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.     

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.     

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     

Stomatal response to leaf water potential in almond trees under drip irrigated and non irrigated conditions

Almond plants (Amygdalus communis L. cv. Garrigues) were grown in the field under drip irrigated and non irrigated conditions. Leaf water potential () and leaf conductance (g₁) were determined at three different times of the growing season (spring, summer and autumn). The relationships between and g₁ in both treatments showed a continuous decrease of g₁ as decreased in spring and summer. Data from the autumn presented a threshold value of (approx. –2.7 MPa in dry treatment, and approx. –1.4 MPa in wet treatment) below which leaf conductance remained constant.     

Water status and development of tropical trees during seasonal drought

Summary Bud break, shoot growth and flowering of trees involve cell expansion, known to be inhibited by moderate water deficits. In apparent contradiction to physiological theory, many trees flower or exchange leaves during the 6 month-long, severe dry season in the tropical dry forest of Guanacaste, Costa Rica. To explore this paradox, changes in tree water status during the dry season were monitored in numerous trees. Water potential of stem tissues (_stem) was obtained by a modification of the pressure chamber technique, in which xylem tension was released by cutting defoliated branch samples at both ends. During the early dry season twigs bearing old, senescent leaves generally had a low leaf water potential (_leaf), while _stem varied with water availability. At dry sites, _stem was very low in hardwood trees (<–4 MPa), but near saturation (>–0.2 MPa) in lightwood trees storing water with osmotic potentials between –0.8 and –2.1 MPa. At moist sites trees bearing old leaves rehydrated during drought; their _stem increased from low values (<–3 MPa) to near saturation, resulting in differences of 3–4 MPa between _stem and _leaf. Indirect evidence indicates that rehydration resulted from osmotic adjustment of stem tissues and improved water availability due to extension of roots into moist subsoil layers. In confirmation of physiological theory, elimination of xylem tension by leaf shedding and establishment of a high solute content and high _stem were prerequisites for flowering and bud break during drought.     

Diurnal and Seasonal Patterns of Water Potential,Photosynthesis, Evapotranspiration and Water Use Efficiency of Clusterbean

Diurnal patterns of leaf water potential (_W), canopy net photosynthetic rate (P _N), evapotranspiration rate (E), canopy temperature (T_c), and water use efficiency (WUE) of clusterbean [Cyamopsis tetragonoloba (L.) Taub., cv. Desi] were studied at six phenological stages of plant development under field conditions at CCS Haryana Agricultural University, Hisar. The highest P _N, E, and WUE were observed at pod initiation stage (61 DAS). Daily maxima of P _N were usually between 11:00 to 14:00 h while those of E and WUE between 12:30 and 16:00 h. P _N was mainly dependent on photosynthetically active radiation and E on air temperature (T_a) but the relationships varied at different growth stages. WUE declined with the increase in T _a. At mid-day, _W was highest during pod initiation.     

Drought-induced variations of water relations parameters in Olea europaea

The effects of water stress on water potential components, tissue water content, mean elastic modulus and the osmoregulation capacity of olive (Olea europaea L. cv. Coratina) leaves was determined. Artificial rehydration of olive leaf tissues altered the P-V relationships so that a plateau phenomenon occurred. Points in the P-V curve in the region affected by the plateau, generally up to –0.5 MPa, were corrected for all the samples analyzed. In the corrected P-V relationship, an osmotic adjustment was found in drought-stressed leaf tissues. Osmotic potentials at full turgor (_{0 (sat)}) and osmotic potential at turgor-loss (_{0 (TVT)}) decreased from –2.06±0.01 MPa and –3.07±0.16 MPa in controls to –2.81±0.03 MPa and –3.85±0.12 MPa in most stressed plants. Osmotic adjustment values obtained from the P-V curves agreed with those obtained using an osmometer. An active osmotic adjustment of 1.42 MPa was also observed in 1–4 mm- diameter roots. Mannitol is the main carbohydrate involved in osmotic potential decrease in all treatments. The maximum elastic modulus increased from 11.6±0.95 MPa in the controls to 18.6±0.61 MPa in the most stressed plants.     

Root and stem xylem embolism,stomatal conductance,and leaf turgor in Acer grandidentatum populations along a soil moisture gradient

The objective of this study was to determine how adjustment in stomatal conductance (g ^s) and turgor loss point (_tlp) between riparian (wet) and neighboring slope (dry) populations of Acer grandidentum Nutt. was associated with the susceptibility of root versus stem xylem to embolism. Over two summers of study (1993–1994), the slope site had substantially lower xylem pressures (_px) and g ^s than the riparian site, particularly during the drought year of 1994. The _tlp was also lower at the slope (-2.9±0.1 MPa; all errors 95% confidence limits) than at riparian sites (-1.9±0.2 MPa); but it did not drop in response to the 1994 drought. Stem xylem did not differ in vulnerability to embolism between sites. Although slope-site stems lost a greater percentage of hydraulic conductance to embolism than riparian stems during the 1994 drought (46±11% versus 27±3%), they still maintained a safety margin of at least 1.7 MPa between midday _px and the critical pressure triggering catastrophic xylem embolism (_pxCT). Root xylem was more susceptible to embolism than stem xylem, and there were significant differences between sites: riparian roots were completely cavitated at -1.75 MPa, compared with -2.75 MPa for slope roots. Vulnerability to embolism was related to pore sizes in intervessel pit membranes and bore no simple relationship to vessel diameter. Safety margins from _pxCT averaged less than 0.6 MPa in roots at both the riparian and slope sites. Minimal safety margins at the slope site during the drought of 1994 may have led to the almost complete closure of stomata (g _s=9±2 versus 79±15 mmol m^-2 s^-1 at riparian site) and made any further osmotic adjustment of _tlp non-adaptive. Embolism in roots was at least partially reversed after fall rains. Although catastrophic embolism in roots may limit the minimum for gas exchange, partial (and reversible) root embolism may be adaptive in limiting water use as soil water is exhausted.     

Effect of nitrogen on growth and water relations of radiata pine families

The effect of nitrogen fertilisation on growth, foliar nutrients and water relations of four families of radiata pine (Pinus radiata, D. Don) currently in the Australian breeding program was examined from age six to 11 years. At this stage, the stand was ready for commercial thinning. The annual rainfall at the site varied from 563 to 733 mm.Application of nitrogen fertiliser resulted in stem wood volume at age nine years of 178 m³ ha^-1 in the controls, compared with 228 m³ ha^-1 in plots treated with 600 kg N ha^-1. Pre-dawn needle water potential () measured in three consecutive summers (when rainfall ranged from 53 to 106 mm) were consistently higher (less water stress) in nitrogen fertilised than in control trees. Similarly, the water stress integral (S) decreased consistently with increasing levels of nitrogen, although total water use in fertilised trees would have been substantially higher because fertiliser application increased the leaf area index. The relationship between S and basal area was strong and paralleled that of foliar nitrogen concentration and basal area growth. Therefore, nitrogen application increased growth rates of trees by improving the nutrient status of trees and lowering the water stress on trees in summer.Families showed markedly different responses of basal area growth to nitrogen, ranging from an increase of 9.4% over three years for the least responsive family to 99.0% for the most responsive. There was no nitrogen × family interaction on or S suggesting that the large genetic variation in the growth response to nitrogen is mediated by factors other than water relations. These results have implications for managing highly productive plantations grown in an environment where rainfall is low compared to potential evapotranspiration.     

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     

Pioneer and late stage tropical rainforest tree species (French Guiana) growing under common conditions differ in leaf gas exchange regulation,carbon isotope discrimination and leaf water potential

Leaf gas exchange rates, predawn _wp and daily minimum _wm leaf water potentials were measured during a wet-to-dry season transition in pioneer (Jacaranda copaia, Goupia glabra andCarapa guianensis) and late stage rainforest tree species (Dicorynia guianensis andEperua falcata) growing in common conditions in artificial stands in French Guiana. Carbon isotope discrimination () was assessed by measuring the stable carbon isotope composition of the cellulose fraction of wood cores. The values were 2.7 higher in the pioneer species than in the late stage species. The calculated time integratedC _i values derived from the values averaged 281 mol mol^–1 in the pioneers and 240 mol mol^–1 in the late stage species. The corresponding time-integrated values of intrinsinc water-use efficiency [ratio CO₂ assimilation rate (A)/leaf conductance (g)] ranged from 37 to 47 mmol mol^–1 in the pioneers and the values were 64 and 74 mmol mol^–1 for the two late stage species. The high values were associated—at least inJ. copaia—with high maximumg values and with high plant intrinsinc specific hydraulic conductance [Cg/(_wm–_wp], which could reflect a high competitive ability for water and nutrient uptake in the absence of soil drought in the pioneers. A further clear discriminating trait of the pioneer species was the very sensitive stomatal response to drought in the soil, which might be associated with a high vulnerability to cavitation in these species. From a methodological point of view, the results show the relevance of for distinguishing ecophysiological functional types among rainforest trees.     

Characteristic moisture curves and maximum water content of two crop residues

Limited data are available relating water potential () to crop residue water content (), although this relationship is important to study decomposition and moisture retention of the residue layer in no-till systems and other agricultural situations where residues are used. The objectives of this study were (i) to determine the characteristic moisture curves of rye (Secale cereale L.) and clover residues (Trifolium incarnatum L.), and (ii) to determine residue characteristics that can predict maximum water content of crop residues. Air-dried residues were separated into leaves and stems, cut into 0.5 cm length pieces and saturated with distilled water. Pieces of the drained residues were dried to various water contents in the laboratory and then transferred into thermocouple psychrometer chambers. Characteristic moisture functions of the type = a ^–b, where a and b are empirical constants, were fitted to the data. The characteristic moisture curves had a similar shape to that of a Cecil sandy loam soil used as an example; however, while plant residues were able to retain up to 4.3 g H₂O g^–1, the mineral soil retained only 0.22 g H₂O g^–1. Soluble carbohydrate concentration can be used as a practical index to estimate maximum water content of residues, given the good relationship between both variables (R ² = 0.92).     

Monitoring the mitochondrial transmembrane potential with the JC-1 fluorochrome in programmed cell death during mesophyll leaf senescence

Summary. Analysis of the mitochondrial transmembrane potential (_m) with the help of the JC-1 fluorochrome (5,5,6,6-tetrachloro-1,1,3,3-tetraethylbenzimidazolcarbocyanine iodide) during mesophyll leaf senescence was performed in order to determine whether a reduction of _m takes place during mesophyll senescence and whether plant mitochondria, like mammalian ones, might be involved in the induction of programmed cell death. Fluorescence analysis of mesophyll protoplasts of Pisum sativum in a confocal microscope, fluorescent spectra analysis and time dependence of fluorescence intensity of monomers and of J-aggregates revealed that JC-1 is incorporated and accumulated specifically in plant mitochondria. Analysis of _m during mesophyll protoplast senescence revealed that two subpopulations of mitochondria which differ in _m exist in all analyzed stages of leaf senescence. The first subpopulation contains mitochondria with red fluorescence of J-aggregates due to an unperturbed high _m. The second subpopulation comprises mitochondria with green fluorescence of monomers due to a low _m, proving total depolarization of mitochondrial membranes. Fluorescence analysis demonstrated that even in the latest analyzed stages of leaf senescence, mitochondria with a high _m still exist. Fluorometric measurements revealed that the fluorescence intensity of J-aggregates decreases with the age of plants, which indicates that a reduction of _m during the mesophyll senescence process takes place; however, it does not take place within the whole population of mitochondria of the same protoplast. The reason of this can be due to a dramatic reorganization of mitochondria in mesophyll cells and the appearance of large mitochondria with local heterogeneity of _m in the oldest analyzed stages. All mitochondria in every stage of senescence maintained their membrane organization even when their size, distribution, and spatial organization in protoplasts changed dramatically. We stated that the reduction of _m does not directly induce programmed cell death in mesophyll cells, as opposed to animal apoptosis.Correspondence and reprints: Department of Plant Anatomy and Cytology, Institute of Experimental Biology of Plants, Warsaw University, Miecznikowa 1, 02-096 Warszawa, Poland.     

Effects of Osmotic Drought Stress Induced by a Combination of NaCl and Polyethylene Glycol on Leaf Water Status,Photosynthetic Gas Exchange,and Water Use Efficiency of Pistacia khinjuk and P. mutica

Leaf water potential, leaf osmotic potential, chlorophyll a and b contents, stomatal conductance, net photosynthetic rate, and water use efficiency were determined in two pistachio species (Pistacia khinjuk L. and P. mutica L.) grown under osmotic drought stress induced by a combination of NaCl and polyethylene glycol 6000. A decrease in values for all mentioned variables was observed as the osmotic potential of the nutrient solution (_s) decreased. The osmotic adjustment () of the species increased by decreasing _s. Thus P. khinjuk had a higher osmotic drought stress tolerance than P. mutica.     

On the dependence of salt tolerance of beans (Phaseolus vulgaris L.) on soil water matric potentials

Summary Bean plants (Kora cv) were grown in potted soil artificially salinized by adding NaCl and CaCl₂ to the irrigation water to obtain an electrical conductivity of the soil saturation extract (EC_e) thirty days after emergence of 0.1, 0.3, 0.5 and 0.7 S/m at 25°C and a sodium adsorption ratio (SAR) of 4 (mmol/l)². Thereafter, plants were irrigated when soil water matric potential (_M) was in the range of –20 to –30 kPa (wet treatment) and when _M was in the range of –40 to –60 kPa (dry treatment).Transpiration rates (Tr) and leaf extension rates (LER) per plant or per unit of leaf area were decreased by increasing soil salinity and by decreasing soil moisture. However, a given decrement of _M produced a considerable larger decrement in Tr of LER than an equivalent decrement of soil water osmotic potential (₀). Absolute yields of green pods under wet treatments were from twice to one and a half time as large under the wet than under the dry treatment at equivalent values of ₀. Relative yields were reduced by 25% when EC_e were about 0.5 S/m and 0.7 S/m in the dry and wet treatment respectively. Salt tolerance data of crops may not have a quantitative interest when soil irrigation regimes under which they were obtained are not specified.     

Seasonal Changes in Photosynthesis and Stomatal Conductance of Five Plant Species from a Semiarid Ecosystem

In order to determine whether stomatal closure alone regulates photosynthesis during drought under natural conditions, seasonal changes in leaf gas exchange were studied in plants of five species differing in life form and carbon fixation pathway growing in a thorn scrub in Venezuela. The species were: Ipomoea carnea, Jatropha gossypifolia, (C3 deciduous shrubs), Alternanthera crucis (C4 deciduous herb), and Prosopis juliflora and Capparis odoratissima (evergreen phreatophytic trees). Xylem water potential () of all species followed very roughly the precipitation pattern, being more closely governed by soil water content in I. carnea and A. crucis. Maximum rate of photosynthesis, Pmax, decreased with in I. carnea, J. gossypifolia, and A. crucis. In I. carnea and J. gossypifolia stomatal closure was responsible for a 90 % decline in net photosynthetic rate (PN) as decreased from -0.3 to -2.0 MPa, since stomatal conductance (gs) was sensitive to water stress, and stomatal limitation on PN increased with drought. In A. crucis, PN decreased by 90 % at a much lower (-9.3 MPa), and gs was relatively less sensitive to . In P. juliflora and C. odoratissima, Pmax, gs, and intercellular CO2 concentration (Ci) were independent of soil water content. In the C3 shrubs stomatal closure was apparently the main constraint on photosynthesis during drought, Ci declining with in I. carnea. In the C4 herb, Ci was constant along the range of values, which suggested a coordinated decrease in both gs and mesophyll capacity. In P. juliflora Ci showed a slow decrease with which may have been due to seasonal leaf developmental changes, rather than to soil water availability.     

Gas Exchange,Photochemical Efficiency,and Leaf Water Potential in Three Salix Species

Gas exchange, photochemical efficiency, and leaf water potential (_l) of Salix matsudana (non-indigenous species), S. microstachya and S. gordejevii (indigenous species) were studied in Hunshandak Sandland, China. _l of all the three species decreased from 06:00 to 12:00, and increased afterwards. S. matsudana showed higher values of _l than others. Net photosynthetic rate (P _N) and stomatal conductance (g _s) of S. matsudana were the lowest among all, with the maximum P _N at 10:00 being 75% of that of S. gordejevii. Compared with the indigenous species, the non-indigenous S. matsudana had also lower transpiration rate (E) and water use efficiency (WUE). The values of F_v/F_m in all the species were lower from 06:00 to 14:00 than those after 14:00, indicating an obvious depression in photochemical efficiency of photosystem 2 in both non-indigenous and native species. However, it was much more depressed in S. matsudana, the non-indigenous tree. P _N was positively correlated to g _s and negatively related to _l. The relationship between g _s and vapour pressure difference (VPD) was exponential, while negative linear correlation was found between g _s and _l.     

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     

Current-voltage relations of sodium-coupled sugar transport across the apical membrane ofNecturus small intestine

Summary The current-voltage (I–V) relations of the rheogenic Na-sugar cotransport mechanism at the apical membrane ofNecturus small intestine were determined from the relations between the electrical potential difference across the apical membrane, ^mc , and that across the entire epithelium, ^ms , when the latter was varied over the range ±200 mV, (i) under steady conditions in the presence of galactose and (ii) after the current across the apical membrane carried by the cotransporter,I _SNa ^m , is blocked by the addition of phloridzin to the mucosal solution.I _SNa ^m was found to be strongly dependent upon ^mc over the range –50 mV < ^mc <E _SNa ^m whereE _SNa ^m is the zero current or reversal potential. Over the range of values of ^mc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so thatI _SNa ^m =g _SNa ^m (E _SNa ^m – ^mc ) whereg _SNa ^m is the contribution of this mechanism to the conductance of the apical membrane and is near constant. In several instancesI _SNa ^m saturated at large hyperpolarizing or depolarizing values of ^mc .The values ofE _SNa ^m determined in the presence of 1, 5, and 15mm galactose strongly suggest that if the Na-galactose cotransporters are kinetically homogeneous, the stoichiometry of this coupled process is unity.Finally, the shapes of the observedI–V relations are consistent with the predictions of a simple kinetic model which conforms with current notions regarding the mechanico-kinetic properties of this cotransport process.     

Osmoregulation and the control of phloem-sap composition in Ricinus communis L.

Phloem-sap composition was studied in plants of Ricinus communis L. grown on a waterculture medium. The sap possessed a relatively high K⁺:Na⁺ ratio and low levels of Ca²⁺ and free H⁺. Sucrose and K⁺ (together with its associated anions) accounted for 75% of the phloem-sap solute potential (_s). In plants kept in continuous darkness, a decrease in phloem-sap sucrose levels over 24h was accompanied by an increase in K⁺ levels. Measurements of phloem-sap _s and xylem water potential () indicated that this resulted in a partial maintenance of phloem turgor pressure _p. In darkness there was also a marked decrease in the malate content of the leaf tissue, and it is possible that organic carbon from this source was mobilized for export in the phloem. The results support the concept of the phloem sap as a symplastic phase. We interpret the increase in K⁺ levels in the phloem in darkness as an osmoregulatory response to conditions of restricted solute availability. This reponse can be explained on the basis of the sucrose-H⁺ co-transport mechanism of phloem loading.Abbreviations water potential - _s solute potential - _p pressure potential