位下,植株的蒸腾速率、耗水量、木质部汁液流速和生物量的顺序均为混合氮>硝态氮>铵态氮依次,但单一铵态氮处理植株的ABA浓度较高,水分利用效率较高.研究表明,同一氮形态下水氮同区供应植株生长较好、水分吸收能力较强,但水氮异区供应下植株的水分利用效率较高;同一供氮部位下,植株生长和水分吸收能力的顺序为混合氮>硝态氮>铵态氮,但单供铵态氮植株的水分利用效率较高.     

Transpiration Induces Radial Turgor Pressure Gradients in Wheat and Maize Roots

Previous studies have shown both the presence and the absence of radial turgor and osmotic pressure gradients across the cortex of roots. In this work, gradients were sought in the roots of wheat (Triticum aestivum) and maize (Zea mays) under conditions in which transpiration flux across the root was varied This was done by altering the relative humidity above the plant, by excising the root, or by using plants in which the leaves were too young to transpire. Roots of different ages (4-65 d) were studied and radial profiles at different distances from the tip (5-30 mm) were measured. In both species, gradients of turgor and osmotic pressure (increasing inward) were found under transpiring conditions but not when transpiration was inhibited. The presence of radial turgor and osmotic pressure gradients, and the behavior of the gradient when transpiration is interrupted, indicate that active membrane transport or radial solvent drag may play an important role in the distribution of solutes across the root cortex in transpiring plants. Contrary to the conventional view, the flow of water and solutes across the symplastic pathway through the plasmodesmata cannot be inwardly directed under transpiring conditions.     

Turgor Regulation in a Brackish Charophyte, Lamprothamnium succinctum II. Changes in K$, Na$ and Cl- Concentrations, Membrane Potential and Membrane Resistance during Turgor Regulation

A brackish Characeae, Lamprothamnium succinctum, regulates intracellularosmotic pressure in response to changes in the external salinityand keeps the turgor pressure constant. The osmotic pressureof the vacuole was found to be mostly due to K^$, Na^$ and Cl^–.But in the cytoplasm, the sum of their concentrations was muchlower than the cellular osmotic pressure. Electroneutralitywas maintained among the analyzed inorganic ions in the vacuolebut a strong anion deficiency was detected in the cytoplasm,supporting the existence of organic anions to balance excesspositive charges. During turgor regulation, concentrations of inorganic ions inthe vacuole changed just enough to accommodate the osmotic pressurechange, while those in the cytoplasm remained almost constant.Since the cytoplasmic volume was almost constant during turgorregulation, some organic molecule(s) may have contributed tothe osmoregulation of the cytoplasm. The membrane potential and resistance at steady state underdifferent salinities were almost constant. Hypotonic treatmentcaused a sudden depolarization of the membrane potential anda drastic decrease in membrane resistance. Hypertonic treatmentcaused a slow hyperpolization of membrane potential but didnot significantly affect the membrane resistance. The energeticsof K^$ and Cl^– movements across the plasma membrane isdiscussed based upon the electrochemical potential gradients. (Received November 28, 1983; Accepted March 14, 1984)     

Mechanical Stress in the Shoot Apices of Euphorbia, Lycopersicon, and Pisum under controlled Turgor

Cuts were made in the surface of the shoot apices of Euphorbialathyris, tomato (Lycopersicon esculentum), and Pea (Pisum sativum)while they were completely immersed in water or aqueous mannitolat various concentrations, or in near-saturated air. Gapingoccurred all over the apical dome of Euphorbia and on the tomatoapex at the site of emergence of the primordial bulge. Maximumgaping occurred in near-saturated air and under water, and wasprogressively reduced with increasing osmotica. It is concludedthat the gaping results from tension in the surface cells andis not caused by superficial drying out. No gaping occurred in the axil of the newly formed primordiumof the tomato nor anywhere in the apex of the pea. Histologicalevidence suggests that these tissues are under lateral compression. The mechanical stresses involved are discussed in relation tothe morphology of the apices together with existing data onthe distribution of cell division during primordia formation.