Germination and seedling growth of carrots under salinity and moisture stress

Poor crop stand is a common problem in saline areas. Germination and seedling emergence may be depressed as a result of impeded aeration, saline or dry conditions. In this study, we examined the effects of salinity and moisture stress and their interactions on seed germination and seedling growth of carrots. Variable soil matric and osmotic potentials were either obtained by equilibrating soil salinized to different degrees on a 0.5 MPa ceramic plate soil moisture extractor or by adding different amounts of salt solutions to the same mass of air-dried soil, based on a previously determined soil moisture release curve, and allowing to equilibrate for 1 week. Germination decreased significantly in the investigated silty soil (Aquic Ustifluvent) at soil moisture potentials higher than −0.01 MPa, whereas osmotic potentials as low as −0.5 MPa did not influence germination. Matric potentials of −0.3 and −0.4 MPa, respectively, resulted in a strong decrease (35–95%) of germination and delayed germination by 2 to 5 days in the silty soil to which different amounts (18 and 36%, respectively) and sizes (0.8–1.2 mm and 1.5–2.2 mm, respectively) of sand particles had been added. No effect of sand and grain diameter was detected. Germination was not affected by comparable osmotic potentials. Seedling growth showed a much higher sensitivity than germination to decreasing matric potentials, but was not affected by osmotic potentials ranging from −0.05 to −0.5 MPa. Optimum shoot growth occurred at matric potentials between −0.025 and −0.1 MPa. Shoot and root growth decreased markedly at matric potentials higher than −0.01 MPa. Fresh weight of shoots decreased gradually at matric potentials lower than −0.2 MPa. Root growth was significantly increased at matric potentials of −0.1 to −0.3 MPa, whereas comparable osmotic potentials did not have equivalent effects. It is concluded that germination and seedling growth are differently affected by comparable matric and osmotic stresses and that water stress exerts a more negative effect than salt stress.     

Effect of soil strength on root growth under different water conditions

The objective of this study was to determine the effects of soil water and soil strength on root growth in situations where the individual effects of both of these factors were important. Three grain legumes were grown from pre-germinated seeds for five days on 50-mm compacted columns of two major soils of Sri Lanka. Four or five levels of bulk density (1.1 to 1.8 Mg.m^–3) and five or six levels of matric potential (–0.02 to–2.0 MPa) were used.Soil strength and matric potential effects on root growth were independently significant for most crop and soil combinations. Under high (wet) matric potential (>–0.77 MPa) soil conditions, the effect of soil water on root growth was evident only in its effect on soil strength. Bulk density had a significant effect on root growth independent of soil strength and matric potential in three cases.For all crops and soils, root penetration was 80% of the maximum or greater when the average soil strength (soil water not limiting) was 0.75 MPa or less, and when the average matric potential (soil strength not limiting) was –0.77 MPa or greater (wetter). Root penetration was 20% of the maximum or less when the soil strength was greater than 3.30 MPa (soil water not limiting), and when matric potential (soil strength not limiting) was less than –3.57 MPa. The use of pre-germinated seeds, which contained imbibed water, combined with a lack of water loss from the closed chambers containing the plants is the probable cause for the very low (–3.57 MPa) matric potential that allowed root growth at 20% of the maximum.     

Toluene diffusion and reaction in unsaturated Pseudomonas putida biofilms

Biofilms are frequently studied in the context of submerged or aquatic systems. However, much less is known about biofilms in unsaturated systems, despite their importance to such processes as food spoilage, terrestrial nutrient cycling, and biodegradation of environmental pollutants in soils. Using modeling and experimentation, we have described the biodegradation of toluene in unsaturated media by bacterial biofilms as a function of matric water potential, a dominant variable in unsaturated systems. We experimentally determined diffusion and kinetic parameters for Pseudomonas putida biofilms, then predicted biodegradation rates over a range of matric water potentials. For validation, we measured the rate of toluene depletion by intact biofilms and found the results to reasonably follow the model predictions. The diffusion coefficient for toluene through unsaturated P. putida biofilm averaged 1.3 x 10(7) cm(2)/s, which is approximately two orders of magnitude lower than toluene diffusivity in water. Our studies show that, at the scale of the microbial biofilm, the diffusion of toluene to biodegrading bacteria can limit the overall rate of biological toluene depletion in unsaturated systems. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 656-670, 1997.     

The gradient between pre-dawn rhizoplane and bulk soil matric potentials, and its relation to the pre-dawn root and leaf water potentials of four species

Uptake of soil water by plants may result in significant gradients between bulk soil and soil in the vicinity of roots. Few experimental studies of water potential gradients in close proximity to roots, and no studies on the relationship of water potential gradients to the root and leaf water potentials, have been conducted. The occurrence and importance of pre-dawn gradients in the soil and their relation to the pre-dawn root and leaf water potentials were investigated with seedlings of four species. Pre-germinated seeds were grown without watering for 7 and lid in a silt loam soil with initial soil matric potentials of -0.02, -0.1 and -0.22 MPa. Significant gradients, independent of the species, were observed only at pre-dawn soil matric potentials lower than -0.25 MPa; the initial soil matric potentials were -0.1 MPa. At an initial bulk soil matric potential of -0.22 MPa, a steep gradient between bulk and rhizoplane soil was observed after 7 d for maize (Zea mays L. cv. Issa) and sunflower (Helianthus annuus L. cv. Nanus), in contrast to barley (Hordeum vulgare L. cv. Athos) and wheat (Triticum aestivum L. cv. Kolibri). Pre-dawn root water potentials were usually about the same as the bulk soil matric potential and were higher than the rhizoplane soil matric potential. Pre-dawn root and leaf water potentials tended to be much higher than rhizoplane soil matric potentials when the latter were lower than -0.5 MPa. It is concluded that plants tend to become equilibrated overnight with the wetter bulk soil or with wetter zones in the bulk soil. Plants can thus circumvent negative effects of localized steep pre-dawn soil matric potential gradients. This may be of considerable importance for water uptake and growth in drying soil.     

Influence of soil water potential on respiration and nitrogen fixation ofAzotobacter vinelandii

Summary Respiration and N₂-fixation (acetylene reduction) ofAzotobacter vinelandii have been studied at a variety of soil water potentials. Both processes were strictly linked and strongly reduced at water potentials between –0.6 and –1.3 MPa. Complete inhibition occurred below –2.1MPa. Osmotic potentials in soil compared to matric potentials of the same value were less inhibitory to respiration and acetylene reduction by Azotobacter. The N₂-fixing efficiency (mg N/g glucose) was not influenced by water potentials ranging from –0.1 to –2.1 MPa.     

Water availability affects the growth,accumulation of compatible solutes and the viability of the biocontrol agent Epicoccum nigrum

Growth of the biocontrol fungus Epicoccum nigrum was more sensitive to ionic solute water stress (NaCl) than non-ionic (glycerol) on potato dextrose-based media at –0.5, –3.0 and –5.5 MPa water potentials. Subsequent physiological manipulation of growth of E. nigrum in glycerol-modified media to –3.0 MPa water potential resulted in a significant increase in the accumulation of compatible solutes in both mycelial liquid cultures and spores, but no enhanced accumulation of the desiccation protectant trehalose, when compared to unmodified media (–0.5MPa). The main solute accumulated was glycerol, followed by arabitol. In temporal studies over 20 days maximum accumulation of glycerol occurred in 5-d old cultures with water stressed cultures having 250× greater amounts than those from unmodified medium. The arabitol content was also higher in mycelium and spores produced under water stress. The difference was maximum after 15 days growth. Glucose content decreased over time in mycelial colonies but increased in spores. The germination of conidia from the two treatments was similar, regardless of compatible solute content, even at –9.25 MPa water potential stress. However, germ tube extension was significantly increased at this water potential level. The production of E. nigrum spores at –3.0 MPa water potential resulted in improved survival when stored fresh at 4 and 25 °C. However, freeze-drying severely affected the viability of spores produced on both media (–0.5 or 3.0 MPa). Accumulation of compatible solutes may assist the fungus in better ecological competence and establishment in the phyllosphere, where water availability is often limited.This revised version was published online in October 2005 with corrections to the Cover Date.     

Physical properties of two-component growth media based on Sphagnum peat and their implications for plant-available water and aeration

The physical properties, in particular the water retention characteristics, of two-component growth media based on low-humified Sphagnum peat were studied. The high water retention of pure peat, which is further increased by shrinkage of the medium at desorption, yielded low air-filled porosity at high matric potentials ( –1 kPa). The addition of coarse perlite to peat decreased the shrinkage markedly and also tended to increase the low saturated hydraulic conductivity of peat, which had initially been rather low. In all media studied, the amount of water that is easily available to plants (water content retained between –1 and –10 kPa matric potential) was relatively high. In peat that contained half repellent rockwool or hydrogel, this water retention was, however, markedly lower. Between –10 and –50 kPa matric potential, water retention was rather low in all media (< 10%). Within the lowest matric potential range studied (–50 to –1500 kPa), water retention was considerably elevated in peat that contained half hydrogel. The implications of the physical properties of the media for plant-available water and aeration in the media are discussed.     

Impact of osmotic and matric water stress on germination, growth, mycelial water potentials and endogenous accumulation of sugars and sugar alcohols in Fusarium graminearum

Studies were conducted to determine the effect of osmotic (NaCl, glycerol) and matric (PEG 8000) water stress on temporal germination and growth of two F. graminearum strains over the water potential range of -0.7 to -14.0 MPa at 15 and 25 C. The effect on endogenous water potentials and accumulation of sugars and sugar alcohols also were measured. For both strains, germination occurred rapidly over the same range of osmotic or matric potential of -0.7 to -5.6 MPa after 4-6 h incubation. At lower osmotic and matric potentials (-7.0 to -8.4 MPa), there was a lag of up to 24 h before germination. Optimum germ-tube extension occurred between -0.7 and -1.4 MPa for both strains but varied with the solute used. Growth was optimal at -1.4 MPa and 25 C in response to matric stress, with the minimum being about -8.0 and -11.2 MPa at 15 and 25 C, respectively. In contrast, F. graminearum grew fastest at -0.7 MPa and was more tolerant of solute stress modified with either glycerol or NaCl with a minimum of about -14.0 MPa at 15 and 25 C. A decrease in the osmotic/matric water potential of the media caused a large decrease in the mycelial water potential (Ψ(c)) as measured by thermocouple psychrometry. In general, the concentration of total sugar alcohols in mycelia increased as osmotic and matric potential were reduced to -1.2 MPa. However, this increase was more evident in mycelia from glycerol-amended media. The quality of the major sugar alcohol accumulated depended on the solute used to generate the water stress. The major compounds accumulated were glycerol and arabitol on osmotically modified media and arabitol on matrically modified media. In response to matric stress, the concentration of trehalose in colonies generally was higher in the case of osmotic stress. In each water-stress treatment there was a good correlation between Ψ(c) and total sugar alcohol content.     

Microtensiometer technique for in situ measurement of soil matric potential and root water extraction from a sandy soil

The suitability of microtensiometers to measure the spatial variation of soil matric potential and its diurnal change was tested in a pot experiment with pearl millet (Pennisetum americanum [L.] Leeke) in a sandy soil as the soil dried out.The temporal and spatial resolution of this technique allowed precise measurement of soil matric potential and thus estimation of soil water extraction from different compartments as well as from the whole rooting zone. The technique also allowed the measurement of rehydration of plants at night and root water uptake rate per unit soil volume or per unit root length. The precision of determination of root water uptake depended greatly on the accuracy of the estimate of hydraulic conductivity, which was derived from a bare soil and might be different for a cropped soil owing to aggregation induced by the root system. A linear relationship between root length and water uptake was found (r²=0.82), irrespective of variation in soil water content between compartments and despite the variation in root age, xylem differentiation and suberin formation expected to exist between different compartments of the rooting zone. As the experiment was carried out in a range of soil matric potentials between –4 and –30 kPa, drought stress did not occur. Further information at lower soil matric potentials are required, to address questions such as the importance of soil resistance for water uptake, or which portion of the root system has to be stressed to induce hormonal signals to the shoot. The microtensiometer technique can be applied to soil matric potentials up to –80 kPa.     

Water potential of standing-dead shoots of an intertidal grass

Summary Dead stems and leaves of smooth cordgrass (Spartina alterniflora Loisel.) undergo substantial fungal decay in the standing position. We conducted a series of experiments to determine the probable range of water availabilities that cordgrass-fungal decomposers experience; we determined the effects of duration of wetting and drying, site on shoots, leaching of osmotica, and stage of decomposition on water potential () of cordgrass shoots. Dried pieces of cordgrass shoots took up water rapidly when submerged, rising from values which were probably less than –150 megapascals (MPa) water potential to about –3.5 (soaked in seawater) or –1.5 (soaked in tapwater) within 5 min. Air-drying resulted in a return to low (<–7.5 MPa) within an hour. Literature reports show that most litter and wood-decomposing fungi which have been tested cannot grow at more negative than –6 MPa. The more lignified stems showed a more negative matric than leaves at water contents greater than 25% fresh weight. As leaves decomposed in the marsh, their increased (from –1.7 to –0.5 MPa, under standard conditions of 30 min freshwater soaking and 30 min air-drying), similar to what other investigators have found for wheat and forest litter. The water content at which cordgrass leaves reached –6 MPa was about 23% fresh weight, within the range (15–32%) found for wheat straw and forest litter.     

Effect of water potential on the development of an haploid strain of Sporisorium reilianum f.sp. zeae

Sporisorium reilianum f.sp. zeae, the causal agent of head smut, infects the roots of the maize plantlets. Little information is available concerning the development of the fungus in soil, although this saprophytic phase is an important part of the life cycle. This paper reports that water potential also affects hyphal induction, and this effect on the fungus may influence disease transmission. In response to a decrease in water potential from 0 to –1.52 MPa in presence of variable molecular weight polyethylene glycols, haploid hyphae develop from the haploid yeast. Hyphal extension is fastest at low water potentials (–1.2 MPa) controlled with high molecular weight polyethylene glycols, PEG-3350 and PEG-8000. Formation of parasitic dikaryotic hyphae following fusion between haploid hyphae was possible at low water potential (–1.2 MPa) and was not inhibited by water stress. These results are consistent with the hypothesis that the effects of low soil water potential on yeast–hyphal transition and hyphal growth facilitate the convergence of compatible haploid strains, and that this may increase disease severity.     

Growth and Nodulation Responses of Rhizobium meliloti to Water Stress Induced by Permeating and Nonpermeating Solutes

Isolates of Rhizobium meliloti, representing antigenically distinct indigenous serogroups 31 and 17, were grown in yeast extract-mannitol broth (YEM) containing NaCl or polyethylene glycol (PEG) to provide external water potentials ranging from −0.15 to −1.5 MPa. Several differences were found between representatives of the two groups in their abilities to adapt to water stress induced by the nonpermeating solute PEG. At potentials below −0.5 MPa, strain 31 had a lower specific growth rate than strain 17 and an irregular cell morphology. In contrast, neither growth nor cell morphology of either strain was affected significantly over the same range of water potentials created by a permeating solute, NaCl. Despite the superior growth of strain 17 at the low water potentials imposed by PEG, upshock of water-stressed cells (−1.0 MPa; PEG) into normal YEM (−0.15 MPa) resulted in a faster recovery of growth by strain 31 than by strain 17. Different responses of the two strains to a water potential increase were also revealed in nodulation studies. Strain 31 required significantly fewer days to nodulate alfalfa than strain 17 did when the strains were transferred from YEM with PEG at −1.0 MPa onto the roots of alfalfa seedlings in plant growth medium (−0.1 MPa). The addition of supplemental calcium (0.1 mM) to growth medium with PEG (−1.0 MPa) reduced the differences between strains in their responses to water stress. The severe growth restriction and morphological abnormalities shown by strain 31 were corrected, and the prolonged recovery time shown by water-stressed cells (−1.0 MPa; PEG) of strain 17 upon transfer to normal YEM was shortened. The latter strain also nodulated earlier and more rapidly after growth in PEG medium at −1.0 MPa in the presence of supplemental calcium ions. These results indicate that the efficacy of osmoregulation can vary among strains of the same species and that the mechanism of osmoregulation may differ depending on the nature of the water stress.     

Water potential affects Coniothyrium minitans growth, germination and parasitism of Sclerotinia sclerotiorum sclerotia

Water availability is an important environmental factor which has major effects on fungal activity. The effects of osmotic (KCl amended agar) and matric Polyethylene glycol ((PEG) 8000 amended agar) potentials over the range -0.1 to -5.0MPa on mycelial growth and conidial germination of eight isolates of the sclerotial parasite Coniothyrium minitans was assessed. The influence of soil water potential on the ability of three selected isolates (LU112, LU545, and T5R42i) to parasitise sclerotia of the plant pathogen Sclerotinia sclerotiorum was determined. For all eight C. minitans isolates, decreasing osmotic and matric potentials caused a reduction in mycelial growth and conidial germination. Isolates were more sensitive to decreasing matric potential than osmotic potential. Across the isolates, growth at an osmotic potential of -5.0MPa was 30-70% of the growth seen in the control, whereas less than 20% of the control growth was seen at the corresponding matric potential. Across all isolates no conidial germination was seen at matric potential of -5.0MPa. The C. minitans isolates varied in their sensitivity to decreasing water potentials. Mycelial growth and conidial germination of three isolates (LU112, Conio, and CH1) were more tolerant of low osmotic potential and matric potential with respect to mycelial growth. Isolates T5R42i and LU430 were least tolerant. In contrast, conidial germination of isolates Conio, LU545, and T5R42i were less sensitive to decreasing matric potential. Soil water potential was seen to affect infection and viability of sclerotia by the three C. minitans isolates. Isolate LU545 reduced sclerotial viability over a wider water potential range (-0.01 to -1.5MPa) compared with LU112 (-0.01 to -1.0MPa), with isolate T5R42i being intermediate. Indigenous soil fungi (Trichoderma spp. and Clonostachys rosea) were recovered from sclerotia but did not result in reduction in sclerotial viability. The relevance of these results in relation to biocontrol activity of C. minitans in soil is discussed.     

Drought avoidance and the effect of local topography on trees in the understorey of Bornean lowland rain forest

The water relations of two tree species in the Euphorbiaceae werecompared to test in part a hypothesis that the forest understorey plays anintegral role in drought response. At Danum, Sabah, the relatively commonspecies Dimorphocalyx muricatus is associated with ridgeswhilst another species, Mallotus wrayi, occurs widely bothon ridges and lower slopes. Sets of subplots within two 4 -hapermanent plots in this lowland dipterocarp rain forest, were positioned onridges and lower slopes. Soil water potentials were recorded in1995–1997,and leaf water potentials were measured on six occasions. Soil water potentialson the ridges (–0.047 MPa) were significantly lower than onthe lower slopes (–0.012 MPa), but during the driest periodin May 1997 they fell to similarly low levels on both sites (–0.53MPa). A weighted 40-day accumulated rainfall index was developedtomodel the soil water potentials. At dry times, D.muricatus(ridge) had significantly higher pre-dawn (–0.21 v.–0.57 MPa) and mid-day (–0.59 v.–1.77 MPa) leaf water potentials than M.wrayi (mean of ridge and lower slope). Leaf osmotic potentials ofM. wrayi on the ridges were lower (–1.63MPa) than on lower slopes (–1.09 MPa), withthose for D. muricatus being intermediate (–1.29MPa): both species adjusted osmotically between wet and dry times.D. muricatus trees were more deeply rooted thanM. wrayi trees (97 v. 70cm). M. wrayi trees had greaterlateral root cross-sectional areas than D. muricatus treesalthough a greater proportion of this sectional area for D.muricatus was further down the soil profile. D.muricatus appeared to maintain relatively high water potentialsduring dry periods because of its access to deeper water supplies and thus itlargely avoided drought effects, but M. wrayi seemed to bemore affected yet tolerant of drought and was more plastic in its response. Theinteraction between water availability and topography determines these species'distributions and provides insights into how rain forests can withstandoccasional strong droughts.     

Water relations and photosynthesis in the cryptoendolithic microbial habitat of hot and cold deserts

Two cryptoendolithic microbial communities, lichens in the Ross Desert of Antarctica and cyanobacteria in the Negev Desert, inhabit porous sandstone rocks of similar physical structure. Both rock types adsorb water vapor by physical mechanisms unrelated to biological processes. Yet the two microbial communities respond differently to water stress: cryp-toendolithic lichens begin to photosynthesize at a matric water potential of –46.4 megaPascals (MPa) [70% relative humidity (RH) at 8°C], resembling thallose desert lichens. Cryptoendolithic cyanobacteria, like other prokaryotes, photosynthesize only at very high matric water potentials [> –6.9 MPa, 90% RH at 20°C].     

Water Content Mediated Microaerophilic Toluene Biodegradation in Arid Vadose Zone Materials

We investigated the conditions promoting toluene biodegradation for gasoline-contaminated near-surface (0.6 m depth) and subsurface (4.7 to 5.0 m depth) vadose zone soils sampled from an arid environment. At both depths, water addition was required for toluene biodegradation to occur. In near-surface samples, no inorganic nutrient addition was necessary and (i) biodegradation was fastest at 0.0 MPa, (ii) biodegradation rates decreased with decreasing water potential down to ?1.0 MPa, and (iii) biodegradation was undetectable at ?1.5 MPa. For subsurface material, toluene depletion was stimulated either by slurrying with a nutrient solution or by adjusting the moisture content to 20% (0.0 MPa) with nutrient solution and lowering the oxygen concentration (to effectively 1 mg L-1 in the aqueous phase). Thus, in the subsurface material, toluene depletion was microaerobic and nutrient-limited, occurring only under low oxygen and with inorganic nutrient addition. Our studies implicate microaerophily as an important characteristic of the toluene-degrading communities in these dry soils, with soil water as a primary controller of oxygen availability.     

Daily and seasonal variation in water relations of macchia shrubs and trees in France (Montpellier) and Turkey (Antalya)

Based upon two different research studies in the mediterranean regions of France and Turkey, drought resistance strategies were investigated in a broad group of species. The diurnal and seasonal patterns of the water relations of different lifeforms from the thermo-mediterranean to submediterranean lifezones were compared. Three sites near Montpellier, in Southern France, and five sites near Antalya, Turkey were used for this comparison. Xylem pressure potential and relative stomatal aperture were the key water relations parameters collected in France while these parameters as well as osmotic potential and leaf conductance were studied in Turkey.From the 26 different study species investigated in France, 7 distinct types of stomatal control were observed, with the deciduous lifeforms showing the least control, the sclerophyllous and coniferous evergreens the greatest control and the malacophyllous shrublets intermediate levels of control. Predawn water potential values provided a means of classifying species according to their temporal and spatial utilization of site water reserves. The comparison of turgor potentials (difference between water and osmotic potentials) gave an insight into leaf adaptations to site moisture. Species with high predawn water potentials generally maintain positive turgor even at midday during the summer, whereas species with low predawn values were frequently at zero turgor even at predawn. Phlomis grandiflora was the most extreme species with mid-summer predawns and midday water potentials of –6 MPa and osmotic potentials never more negative than –2.4 MPa.     

Water relations of the root hemiparasite Olax phyllanthi (Labill) R.Br. (Olacaceae) and its multiple hosts

Summary Water relations of the root hemiparasite Olax phyllanthi were compared with those of its major species of hosts in natural habitat in coastal heath near Denmark, SW Australia. Leaf water potentials of Olax during winter were 0.4 to 1.4 MPa lower (more negative) than those of all (29) non parasitic host species examined. During the dry summer months (January to March), shallow-rooted hosts developed water potentials up to 3 MPa lower than those of Olax, and were accordingly rated as no longer accessible as a source of water to the hemiparasite. By contrast, deep-rooted hosts, with access to the water table, showed water potentials less negative than Olax, and haustorial contacts retained with these apparently enabled continued extraction of water and nutrients throughout the summer. Three other species of root hemiparasites parasitized by Olax, but not themselves parasitizing Olax, showed leaf water potentials throughout the year very close to, and mostly slightly more negative than those of Olax. Nocturnal measurements of leaf water potential in winter (July and August) in soil at field capacity (water potential –0.006 MPa) showed maintenance of a 0.5–0.8 MPa potential difference between Olax and a range of common host species. By dawn most hosts had equilibrated with the water potential of the soil, whereas both exposed and bagged Olax plants recorded potentials of –0.8 MPa. Daytime rates of transpiration and photosynthesis of Olax were less than those of a range of common hosts, but water use efficiencies were not consistently different between hemiparasite and hosts. This was reflected in almost identical mean values for carbon isotope ratio (¹³C/¹²C) between Olax (mean value –27.0) and thirteen frequently exploited hosts ( value –27.1). The results are discussed in relation to published information on other angiosperm hemiparasites.     

Seasonal variations of leaf water potential and growth in fertigated carob-trees (Ceratonia siliqua L.)

Variations of predawn and midday leaf water potential and relative growth rates were studied in mature carob trees (Ceratonia siliqua L. cv Mulata) submitted to a fertigation experiment. Three levels of irrigation were tested: 0%, 50% and 100%, based on daily standard evaporation values. For each irrigation level two nitrogen amounts were applied –21 and 63 kg N ha^-1 year^-1 as ammonium nitrate. The experiment was run between July 91 and August 1993. Measurements of leaf water potential and absolute branch length increments were made at monthly intervals, during the entire experimental period or during seasonal growth, respectively. Leaf water potential was related to soil volumetric water content, maximum and minimum air temperature and daily evaporation. Predawn leaf water potentials were always higher than –1.1 MPa. Midday leaf water potential values presented very large seasonal variations and very low values independent of treatments. The low leaf water potentials observed for the fertigated trees during summer, suggest that this parameter may be related not only to the evaporative demand but also to growth investment. The amount of fertigation was positively correlated with vegetative growth increment and fruit production. Practical implications for irrigation schedules of leaf water potential patterns together with drought adaptation mechanisms of carob tree are discussed.     

Salinity,water use and yield of maize: Testing of the mathematical model ecosys

There is a need to establish how root water uptake should be calculated under saline conditions, and to test calculated uptake against experimental data recorded under documented site conditions. In this study, the ecosystem simulation model ecosys was expanded to include an ion transfer-equilibrium-exchange model used to calculated electrical conductivity and osmotic potential. This expanded model was tested against experimental data for maize growth and water use reported under different irrigation and salinity levels at four different sites in the western U.S. to determine if salinity effects on crop growth and water use could be modelled from the effects of salinity on soil osmotic potential. The model was able to reproduce reductions in water use and phytomass yields on salinized (10 g total salts kg^–1 water) soils that ranged from 10 to 50% of those on non-salinized controls. In general, these reductions increased with increasing irrigation deficits. These reductions arose in the model from reduced canopy water potentials and conductances caused by reduced osmotic potentials in the saline soils. The hypothesis that salinity effects on crop growth and water use are caused by salinity effects on soil osmotic potential appear to be supported under the range of conditions included in this study. Models such as ecosys that are based on general hypotheses for the effects of salinity upon biological activity may be well adapted for general use in assessing the effects of salinity on crop growth and water use with different soils, managements and climates.