Influence of Phosphorus Application on Water Relations, Biochemical Parameters and Gum Content in Cluster Bean Under Water Deficit

Relative water content (RWC), leaf water potential (_w) and osmotic potential (_s), contents of chlorophyll (Chl) a, Chl b, soluble sugars, and seed quality (gum content) were used to evaluate the role of phosphorus in alleviation of the deleterious effect of water deficit in clusterbean (Cyamopsis tetragonoloba L. Taub). Under water stress, _w, _s, and Chl and gum contents decreased and soluble sugar contents increased. Phosphorus application increased Chl and sugar contents in control plants and ameliorated negative effects of water stress.     

Drought resistance of 2-year-old saplings of Mediterranean forest trees in the field: relations between water relations,hydraulics and productivity

Drought resistance was examined for 2-year-old saplings of Ceratonia siliqua L., Olea oleaster Hoffmgg. et Link., Quercus suber L. and Q. pubescens Willd. growing in the field in Sicily, with the aim of testing their possible use in the reforestation of degraded areas. To this purpose, leaf conductance to water vapour (g _L), transpiration rate (E _L), relative water content (RWC) and water potential (_L) were measured between pre-dawn and sunset, monthly from May to November. Parallel measurements of loss of hydraulic conductance of twigs of the current year (PLC) were made together with an estimate of whole-plant hydraulic conductance (K _PLANT) on the basis of the ratio of maximum E _L to (_PD-_MIN), where _PD is pre-dawn _L and _MIN is the minimum diurnal _L. C. siliqua saplings maintained high g _L throughout the study period with high RWC (over 90%) and _L. They grew rapidly and increased their foliage area (A _L) by over 60% from May to winter rest. This was accompanied by low twig PLC (about 20% in September) resulting in high K _PLANT all through the study period. In contrast, O. oleaster saplings underwent distinct dehydration in July, i.e. they showed stomatal closure due to a drop in RWC (to 75%) and _L (to the turgor-loss point). This was apparently due to twig cavitation, resulting in a strong decrease of K _PLANT. Plants, however, tolerated summer drought, and showed growth (A _L increased by 15% from May to November). Saplings of Q. suber and Q. pubescens were much more vulnerable to twig cavitation (PLC was 35–48% from June to November) than the other two species, and their average K _PLANT was lower. Saplings of Q. suber and Q. pubescens grew only during the wet spring period, and no new foliage was produced thereafter. Changes in twig hydraulic conductance played a dominant role in determining changes in K _PLANT in that the two variables were well correlated to each other (r=0.68 for P=0.001). We conclude that C. siliqua is an ideal candidate for reforestation of Sicilian degraded areas as is O. oleaster that, however, requires some additional water supply in the summer, at least during the early years after plantation. In wetter locations of Sicily, to an altitude between 0 and 500 m, Q. suber and Q. pubescens can be used for reforestation with expected higher competitiveness of the latter over the former species.     

Seasonal water potential changes and proline accumulation in mediterranean shrubland species

We studied the water relations of 6 shrub and 3 tree species typical of the mediterranean climate region of central Spain to identify differential responses to water stress between and within species, and to determine if free proline concentration in leaves could be used as a water stress indicator. Predawn and midday water potentials (w) on a seasonal basis, relative water content (RWC), leaf mass per area, foliar nitrogen and free proline concentrations were measured. The lowest water potentials were observed at the end of the summer, with recovery to higher water potentials in the fall and winter seasons. Species differed regarding the annual w fluctuation. Thymus zygis, Halimium viscosum, Genista hirsuta and Juniperus oxycedrus exhibited the most negative midday and predawn w (both less than -6 MPa) with a large magnitude of response to changing conditions in soil moisture of the upper horizon of the soil. Lavandula pedunculata and Cistus ladanifer showed a moderate response. Quercus rotundifolia, Quercus faginea and Retama sphaerocarpa showed a modest response. The w of different size individuals of Quercus rotundifolia and Cistus ladanifer were compared. The annual w fluctuation was greater in small individuals as compared to large individuals. In every species, there was an increase in proline concentration of bulk leaf tissues when predawn w dropped below -5 MPa. Small plants of Cistus ladanifer reached lower water potentials and also higher concentration of proline than bigger plants. Proline could possibly be used as a drought stress indicator in every species except Q. rotundifolia. It is suggested that in addition to water stress avoidance due to deep root systems, some mechanisms of water stress tolerance may operate among shrub and tree species of central Spain.     

Leaf Anatomy,Inclination, and Gas Exchange Relationships in Evergreen Sclerophqldous and Drought Semideciduous Shrub Species

There are significant differences in leaf life-span among evergreen sclerophyllous species and drought semideciduous species growing in the Mediterranean maquis. Cistus incamus, which has a leaf life-span of four-eight months, was characterised by the highest net photosynthetic rates (P _N), while Quercus ilex and Phillyrea latifolia, which maintain their leaves two-three and two-four years, respectively, had a lower P _N. The longer leaf life-span of the two evergreen sclerophyllous species may be justified to cover the high production costs of leaf protective structures such as cuticle, hairs, and sclereids: cuticle and hairs screen radiation penetrating into the more sensitive tissues, and sclereids have a light-guiding function. Q. ilex and P. latifolia have the highest leaf mass/area ratio (LMA = 209 g m^-2) and a mesophyll leaf density (2065 cells per mm² of leaf cross section area) about two times higher than C. incanus. In the typical evergreen sclerophyllous species the steepest leaf inclination ( = 56°) reduces 42 % of radiation absorption, resulting in a reduced physiological stress at leaf level, particularly in summer. C. incanus, because of its low leaf life-span, requires a lower leaf investment in leaf protective structures. It exhibits a drastic reduction of winter leaves just before summer drought, replacing them with smaller folded leaves. The lower leaf inclination ( = 44°) and the lower LMA (119 g m^-2) of C. incanus complement photosynthetic performance. Water use efficiency (WUE) showed the same trend in Q. ilex, P. latifolia, and C. incanus, decreasing 60 % from spring to summer, due to the combined effects of decreased CO₂ uptake and increased transpirational water loss.     

Leaf water relations characteristics of Lupinus angustifolius and L. cosentinii

Summary Lupins (Lupinus angustifolius and L. cosentinii) growing in 321 containers in a glasshouse were exposed to drought by withholding water. Leaf water potential (₁), and leaf osmotic potential (_s) were measured daily as soil water became depleted. Leaf water relations were further assessed by a pressure-volume technique and by measuring _s and relative water content of leaves after rehydration. Analysis by pressure-volume or cryoscopic techniques showed that leaf osmotic potential at saturation (_s100) decreased from -0.6 MPa in well watered to -0.9 MPa in severely droughted leaves, and leaf water potential at zero turgor (_zt) decreased from about -0.7 to -1.1 MPa in well watered and droughted plants, respectively. Relative water content at zero turgor (RWC_zt) was high (88%) and tended to be decreased by drought. The ratio of turgid leaf weight to dry weight was not influenced by drought and was high at about 8.0. The bulk elastic modulus () was approximately halved by drought when related to leaf turgor potential (_p) and probably mediated turgor maintenance during drought. The latter was found to be negatively influenced by rate of drought. Supplying the plants with high levels of K salts did not promote adjustment or turgor maintenance.     

Comparative Field Summer Stress of Three Tree Species Co-occurring in Mediterranean Coastal Dunes

Chlorophyll a fluorescence, water potential (_s), and root system of Juniperus oxycedrus ssp. macrocarpa, Juniperus phoenicea ssp. turbinata, and Pinus pinea were studied in Mediterranean coastal dunes of SW Spain during summer drought and after fall rains in 1999, the driest year in the 90's. A strong and reversible depression in the photochemical efficiency of photosystem 2 of the three species was recorded, which happened concomitantly with the diurnal increase and decrease in radiation. J. phoenicea, with superficial root system, was the most affected species by summer drought. It showed high rates of down-regulation of photosynthesis by photoinhibition and positive correlation between _s and F_v/F_p, with _s lower than -7 MPa. However, it tolerated this high stress, showing a fast recovery of its physiological state after fall rains. On the other hand, J. oxycedrus and P. pinea, both with deep root systems, kept their _s values up to -3 MPa, showing lower stress during summer drought. On the other hand, J. oxycedrus and J. phoenicea were more sensible to changes in edaphic water content than P. pinea. These specific responses to summer drought would be determined by their root distributions and stomatal control of transpiration, conditioning the efficiency in getting and using the available water resources. Ecophysiological responses indicate that these species are well-adapted to long periods of drought in Mediterranean climate areas, developing different strategies: J. phoenicea tolerates high stress with a fast recovery after fall rains, while J. oxycedrus and P. pinea are less affected by summer drought since their deep root systems would allow them to reach deep water resources.     

Adaptive photosynthetic strategies of the Mediterranean maquis species according to their origin

In consideration of their origin the adaptive strategies of the evergreen species of the Mediterranean maquis were analysed. Rosmarinus officinalis L., Erica arborea L., and Erica multiflora L. had the lowest net photosynthetic rate (P_N) in the favourable period [7.8±0.6 mol(CO₂) m^–2s^–1, mean value], the highest P_N decrease (on an average 86 % of the maximum) but the highest recovery capacity (>70 % of the maximum) at the first rainfall in September. Cistus incanus L. and Arbutus unedo L. had the highest P_N during the favourable period [15.5±5.2 mol(CO₂) m^–2s^–1, mean value], 79 % decrease during drought, and a lower recovery capacity (on an average 54 %). Quercus ilex L., Phillyrea latifolia L., and Pistacia lentiscus L. had an intermediate P_N in the favourable period [9.2±1.3 mol(CO2) m^–2s^–1, mean value], a lower reduction during drought (on an average 63 %), and a range from 62 % (Q. ilex and P. latifolia) to 39 % (P. lentiscus) of recovery capacity. The Mediterranean species had higher decrease in P_N and stomatal conductance during drought and a higher recovery capacity than the pre-Mediterranean species. Among the pre-Mediterranean species, P. latifoliahad the best adaptation to long drought periods also by its higher leaf mass per area (LMA) which lowered leaf temperature thus decreasing transpiration rate during drought. Moreover, its leaf longevity determined a more stable leaf biomass during the year. Among the Mediteranean species, R. officinalis was the best adapted species to short drought periods by its ability to rapidly recover. Nevertheless, R. officinalis had the lowest tolerance to high temperatures by its P_N dropping below half its maximum value when leaf temperature was over 33.6°C. R. officinalismay be used as a bioindicator species of global change.This revised version was published online in March 2005 with corrections to the page numbers.     

The effect of humidity and light on cellular water relations and diffusion conductance of leaves ofTradescantia virginiana L.

Turgor (_p) and osmotic potential (_s) in epidermal and mesophyll cells, in-situ xylem water potential (-xyl) and gas exchange were measured during changes of air humidity and light in leaves ofTradescantia virginiana L., Turgor of single cells was determined using the pressure probe. Sap of individual cells was collected with the probe for measuring the freezing-point depression in a nanoliter osmometer. Turgor pressure was by 0.2 to 0.4 MPa larger in mesophyll cells than in epidermal cells. A water-potential gradient, which was dependent on the rate of transpiration, was found between epidermis and mesophyll and between tip and base of the test leaf. Step changes of humidity or light resulted in changes of epidermal and mesophyll turgor (_p-epi, _p-mes) and could be correlated with the transpiration rate. Osmotic potential was not affected by a step change of humidity or light. For the humidity-step experiments, stomatal conductance (g) increased with increasing epidermal turgor.g/_p-epi appeared to be constant over a wide range of epidermal turgor pressures. In light-step experiments this type of response was not found and stomatal conductance could increase while epidermal turgor decreased.Symbols E transpiration - g leaf conductance - w leaf/air vapour concentration difference - -epi water potential of epidermal cells - -mes water potential of mesophyll cells - -xyl water potential of xylem - _p-epi turgor pressure of epidermal cells - _p-mes turgor pressure of mesophyll cells - _s-epi osmotic potential of epidermal cells - _s-mes osmotic potential of mesophyll cells     

Increased photosynthesis and water potentials in Silphium integrifolium galled by cynipid wasps

Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. We present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (). Preliminary data showed that in drought-stressed plants galled shoots had 36% greater A, and 10% greater stem than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. We hypothesized that 1) galled shoots have higher , g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and , but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO₂ concentration relationships were consistent with the explanation that increased allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationships or by removing low- limitations on non-stomatal components of A. Our working hypothesis is that galls increase and A by reducing the shoot: root ratio so that the plant is exploiting a greater soil volume per unit leaf area. We argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased and A may protect gall insects from variation in resource availability caused by periodic drought stress, potentially reducing negative effects of drought on plant quality and on gall insect populations.     

Tissue-water relations of two co-occurring evergreen Mediterranean species in response to seasonal and experimental drought conditions

Tissue-water relations were used to characterize the responses of two Mediterranean co-occurring woody species (Quercus ilex L. and Phillyrea latifolia L.) to seasonal and experimental drought conditions. Soil water availability was reduced 15% by partially excluding rain throughfall and lateral flow (water runoff). Seasonal and experimental drought elicited physiological and morphological adaptations other than osmotic adjustment: both species showed large increases in cell-wall elasticity and decreased saturated-to-dry-mass ratio. Increased elasticity (lower elastic modulus) resulted in concurrent decreases in relative water content at turgor loss. In addition, P. latifolia showed significant increases in apoplastic water fraction. Decreased saturated-to-dry-mass ratio and increased apoplastic water fraction were accompanied by an increased range of turgor maintenance, which indicates that leaf sclerophyllous traits might be advantageous in drier scenarios. In contrast, the degree of sclerophylly (as assessed by the leaf mass-to-area ratio) was not related to tissue elasticity. An 15% reduction in soil water availability resulted in significant reductions in diameter growth when compared to control plants in both species. Moreover, although P. latifolia underwent larger changes in tissue water-related traits than Q. ilex in response to decreasing water availability, growth was more sensitive to water stress in P. latifolia than in Q. ilex. Differences in diameter growth between species might be partially linked to the effects of cell-wall elasticity and turgor pressure on growth, since Q. ilex showed higher tissue elasticity and higher intrinsic tolerance to water deficit (as indicated by lower relative water content at turgor loss) than P. latifolia.     

Responses to drought of five Brachiaria species. II. Water relations and leaf gas exchange

Neotropical savannas are exposed to recurrent dry periods of varied duration, and forage grasses must be able to cope with such temporal stresses to maintain productive pastures. This study compared leaf water relations and net photosynthesis under drought of five perennial Brachiaria species: the tufted B. brizantha (CIAT 6780), the semi-stoloniferous B. decumbens (CIAT 606) and B. mutica, and the stoloniferous B. humidicola (CIAT 679) and B. dictyoneura (CIAT 6133). Plants of the five grasses were grown in large pots and subjected to drought by suspending watering until first wilting symptoms (14 days for B. brizantha, B. decumbens and B. mutica, and 29 days for B. humidicola and B. dictyoneura). Afterwards, they were re-watered and a second soil dry cycle was imposed. Time trends in leaf water potential (_l), relative water content (RWC), osmotic potential at full turgor (₀ ¹⁰⁰), stomatal conductance (Gs) and net photosynthesis (A) of stressed (DT) plants were compared to those of well-irrigated (CT) plants. Predawn _l in DT plants decreased to a minimum of –1.5 and –2.0 MPa in B. brizantha and B. mutica, compared to –2.5 to –3.0 MPa in B. decumbens, B. humidicola and B. dictyoneura. RWC decreased up to 50% in B. brizantha, compared to 75% in the other species. In B. humidicola, B. dictyoneura and in a lesser extent, B. decumbens, leaves of DT plants adjusted osmotically, by an apparent accumulation of nutrient solutes, at a rather constant ratio of turgid to dry weight of the tissue. Calculated osmotic adjustment ranged between 0.38 (B. decumbens) to 0.87 MPa (B. humidicola). This adjustment in ₀ ¹⁰⁰ was in some cases maintained 7 days after re-watering. In B. brizantha and B. mutica, Gs and A were significantly affected by drought, with maximum reduction percentages at the second drought period of 65 and 80%, respectively. The corresponding reduction in B. decumbens was 53 and 55%, respectively; whereas in B. humidicola and B. dictyoneura Gs and A were reduced less than 20%. In all species, re-watering allowed for the water relations (except ₀ ¹⁰⁰) and photosynthetic activity of leaves of DT plants to reach values comparable to those of CT plants. Results are discussed in term of root morphology and soil water extraction pattern, as well as leaf traits that may contribute to withstand drought under moderate soil water stress.     

Acclimation of photosynthesis in Zea mays to low water potentials involves alterations in protoplast volume reduction

Effects of water-stress treatment of Zea mays L. plants on protoplast volume and photosynthesis in leaf slices exposed to solutions of different osmotic potential ( _s) were studied. Decreased photosynthetic capacity in the leaf slices at low tissue _w was associated with dehydration-induced protoplast-volume reduction. Leaf slices from plants exposed to in-situ water deficits exhibited greater photosynthetic capacity and relative protoplast volume at low water potential ( _w) invitro than tissue from control plants.In-situ water stress induced osmotic adjustment of the leaf tissue as determined by pressure/volume analysis. It is concluded that plant acclimation to low leaf _w may involve a reduced degree of cell shrinkage at a given _w. This acclimation would allow for the maintenance of relatively higher photosynthetic capacity at low water protentials.Symbols _s Osmotic potential - _w water potential New Jersey Agricultural Experiment Station Publication No. 12149-6-87     

Osmotic adjustment and the inhibition of leaf,root, stem and silk growth at low water potentials in maize

The expansion growth of plant organs is inhibited at low water potentials ( _w), but the inhibition has not been compared in different organs of the same plant. Therefore, we determined elongation rates of the roots, stems, leaves, and styles (silks) of maize (Zea mays L.) as soil water was depleted. The _w was measured in the region of cell expansion of each organ. The complicating effects of transpiration were avoided by making measurements at the end of the dark period when the air had been saturated with water vapor for 10 h and transpiration was less than 1% of the rate in the light. Growth was inhibited as the _w in the region of cell expansion decreased in each organ. The _w required to stop growth was-0.50,-0.75, and-1.00 MPa, in this order, in the stem, silks, and leaves. However, the roots grew at these _w and ceased only when _w was lower than-1.4 MPa. The osmotic potential decreased in each region of cell expansion and, in leaves, roots and stems, the decrease was sufficient to maintain turgor fully. In the silks, the decrease was less and turgor fell. In the mature tissue, the _w of the stem, leaves and roots was similar to that of the soil when adequate water was supplied. This indicated that an equilibrium existed between these tissues, the vascular system, and the soil. At the same time, the _w was lower in the expanding regions than in the mature tissues, indicating that there was a _w disequilibrium between the growing tissue and the vascular system. The disequilibrium was interpreted as a _w gradient for supplying water to the enlarging cells. When water was withheld, this gradient disappeared in the leaf because _w decreased more in the xylem than in the soil, indicating that a high flow resistance had developed in the xylem. In the roots, the gradient did not decrease because vascular _w changed about the same amount as the soil _w. Therefore, the gradient in _w favored water uptake by roots but not leaves at low _w. The data show that expansion growth responds to low _w differently in different growing regions of the plant. Because growth depends on the maintenance of turgor for extending the cell walls and the presence of _w gradients for supplying water to the expanding cells, several factors could have been responsible for these differences. The decrease of turgor in the silks and the loss of the _w gradient in the leaves probably contributed to the high sensitivity of these organs. In the leaves, the gradient loss was so complete that it would have prevented growth regardless of other changes. In the roots, the maintenance of turgor and _w gradients probably allowed growth to continue. This difference in turgor and gradient maintenance could contribute to the increase in root/shoot ratios generally observed in water-limited conditions.Symbols _s osmotic potential - _w water potential     

Effect of sodium sulfate on in vitro organogenesis of tobacco callus

Callus of tobacco (Nicotiana tabacum L. cv. Wisconsin 38) was grown on callus-proliferating (CP) and shoot-forming (SF) media with elevated sodium sulfate (Na₂SO₄) concentrations either in the light or dark for more than one year. An increase in Na₂SO₄ concentration resulted in a decrease in callus growth index, an increase in percent dry weight of callus tissues grown on both media, and a decrease in both number of calli forming shoots and number of shoots per callus in SF medium. The CP callus grown in the light spontaneously began to form shoots after the 5th monthly transfer, and spontaneous root formation occured after the 16th transfer in the presence of 0.75 and 1.0% Na₂SO₄. Both water () and osmotic (_s) potentials of the callus increased with increasing Na₂SO₄ concentration; and callus exhibited greater and _s in the light than dark for both CP and SF media.     

Stomatal conductance in a tropical xerophilous shrubland at a lava substratum

Diurnal variation in leaf stomatal conductance (g _s) of three xerophilous species (Buddleia cordata, Senecio praecox and Dodonaea viscosa) was measured over a 10-month period during the dry and wet seasons in a shrubland that is developing in a lava substratum in Mexico. Averaged stomatal conductances were 147 and 60.2 (B. cordata), 145 and 24.8 (D. viscosa) and 142.8 and 14.1 mmol m^–2 s^–1 (S. praecox) during the wet and dry season respectively. Leaf water potential () varied in a range of –0.6 to –1.2 (S. praecox), –0.6 to –1.8 (B. cordata) and –0.9 to –3.4 MPa (D. viscosa) during the same measurement periods. Stomata were more sensitive to changes in irradiance, air temperature and leaf–air vapour pressure difference in the rainy season than the dry season. Although stomatal responses to were difficult to distinguish in any season (dry or rainy), data for the entire period of measurement showed a positive correlation, stomata tending to open as increased, but there is strong evidence of isohydric behaviour in S. praecox and B. cordata. A multiplicative model relating g _s to environmental variables and to accounted for 79%–83% of the variation of g _s in three sites (pooled data); however, the performance of the model was poorer (60%–76%) for individual species from other sites not included in the pooled data.     

Control of the rate of cell enlargement: Excision,wall relaxation,and growth-induced water potentials

A new guillotine thermocouple psychrometer was used to make continuous measurements of water potential before and after the excision of elongating and mature regions of darkgrown soybean (Glycine max L. Merr.) stems. Transpiration could not occur, but growth took place during the measurement if the tissue was intact. Tests showed that the instrument measured the average water potential of the sampled tissue and responded rapidly to changes in water potential. By measuring tissue osmotic potential ( _s ), turgor pressure ( _p ) could be calculated. In the intact plant, _s and _p were essentially constant for the entire 22 h measurement, but _s was lower and _p higher in the elongating region than in the mature region. This caused the water potential in the elongating region to be lower than in the mature region. The mature tissue equilibrated with the water potential of the xylem. Therefore, the difference in water potential between mature and elongating tissue represented a difference between the xylem and the elongating region, reflecting a water potential gradient from the xylem to the epidermis that was involved in supplying water for elongation. When mature tissue was excised with the guillotine, _s and _p did not change. However, when elongating tissue was excised, water was absorbed from the xylem, whose water potential decreased. This collapsed the gradient and prevented further water uptake. Tissue _p then decreased rapidly (5 min) by about 0.1 MPa in the elongating tissue. The _p decreased because the cell walls relaxed as extension, caused by _p , continued briefly without water uptake. The _p decreased until the minimum for wall extension (Y) was reached, whereupon elongation ceased. This was followed by a slow further decrease in Y but no additional elongation. In elongating tissue excised with mature tissue attached, there was almost no effect on water potential or _p for several hours. Nevertheless, growth was reduced immediately and continued at a decreasing rate. In this case, the mature tissue supplied water to the elongating tissue and the cell walls did not relax. Based on these measurements, a theory is presented for simultaneously evaluating the effects of water supply and water demand associated with growth. Because wall relaxation measured with the psychrometer provided a new method for determining Y and wall extensibility, all the factors required by the theory could be evaluated for the first time in a single sample. The analysis showed that water uptake and wall extension co-limited elongation in soybean stems under our conditions. This co-limitation explains why elongation responded immediately to a decrease in the water potential of the xylem and why excision with attached mature tissue caused an immediate decrease in growth rate without an immediate change in _p Abbreviations and symbols L tissue conductance for water - m wall extensibility - Y average yield threshold (MPa) - _o water potential of the xylem - _p turgor pressure - _s osmotic potential - _w water potential of the elon gating tissue     

Graphical evaluation and partitioning of turgor responses to drought in leaves of durum wheat

The relationship between relative water content (R) and turgor potential (_p) may be derived from pressure-volume (PV) curves and analyzed in various ways. Fifty PV curves were measured with the pressure chamber on leaves of durum wheat (Triticum durum L.). The plots of _p versus R were highly variable and could not be adequately described by a single mathematical function. The area below the curve was therefore determined by means of an area meter. This procedure gave the integral of turgor from full saturation to the turgor-loss point. Responses to drought treatment could thus be quantified and partitioned into effects of osmotic adjustment and elastic adjustment. These two adjustment responses, which are probably of different metabolic origin, together improve turgor maintenance in durum wheat considerably.Abbreviations and symbols PV pressure-volume - R relative water content - T_i turgor integral between full saturation and turgor-loss point - _p turgor (pressure) potential     

An investigation of the role of solutes in the xylem sap and in the xylem parenchyma as the source of root pressure

Summary Solute osmotic potentials (_x) in the vessels of hydroponically grown maize roots were measured to assess the osmotic-xylem-sap mechanism for generating root pressure (indicated by guttation). Solutes in vessels were measured in situ by X-ray microanalysis of plants frozen intact while guttating. Osmotic potentials outside the roots (_o) were changed by adding polyethylene glycol to the nutrient solution. Guttation rate fell when _o was decreased, but recovered towards the control value during 3–5 days when _o was greater than or equal to –0.3 MPa, but not when _o was equal to –0.4 MPa. In roots stressed to _o = –0.3 MPa, _x, was always more positive than _o, and _x changed only slightly (ca. 0.05 MPa). Thus the adjustment in the roots which increased root pressure cannot be ascribed to _x, contradicting the osmotic-xylem-sap mechanism. An alternative driving force was sought in the osmotic potentials of the vacuoles of the living cells (_v), which were analysed by microanalysis and estimated by plasmolysis. _v showed larger responses to osmotic stress (0.1 MPa). Some plants were pretreated with abundant KNO₃ in the nutrient solution. These plants showed very large adjustments in _v (0.4 MPa) but little change in _x (0.08 MPa). They guttated by 4 h after _o was lowered to –0.4 MPa. It is argued that turgor pressure of the living cells is a likely alternative source of root pressure. Published evidence for high solute concentrations in the xylem sap is critically assessed.Abbreviations _o external water potential - _x osmotic potential of xylem sap - _v osmotic potential of vacuolar sap - EDX energy dispersive X-ray microanalysis - CSEM cryo-scanning electron microscope - LN₂ liquid nitrogen - PEG polyethylene glycol     

Tree growth,mortality, and above-ground biomass accumulation in a holm oak forest under a five-year experimental field drought

A holm oak forest was exposed to an experimental drought during 5 years to elucidate the growth responses of the dominant species Quercus ilex, Arbutus unedo and Phillyrea latifolia. Soil water availability was partially reduced, about 15% as predicted for this area for the next decades by GCM and ecophysiological models, by plastic strips intercepting rainfall and by ditch exclusion of water runoff. The stem diameter increment was highly correlated with annual rainfall in all species, and drought treatment strongly reduced the diameter increment of Q. ilex (41%) and specially of A. unedo (63%), the species showing higher growth rates. Stem mortality rates were highly correlated with previous stem density, but drought treatment increased mortality rates in all species. Q. ilex showed the highest mortality rates (9% and 18% in control and drought plots, respectively), and P. latifolia experienced the lowest mortality rates (1% and 3% in control and drought plots, respectively). Drought strongly reduced the increment of live aboveground biomass during these 5 years (83%). A. unedo and Q. ilex experienced a high reduction in biomass increment by drought, whereas P. latifolia biomass increment was insensitive to drought. The different sensitivity to drought of the dominant species of the holm oak forest may be very important determining their future development and distribution in a drier environment as expected in Mediterranean areas for the next decades. These drier conditions could thus have strong effects on structure (species composition) and functioning (carbon uptake and biomass accumulation) of these Mediterranean forests.     

CO2 Assimilation and Water Relations of Almond Tree (Prunus amygdalus Batsch) Cultivars Grown Under Field Conditions

Gas exchanges and leaf water potential (_w) of six-years-old trees of fourteen Prunus amygdalus cultivars, grafted on GF-677, were studied in May, when fruits were in active growing period, and in October, after harvesting. The trees were grown in the field under rain fed conditions. Predawn _w showed lower water availability in October compared with May. The lowest _w values at midday in May increased gradually afterwards, while in October they decreased progressively until night, suggesting a higher difficulty to compensate the water lost by transpiration. However, relative water content (RWC) measured in the morning was similar in both periods, most likely due to some rainfall that occurred in September and first days of October that could be enough to re-hydrate canopy without significantly increasing soil water availability. The highest net photosynthetic rate (P _N) was found in both periods early in the morning (08:00–11:00). Reductions in P _N from May to October occurred in most cultivars except in José Dias and Ferrastar. In all cultivars a decrease in stomatal conductance (g _s) was observed. Photosynthetic capacity (P _max) did not significantly change from spring to autumn in nine cultivars, revealing a high resistance of photosynthetic machinery of this species to environmental stresses, namely high temperature and drought. Osmotic adjustment was observed in some cultivars, which showed reductions of ca. 23 % (Duro d' Estrada, José Dias) and 15 % (Tuono) in leaf osmotic potential (). Such decreases were accompanied by soluble sugars accumulation. The Portuguese cultivar José Dias had a higher photosynthetic performance than the remaining genotypes.