Water balance in leaf tissue

Samples of the leaf tissue (14cm²) were placed in a plexiglass chamber which consisted of three parts. Water absorbed by the leaf tissue on one side of the sample was transported through the middle part of the sample to the opposite side and was transpirated there. The intensity of transpiration the intensity of water absorption and water saturation deficit (w.s.d.) were determined simultaneously in this tissue by gravimetry. Water balance was studied either in saturated samples of leaf tissue or in tissue where w.s.d. (10%, 20%, 30%, 40%) was established in advance. Although conditions for water absorption in leaf segments were optimal, w.s.d. originated in the saturated leaf tissue under all given external conditions (evaporation from 41.7 to 17.8 mg cm^?2 h^?1). W.s.d. which was established in advance for the most part increased during the experiment and reached even high values (more than 60%). the equilibration was reached only under conditions of low evaporation and initial w.s.d. higher than 20% in young leaves and higher than 30% in adult leaves. A positive correlation between the ratio of the intensity of water absorption to the intensity of transpiration and w.s.d. was found only under conditions of lower evaporation (17.8 and 23.2mg cm^?2h^?1). The maximal values of w.s.d. were limited in this way. Water balance was studied: 1. in leaf tissue of upper, middle and lower leaves of fodder cabbage, 2. in leaf tissue of middle leaves of young and adult plants of fodder cabbage, 3. in leaf tissue of dicots (fodder cabbage) with different vessel orientation in respect to water transport, 4. in leaf tissue of monocots (banana-tree) with water transport upright to the vessel orientation. Considerable change of water balance was observed when the water transport was prolonged by two incisions in the middle part of the sample. Results of all these experiments revealed the possibility of water stress origin even in leaf tissue sufficiently supplied with water.     

Determination of relationships between water potential and water saturation deficit in leaf tissue

The mutual relationship between the water potential and water saturation deficit (w.s.d.) was studied on samples of leaf tissue of fodder cabage. Definite values of water potential were obtained by long-term exposure of plant material to an atmosphere with definite constant pressure of water vapour. The resulting w.s.d. values were determined gravimetrically. Water saturation deficit varies indirectly with the water potential. This dependence was linear for values of water potential from ?4·4 to ?43·9 atm. Since the stabilization of equilibrum of water potential between the leaf tissue and surrounding atmosphere was very slow the relationship between water potential and w.s.d. was influenced by the size of samples and by the length of exposure. Therefore this method was more suitable for relative than for absolute measurement.     

Stem water storage capacity and efficiency of water transport: their functional significance in a Hawaiian dry forest

We investigated the contribution of internal water storage and efficiency of water transport to the maintenance of water balance in six evergreen tree species in a Hawaiian dry forest. Wood‐saturated water content, a surrogate for relative water storage capacity, ranged from 70 to 105%, and was inversely related to its morphological correlate, wood density, which ranged between 0·51 and 0·65 g cm^?3. Leaf‐specific conductivity (k_L) measured in stem segments from terminal branches ranged from 3 to 18 mmol m^?1 s^?1 MPa^?1, and whole‐plant hydraulic efficiency calculated as stomatal conductance (g) divided by the difference between predawn and midday leaf water potential (Ψ_L), ranged from 70 to 150 mmol m^?2 s^?1 MPa^?1. Hydraulic efficiency was positively correlated with k_L (r² = 0·86). Minimum annual Ψ_L ranged from ? 1·5 to ? 4·1 MPa among the six species. Seasonal and diurnal variation in Ψ_L were associated with differences among species in wood‐saturated water content, wood density and k_L. The species with higher wood‐saturated water content were more efficient in terms of long‐distance water transport, exhibited smaller diurnal variation in Ψ_L and higher maximum photosynthetic rates. Smaller diurnal variation in Ψ_L in species with higher wood‐saturated water content, k_L and hydraulic efficiency was not associated with stomatal restriction of transpiration when soil water deficit was moderate, but avoidance of low minimum seasonal Ψ_L in these species was associated with a substantial seasonal decline in g. Low seasonal minimum Ψ_L in species with low k_L, hydraulic efficiency, and wood‐saturated water content was associated with higher leaf solute content and corresponding lower leaf turgor loss point. Despite the species‐specific differences in leaf water relations characteristics, all six evergreen tree species shared a common functional relationship defined primarily by k_L and stem water storage capacity.     

The relationship between the resistance to water transport and the water saturation deficit in leaf tissue of kale and tobacco

Positive linear correlation between the resistance to water transport in liquid phase and water saturation deficit (w.s.d.) in the tissue ofBrassica oleracea andNicotiana tabacum leaves was observed. At the same values of w.s.d. corresponding values of the resistance to water transport were higher when dehydration of the leaf tissue occurred during the experiment and lower when water balance was in equilibrium or resaturation of the leaf tissue occurred.     

Effect of decreased rate of water absorption on water balance of leaf tissue

The influence of decrease of water absorption rate on the transpiration rate and the development of water saturation deficit (WSD) was studied on the leaf segments of kale. Solutions of polyethyleneglycol (0-25m, 0-50m, O-75m and 1-00m) and mannitol (0-50m) were used as osmotic agents. The rate of water absorption decreased to zero when the concentration of polyethyleneglycol was 0-25m. At a concentration higher than 0-50m, water from the tissue diluted the external solution. The transpiration rate of samples affected by polyethyleneglycol or mannitol was only a little lower than that of control samples. WSD was noticeably increased only in the absorbing part of segment; in the adjoining, transporting part of segment WSD was practically the same in all variants. WSD in the transpiring part was slightly increased only in some cases. Due to decrease of osmotic potential of the external solution also the gradient of water potential changed. In the case of absorption from pure water, water potential gradually decreased from the absorbing to the transpiring part. Under the influence of polyethyleneglycol or mannitol solution the highest water potential was in the transporting part and from this point water potential decreased to both opposite sides.     

Stomatal and hydraulic conductance in growing sugarcane: stomatal adjustment to water transport capacity*

Abstract Stomatal conductance per unit leaf area in well-irrigated field- and greenhouse-grown sugarcane increased with leaf area up to 0.2 m² plant ¹, then declined so that maximum transpiration per plant tended to saturate rather than increase linearly with further increase in leaf area. Conductance to liquid water transport exhibited parallel changes with plant size. This coordiantion of vapour phase and liquid phase conductances resulted in a balance between water loss and water transport capacity, maintaining leaf water status remarkably constant over a wide range of plant size and growing conditions. The changes in stomatal conductance were not related to plant or leaf age. Partial defoliation caused rapid increases in stomatal conductance, to re-establish the original relationship with remaining leaf area. Similarly, pruning of roots caused rapid reductions in stomatal conductance, which maintained or improved leaf water status. These results suggest that sugarcane stomata adjusted to the ratio of total hydraulic conductance to total transpiring leaf area. This could be mediated by root metabolites in the transpiration stream, whose delivery per unit leaf area would be a function of the relative magnitudes of root system size, transpiration rate and leaf area.     

Through-flow of water in leaves of a submerged plant is influenced by the apical opening

Submerged aquatic higher plants maintain acropetal water transport to the young leaves in active growth to satisfy their demand for nutrients and hormones derived from the roots. We here present the first measurements of hydraulic properties for a submerged plant, the monocotyledon Sparganium emersum Rehman. The hydraulic conductance per unit length, K_h, was measured in leaf segments without the leaf tip and shown to be greater in old, fully developed leaves (1.5 · 10⁻¹⁰ · m⁴ · MPa⁻¹ · s⁻¹) than in young leaves (1.0 · 10⁻¹⁰ · m⁴ · MPa⁻¹ · s⁻¹). In leaves with intact leaf tips, however, K_h was significantly greater in the youngest leaves, which suggests that the leaf tip with the hydathode influences resistance and thus flow. Microscopy confirmed that the hydathodal area, which is an apical opening, undergoes structural changes with leaf age; a matrix of microorganisms develops in the older leaves and probably restricts water flow by clogging the hydathodes. The leaf specific conductivity expressing transport capacity relative to the leaf area supplied, of S. emersum (0.1 · 10⁻⁸ to 9 · 10⁻⁸ · m² MPa⁻¹· s⁻¹) was within the same range as for various species of terrestrial ferns, vines and trees. This finding does not support the traditional concept of functionally reduced vascular transport in Received: 15 July 1996 / Accepted: 30 November 1996     

Relative rates of delivery of xylem solute to shoot tissues: Possible relationship to sequential leaf senescence

The rates of delivery of regulatory solutes such as cytokinins and mineral ions from the roots to competing shoot tissues can influence rates of metabolism and development. A 15 min pulse of a synthetic xylem mobile and phloem-immobile solute, acid fuchsin, was used to quantify relative rates of solute delivery to competing organs on excised transpiring bean shoots (Phaseolus vulgaris L. cv. Contender) at different stages of development. Stem, flower and fruit tissues received comparatively low rates of solute delivery. The relative rate of solute delivery to newly opened leaves was initially low, but increased during rapid leaf expansion and then declined progressively as the leaves exceeded 70% of their final area. The relative rate of solute delivery to tissues of the basal primary leaves declined progressively from 2 weeks onwards. This decline appeared to be caused by progressive internally regulated increases in both stomatal resistances and hydraulic resistances to xylem flow up to and into the leaf blade. Thus combined abaxial and adaxial stomatal resistance values in the primary leaves (Rs) increased from 3 to ≥ 7 s cm^?1 between 2 and 5 weeks. Similarly, mean values for the connection resistances (Rc) to hydraulic flow into the primary leaves rose from 7 to 13 TPa · s · m^?1 between 2 and 4 weeks. In the same period pathway resistance from stem to primary leaf petioles (Rp), as determined by direct pressure flow assay, increased from 7 to 15 TPa · s · m^?1. Senescence-associated declines in protein and chlorophyll levels in the primary leaves were initiated in parallel with, or after, declines in relative rates of solute delivery. The provision of extra illumination at the basal leaf level between 2 and 5 weeks did not prevent declines in chlorophyll and soluble protein or increases in stomatal resistance. We suggest that internally programmed changes in the hydraulic architecture of the plant progressively divert xylem-transported root supplies of nutrients and cytokinins from basal to more apical leaves and thus regulate the progressive senescence of leaves from the base upwards.     

Joint effects of acetochlor and urea on germinating characteristics of crop seeds

In order to evaluate ecological risk of agrochemicals in common use, joint toxic effects of acetochlor and urea on germinating characteristics of Chinese cabbage (Brassica Pekinensis Rupr) seeds were investigated using the water-culture method and the soil-culture method. The results indicated that excessive application of acetochlor and urea, when the concentrations were higher than 31.3 mg · kg^?1 for acetochlor and 500 mg · kg^?1 for urea, had strong inhibitory effects on the rate of seed germination, root elongation and hypocotyl length of Chinese cabbage. The inhibitory rate of the germinating characteristics of Chinese cabbage seeds was significantly increased with an increase in the concentration of acetochlor or urea. The two agrochemicals in water had a stronger toxicity than these in the soil at the same concentration. Among the three indexes, hypocotyl length was the most sensitive to the toxicity of acetochlor and urea.     

Hydraulic architecture and water flow in growing grass tillers (Festuca arundinacea Schreb.)

The water relations and hydraulic architecture of growing grass tillers (Festuca arundinacea Schreb.) are reported. Evaporative flux density, E (mmol s^?1 m^?2), of individual leaf blades was measured gravimetrically by covering or excision of entire leaf blades. Values of E were similar for mature and elongating leaf blades, averaging 2·4 mmol s^?1 m^?2. Measured axial hydraulic conductivity, K_h (mmol s^?1 mm MPa^?1), of excised leaf segments was three times lower than theoretical hydraulic conductivity (K_t) calculated using the Poiseuille equation and measurements of vessel number and diameter. K_t was corrected (K_t*) to account for the discrepancy between K_h and K_t and for immature xylem in the basal expanding region of elongating leaves. From base to tip of mature leaves the pattern of K_t* was bell‐shaped with a maximum near the sheath–blade joint (≈ 19 mmol s^?1 mm MPa^?1). In elongating leaves, immature xylem in the basal growing region led to a much lower K_t*. As the first metaxylem matured, K_t* increased by 10‐fold. The hydraulic conductances of the whole root system, (mmol s^?1 MPa^?1) and leaf blades, (mmol s^?1 MPa^?1) were measured by a vacuum induced water flow technique. and were linearly related to the leaf area downstream. Approximately 65% of the resistance to water flow within the plant resided in the leaf blade. An electric‐analogue computer model was used to calculate the leaf blade area‐specific radial hydraulic conductivity, (mmol s^?1 m^?2 MPa^?1), using , K_t* and water flux values. values decreased with leaf age, from 21·2 mmol s^?1 m^?2 MPa^?1 in rapidly elongating leaf to 7·2 mmol s^?1 m^?2 MPa^?1 in mature leaf. Comparison of and values showed that ≈ 90% of the resistance to water flow within the blades resided in the liquid extra‐vascular path. The same algorithm was then used to compute the xylem and extravascular water potential drop along the liquid water path in the plant under steady state conditions. Predicted and measured water potentials matched well. The hydraulic design of the mature leaf resulted in low and quite constant xylem water potential gradient (≈ 0·3 MPa m^?1) throughout the plant. Much of the water potential drop within mature leaves occurred within a tenth of millimetre in the blade, between the xylem vessels and the site of water evaporation within the mesophyll. In elongating leaves, the low K_t* in the basal growth zone dramatically increased the local xylem water potential gradient (≈ 2·0 MPa m^?1) there. In the leaf elongation zone the growth‐induced water potential difference was ≈ 0·2 MPa.     

Manganese application alleviates the water deficit-induced decline of N2 fixation

Water deficit is a very serious constraint on N₂ fixation rates and grain yield of soybean (Glycine max Merr.). Ureides are transported from the nodules and they accumulate in the leaves during soil drying. This accumulation appears responsible for a feedback mechanism on nitrogen fixation, and it is hypothesized to result from a decreased ureide degradation in the leaf. One enzyme involved in the ureide degradation, allantoate amidohydrolase, is manganese (Mn) dependent. As Mn deficiency can occur in soils where soybean is grown, this deficiency may aggravate soybean sensitivity to water deficit. In situ ureide breakdown was measured by incubating soybean leaves in a 5 mol m ^{? 3} allantoic acid solution for 9 h before sampling leaf discs in which remnant ureide was measured over time. In situ ureide breakdown was dramatically decreased in leaves from plants grown without Mn. At the plant level, allantoic acid application in the nutrient solution of hydroponically grown soybean resulted in a higher accumulation of ureide in leaves and lower acetylene reduction activity (ARA) by plants grown with 0 mol m ^{? 3} Mn than those grown with 6·6 mol m ^{? 3} Mn. Those plants grown with 6·6 mol m ^{? 3} Mn in comparison with those grown with 52·8 mol m ^{? 3} Mn had, in turn, higher accumulated ureide and lower ARA. To determine if Mn level also influenced N₂ fixation sensitivity to water deficit, a dry‐down experiment was carried out by slowly dehydrating plants that were grown in soil under four different Mn nutritions. Plants receiving no Mn had the lowest leaf Mn concentration, 11·9 mg kg ^{? 1}, and had N₂ fixation more sensitive to water deficit than plants treated with Mn in which leaf Mn concentration was in the range of 21–33 mg kg ^{? 1}. The highest Mn treatments increased leaf Mn concentration to 37·5 mg kg ^{? 1} and above but did not delay the decline of ARA with soil drying, although these plants showed a significant increase in ARA under well‐watered conditions.     

The Effect of Leaf Water Potential, Leaf Temperature and Light Intensity on Leaf Diffusion Resistance and the Transpiration of Leaves of Malus sylvestris

The relationship between leaf resistance to water vapour diffusion and each of the factors leaf water potential, light intensity and leaf temperature was determined for leaves on seedling apple trees (Malus sylvestris Mill. cv. Granny Smith) in the laboratory. Leaf cuticular resistance was also determined and transpiration was measured on attached leaves for a range of conditions. Leaf resistance was shown to be independent of water potential until potential fell below — 19 bars after which leaf resistance increased rapidly. Exposure of leaves to CO₂-free air extended the range for which resistance was independent of water potential to — 30 bars. The light requirement for minimum leaf resistance was 10 to 20 W m^?2 and at light intensities exceeding these, leaf resistance was unaffected by light intensity. Optimum leaf temperature for minimum diffusion resistance was 23 ± 2°C. The rate of change measured in leaf resistance in leaves given a sudden change in leaf temperature increased as the magnitude of the temperature change increased. For a sudden change of 1°C in leaf temperature, diffusion resistance changed at a rate of 0.01 s cm^?1 min^?1 whilst for a 9°C leaf temperature change, diffusion resistance changed at a rate of 0.1 s cm^?1 min^?1. Cuticular resistance of these leaves was 125 s cm^?1 which is very high compared with resistances for open stomata of 1.5 to 4 s cm^?1 and 30 to 35 s cm^?1 for stomata closed in the dark. Transpiration was measured in attached apple leaves enclosed in a leaf chamber and exposed to a range of conditions of leaf temperature and ambient water vapour density. Peak transpiration of approximately 5 × 10^?6 g cm^?2 s^?1 occurred at a vapour density gradient from the leaf to the air of 12 to 14 g m^?3 after which transpiration declined due presumably to increased stomatal resistance. Leaves in CO₂-free air attained a peak transpiration of 11 × 10^?6 g cm^?2 s^?1 due to lower values of leaf resistance in CO₂ free air. Transpiration then declined in these leaves due to development of an internal leaf resistance (of up to 2 s cm^?1). The internal resistance was masked in leaves at normal CO₂ concentrations by the increase in stomatal resistance.     

Insight into the physiological role of water absorption via the leaf surface from a rehydration kinetics perspective

Soil water transported via the petiole is a primary rehydration pathway for leaves of water‐stressed plants. Leaves may also rehydrate by absorbing water via their epidermal surfaces. The mechanisms and physiological relevance of this water pathway, however, remain unclear, as the associated hydraulic properties are unknown. To gain insight into the foliar water absorption process, we compared rehydration kinetics via the petiole and surface of Prunus dulcis and Quercus lobata leaves. Petiole rehydration could be described by a double exponential function suggesting that 2 partly isolated water pools exist in leaves of both species. Surface rehydration could be described by a logistic function, suggesting that leaves behave as a single water pool. Whereas full leaf rehydration via the petiole required approximately 20 min, it took over 150 and 300 min via the surface of P. dulcis and Q. lobata , respectively. Such differences were attributed to the high resistance imposed by the leaf surface and especially the cuticle. The minimum resistance to surface rehydration was estimated to be 6.6 × 10² (P. dulcis ) and 2.6 × 10³ MPa·m²·s·g^?1 (Q. lobata ), which is remarkably higher than estimated for petiole rehydration. These results are discussed in a physiological context.     

Temporal and spatial variations in the oxygen-18 content of leaf water in different plant species

Temporal variations in the δ¹⁸ oxygen (δ¹⁸O) content of water transpired by leaves during a simulated diurnal cycle fluctuated around the δ¹⁸O content of the source water. Reconstructed variations in the δ¹⁸O values of leaf water differed markedly from those predicted by conventional models. Even when transpiring leaves were maintained under constant conditions for at least 3 h, strict isotopic steady-state conditions of leaf water (equality of the ¹⁸O/¹⁶O ratios in the input and transpired water) were rarely attained in a variety of plant species (Citrus reticu-lata, Citrus paradisi, Gossypium hirsutum, Helianthus annuns, Musa musaceae and Nicotinia tabacum). Isotopic analysis of water transpired by leaves indicated that leaves approach the isotopic steady state in two stages. The first stage takes 10 to 35 min (with a rate of change of about 3–3%h^?1), while in the second stage further approach to the isotopic steady state is asymptotic (with a rate of change of about 0–4% h^?1), and under conditions of low transpiration leaves can last for many hours. Substantial spatial isotopic heterogeneity was maintained even when leaves were at or near isotopic steady state. An underlying pattern in this isotopic heterogeneity is often discerned with increasing ¹⁸O/¹⁶O ratios from base to tip, and from the centre to the edges of the leaves. It is also shown that tissue water along these spatial isotopic gradients, as well as the average leaf water, can have ¹⁸O/¹⁶O ratios both lower and higher than those predicted by the conventional Craig and Gordon model. We concluded, first, that at any given time during the diurnal cycle of relative humidity the attainment of an isotopic steady state in leaf water cannot be assumed a priori and, secondly, that the isotopic enrichment pattern of leaf water reflects gradual enrichment along the water-flow pathway (e.g. as in a string of pools), rather than a single-step enrichment from source water, as is normally assumed.     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Effects of nitrogen nutrition and root medium water potential on growth, nitrogen uptake and osmotic adjustment of rice

The effects of nitrogen (N) nutrition on growth, N uptake and leaf osmotic potential of rice plants (Oryza sativa L. ev. IR 36) during simulated water stress were determined. Twenty-one-day-old seedlings in high (28.6 × 10 ^?4M) and low (7.14 × 10 ⁴M) N levels were exposed to decreased nutrient solution water potentials by addition of polyethylene glycol 6000. The roots were separated from the solution by a semi-permeable membrane. Nutrient solution water potential was ?0.6 × 10⁵ Pa and was lowered stepwise to ?1 × 10⁵, ?2 × 10⁵, ?4 × 10⁵ and ?6 × 10⁵ Pa at 2-day intervals. Plant height, leaf area and shoot dry weight of high and low nitrogen plants were reduced by lower osmotic potentials of the root medium. Osmotic stress caused greater shoot growth reduction in high N than in low N plants. Stressed and unstressed plants in 7.14 × 10⁴M N had more root dry matter than the corresponding plants in 28.6 × 10⁴M N. Dawn leaf water potential of stressed plants was 1 × 10⁵ to 5.5 × 10⁵ Pa lower than nutrient solution water potential. Nitrogen-deficient water-stressed plants, however, maintained higher dawn leaf water potential than high nitrogen water-stressed plants. It is suggested that this was due to higher root-to-shoot ratios of N deficient plants. The osmotic potentials of leaves at full turgor for control plants were about 1.3 × 10⁵ Pa higher in 7.14 × 10^?4M than in 28.6 × 10^?4M N and osmotic adjustment of 2.6 × 10⁵ and 4.3 × 10⁵ Pa was obtained in low and high N plants, respectively. The nitrogen status of plants, therefore, affected the ability of the rice plant to adjust osmotically during water stress. Plant water stress decreased transpiration and total N content in shoots of both N treatments. Reduced shoot growth as a result of water stress caused the decrease in amount of water transpired. Transpiration and N uptake were significantly correlated. Our results show that nitrogen content is reduced in water-stressed plants by the integrated effects of plant water stress per se on accumulation of dry matter and transpiring leaf area as well as the often cited changes in soil physical properties of a drying root medium.     

The effect of brackish water irrigation on the above- and below-ground development of pollarded Acacia saligna shrubs in an arid environment

The regrowth capacity after pollarding of a short-rotation plantation of Acacia saligna (Labill.) H. Wendl. was investigated in a field trial. This shrub has been proposed as a provider of biomass (fuelwood and fodder) in an arid environment, using local marginal water resources such as surface runoff and brackish groundwater. The specific objective of this study was to examine the effects of water quality, irrigation frequency and annual runoff flooding onthe above- and belowground development of the pollarded shrubs. Treatments consisted of drip-irrigation with freshwater or brackish water, at low (twice a month) or high (weekly) frequency, with or without annual freshwater flooding, and on a well-watered basis (twice a week) without flooding. Each 15?×?5 m² plot contained four rows of four shrubs. After 5 years of growth, the shrubs were pollarded to a height of 1.5 m and during the subsequent year of regrowth, root development was monitored non-destructively using the minirhizotron, shoot growth was estimated from trunk cross-sectional area and allometric equations (obtained at the end of the measuring period by measurements and destructive sampling), and plant water status was monitored by measuring pre-dawn leaf water potential. Dry fodder (leaves and thin branches) production was between 3.50 and 9.75 t ha^?1 and dry wood was between 3.50 and 15.50 t ha^?1. The highest biomass production was obtained in the well-watered freshwater treatment, which also had the highest number of roots and highest predawn leaf water potential throughout the year. Shrubs irrigated with brackish water at low frequency without supplemental flooding produced the lowest yields. Water quality significantly affected shoot development only in the well-watered treatments although root development was reduced wherever brackish water was applied. Flooding the plots with freshwater once a year led to an increase in the number of roots outside the drip-irrigation zone, especially in brackish water treatments. A continued root growth with time was observed in all treatments even though the shoots were pollarded. In fact total root increments and aboveground biomass production were positively linearly related. Moreover the linear response of shoot and root increments to increasing water availability and not to water quality suggests that irrigation frequency was the main factor determining the regrowth capacity and amount of above- and belowground biomass production. Based on the above, runoff water and brackish groundwater could be used in a complementary manner for the sustainable production of fuelwood and fodder in a short-rotation plantation of this shrub.     

Differences in the stimulation of cyclic electron flow in two tropical ferns under water stress are related to leaf anatomy

Cyclic electron flow (CEF) plays an important role in photoprotection for angiosperms under environmental stresses. However, ferns are more sensitive to drought and their water transport systems are not as efficient as those of angiosperms, it is unclear whether CEF also contributes to photoprotection in these plants. Using Microsorum punctatum and Paraleptochillus decurrens, we studied the electron fluxes through both photosystem I (PSI) and photosystem II (PSII) under water stress and their leaf anatomies. Our goal was to determine if CEF functions in the photoprotection of these ferns and, if so, whether CEF stimulation is related to leaf anatomy. Compared with P. decurrens, M. punctatum had thicker leaves and cuticles and higher water storage capacity, but lower stomatal density and slower rate of water loss. During induced drought, the decrease in leaf water potential (Ψ_leaf) was more pronounced in P. decurrens than in M. punctatum. For both species, the decline in Ψ_leaf was associated with a lower effective PSII quantum yield, photochemical quantum yield of PSI and electron transport rate (ETR), whereas increases were found in the quantum yield of regulated energy dissipation, CEF and CEF/ETR(II) ratio. Values for CEF and the CEF/ETR(II) ratio peaked in M. punctatum at a light intensity of 500–600 µmol m^?2 s^?1 vs only 150–200 µmol m^?2 s^?1 in P. decurrens. Therefore, our results indicate that the stimulation of CEF in tropical ferns contributes to their photoprotection under water stress, and is related to their respective drought tolerance and leaf anatomy.     

Water availability – a physiological constraint on nitrate utilization in plants of Australian semi-arid muiga woodlands

Nitrate was found to be the predominant form of available nitrogen in mulga soils. Nitrate reductase activities on a fresh mass basis of a range of plants from eastern (Queensland) mulga ecosystems 2 weeks after partial relief from drought were uniformly low for both herbaceous species (165 ± 25 pkat g^?1) and woody perennials (77 ± 14 pkat g^?1). Supply of nitrate for 24 h to cut transpiring shoots of woody species or application of nitrate solution to the rooting zone of herbaceous species promoted little further increase in mean shoot nitrate reductase activities. Most species exhibited high tissue nitrate concentrations during water stress and soluble organic N profiles were in many cases dominated by the osmoprotective compounds, proline or glycine betaine. Species with low levels of proline or glycine betaine showed high foliar concentrations of other compatible osmotica such as polyols or sugars. Effects of relieving water stress on nitrate reductase activity, proline, glycine betaine and nitrate levels were followed over, 3d of irrigation. Available soil nitrate rose 10-fold immediately and, following rapid restoration of leaf water status of the eight study species, a 4-fold increase occurred in mean nitrate reductase activity together with progressive decreases in mean tissue concentrations of nitrate, proline and glycine betaine over the 3 d period. Similar changes in soil nitrate, nitrate reductase activity, proline and tissue nitrate were observed in the same ecosystem following a natural rainfall event and in western (S.W. Australia) mulga following irrigation. It is concluded that, although nitrate nitrogen is present at high concentrations and is the predominant inorganic nitrogen source in soils of the mulga biogeographic region, its assimilation by perennial and ephemeral vegetation is limited primarily by water availability. A scheme is presented depicting interrelated physiological and biochemical events in typical mulga species following a rain event and subsequent drying out of the habitat.