Reduced Leaf Growth Research Articles

Plant age-related changes in cytokinins, leaf growth and pigment accumulation in juvenile mastic trees

Despite plant age is a key factor to understand the performance and survival of any given species in the field, little is known about its influence on the endogenous levels of cytokinins in juvenile individuals of perennial plants. In this study, we hypothesized that plant age in juvenile mastic trees may affect cytokinin levels and lead to significant changes in cytokinin-related processes, such as leaf growth and pigment accumulation. Two groups of juvenile plants aged 2 and 5 years old were grown under Mediterranean field conditions to evaluate the age-related differences in the endogenous levels of cytokinins and auxins, leaf biomass and area, pigments levels and the efficiency of PSII photochemistry (Fv/Fm ratio). Furthermore, regression analyses between cytokinin levels and several indicators of stress performance were carried out in a population of juvenile mastic trees. Results showed that endogenous levels of cytokinins, particularly 2-isopentenyladenine (2-iP), increased during periods of intense leaf growth. However, plant age did not affect 2-iP levels, but those of trans-zeatin (Z); 2-year-old plants showing 2-fold higher levels of Z compared to 5-year-old plants, the latter having 11-fold higher total plant biomass. Plant age-related reductions in cytokinins were not associated with reductions in leaf growth in the largest individuals, but with an increased chlorophyll (Chl) a/b ratio. Further regression analyses revealed that although enhanced Chl a/b ratios predispose leaves to suffer photooxidative stress, as indicated by increased anthocyanins levels and lowered Fv/Fm ratios in juvenile mastic trees, individuals of different ages showed a similar degree of stress tolerance. It is concluded that plant age may alter the endogenous composition of cytokinins and the pattern of pigment accumulation in leaves, but this does not reduce leaf growth or stress tolerance in this species.

Overexpression of stress-related genes, BrERF4 and AtMYB44, in Arabidopsis thaliana alters cell expansion but not cell proliferation during leaf growth

We have previously shown that overexpression of BrERF4 (BrassicarapaETHYLENE-RESPONSIVEFACTOR4) increases salt and drought tolerance in Arabidopsis plants, and also retarded organ growth. In the present study, we investigated in detail the leaf growth retardation phenotype at the cellular level. Results showed that BrERF4-overexpressing Arabidopsis plants developed small leaves by reducing their cell size but not the cell number. Detailed kinematic analysis revealed that changes in cell size appeared from the very early stages of leaf development, directly affecting the size of leaf organs. RT-PCR analysis showed that expression of expansin genes was reduced in the overexpressors, whereas expression of the cell cycle gene, CYCB1;1, was not altered at all. In addition, overexpression of AtMYB44, another stress-related transcription factor gene, reduced leaf growth, which also resulted from reduction in cell size but not in cell number. These results suggest that overexpression of those transcription factors negatively affects cell expansion during leaf growth without altering cell number. We discuss about the advantages that the BrERF4- or AtMYB44-induced cell expansion retardation confers on plants under natural environmental adversity.

Is the control of green area index of barley crops by application of alkaline materials possible in field conditions?

The risk of excessive leaf canopy growth in winter barley (Hordeum vulgare L.) grown in the UK is usually reduced by avoiding very early sowing at high seed rates, and in spring by delaying and reducing nitrogen fertilizer application. A preliminary study was conducted of a possible additional agronomic method for retarding winter barley leaf growth in spring. The study was based on previous plant physiological experiments showing that drought-induced reduction in leaf growth of barley is mediated by increasing alkalinity of the xylem sap. Leaf extension was measured in plants detached from the roots and fed artificial xylem sap adjusted to a wide range of pH. A field experiment tested the effects on barley canopy growth of solid calcium carbonate and alkaline pH buffer solution application to the foliage. Rate of leaf extension in the detached plant experiments decreased linearly with increasing artificial xylem sap pH, confirming the previous results in a different cultivar. Rate of canopy growth in the field experiment was halved by application of 1000kg/ha of calcium carbonate up to four days after application. In contrast at six and eight days after application, following rain, compensatory growth occurred and the rate of increase in canopy size was larger with increasing calcium carbonate application rate. A pH buffer spray gave similar results but was less effective at reducing canopy growth. An improvement in persistence of the reduction in canopy growth by alkaline materials is needed before the potential for retarding leaf growth as an additional agronomic practice can be assessed.

Do pH changes in the leaf apoplast contribute to rapid inhibition of leaf elongation rate by water stress? Comparison of stress responses induced by polyethylene glycol and down‐regulation of root hydraulic conductivity

We have dissected the influences of apoplastic pH and cell turgor on short-term responses of leaf growth to plant water status, by using a combination of a double-barrelled pH-selective microelectrodes and a cell pressure probe. These techniques were used, together with continuous measurements of leaf elongation rate (LER), in the (hidden) elongating zone of the leaves of intact maize plants while exposing roots to various treatments. Polyethylene glycol (PEG) reduced water availability to roots, while acid load and anoxia decreased root hydraulic conductivity. During the first 30 min, acid load and anoxia induced moderate reductions in leaf growth and turgor, with no effect on leaf apoplastic pH. PEG stopped leaf growth, while turgor was only partially reduced. Rapid alkalinization of the apoplast, from pH 4.9 ± 0.3 to pH 5.8 ± 0.2 within 30 min, may have participated to this rapid growth reduction. After 60 min, leaf growth inhibition correlated well with turgor reduction across all treatments, supporting a growth limitation by hydraulics. We conclude that apoplastic alkalinization may transiently impair the control of leaf growth by cell turgor upon abrupt water stress, whereas direct hydraulic control of growth predominates under moderate conditions and after a 30-60 min delay following imposition of water stress.

Reductions in biomass accumulation, photosynthesis in situ and net carbon balance are the costs of protecting Vitis vinifera 'Semillon' grapevines from heat stress with shade covering.

Covering whole vines with shade cloth is used to protect the vines from heat stress, but may have costs on vine productivity through reduced light availability. Our aim was to assess the carbon balance of vines growing with and without shade to quantify the impact of the covering. Whole vines were covered with 70 % shade cloth, and shoot leaf area and leaf, stem and bunch growth were followed over two growing seasons. Photosynthesis was measured in situ in all leaves along selected shoots over the growing season. A carbon balance was constructed from the difference in acquisition of carbon and the sequestration of carbon as biomass across the growing seasons. Shade covering had no initial impact on shoot growth but later reduced leaf growth and later still bunch growth. Stem growth was unaffected. Photosynthetic properties were characteristic of shade leaves, with lower rates and lower light saturation compared with well-exposed leaves. Overall, net photosynthesis was reduced by 40 % by the shade covering and was attributed to the reduced photon flux densities. From the carbon balance, vines were reliant on carbon reserves over 6 weeks after budbreak until current photosynthate increased sufficiently to supply the growth. Shade covering impacted most on biomass accumulation to leaves and bunches at the stage when the vines became autotrophic, consistent with the reduction in carbon acquisition. The markedly high carbon demand by bunches caused a mid-season negative carbon balance, implying that shoots had to draw further on reserves to supply the carbon. Shade covering over whole grapevines exacerbated the imbalance between the supply of and demand for carbon and greatly reduced vine biomass, especially reproductive allocation. Covering vines with shade cloth to protect the vines from heat events, therefore, had major costs in the carbon economy.

Light regulation of endogenous salicylic acid levels in hypocotyls of Helianthus annuus seedlings

Plants growing in canopy shade typically exhibit increased stem elongation and reduced leaf growth. This is as a result of direct interactions between plant photoreceptors sensing the change (reduction) in the ratio of red to far-red (R/FR) light and photosynthetically active radiation (PAR) and plant hormones, and regulating these morphological traits. The effect of the varying light conditions found in shade on endogenous salicylic acid (SA) content was tested, and the possible role of SA in shade avoidance by sunflower ( Helianthus annuus L.) hypocotyls was examined. A logarithmic increase in PAR irradiance levels increased endogenous SA levels roughly 10-fold. Separation of individual light wavelengths (R, FR, and blue) constituting the PAR irradiance of sunlight, established that only FR light had significant and positive effects on endogenous SA levels. Further, a low R/FR ratio significantly increased the endogenous SA content in hypocotyls compared with normal and high R/FR ratios. Uncoupling the effect of R/FR ratio and PAR irradiance on endogenous SA content demonstrated that PAR irradiance is a much stronger signal than FR light-enrichment. Thus, while a low R/FR ratio increases the SA content in sunflower hypocotyls, low PAR, the other component of canopy shade, decreases the SA content much more effectively than low R/FR ratio increases it. Therefore, it appears that SA probably has no direct role in shade avoidance effects.

Sensitivity of the seagrass Cymodocea nodosa to hypersaline conditions: A microcosm approach

Marine water desalination is an emergent strategy to satisfy water demand in coastal regions. The brine originated as a by-product (60–90 psu) can have a significant impact on important marine ecosystems, yet the tolerance thresholds of most key species remain unknown. Therefore, with the aim of filling this gap and providing useful information to mitigate such impacts, we have experimentally assessed the response to hypersaline conditions of the seagrass Cymodocea nodosa. We exposed plants in microcosms to 4 treatments: control (37.2 psu), 44 psu, 54 psu and 62 psu, with three replicate aquaria per treatment for 17 days, and we measured plant response and status. We found that the seagrass shoots endured 44 psu without any apparent damage, but vitality decreased in 54 and 62 psu treatments. Plants at both 54 and 62 psu displayed reduced leaf growth and higher incidence of leaf necrosis, relative to control conditions. In addition, in plants at 62 psu, photosynthetic performances (measured as leaf quantum efficiency) dropped shortly after salt addition, and by the end of the experiment they also suffered substantial mortality. These findings should be viewed with caution, due to the relatively short time-span of the experiment, as in the long-term plants may be less tolerant than reported here. However, our results highlight the need for careful management of brine disposal to prevent deterioration of seagrass habitats; salinity should never exceed 44 psu inside the meadows, to prevent deterioration of C. nodosa habitats.

Metabolomic and physiological responses reveal multi‐phasic acclimation of Arabidopsis thaliana to chronic UV radiation

Biochemical changes in vivo and pathway interactions were investigated using integrated physiological and metabolic responses of Arabidopsis thaliana L. to ultraviolet (UV) radiation (280-400 nm) at 9.96 kJ m(-2) d(-1) over the entire life cycle from seed to seed (8 weeks). Columbia-0 (Col-0) and a UV-B sensitive accession (fah-1) showed significant (P < 0.001) reductions in leaf growth after 6 weeks. Col-0 recovered growth after 8 weeks, with recovery corresponding to a switch from production of phenylpropanoids to flavonoids. fah-1 failed to recover, indicating that sinapate production is an essential component of recovery. Epidermal features show that UV radiation caused significant (P < 0.001) increases in trichome density, which may act as a structural defence response. Stomatal indices showed a significant (P < 0.0001) reduction in Col-0 and a significant (P < 0.001) increase in fah-1. Epidermal cell density was significantly increased under UV radiation on the abaxial leaf surface, suggesting that that a fully functioning phenylpropanoid pathway is a requirement for cell expansion and leaf development. Despite wild-type acclimation, the costs of adaptation lead to reduced plant fitness by decreasing flower numbers and total seed biomass. A multi-phasic acclimation to UV radiation and the induction of specific metabolites link stress-induced biochemical responses to enhanced acclimation.

Impact of a Horizontally Transmitted Endophyte,Balansia henningsiana, on Growth and Drought Tolerance ofPanicum rigidulum

The impact of the horizontally transmitted endophyte Balansia henningsiana on the growth of C4 grass Panicum rigidulum was examined in the greenhouse under control and drought‐stress conditions. Tiller number, leaf number, and the root:shoot ratio were higher and shoot height and leaf area were lower for infected (E+) than for uninfected (E−) plants. Under control conditions, the total biomass was similar for E+ and E− plants. Under drought‐stress conditions, however, E+ plants had less biomass. In response to drought stress, E+ plants reduced leaf growth and increased leaf senescence more than E− plants. After recovery, E+ plants responded by more rapid leaf growth and allocated more resources to the leaf than to the root. Under control conditions, the photosynthetic pigment content of E+ leaves tended to be higher than that of E− leaves. With decreasing leaf water content, the difference in pigment content between E+ and E− plants became smaller. Endophyte infection had no effect on net CO2 assimilation rate or transpiration rate and thus on water use efficiency, regardless of water status. Overall, B. henningsiana was not beneficial to host growth during drought stress. However, infection facilitated rapid leaf regrowth of the host grass after soil water became available.

Additional nitrogen fertilization affects salt tolerance of lemon trees on different rootstocks

Irrigation with saline water is one of the major problems in citrus crop in arid and semi-arid regions. Because rootstock and fertilization play an important role in citrus salt tolerance, we investigated the influence of the nitrogen fertilization and rootstock on salt tolerance of 2-year-old potted Fino 49 lemon trees. For that, trees grafted on Citrus macrophylla (M) or Sour orange (SO) rootstocks were watered for 12 weeks with complete nutrient solution containing either 0 mM NaCl (control, C), 50 mM NaCl (S), 50 mM NaCl with an additional 10 mM potassium nitrate (S + N), or 50 mM NaCl with a 1% KNO 3 (S + Nf) foliar spray application. Trees on M were more vigorous than trees on SO and saline treatments reduced leaf growth similarly in trees on both rootstocks. Trees on SO had a lower leaf Cl − and Na + concentration than those on M. Additional soil nitrogen (S + N) decreased leaf Cl − concentration and increased leaf K + concentration in salinized trees on both rootstocks. However, the salinity-induced reduction leaf growth was similar in S + N and S trees. This was due to osmotic effect, beside leaf Cl − and Na + toxicity, played an important role in the growth response of Fino 49 lemon to the salt stress. Additional foliar nitrogen in the S + Nf treatment also reduced leaf Cl − concentration relative to the S treatment but trees from S + Nf treatment had the lowest leaf growth. Net assimilation of CO 2 ( A C O 2 ), stomatal conductance ( g s) and plant transpiration were reduced similarly in all three salt treatments, regardless rootstock. Salinity reduced leaf water and osmotic potential such that leaf turgor was increased. Thus, the salinity-induced A C O 2 reductions were not due to loss of turgor but rather due to high salt ion accumulation in leaves.

The apoplastic pH and its significance in adaptation to salinity in maize (Zea mays L.): Comparison of fluorescence microscopy and pH-sensitive microelectrodes

The apoplastic ionic milieu contains essential determinants for cell expansion and plant growth. Since pH is a multifunctional basic component of this extracellular space, the knowledge of its behaviour during stress situations is of major importance. In detached leaves of maize (Zea mays L. cvs. Pioneer 3906 and SR 03) the effect of salinity on apoplastic pH was measured to investigate its adaptive role to salt stress applying two different methods: an optical approach using pH-sensitive fluorescent dyes (fluorescein isothiocyanate-dextran (FITC), fluorescein tetramethylrhodamine-dextran (FTMR) and Oregon Green® 488), and an electrophysiological technique, pH-sensitive microelectrodes. Both approaches yielded similar results. In the presence of 100mM NaCl, which was added to the growth medium, apoplastic pH of the salt-sensitive maize genotype Pioneer 3906 leaves increased in maximum by 0.4 units (pH microelectrodes) and by 0.3 units (fluorescent dyes); the salt-resistant SR 03 hardly responded. The same treatment reduced leaf growth by 60% in Pioneer 3906, but only by 40% in SR 03. Since according to acid growth considerations apoplastic pH is an important factor in elongation growth, we suggest that this pH increase is a main cause for reduced leaf growth under salt stress conditions.

Ultraviolet Radiation as a Limiting Factor in Leaf Expansion and Development

Reductions in leaf growth are a commonly observed response to ultraviolet radiation, but the underlying mechanisms remain poorly defined. This study examined the response of leaves exposed to a UV environment across a range of organizational scales, including leaf expansion rate, epidermal cell size and number, biomechanical properties, leaf-water relations and activity of cell-wall peroxidases. Two experimental approaches were used; Lettuce (Lactuca sativa L.) plants were propagated under (a) supplementary UV-B (9 kJ m(-2) day(-1)) in controlled environment (CE) conditions, and (b) field conditions, where plants were placed under three horticultural films with differing UV transmissions. In both experiments, UV-B caused the greatest reductions in leaf expansion and final leaf size, with some reductions attributable to UV-A wavelengths. In supplementary UV-B conditions, adaxial cell size was reduced, while in field plants, both cell size and cell number were lower in an increased UV environment, as was the case with abaxial cells in CE plants. Although leaf turgor and leaf extensibility were not affected by UV wavelengths, breaking strain of leaf tissue was decreased under supplementary UV-B. Cell-wall peroxidase activity was increased in both supplementary UV conditions and in the field, where only a zero UV environment showed no upregulation of cell-wall peroxidase.

Orange varieties as interstocks increase the salt tolerance of lemon trees

SummaryWe investigated the ability of interstocks to increase salt tolerance in lemon trees. We compared 2-year-old ‘Verna’ lemon trees [Citrus limon (L.) Burm.; VL] grafted on Sour Orange (C. aurantium L.; SO) rootstock either without an interstock (VL/SO), or interstocked with ‘Valencia’ orange (C. sinensis Osbeck; VL/V/SO), or with ‘Castellano’ orange (C. sinensis Osbeck; VL/C/SO). Trees were grown under greenhouse conditions and supplied with nutrient solutions containing 0, 30, or 60 mM NaCl. Reductions in leaf growth caused by salt treatment were greatest in non-interstocked (VL/SO) trees, followed by VL/C/SO trees, and were the least in VL/V/SO trees. Although the levels of Cl− and Na+ ions in the roots and stems were not affected by either interstock, leaf concentrations of Cl− and Na+ were higher in VL/SO trees than in VL/C/SO or VL/V/SO trees, suggesting that an interstock in Citrus trees could limit the uptake and transport of such ions to the shoots. Saline-treated VL/SO trees also tended to have the lowest shoot:root (S:R) ratios; so, overall, there was a negative relationship between S:R ratio and leaf Cl- ion concentration. Leaf transpiration (Eleaf) may also be involved in the reduction in leaf Cl− concentration, as interstocked trees had lower Eleaf values at mid-day than non-interstocked trees. Salinity increased leaf concentrations of Ca2+ in VL/C/SO trees and increased both leaf K+ and N concentrations in all trees, regardless of interstock. Salinity reduced leaf water potentials and osmotic potentials, such that leaf turgor was increased in all trees.

Growth of European beech ( Fagus sylvatica L.) saplings is limited by elevated atmospheric vapour pressure deficits

There is an ongoing debate about how European beech might be affected by a future drier climate. While numerous studies have examined the effects of soil drought on beech growth and development, studies investigating the effects of elevated atmospheric water vapour pressure deficit (VPD) are lacking. By increasing VPD in climate and open-top chamber experiments, with moisture in the rooting medium near optimum, we tested the hypothesis that increased VPD negatively affects the growth and development of European beech saplings. In the climate chambers, a reduction in relative air humidity by 40% resulted in a 68% reduction in productivity. Similarly, in the open-top chamber experiment conducted on the forest floor, biomass declined by 30% when relative air humidity was 15% lower. The reduction in biomass was mainly a consequence of a dramatically reduced leaf growth of beech in the elevated VPD treatments. Our results show that growth and development of beech saplings strongly depend not only on soil moisture but also on the prevailing VPD level. We conclude that the vapour pressure deficit is a widely ignored factor which influences the growth and vitality, and possibly also the distribution of European beech. Future forest management schemes under an altered climate should take this factor into account.

Genetic variability for leaf growth rate and duration under water deficit in sunflower: analysis of responses at cell, organ, and plant level

Plants under water deficit reduce leaf growth, thereby reducing transpiration rate at the expense of reduced photosynthesis. The objective of this work was to analyse the response of leaf growth to water deficit in several sunflower genotypes in order to identify and quantitatively describe sources of genetic variability for this trait that could be used to develop crop varieties adapted to specific scenarios. The genetic variability of the response of leaf growth to water deficit was assessed among 18 sunflower (Helianthus annuus L.) inbred lines representing a broad range of genetic diversity. Plants were subjected to long-term, constant-level, water-deficit treatments, and the response to water deficit quantified by means of growth models at cell-, leaf-, and plant-scale. Significant variation among lines was found for the response of leaf expansion rate and of leaf growth duration, with an equal contribution of these responses to the variability in the reduction of leaf area. Increased leaf growth duration under water deficit is usually suggested to be caused by changes in the activity of cell-wall enzymes, but the present results suggest that the duration of epidermal cell division plays a key role in this response. Intrinsic genotypic responses of rate and duration at a cellular scale were linked to genotypic differences in whole-plant leaf area profile to water deficit. The results suggest that rate and duration responses are the result of different physiological mechanisms, and therefore capable of being combined to increase the variability in leaf area response to water deficit.

Root growth of Nicotiana attenuata is decreased immediately after simulated leaf herbivore attack

Image-based non-destructive methods were used to quantify root growth reactions happening within hours following simulated leaf herbivore attack.1 The induction of wound reactions in leaves of seedlings of Nicotiana attenuata led to transiently decreased root growth rates: Upon application of the oral secretions and regurgitants of the specialist herbivore Manduca sexta, a transient decrease in root growth was observed that was more pronounced than if a mere mechanical wounding was imposed. Root growth reduction was more severe than leaf growth reduction and the timing of the transient growth reduction coincided with endogenous bursts of jasmonate (JA) and ethylene emissions reported in literature. The reaction of root growth was superimposed by a strong diel variation of root growth, which was caused by the fluctuating temperature to which the plants were exposed. Apart from the observed root growth reaction, other defense-related traits such as increased nicotine concentration, trichome length and density were activated within 72 h after wounding. Further experiments indicated that the response was elicited by fatty acid-amino acid conjugates that are contained in the oral secretions and that JA signalling is crucial for root-shoot communication here.

Shoot and root growth of hydroponic maize (Zea mays L.) as influenced by K deficiency

Potassium (K) has major biophysical and biochemical functions in plant physiology. However, plant responses to K deficiency at the whole plant level are not always clearly related to these well-known functions of K at the cellular level. The objective of this study was to investigate the morphological response of maize to increasing K deficiency and test to what extent this morphological response can be interpreted in the light of the simple model proposed by Leigh and Wyn Jones, suggesting that biophysical functions are affected first. Maize was grown in a greenhouse under hydroponic conditions. For half of the plants, K was removed from the nutrient solution from the 4th visible leaf stage. The K content in the starved plants dropped from 100 to 30 mM, and was not fully compensated by an increase in other cations. Leaf elongation rates were reduced on K-deprived plants, whereas axile root elongation rates were slightly increased between 45°C days and 75°C days after starvation, and reduced thereafter. During the first part of the starvation period, i.e. under moderate K deficiency (K concentration above 40 mM), all measured variables suggest that the whole plant response may be interpreted as the consequence of the reduced leaf growth, probably due to insufficient turgor pressure or cell-wall extensibility. This general pattern of response is in agreement with the model of Leigh and Wyn Jones. However, during the second part of the starvation period, i.e. under more severe K deficiency (K concentration below 40 mM), malfunction of additional physiological processes (mostly related to biochemical functions like photosynthetic processes) must be considered to explain the plant morphological response.

Root growth dynamics of Nicotiana attenuata seedlings are affected by simulated herbivore attack

Many studies demonstrate resource-based trade-offs between growth and defence on a large timescale. Yet, the short-term dynamics of this growth reaction are still completely unclear, making it difficult to explain growth-defence trade-offs mechanistically. In this study, image-based non-destructive methods were used to quantify root growth reactions happening within hours following simulated herbivore attack. The induction of wound reactions in Nicotiana attenuata in the seedling stage led to transiently decreased root growth rates. Application of the oral secretion of the specialist herbivore Manduca sexta to the leaves led to a transient decrease in root growth that was more pronounced than if a mere mechanical wounding was imposed. Root growth reduction was more pronounced than leaf growth reduction. When fatty acid-amino acid conjugates (FACs) were applied to wounds, root growth reduction occurred in the same intensity as when oral secretion was applied. Timing of the transient growth reduction coincided with endogenous bursts of jasmonate (JA) and ethylene emissions reported in literature. Simulation of a wound response by applying methyl jasmonate (MeJA) led to more prolonged negative effects on root growth. Increased nicotine concentrations, trichome lengths and densities were observed within 72 h in seedlings that were treated with MeJA or that were mechanically wounded. Overall, these reactions indicate that even in a very early developmental stage, the diversion of plant metabolism from primary (growth-sustaining) to secondary (defence-related) metabolism can cause profound alterations of plant growth performance.

Nitrate signalling to stomata and growing leaves: interactions with soil drying, ABA, and xylem sap pH in maize

Increasing the nitrate (N) concentration in the rooting substrate above deficiency decreased stomatal conductance and leaf growth rate compared with sufficient N in maize seedlings (Zea mays L.) growing in drying substrate. Novel effects were detected when N in the non-deficient range was supplied directly to the xylem of detached shoots: concentrations above 2.0 mol m-3 KNO3 reduced transpiration, and concentrations above 12 mol m-3 KNO3 reduced leaf growth rate. Evidence is provided that the novel effects of N on transpiration and growth were mediated by pH-based ABA redistribution. ABA at 0.05 mol m-3, whilst ineffective alone, sensitized leaf growth to increases in KNO3 concentration (from 3.0 mol m-3), and the capacity of higher concentrations of ABA to reduce growth was enhanced by KNO3. Transpiration was sensitively reduced by KNO3, ABA, or buffers adjusted to pH 6.7-7.0 (compared with buffers adjusted to pH 5.0) alone. Nevertheless, a synergistic effect of KNO3 and either ABA or buffers adjusted to pH 6.7-7.0 was observed. Buffers of pH 5.6 supplied to detached shoots alleviated the depression of transpiration caused by 12 mol m-3 KNO3. Buffers adjusted to pH 6.7 increased the sensitivity of growth to KNO3. Xylem sap extracted from intact seedlings growing in drying soil exhibited an initial increase in N concentration, followed by a decrease at progressively lower soil water potentials. The importance for novel N signalling above deficiency is discussed with reference to the generality of fluctuations in soil and xylem N concentration within this range.

Growth, abscisic acid content, and carbon isotope composition in wheat cultivars grown under different soil moisture

Changes in dry matter accumulation and allocation, abscisic acid content and carbon isotope composition of three wheat cultivars from dry, middle and wet climate regions were recorded at full maturity after exposure to different watering regimes (100, 50 and 25 % field capacity). Compared with the wet climate cultivar, the dry climate cultivar showed lower stem height, total leaf area, total dry biomass and total grain dry mass, and higher root/shoot ratio, abscisic acid content and carbon isotope composition under all watering regimes. Both water-limited treatments significantly reduced leaf growth and increased dry matter allocation into the roots leading to a significant raise of root/shoot ratio in all cultivars tested. In addition, drought affected morphological and physiological properties more in the dry climate cultivar than in the wet climate cultivar.