Decreasing Soil Water Availability Research Articles

Changes in physiological attributes of ponderosa pine from seedling to mature tree.

Plant physiological models are generally parameterized from many different sources of data, including chamber experiments and plantations, from seedlings to mature trees. We obtained a comprehensive data set for a natural stand of ponderosa pine (Pinus ponderosa Laws.) and used these data to parameterize the physiologically based model, TREGRO. Representative trees of each of five tree age classes were selected based on population means of morphological, physiological, and nearest neighbor attributes. Differences in key physiological attributes (gas exchange, needle chemistry, elongation growth, needle retention) among the tree age classes were tested. Whole-tree biomass and allocation were determined for seedlings, saplings, and pole-sized trees. Seasonal maxima and minima of gas exchange were similar across all tree age classes. Seasonal minima and a shift to more efficient water use were reached one month earlier in seedlings than in older trees because of decreased soil water availability in the rooting zone of the seedlings. However, carbon isotopic discrimination of needle cellulose indicated increased water-use efficiency with increasing tree age. Seedlings had the lowest needle and branch elongation biomass growth. The amount of needle elongation growth was highest for mature trees and amount of branch elongation growth was highest for saplings. Seedlings had the highest biomass allocation to roots, saplings had the highest allocation to foliage, and pole-sized trees had the highest allocation to woody tissues. Seedlings differed significantly from pole-sized and older trees in most of the physiological traits tested. Predicted changes in biomass with tree age, simulated with the model TREGRO, closely matched those of trees in a natural stand to 30 years of age.

Measurement and modelling of the transpiration of a Mediterranean vineyard

This paper presents the results on the spatio-temporal variation in vine transpiration at the scale of a vineyard and derives a simple model relating daily vine transpiration to soil water storage. An experiment was conducted during summers of 1994 and 1995 in a 1 ha vineyard of Southern France in which five monitoring plots had been installed. In each plot, soil moisture was monitored with neutron probes down to 260 cm under the row and between the rows to take into account the heterogeneity of the rooting system. Transpiration was monitored by the stem heat balance method on two vinestocks per plot. The soil water retention characteristics were measured on undisturbed samples with a pressure plate extractor. With decreasing soil water availability, vines react similarly to most crops: in a first stage up to mid-July transpiration remains at a maximum and in a second stage transpiration is reduced due to soil water depletion. Although there is a large variation in the transpiration intensity between vinestocks, the temporal variation of transpiration is predominantly similar over the vineyard. This justifies a uniform modelling approach to simulating vineyard transpiration. Accordingly, we derived a bilinear relationship relating the ratio between the actual transpiration and the potential transpiration of the vineyard to the field average soil water storage. The parameters of the relationship could be related to laboratory determined values of soil moisture content at −0.1 and −1.5 MPa.

Interspecific variation in sapling mortality in relation to growth and soil moisture

To examine the causes of landscape variation in forest community composition, we have quantified sapling mortality as a function of growth and soil moisture for seven dominant species in transition oak‐northern hardwood forests of the northeastern USA. We located saplings in sites that encompassed a wide range of variation in soil moisture and light availability. In mesic conditions, the probability of mortality decays rapidly with increasing growth among shade tolerant species and more gradually among shade intolerant species: the rank order of survivorship at low growth rates is Tsuga canadensis>Fagus grandifolia>Acer saccharum>Fraxinus americana>Acer rubrum>Quercus rubra>Pinus strobus. The relationship between probability of mortality and growth does not vary with soil moisture among species insensitive to drought: Tsuga canadensis, Quercus rubra, and Pinus strobus. However, probability of mortality increases substantially with decreasing soil water availability for the other four species. Acer saccharum and Fagus grandifolia have high mortality rates under xeric conditions even when their growth is not suppressed. Acer rubrum and Fraxinus americana exhibited a steady but more gradual increase in the probability of mortality with decreasing soil moisture. Among the five deciduous hardwood species we examined there is a weak inverse relationship between the ability to survive growth suppression, a measure of shade tolerance, and the ability to survive in xeric conditions, a measure of drought tolerance. Tsuga canadensis, however, is tolerant of growth suppression and exhibits high survivorship in xeric conditions, while Pinus strobus is intolerant of growth suppression but insensitive to soil moisture. Species differences in water‐dependent mortality are consistent with the species distributions across landscape gradients of soil water availability.

SAP FLOW TRANSPIRATION MEASUREMENTS IN A TABLE GRAPE VINEYARD GROWING IN SOUTHERN ITALY

Optimising crop irrigation management is a general concern in semi-arid climates. Actual crop water use in the field has to be determined as a function of microclimate and of soil water availability. The work described here studies stomatal conductance, transpiration and evapotranspiration of an irrigated vineyard to elucidate their relations with microclimate at decreasing soil moisture availability and to assess their usefulness in determining plant water consumption in the field. The experiment was carried out in a vineyard, trained by the overhead system (“Tendone”), in Southern Italy, in July 1998 for six days in the interval between two irrigations. Stem heat balance gauges were installed on six plants and transpiration flow of one shoot per plant was measured; daily course of stomatal conductance was determined by porometry, and actual evapotranspiration was measured by the Bowen Ratio technique. Results can be summarised as follows: i) at decreasing soil water availability stomatal conductance and transpiration rate decreased and modified their daily pattern, shifting their peaks from noon to mid-morning; ii) sap flow transpiration rates per unit leaf area showed considerable variability among shoots; iii) the daily transpiration course measured by sap flow was in good agreement with actual evapotranspiration measured by the Bowen Ratio method.

Control of crops leaf growth by chemical and hydraulic influences

Three species of forage grasses (Festuca arundinacea, Eragrostis curvula, Sporobolus stapfianus) commonly grown in the Mediterranean region were subjected to a soil drying treatment. Leaf growth rate in F. arundinacea was highly sensitive to soil drying and low growth rates were associated with high laminar turgors. The production of ABA was stimulated by soil drying and there was a clear relation between increasing ABA accumulation and reduction in leaf growth. Leaf growth of E. cutvula, a C4 warm season grass, was relatively insensitive to soil drying which was not accompanied by a substantial increase in leaf ABA content. S. stapfianus, a resurrection plant, was highly sensitive to decreasing soil water availability. In these two latter species, leaf growth was substantially restricted before ABA accumulation occurred. It is suggested that reductions in laminar turgor of E. curvula and S. stapfianus may be limiting leaf growth as soil dries. The results indicated a different mechanism of sensing and responding to reduction in soil water availability for the three species studied. The relative importance of the chemical and hydraulic control of leaf growth is discussed.

Grapevine growth of shoots and fruit linearly correlate with water stress indices based on root-weighted soil matric potential

Over two consecutive seasons, six deficit irrigation regimes were imposed on Colombard vines on Ramsey rootstock. The soil water availability was quantified by logarithmic transformation of the absolute value of a root-weighted measure of soil matric potential. Stem water potential declined linearly with decreasing soil water availability. Water stress indices were calculated by integrating the daily values of soil water availability over specific periods. Declines in both fruit and vegetative growth were linearly correlated with increases in the values of water stress indices during the respective growth periods. In the berries, concentrations of total soluble solids, tartrate and potassium were not correlated with the water stress index during the period of fruit growth; malate concentration declined linearly with increasing values of this index. Responses of pH and titratable acidity to variation in the water stress index were not consistent in the two seasons.

Gas exchange and resource‐use efficiency of Leymus cinereus (Poaceae): diurnal and seasonal responses to naturally declining soil moisture

We examined factors that limit diurnal and seasonal photosynthesis in Leymus cinereus, a robust tussock grass from shrub‐steppes of western North America. Data from plants in a natural stand and in experimental field plots indicate that this bunchgrass has 1) a high photosynthetic capacity, 2) high leaf nitrogen content and high nitrogen‐use efficiency, 3) a steep leaf‐to‐air diffusion gradient for carbon dioxide, which enhances intrinsic water‐use efficiency, and 4) photosynthetic tissues that tolerate severe water stress and recover quickly from moderate water stress. Midday depressions of CO2 assimilation (A) and stomatal conductance were slight in plants with plentiful water, but marked in plants subject to moderate water stress. Midday stomatal closure in moderately stressed plants reduced intercellular carbon dioxide concentration (ci) by ≈40 μl liter‐1. The maximum rate of A achieved during the day for severely stressed plants (predawn water potential = ‐4 MPa) was one‐third and daily carbon gain per unit leaf area was about one‐fourth that of well‐watered plants. For plants in the natural stand, CO2‐saturated photosynthesis declined almost linearly with decreasing soil water availability over the growing season, whereas there was little effect on A at CO2 ambient levels or on carboxylation efficiency until predawn water potentials reached ‐1.8 MPa. Nitrogen‐use efficiency declined with diminishing soil moisture, but there was no seasonal change in stomatal limitation or instantaneous water‐use efficiency as estimated from A vs. ci curves at optimal leaf temperature and moderate atmospheric evaporative demand. Thus, reduced stomatal conductance in response to increased evaporative demand may increase stomatal limitation diumally, but over the growing season, stomatal limitation estimated from A vs. ci curves is relatively constant because maximum stomatal conductance is closely tuned to the CO2 assimilatory capacity of the mesophyll.

Stomatal conductance and CO2 assimilation as screening tools for drought resistance in sorghum

Stomatal conductance and CO2 assimilation were evaluated at three different levels of soil water availability as tools for estimating relative drought resistance in sorghum (Sorghum bicolor (L.) Moench) at vegetative stage (preanthesis). Four genotypes differing in drought resistance in the field (A Texas line TX 622; Oklahoma lines, BOK 11 and BOK 111; and IN-15, a line from the Sudan of Africa) were investigated. Plants were grown in a growth chamber at 27 °C, (day/night) day length of 14 h and photosynthetic photon flux density of 350 μmol m−2 S−1. Three weeks after germination, plants were divided into control, 50% and 25% soil water saturation groups. At the end of the second week of treatment, simultaneous measurements were made of stomatal conductance, CO2 assimilation, leaf water potential and transpiration, on the youngest most mature leaf. After gas exchange measurements were taken, leaf water potential was measured. In a separate experiment, survival rate under water stress conditions was also determined on plants grown under the same conditions as above. Four weeks after germination, the plants were subjected to two cycles of drought and the survival rate and growth of each genotype determined. Survival rate and growth throughout dought cycles I and II was, in the decreasing order of, IN-15, BOK 111, TX 622 and BOK 11. Water potential, stomatal conductance and CO2 assimilation of each genotype declined with decreased soil water availability; the smallest decrease was observed in IN-15 and the largest decreases in BOK 111, followed by TX 622 and BOK 11. Water use efficiency of each genotype was increased, in the same order as above, with decreased soil water availability. This correlates with the ranking obtained in the artificial drought test. In conclusion, stomatal conductance and CO2 assimilation appear to be useful tools for screening sorghum genotypes at vegetative stage (preanthesis) of growth for drought resistance. Key words: Water potential, water use efficiency (WUE), Sorghum bicolor (L.) Moench

Field water relations of three temperate vines.

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

Field Water Relations of Sonoran Desert Annuals

Field water relations were examined in winter and summer annuals of the Sonoran Desert. Both groups were characterized by low belowground biomass allocation. Winter annuals had large interspecific variation in water relations parameters, but their leaves generally had high conductances of water vapor and high water potentials during early growth periods. These high values were very short—lived and were reduced by peak flowering periods. Summer annuals were measured midway through the growing season; their leaves showed less interspecific variability in water vapor conductances and had values similar to the winter annuals for this stage of the growing season. Midday and diurnal courses of photosynthesis in the winter annuals exhibited lower rates than values previously reported for plants grown under unlimited water and nutrient regimes in glasshouses. This was probably due to the decreased leaf water potentials and increased vapor pressure deficits present in field situation. Both the winter and the summer annuals showed a dichotomy among species in leaf water potentials, associated with either variations in soil water availability or the ability of leaves to adjust osmotically to decreased soil water availability. Although winter and summer annuals are ephemeral, some species are capable of tolerating low leaf water potentials and therefore are not drought evading in the traditional sense.

Use of Leaf Water Potential to Determine Water Stress in Field-grown Tomato Plants1

Abstract Tomato plants (Lycopersicon esculentum Mill.) were grown under 4 irrigation treatments. In the first three an attempt was made to maintain soil water at a different level for each treatment. The fourth treatment was an approximation of the varied moisture levels that would be encountered with furrow irrigation. Leaf water potential (ΨL) was affected more during a day by atmospheric factors than by soil water availability. Hourly changes in ΨL and air water potential (Ψ air) were highly correlated (range .94*** and .99***). ΨL decreased as the plant aged, apparently due to decreasing soil water availability, decreasing root activity, and increasing resistance to water flow in stems and leaves. The stress factor, (which is an integration of the area below −6 bars; a critical level for tomatoes) was determined from the second degree polynomial regression of time versus ΨL and is proposed as a useful integration of ΨL. By using stepwise regression it was found that plant water status as evaluated by tensiometer reading, stress factor, the ratio of soil water/soil water at field capacity, and daily pan evaporation had the greatest effect on yield and total soluble solids content of tomato fruit.