The Effect of Rapid Temperature Increase on the Growth Rate of Wheat Leaves

The growth rate of the first leaf of eight-day-old wheat plants was measured using a DLT-2 highly sensitive linear displacement transducer. Leaf extensibility was evaluated from the growth rate under the increase in the pulling force by 2 g. An increase in the air temperature resulted in the doubling of the transpiration rate and immediate slowing of the leaf growth followed by the leaf shrinkage. However, growth was later resumed almost completely. Heat treatment did not induce any changes in the leaf extensibility, indicating that cell-wall mechanical properties were not changed. Growth retardation was supposed to result from a decrease in the water content in the leaf tissues because the balance between water influx from roots and its loss through transpiration was shifted toward the water loss. An initial drop in the relative water content (RWC) indicates such a misbalance. Subsequent growth resumption coincided with a decreased water deficiency. Since the rate of transpiration was not reduced, RWC and growth rate restoring evidently occurred due to the activated water uptake by roots, which can be explained by the increased hydraulic permeability detected in our experiments.     

The Role of Hormones in Fast Growth Responses of Wheat Plants to Osmotic and Cold Shocks

The effects of nutrient-solution cooling and PEG addition to the nutrient solution on the phytohormone content, the rate of leaf growth, leaf extensibility under the influence of external mechanical action, osmotic potential, and transpiration were studied in seven-day-old wheat plants. Leaf growth rapidly ceased, and the transpiration rate was reduced in both treatments. Growth cessation induced by PEG was transient, and growth resumption was preceded by an increase in the leaf extensibility. The functional role of auxin accumulation in plant shoots in the control of extensibility as well as the relationship between the ABA accumulation and a decrease in the cytokinin content, on the one hand, and reduced transpiration, on the other hand, under stress conditions are discussed.     

Water-stressed maize, barley and rice seedlings show species diversity in mechanisms of leaf growth inhibition

The possible occurrence of species diversity in mechanisms underlying leaf-growth inhibition by water stress, was investigated in related cereal plants. Water stress was generated by additions of the osmoticum polyethylene glycol 6000 to the root medium. Effects of external water potentials ranging from 0 to -0.6MPa, on early growth parameters of emerging leaves were measured under controlled environment conditions, using pairs of maize, barley or rice genotypes with differing resistance to water stress under field conditions. Water potentials of -0.4 MPa for 24 h, similarly reduced leaf growth, comparative production rates of leaf epidermal cells and cell size in all genotypes. These reductions did not appear to be caused by reductions in the osmotic potential gradients between the expanding leaf cells and their external water source. However, growth inhibition in maize and barley, was accompanied by significant reductions in comparative leaf and cell wall extensibility. Moreover, regression plots revealed good linear correlations (r=0.83** for maize and r=0.77** for barley) between the reductions in leaf growth induced by a series of water potentials and associated reductions in leaf extensibility. In contrast, the reduction in growth of rice leaves, was not accompanied by any significant changes in leaf or cell wall extensibility. Similarly, regression plots revealed poor correlations between leaf growth and leaf extensibility in both paddy and upland rice (r=0.17 and r=0.07, respectively). Thus, despite numerous inter-species similarities, biophysical changes associated with stress-induced leaf growth inhibition in maize and barley, differed from those in rice.Key words: Cell walls, extensibility, water stress, cereal diversity, leaf growth.      

Rapid and tissue-specific changes in ABA and in growth rate in response to salinity in barley leaves

The addition of 100 mM NaCl to the root medium of barley plantscaused the rapid cessation of elongation of the growing leafthree, followed by a sudden resumption of growth during thefollowing hour. The idea that resumption of growth is precededand mediated by rapid and tissue-specific changes in ABA concentrationand by changes in transpiration was tested. Leaf elongationvelocity was recorded continuously using linear variable displacementtransducers (LVDT), ABA was determined by immunoassay, and transpirationand stomatal conductivity were measured gravimetrically andby porometry, respectively. Within 10 min following additionof salt, ABA increased 6-fold in the distal portion of the leafelongation zone; in the proximal portion, ABA accumulated witha delay. In the portion of the growing blade that had emergedABA increased 3-fold and remained elevated during the following20 min. This preceded a decrease in transpiration and stomatalconductivity, which, in turn, coincided with growth resumption.Twenty hours following the addition of salt, the ABA concentrationshad returned to the level before stress. Leaf elongation velocitywas still reduced. It is concluded that NaCl causes a rapidincrease in ABA in the transpiring portion of the growing leaf.This leads to a decrease in transpiration. As a result, xylemwater potential is expected to rise. The moment that the waterpotential gradient between the xylem and the peripheral cellsin the growth zone favours water uptake again into the latter,leaf elongation resumes. The results suggest that ABA causesdifferent responses in different leaf regions, all aimed atpromoting the resumption of leaf growth. Key words: Abscisic acid, cell elongation, Hordeum vulgare, leaf growth, salinity, water relations.     

Acclimation of leaf growth to low water potentials in sunflower

Abstract Leaf growth is one of the most sensitive of plant processes to water deficits and is frequently inhibited in field crops. Plants were acclimated for 2 weeks under a moderate soil water deficit to determine whether the sensitivity of leaf growth could be altered by sustained exposure to low water potentials. Leaf growth under these conditions was less than in the controls because expansion occurred more slowly and for less of the day than in control leaves. However, acclimated leaves were able to grow at leaf water potentials (Ψ₁) low enough to inhibit growth completely in control plants. This ability was associated with osmotic adjustment and maintenance of turgor in the acclimated leaves. Upon rewatering, the growth of acclimated leaves increased but was less than the growth of controls, despite higher concentrations of cell solute and greater turgor in the acclimated leaves than in controls. Therefore, factors other than turgor and osmotic adjustment limited the growth of acclimated leaves at high ψ₁ Four potentially controlling factors were investigated and the results showed that acclimated leaves were less extensible and required more turgor to initiate growth than control leaves. The slow growth of acclimated leaves was not due to a decrease in the water potential gradient for water uptake, although changes in the apparent hydraulic conductivity for water transport could have occurred. It was concluded that leaf growth acclimated to low ψ₁, by adjusting osmotically, and the concomitant maintenance of turgor permitted growth where none otherwise would occur. However, changes in the extensibility of the tissue and the turgor necessary to initiate growth caused generally slow growth in the acclimated leaves.     

Salinity Stress Inhibits Bean Leaf Expansion by Reducing Turgor, Not Wall Extensibility

Treatment of bean (Phaseolus vulgaris L.) seedlings with low levels of salinity (50 or 100 millimolar NaCl) decreased the rate of light-induced leaf cell expansion in the primary leaves over a 3 day period. This decrease could be due to a reduction in one or both of the primary cellular growth parameters: wall extensibility and cell turgor. Wall extensibility was assessed by the Instron technique. Salinity did not decrease extensibility and caused small increases relative to the controls after 72 hours. On the other hand, 50 millimolar NaCl caused a significant reduction in leaf bulk turgor at 24 hours; adaptive decreases in leaf osmotic potential (osmotic adjustment) were more than compensated by parallel decreases in the xylem tension potential and the leaf apoplastic solute potential, resulting in a decreased leaf water potential. It is concluded that in bean seedlings, mild salinity initially affects leaf growth rate by a decrease in turgor rather than by a reduction in wall extensibility. Moreover, longterm salinization (10 days) resulted in an apparent mechanical adjustment, i.e. an increase in wall extensibility, which may help counteract reductions in turgor and maintain leaf growth rates.     

The short-term growth response to salt of the developing barley leaf

Recent results concerning the short-term growth response to salinity of the developing barley leaf are reviewed. Plants were grown hydroponically and the growth response of leaf 3 was studied between 10 min and 5 d following addition of 100 mM NaCl to the root medium. The aim of the experiments was to relate changes in variables that are likely to affect cell elongation to changes in leaf growth. Changes in hormone content (ABA, cytokinins), water and solute relationships (osmolality, turgor, water potential, solute concentrations), gene expression (water channel), cuticle deposition, membrane potential, and transpiration were followed, while leaf elongation velocity was monitored. Leaf elongation decreased close to zero within seconds following addition of NaCl. Between 20 and 30 min after exposure to salt, elongation velocity recovered rather abruptly, to about 46% of the pre-stress level, and remained at the reduced rate for the following 5 d, when it reached about 70% of the level in non-stressed plants. Biophysical and physiological analyses led to three major conclusions. (i) The immediate reduction and sudden recovery in elongation velocity is due to changes in the water potential gradient between leaf xylem and peripheral elongating cells. Changes in transpiration, ABA and cytokinin content, water channel expression, and plasma membrane potential are involved in this response. (ii) Significant solute accumulation, which aids growth recovery, is detectable from 1 h onwards; growing and non-growing leaf regions and mesophyll and epidermis differ in their solute response. (iii) Cuticular wax density is not affected by short-term exposure to salt; transpirational changes are due to stomatal control.     

The different effects of PEG 6000 and NaCI on leaf development are associated with differential inhibition of root water transport

The inhibitory effects of PEG on whole-plant growth can exceed the effects of other osmolytes such as NaCI, and this has been ascribed to toxic contaminants, or to reduced oxygen availability in PEG solutions. We investigated another possibility, namely that PEG has an additional inhibitory effect on root water transport which in turn affects leaf development. The effects on first-leaf growth of applications of PEG 6000 or isoosmotic NaCI to the roots were determined using hydroponically grown maize (Zea mays L.) seedlings. Leaf growth rates were inhibited within minutes of PEG application to the roots and remained inhibited for days. The inhibitory effects on growth of NaCI, and also of KCl and mannitol, were much smaller. The comparative effects of NaCI and PEG on root water transport were determined by assaying pressurized flow through excised roots. PEG induced a 7-fold greater inhibition of flow through live roots than NaCI. Killing of the roots by heat treatment, to reduce cell membrane resistances to solute penetration, nearly doubled the flow rate for roots in NaCI, but not for roots in PEG. We suggest that the greater viscosity of PEG solutions, as compared with NaCI, may be a primary factor contributing to the additional inhibition of water flow through live and killed roots. PEG did not have additional effects on leaf turgor but had a 3 times greater inhibitory effect than NaCI on the irreversible extensibility of the leaves and induced 16 times more leaf accumulation of the growth inhibitory stress hormone abscisic acid (ABA). We conclude that greater inhibition of root water transport by PEG 6000, as compared with NaCI, leads to additional reductions in extensibility, additional ABA accumulation, and a greater inhibition of leaf growth.     

Dynamic Relation between Expansion and Cellular Turgor in Growing Grape (Vitis vinifera L.) Leaves

Measurements of the growth and water relations of expanding grape (Vitis vinifera L.) leaves have been used to determine the relationship between leaf expansion rate and leaf cell turgor. Direct measurement of turgor on the small (approximately 15 micrometer diameter) epidermal cells over the midvein of expanding grape leaves was made possible by improvements in the pressure probe technique. Leaf expansion rate and leaf water status were perturbed by environmentally induced changes in plant transpiration. After establishing a steady state growth rate, a step decrease in plant transpiration resulted in a rapid and large increase in leaf cell turgor (0.25 megapascal in 5 minutes), and leaf expansion rate. Subsequently, leaf expansion rate returned to the original steady state rate with no change in cell turgor. These results indicate that the expansion rate of leaves may not be strongly related to the turgor of the leaf cells, and that substantial control of leaf expansion rate, despite changes in turgor, may be part of normal plant function. It is suggested that a strictly physical interpretation of the parameters most commonly used to describe the relationship between turgor and growth in plant cells (cell wall extensibility and yield threshold) may be inappropriate when considering the process of plant cell expansion.     

Loss of capacity for acid-induced wall loosening as the principal cause of the cessation of cell enlargement in light-grown bean leaves

Cell enlargement in primary leaves of bean (Phaseolus vulgaris L.) can be induced, free of cell divisions, by exposure of 10-d-old, red-light-grown seedlings to white light. The absolute rate of leaf expansion increases until day 12, then decreases until the leaves reached mature size on day 18. The cause of the reduction in growth rate following day 12 has been investigated. Turgor calculated from measurements of leaf water and osmotic potential fell from 6.5 to 3.5 bar before day 12, but remained constant thereafter. The decline of growth after day 12 is not caused by a decrease in turgor. On the other hand, Instron-measured cell-wall extensibility decreased in parallel with growth rate after day 12. Two parameters influencing extensibility were examined. Light-induced acidification of cell walls, which has been shown to initiate wall extension, remained constant over the growth period (days 10–18). Furthermore, cells of any age could be stimulated to excrete H⁺ by fusicoccin. However, older tissue was not able to grow in response to fusicoccin or light. Measurements of acid-induced extension on preparations of isolated cell walls showed that as cells matured, the cell walls became less able to extend when acidified. These data indicate that it is a decline in the capacity for acid-induced wall loosening that reduces wall extensibility and thus cell enlargement in maturing leaves.Abbreviations and symbols FC fusicoccin - P turgor pressure - RL red light - WEx wall extensibility - WL white light - P _w leaf water potential - P _s osmotic potential     

Rapid Response of the Yield Threshold and Turgor Regulation during Adjustment of Root Growth to Water Stress in Zea mays

Responses of cortical cell turgor (P) following rapid changes in osmotic pressure ([pi]m) were measured throughout the elongation zone of maize (Zea mays L.) roots using a cell pressure probe and compared with simultaneously measured root elongation to evaluate: yield threshold (Y) (minimum P for growth), wall extensibility, growth-zone radial hydraulic conductivity (K), and turgor recovery rate. Small increases in [pi]m (0.1 MPa) temporarily decreased P and growth, which recovered fully in 5 to 10 min. Under stronger [pi]m (up to 0.6 MPa), elongation stopped for up to 30 min and then resumed at lower rates. Recoveries in P through solute accumulation and lowering of Y enabled growth under water stress. P recovery was as much as 0.3 MPa at [pi]m = 0.6 MPa, but recovery rate declined as water stress increased, suggesting turgor-sensitive solute transport into the growth zone. Under strong [pi]m, P did not recover in the basal part of the growth zone, in conjunction with a 30% shortening of the growth zone. Time courses showed Y beginning to decrease within several minutes after stress imposition, from about 0.65 MPa to a minimum of about 0.3 MPa in about 15 min. The data concerning Y were not confounded significantly by elastic shrinkage. K was high (1.3 x 10-10 m2 s-1 MPa-1), suggesting very small growth-induced water potential gradients.     

Effect of increased irradiance on the hormone content,water relations,and leaf elongation in wheat seedlings

In wheat (Triticum durum Desf., cv. Bezenchukskaya 139) seedlings, an increase in irradiance from 20 to 400 μmol/(m² s) PAR enhanced transpiration and increased stomatal conductance by three times on the background of reduced relative water content (RWC). After this treatment, leaves quickly ceased to grow and became even shrunk later. In 40 or 50 min, leaf growth was resumed. At this period, we observed an increase in hydraulic conductivity and RWC and also in leaf extensibility. As soon as 10 min after treatment, some changes in hormone content were noted. In the zones of leaf growth and its mature part, zeatin and zeatin riboside were accumulated, whereas ABA accumulation was observed in the zone of leaf growth and in the roots. The results obtained indicate that leaf expansion at increased irradiance was related to changes in cell-wall extensibility and hydraulic conductivity. The first effect could be due to cytokinin accumulation, whereas the second one, to ABA accumulation.     

Productivity of Peach Trees : Tree Growth and Water Stress in Relation to Fruit Growth and Assimilate Demand

The growth of the limbs of peach trees measured by dendrometerswas inhibited during periods when the rate of d. wt accumulationby the fruit was increasing. The diurnal shrinkage of theselimbs measured by the same dendrometers was greatest duringperiods when limb growth was inhibited. The increase in diurnalshrinkage of the limbs coinciding with inhibited limb growth,and increased assimilate demand, was greater than that causedby environmental factors at maximum soil water-potential andgreater than that caused by five days soil drying after irrigation.Leaf water potential and diurnal limb shrinkage were measuredcontinuously during two periods of maximum soil water-potentialand leaf area when the rate of d. wt increase of the fruit wasdecreasing (DW II) and then later when the rate of d. wt increaseof the fruit was increasing (DW III). The leaf water potentialwas lower and limb shrinkage greater during DW 111 than DW II.The hydraulic gradient also increased from 1 bar m^–1 inDW II to 2 bar m^–1 in DW III Environmental conditions during both periods were very similarand the data suggest total water use by the tree increased substantiallyduring periods of high assimilate demand by the fruit.     

Nitrate Supply and the Biophysics of Leaf Growth in Salix viminalis

The influence of nitrogen on leaf area development and the biophysicsof leaf growth was studied using clonal plants of the shrubwillow, Salix viminalis grown with either optimal (High N) orsub-optimal (Low N) supplies of nitrate. Leaf growth rate andfinal leaf size were reduced in the sub-optimal treatment andthe data suggest that in young rapidly growing leaves, thiswas primarily due to changes in cell wall properties, sincecell wall extensibility (% plasticity) was reduced in the LowN plants. The biophysical regulation of leaf cell expansion also differedwith nitrogen treatment as leaves aged. In the High N leaves,leaf cell turgor pressure (P) increased with age whilst in theLow N leaves P declined with age, again suggesting that foryoung leaves, cell wall plasticity limited expansion in theLow N plants. Measurements of cell wall properties showed thatcell wall elasticity (%E) was not influenced by nitrogen treatmentand remained constant regardless of leaf age. Key words: Salix, cell wall extensibility, nitrogen nutrition, biophysics of leaf growth     

夏玉米三叶期持续干旱下不同叶位叶片含水量变化及其与光合作用的关系

植物干物质的累积依赖于群体光合速率,而群体光合速率又与单叶的光合能力密切有关。叶片光合作用与其含水量密切相关,目前关于不同叶位叶片含水量对持续干旱的响应及其与光合作用的关系还未见报道。以华北夏玉米郑单958为材料,设置6个不同灌水处理,模拟不同灌溉量下持续干旱对夏玉米不同叶位叶片生理特性的影响,分析夏玉米顶部开始的第一、三、五叶位叶片的水分变化及其与净光合速率的关系。结果表明:夏玉米不同叶位的叶片最大含水量不同,且随干旱进程的推进叶片含水量的变化速率也不同,第一叶的叶片含水量下降速率高于第三、第五叶,第一叶的最大含水量高于第三、五叶,且可进行光合产物积累的叶片含水量下限随叶位的增加而增大。同时,第一叶的叶片含水量与土壤水分呈显著相关,且与净光合速率的相关性也非常强。第一叶可进行光合产物积累的叶片水分下限(净光合速率为零时的叶片含水量)最小,表明其耐旱性最强,对干旱具有指导意义。研究结果可为提高冠层光合作用模拟的准确性及夏玉米干旱发生发展的监测预警提供参考。     

The Relationship Between Leaf Thickness and Plant Water Potential

Leaf thickness was continuously measured in a wide range ofenvironments using a new type of displacement transducer whichis easy to set-up and automatically compensates for the effectsof temperature. Simultaneous measurements were made of waterpotential using either a psychrometer attached to the leaf petioleor a leaf pressure chamber. Thickness of leaves was a sensitiveindicator of plant water status but calibrations against anindependent method were necessary in every plant for accurateestimates of water potential. The relationship between leafthickness changes and water potential, measured in detachedleaves, was usually curvilinear and was strongly influencedby leaf age, stress history and, in young leaves, by the effectsof leaf growth. Leaf thickness growth was absent in mature cabbageleaves. Key words: Leaf thickness, plant water potential, psychrometer     

Photomodulation of mesocotyl elongation in maize seedlings: Is there a correlative relationship between phytochrome,auxin and cell wall extensibility?

Three h white light irradiation of etiolated maize seedlings ( Zea mays L. cv. Jubilee) inhibited mesocotyl elongation and caused a sharp decrease in cell wall plastic extensibility as measured by the Instron technique. The plastic extensibility following white light irradiation (3 h) was photomodulated by phytochrome. Although the photomodulation of the plastic extensibility was correlated with growth during 20 h, no such correlation was observed at shorter times. The addition of indole-3-acetic acid to light-inhibited intact seedlings, or seedlings from which the coleoptile and inner leaves were excised, resulted in a stimulation of growth. However, none of the IAA concentrations could reverse light inhibition. The possibility of a correlative relationship between phytochrome, auxin and cell wall extensibility is discussed.     

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     

Signal Integration by ABA in the Blue Light-Induced Acidification of Leaf Pavement Cells in Pea (Pisum sativum L. var. Argenteum)

Leaf pavement cell expansion in light depends on apoplastic acidification by a plasma membrane proton-pumping ATPase, modifying cell wall extensibility and providing the driving force for uptake of osmotically active solutes generating turgor. This paper shows that the plant hormone ABA inhibits light-induced leaf disk growth as well as the blue light-induced pavement cell growth in pea (Pisum sativum L.). In the phytochrome chromophore-deficient mutant pcd2, the effect of ABA on the blue light-induced apoplastic acidification response, which exhibits a high fluence phase via phytochrome and a low fluence phase via an unknown blue light receptor, is still present, indicating an interaction of ABA with the blue light receptor pathway. Furthermore, it is shown that ABA inhibits the blue light-induced apoplastic acidification reversibly. These results indicate that the effect of ABA on apoplastic acidification can provide a mechanism for short term, reversible adjustment of leaf growth rate to environmental change.Key Words: ABA, apoplastic acidification, blue light, epidermal pavement cell growth, leaf growth, pea (Pisum sativum L.), signal integration