Range Of Leaf Water Potentials Research Articles

Divergent root water uptake depth and coordinated hydraulic traits among typical karst plantations of subtropical China: Implication for plant water adaptation under precipitation changes

Plantations, as a form of vegetation restoration, play a critical role in the recent greening of the earth. However, the sustainability of the restored vegetation under precipitation changes has rarely been estimated, largely because of a lack of knowledge about the related plant water adaptation. In the current study, six tree species, including evergreen and deciduous, were selected from a typical karst forest types in Southwest China, a hotspot of earth greening. Root water uptake depth and hydraulic-related traits of stem and leaf were studied. Our results showed that, in the wet season, the evergreen species mainly rely on soil water sources from a depth of 0–30 cm (49.74%), while the deciduous use deeper layers of 30–70 cm in depth (47.58%). In the dry season, the deciduous species shed their leaves while the evergreens utilized soil water at a depth of 0–10 cm with an average absorption of 66.96%. The evergreen species exhibit higher xylem-cavitation resistance (reflect by P50) and leaf-turgor maintenance capacity (reflect by Ψtlp) than the deciduous. The significant correlations between plant water uptake depth and water potential, and hydraulic traits and water uptake depth indicate the different water adaptation strategies of evergreen and deciduous. In detail, evergreen species that rely on shallower water sources exhibit a larger diurnal range of leaf water potential, more xylem-cavitation resistant and leaf-turgor maintenance capacity. In contrast, deciduous species, with deeper water uptake, show narrower diurnal range of leaf water potential with lower drought resistance of stem and leaves. Our results highlight the probable divergent response (such as growth restriction and hydraulic failure) of evergreen and deciduous plantation tree species to the changing precipitation patterns under climate change.

Leaf hydraulic parameters are more plastic in species that experience a wider range of leaf water potentials

Abstract Many plant species experience large differences in soil moisture availability within a season, potentially leading to a wide range of leaf water potentials (ΨLEAF). In order to decrease the risk of leaf dehydration, among species, there is a continuum ranging from strict control (isohydry) to little control (anisohydry) of minimum ΨLEAF. In central Texas USA, species are exposed to a range of soil moisture from wet springs to hot, dry summers. There are diverging water management strategies among the four dominant woody species in this system; two of these species are more isohydric (Prosopis glandulosa, Quercus fusiformis) while two others are more anisohydric (Diospyros texana, Juniperus asheii). To maintain leaf turgor and photosynthesis during periods of limited soil moisture, anisohydric species may adjust leaf hydraulic parameters more than isohydric species. To test this hypothesis, we quantified iso/anisohydry from 3 years of ΨLEAF predawn and midday measurements, and we measured the changes in turgor loss points (ΨTLP), osmotic potential at full hydration (Ψπ100), and resistance to leaf hydraulic dysfunction (leaf P50) throughout the spring and summer of 2016. Diospyros and Juniperus experienced more negative ΨLEAF and adjusted ΨTLP and Ψπ100 in response to both drying soils during the summer also in response to rainfall events during September. In contrast, the more isohydric species (Quercus and Prosopis) did not appear to adjust ΨTLP or Ψπ100 in response to soil moisture. The more anisohydric species also adjusted leaf P50 during periods of reduced soil moisture. Our results suggest that species that experience wider ranges of ΨLEAF have a greater ability to alter leaf hydraulic properties. This provides insight on how species with different strategies for water potential regulation may modify properties to mitigate drought effects in the future. A plain language summary is available for this article.

Urban forest resilience through tree selection—Variation in drought tolerance in Acer

It is widely recognized that trees contribute a range of ecosystem services in urban environments. However, the magnitude of their contribution is closely related to their physiological condition and capacity to persist within our towns and cities. Root loss during transplanting, limited soil volume, disruption to soil hydrological processes and impermeable surfaces result in water deficits being major physiological stress limiting the performance of urban trees. The leaf water potential at turgor loss (ΨP0) provides a robust measure of drought tolerance since a more negative ΨP0 allows the leaf to maintain physiological function over a wider range of leaf water potentials and, by implication, soil matric potentials (Ψsoil). In this study, ΨP0 was calculated for 27 maple (Acer) genotypes based on a known linear relationship between the osmotic potential at full turgor (Ψπ100) and ΨP0. In spring, ΨP0 varied between −1.4MPa in Acer carpinifolium and −2.7 in both Acer rubrum ‘Northwood’ and Acer x freemanii ‘Autumn Blaze’. During summer, Acer spicatum had the highest ΨP0 at −1.6MPa and Acer monspessulanum had the lowest ΨP0 at −4.3MPa. Significant differences in ΨP0 were found between cultivars of A. rubrum and Acer saccharum. A highly significant relationship was found between seasonal osmotic adjustment and summer ΨP0 suggesting that osmotic adjustment is a driving force for summer ΨP0 in Acer leaves. These data confirm the wide range of tolerance to water deficits in Acer and give important insight into the potential of species to tolerate periods of low water availability by providing quantitative data not previously available. The technique shows great promise as a screening tool for the drought tolerance of new and traditional plant material. This data will be highly relevant for those selecting trees for urban sites as well as for nurseries seeking to evaluate genotypes for production purposes.

Photosynthesis of cotton near-isogenic lines introgressed with QTLs for productivity and drought related traits

Quantitative trait loci (QTLs) for yield and drought related traits were exchanged via marker-assisted selection (MAS) between elite cultivars of two cotton species, Gossypium barbadense ( GB) cv. F-177 and Gossypium hirsutum ( GH) cv. Siv’on. The resulting near-isogenic lines (NILs) manifested in many cases the expected drought-adaptive traits, but rarely exhibited an advantage in yield relative to the recipient parents. In the current study we characterized the photosynthetic activity of two selected NILs and their recipient parents under dryland and irrigated field conditions. The GB NIL exhibited a stable net rate of CO 2 assimilation ( A) across a wide range of leaf water potentials with a notable advantage over its recipient parent, F-177, under severe drought, associated with lower stomatal limitation, greater maximum velocity for carboxylation of Rubisco and greater electron transport rate. The GH NIL exhibited greater mesophyll conductance under drought conditions than its recipient parent, Siv’on, but these genotypes did not differ in A. Nevertheless, both NILs did not differ from their recipient parents in yield. This study provides further support to our previous conclusion that MAS is a useful means to enhance drought-adaptive traits of cotton but complimentary conventional breeding is required to achieve high yield potential.

Effect of leaf Water potential on cold tolerance of Coffea arabica L.

Young coffee plants from cultivar Mundo Novo of Coffea arabica were grown without irrigation for 32 consecutive days, to evaluate the effect of leaf water potential on damage caused by low temperatures, under controlled conditions. A wide range of leaf water potentials were evaluated, from - 0.45MPa (wet soil) at the beginning of the experimental period, to - 4.8MPa (severe leaf wilting) at the end. Results showed that under moderate water stress, there was a higher frequency of undamaged plants and lower frequency of severely damaged plants. These results help explain part of the regional variability observed after a frost and stress the importance of new studies associating cold and drought tolerance in coffee.

Photosynthesis and Chlorophyll a Fluorescence of two Poplars under Water Stress

In order to estimate drought tolerance in two species of Populus, Populus alba var. pyramidalis Bunge and Populus nigra L. var. thevestina (Dode), widely planted at the southern margin of the Taklimakan Desert, responses of net photosynthesis and chlorophyll a fluorescence to irradiance and water stress were examined under laboratory conditions. Results showed that P. alba exhibited stronger drought tolerance than P. nigra. A linear relationship between net photosynthetic rates (An) and electron transport rates (ETR) was found in both poplars under different irradiance and leaf water potentials. Net photosynthetic rates (An) in the two poplars significantly correlated linearly with the photochemical efficiency of the saturation light-adapted leaves throughout the range of leaf water potentials, suggesting that the leaf photochemical efficiency in saturation light-adapted leaves can be used to estimate leaf photosynthetic capacity and leaf water conditions in the two poplars within a magnitude of air temperature between 20 and 30°C.

082 OFFSETTING EFFECTS OF REDUCED ROOT HYDRAULIC CONDUCTIVITY AND OSMOTIC ADJUSTMENTIN DROUGHT-STRESSED PEACH, OLIVE, CITRUS, AND PISTACHIO

Root hydraulic conductivity (Lp) and osmotic potential (π) were measured in young, drought-stressed and non-stressed peach (Prunus persica), Olive (Olea europea), Citrumelo (Citrus paradisi x Poncirus trifoliata) and Pistachio (Pistachia integerrima) plants. Drought stress reduced Lp 2.5 to 4.2-fold, depending on species, but π was reduced only in expanded citrumelo leaves and unexpanded olive leaves by 0.34 and 1.4 MPa, respectively. A simulation model of plant water uptake and leaf water relations was constructed to quantify the offsetting effects of reduced Lp and osmotic adjustment (OA) on turgor maintenance. For olive data, a 2.5-fold reduction of Lp caused a linear decrease in turgor pressure difference between stressed and non-stressed plants, such that the effect of OA was totally offset at a leaf water potential (stressed) of ≈ -3.0 MPa. For citrumelo, because the degree of OA was lower, the water potential at which the effects of OA and reduced Lp were offsetting with respect to turgor maintenance was ≈ -0.6 MPa. The analysis suggests that some level of stomatal closure would be necessary to extend the water potential range over which stressed plants maintain higher turgor than non-stressed plants for citrumelo. Conversely, no degree of stomatal closure would be required of stressed olive plants to maintain higher turgor than non-stressed counterparts over a physiologically meaningful range of leaf water potential.

Stomatal and Photosynthetic Adjustment to Water Deficit as the Expression of Heterosis in Sunflower

Photosynthetic adjustment is a major mechanism of plant adaptation to drought. The objective was to determine the heterosis response to internal water deficit and to drought acclimation in sunflower (Helianthus annuus L.). Stomatal resistance, net photosynthesis, and water use efficiency were analyzed in relation to the leaf water potential reduction in acclimated and non‐acclimated plants for two F1 hybrids (Flamme and Viki) and their homozygous parent lines. The experiments were carried out in a controlled growth chamber on two series of plants previously acclimated or not acclimated to a 6‐d drought period with a water deficiency resulting in leaf water potentials of −2 MPa. The tolerance to internal water deficit in nonacclimated plants is a genotypic trait, but Stomatal and photosynthetic adjustment appeared as a heterosis response shown by delayed Stomatal closure from −1.50 to −1.91 MPa of the F1 hybrid Viki and from −1.52 to −1.95 MPa for the F1 hybrid Flamme. These F1 hybrids were also characterized after acclimation by higher net photosynthesis and water use efficiency over a wide range of leaf water potentials during the leaf water deficit. Thus, Stomatal adjustment appears to be closely associated with heterosis in sunflower which can develop a tolerance to internal water deficit in F1 hybrids; the tolerance is at least equal to that of the best homozygous lines. The parental line performance thus cannot provide decisive information on the F1 hybrid performance for this trait in breeding programs.

Use of estimated leaf water potential for irrigation timing of wheat

A regression model with which the leaf water potential (LWP) of wheat can be estimated from soil water content and potential evapotranspiration was developed from data generated during 1986. This model was used during the 1988 wheat season to decide when to relieve crop water stress by irrigation. Seven values of LWP, viz. −1.2; −1.4; −1.6; −1.8; −2.0; −2.2 and −2.4 MPa were considered as possible threshold values for irrigation. The soil water content, determined twice weekly, was used to estimate actual evapotranspiration. Midday LWP was determined on selected cloudless days and found to be most suitable as an irrigation timing criterion for several reasons. Firstly, the ratio evapotranspiration/pan evaporation was at least as well related to LWP as to soil water status. Secondly, strong relationships between grain yield, seasonal evapotranspiration, peak evapotranspiration rate and threshold LWP exist, and finally, indications are that yields near the upper limit of the evapotranspiration-yield relationship are ensured. Leaf water potential for irrigation timing purposes could be satisfactorily estimated by the model if calibrated over a sufficiently large range of leaf water potentials. The threshold potential for maximum water use efficiency was −1.82 MPa and for maximum yield −1.44 MPa.

Turgor Maintenance in Rosa rugosa Grown at Three Levels of Nitrogen and Subjected to Drought

Abstract The influence of N fertilization on turgor maintenance was determined in leaves from well-watered and droughted Rosa rugosa L. Plants were fertilized for 60 days with a complete fertilizer with N at levels of 0, 200 or 500 ppm and then subjected to several drought cycles for 22 days. Plants receivmg no N were stunted and chlorotic, but had the greatest full saturation turgor pressure and symplastic osmolality, and the lowest full saturation osmotic potential, after drought. These plants also maintained higher turgor across a range of leaf water potentials and relative water contents. Leaf water content at full turgor and relative water content at the turgor loss point were also lower in plants without N. N treatments of 200 and 500 ppm had similar water relations under both well-watered and drought conditions.

The influence of plant water deficit on photosynthesis and translocation of 14C‐labeled assimilates in cacao seedlings

The effect of plant status on net assimilation and translocation of “C‐labeled assimilates in cacao (Theobroma cacao L.) was evaluated. As plant water potential (ψ) decreased from −0.5 to −1.0 MPa, neither net assimilation nor the rate of label translocation out of the l4CO,‐fed leaf were affected, but as iji fell between −1.0 and −1.5 MPa, net assimilation decreased sharply and label retention increased greatly. Translocation out of source leaves was strongly correlated with net assimilation (r =−0.93). Translocation velocity, assessed by detection of labeled assimilates in sink leaves, was sensitive to plant water deficit, and it declined linearly (r = 0.97) throughout the range of leaf water potentials observed. The results may be explained by reduction in the velocity of assimilate movement within the sieve elements, reduction in supply of labeled assimilates from source leaves, reduction in sink strength or diversion of assimilates to sites of storage or utilization.

Intraspecific variation in the water relations of Salix arctica, an arctic-alpine dwarf willow

The seasonal and diurnal water relations were investigated within arctic and alpine populations of the dwarf willow Salix arctica. Marked differences that were habitat dependent (e.g. xeric vs. mesic) occured both within and between the populations. The environmental variables that most affected plant water balance and the bulk tissue water relations were soil water potential (Ψsoil) and the leafto-air water vapor pressure gradient (Δw), however, low soil temperature (<4.0° C) also had a marked effect in the wet to mesic habitats. The effects of declining Ψsoil and increased ‡w were most pronounced in the plants growing in xiric habitats in both populations. Stomatal response to increased ‡w was two-fold greater in alpine versus arctic plants and is hypothesized to have arisen in response to more frequent exposure to the higher evaporative conditions of alpine existance. Seasonal fluctuations in the osmotic potential closely followed changes in Ψsoil, suggesting that these were active rather that passive changes. Additionally, plants from xeric habiats had a lower bulk tissue elastic modulus (more elastic tissues) in both arctic and alpine populations. The osmotic and elastic properties enhanced turgor maintenance over a broad range of leaf water potentials and during periods when ‡w was high. Turgor maintenance also correlated to continued transpiration despite fluctuations in soil and atmospherically induced water deficits. Arctic habitats have a shorter growing season, lower soil temperatures due to the presence of permafrost, but higher soil water potentials and lower leaf-to-air vapor pressure gradients than alpine habitats. The observed variation in patterns of stomatal conductance and in values of tissue water relations characteristics between arctic and alpine populations of S. arctica is hypothesized to have arisen in response to these different environmental regimes which represent different selective regimes that occur along the arctic-alpine environmental continuum inhabited by this wide ranging species.

Leaf Gas Exchange and Water Relations of Lupins and Wheat. III. Abscisic Acid and Drought-Induced Stomatal Closure

Changes in the content of endogenous abscisic acid (ABA) were followed in glasshouse experiments during stomatal closure induced by drought in leaves of lupin (Lupinus cosentinii Guss. cv. Eregulla) and wheat (Triticum aestivum L. cvv. Gamenya and Warigal), species which differ in stomatal sensitivity to changes in leaf water potential. Increases in bulk leaf ABA concentration were closely correlated with decreases in leaf conductance in both species. In lupin, substantial increases in ABA and decreases in conductance occurred over a very narrow range of leaf water potential. ABA concentrations in wheat leaves were highly negatively correlated with bulk leaf turgor, but there was no significant relationship between ABA and turgor in lupin. However, ABA accumulated progressively in the leaves of both species as soil water content decreased. Stomatal closure in lupin could be induced by supplying exogenous ABA to detached leaves via the transpiration stream at concentrations of 10-4 to 10-2 mol m-3 of (+)-ABA. Abaxial stomata closed more readily than those on the adaxial surface in response to both drought and applied ABA. Stomatal response to ABA was not affected by the presence of the cytokinin zeatin, and zeatin by itself had no effect on conductance. When treatments designed to reduce endogenous cytokinin concentrations were imposed (prolonged leaf detachment or prior drought), stomatal response to low concentrations of ABA was enhanced. However, such treatments did not significantly change the stomatal response to high ABA concentrations, nor affect the stomatal conductance of leaves supplied with water alone. It is concluded that drought-induced stomatal closure could be mediated by ABA in both wheat and lupin, despite the initially small change in leaf water status in the latter species.

Field water relations of three temperate vines.

A two year comparative field water relations study was conducted in central Maryland on three sympatric temperate lianas, Lonicera japonica, Vitis vulpina and Parthenocissus quinquefolia. Seasonal physiological activity was longer by approximately 9 weeks in the evergreen L. japonica, while peak rates of stem elongation were 4-10 fold higher in the two deciduous species. There were marked differences in vascular anatomy and water use patterns among the three species, however all three evidenced varying degrees of stomatal closure in response to decreasing soil water availability and increasing atmospheric evaporative demand. The range of leaf water potentials measured in these species was quite narrow in comparison to other temperate woody species. Two of the species showed no alterations in their tissue water release properties in response to decreased soil water availability, while V. vulpina showed a limited capacity in this regard. Most significant among the species differences in water relations were the conservative water use patterns of P. quinquefolia, and the midday maxima of transpirational water loss measured in L. japonica compared to the morning peaks in traspiration for the two deciduous species. The differences found in anatomy, leaf phenology, climbing mechanics, water relations and canopy development among these three sympatric vines implies a spatial and temporal partitioning of light and water resources and emphasizes the diversity of morphological-physiological suites of characters present among species co-occurring in the same macrohabitat.

Leaf Photosynthesis and Conductance of Selected Triticum Species at Different Water Potentials

Leaf gas exchange characteristics of a desert annual (Triticum kotschyi [Boiss.] Bowden) and the wheat cultivar TAM W-101 (Triticum aestivum L. em Thell) were compared over a range of leaf water potentials from -0.50 to -2.9 megapascals. At an ambient [CO(2)] of 330 microliters per liter, T. kotschyi had higher conductance and CO(2) assimilation (A) at a given water potential than T. aestivum. Under well watered conditions, A versus internal CO(2) concentration (C(i)) response curves for both species were similar in shape and magnitude, and the higher A of T. kotschyi at an ambient [CO(2)] of 330 microliters per liter was mostly related to the higher stomatal conductance of T. kotschyi. The higher conductance of T. kotschyi than T. aestivum under well watered conditions was associated with higher C(i) and lower water use efficiency. Under water deficits, however, C(i) at 330 microliters per liter ambient [CO(2)] did not differ significantly between species. T. kotschyi had higher A under water deficits than T. aestivum primarily because its A versus C(i) response curves had higher A at C(i) values above about 150 microliters per liter. The results show that conductance played an important role in the high A of T. kotschyi under well watered conditions, but under water deficits the high A of T. kotschyi was related more to the maintenance of a higher capacity for mesophyll photosynthesis.

Turgor and osmotic relations of the desert shrub Larrea tridentata

Abstract Leaf water relations characteristics of creosote bush, Larrea tridentata, were studied in view of previous reports that its leaves commonly experience zero or negative turgor under dry conditions. Leaf turgor loss point () was determined by a pressure‐volume method for samples subjected to a hydration procedure and for untreated samples. Hydration caused to increase by as much as 3 M Pa. Hydration of samples also caused changes in other leaf water relations characteristics such as symplastic solute content, tissue elasticity and symplasmic water fraction, but total leaf solute content was unchanged. Comparison of our field plant water potential data with values of obtained by the two methods resulted in predictions of turgor loss during part or all of a diurnal cycle based on hydrated samples, and turgor maintenance (at least 0.3 MPa) based on untreated samples. Pooled data for obtained from both partially hydrated and untreated samples showed that L. tridentata maintains fairly constant levels of turgor over a wide range of leaf water potential. Dilution of cell contents by apoplastic water introduced significant errors in psychrometric determinations of osmotic potential in both frozen and thawed leaf tissue and expressed cell sap. Use of these values of osmotic potential resulted in predictions of zero turgor at all plant water potentials measured in the field.

Effects of water stress and salinity on photosynthesis of pistachio

Responses of Pistachia vera L. cultivar ‘Kerman’ to water stress were typical of those of xerophytic plants. Although photosynthesis declined with decreasing leaf water potential, plants continued to photosynthesize until a leaf water potential of as low as −5 MPa was reached. Some measure of turgor was maintained at very low leaf water potential (−6 MPa), which is a rarity in woody plants. However, at the lower range of leaf water potential, the plants were less efficient in their water use. Salinity levels up to 225 mM Cl − did not have any effect on photosynthetic rate or on stomatal and mesophyll resistances to the diffusion of CO 2. Chlorophyll content and leaf water, osmotic and turgor potentials were also unaffected by this level of salinity, despite significantly higher concentrations of Cl − and Na + in the leaves. When the salinity level was raised to 400 mM Cl −, a significant increase in leaf succulence diluted chloride and sodium concentrations (on a tissue-water basis) to values less than in leaves of plants treated with 225 mM Cl −.

Phenology and Water Relations of Three Plant Life Forms in a Dry Tree‐Line Meadow

The relationship between physiological and phenological adaptations to drought was examined in three species of different life form in a tree—line meadow in the Sierra Nevada. Soil moisture in this meadow is predictably high following snow melt, but decreases through the summer because of infrequent precipitation. There were large differences among the life forms in allocation patterns and in vegetative and reproductive phenologies, but there were no comparable differences in their water relations. The annual Polygonum minimum and the hemicryptophyte Penstemon heterodoxus respond to decreasing soil moisture by reducing conductance; they consequently maintain a longer activity period. The geophyte Saxifraga aprica has little stomatal control of water loss and becomes dormant as soils dry. Penstemon heterodozus is a long—lived perennial with an intermediate root:shoot ratio (1.9), evergreen leaves, and high allocation to vegetative structures. Leaf growth occurs during the first part of the season, while the soil is moist. Flowering beings at the end of shoot growth, and fruits dehisce 3 mo after snow melt. During a dry summer, plants were observed to lose many leaves; 25% of the plants died, and none of the remaining plants flowered the next year. Although Penstemon decreases conductances under a limited range of leaf water potential, it is still not well adapted to long dry periods. Polygonum minimum is a very small annual with a low root:shoot ratio (0.15) and an indeterminate growth pattern. After a period of solely vegetative growth, leaves and floral structures are produced continually until °soil reaches —5 MPa. Seeds dehisce °2 mo after snow melt. Total seed production is a function of the length of the growth period. Regulation of water balance by stomatal control assures adequate time for fruit production. Other annuals in the community had a very similar phenological pattern. Saxifraga aprica has a high root:shoot ratio (5.0) and a small basal rosette of leaves. Leaf growth occurs for only 2 wk and is followed immediately by flowering. Only 5—10% of the plants flower each year. Fruits mature 5 wk after snow melt, and plants become dormant at —0.5 Mpa or within 6 wk of snow melt. This species and the other ephemeral geophytes complete their growth during the period of predictable high soil moisture, and therefore require little ability to regulate water balance. This study provides evidence that high diversity in life forms is maintained in environments with high yearly variability in the lengths of the drought and snow—free periods. No single set of phenological and physiological characteristics is optimally adapted to the variability in the subalpine environment that was studied, and therefore several adaptive strategies co—occur.

Evaluation of the hydraulic press for measurement of leaf water potentials in cassava

SUMMARYIn the application of the Scholander pressure chamber technique to cassava water relations studies, the leaf water potential measured on central lobules was initially compared with that measured on entire leaves (including petiole). Measurements made using both a Campbell-Brewster hydraulic press and a pressure chamber of the leaf water potential in six different cassava clones were also compared. Although the central lobules showed a greater sensitivity to moisture loss after sampling than entire leaves, their leaf water potential was in close agreement with those measured on the entire leaves (r3 = 0·96). Therefore, for routine and field estimates in cassava, measurements made on the central lobules may be used to avoid the large reduction in total leaf area. The Campbell-Brewster hydraulic press satisfactorily estimated leaf water potential in M.Col. 1684 clone, which had the longest and narrowest lobules, but in other clones the leaf water potential was overestimated at high leaf potential (&gt; -12·5) and underestimated at low water potentials (&lt; -12·5). Over a wide range of leaf water potentials, a poor relationship between leaf water potentials estimated with hydraulic press and with the pressure chamber was observed for cassava because press estimates are influenced by lobule length and lobule width.

Kinetics of Tall Fescue Leaf Elongation: Responses to Changes in Illumination and Vapor Pressure1

When measured at high resolution, elongation of tall fescue (Festuca arundinacea Schreb.) leaves is shown to be rapidly and highly responsive to step changes in illumination and atmospheric water potential. Furthermore, elongation rates are seen to be quite unstable under presumably constant conditions; sharp fluctuations and rhythmic oscillations are often observed when no external changes are imposed. The diurnal pattern of leaf elongation (15/9 hour day/night, 25 C) displayed higher nighttime rates, a sharp burst of growth at the beginning of the dark period, and a temporary depression of growth at the beginning of the light period. Lag times for responses to changes in illumination were 5 to 6 min. A decrease in vapor pressure deficit from 1.7 kPa to 0.3 kPa resulted in a nearly immediate (&lt;1 min lag time) acceleration of leaf elongation rate. The response to increased vapor pressure was seen in the light and dark. Plants growing in the light and at vapor pressure deficits of around 1.7 kPa frequently exhibited growth oscillations. The frequency of the oscillations in elongation rates ranged from 2 to 3 cycles cycles hour−1, and the amplitudes ranged from 0.05 to &gt;1 mm hour−1. Concurrent measurements of transpiration and carbon exchange revealed those two processes to be cycling at the same frequency; however, they were out of phase with elongation. Leaf elongation was at or near a cyclic maximum when transpiration and photosynthesis were at a cyclic minimum. It is argued that leaf elongation rate is, a priori an indicator of the flux of water into the cells that drive leaf expansion. Only changes in leaf water status, as modulated by stomata and vapor pressure deficits, could account for the observed rapid, reversible, and rhythmic changes in elongation/water flux. The results suggest that tall fescue leaf elongation is frequently limited by leaf water status, even in well‐watered plants and that stomatal movement and leaf extension are modulated within a common range of leaf water potentials.