Water Relations Measurements Research Articles

Laboratory and field measurements of water relations, photosynthetic parameters, and hydration traits in macrolichens in a tropical lower montane rainforest in Thailand.

Ecophysiological studies of lichens in tropical Asia are rare, and additional studies can increase the understanding of lichen life in this region. The main aim of this study was to observe the relationships between water availability and photosynthetic parameters, as well as hydration trait parameters, in macrolichens during the rainy and dry seasons in a tropical forest. A total of 11 lichen species growing in a lower montane rainforest in Thailand were collected and studied. The results clearly showed that the specific thallus mass (STM), net photosynthetic rate (Pn), the potential quantum yield of primary photochemistry (Fv/Fm), chlorophyll content, and carotenoid content of almost all lichens were lower in the dry season than in the rainy season. Field measurements in the dry season revealed that only the foliose chlorolichen Parmotrema tinctorum was metabolically active and exhibited slight carbon assimilation. In the rainy season, all lichens started their photosynthesis in the early morning, reached maximal values, declined, and ceased when the thalli desiccated. The photosynthetically active period of the lichens was approximately 2-3h in the morning, and the activities of the cyanolichens ended approximately 30min after the chlorolichens. The hydration trait parameters, including the STM, maximal water content (WCmax), and water holding capacity (WHC), were greater in the cyanolichens. In addition, the maximal Pn (Pnmax) and optimal water content (WCopt) for Pn were also greater in the cyanolichens, but the maximal Fv/Fm (Fv/Fmmax) was lower. The cyanolichens compensated for their inability to use humid air to restore photosynthesis by having higher water content and storage, higher photosynthetic rates, and longer photosynthetically active periods. This study provides additional insights into lichen ecophysiology in tropical forests that can be useful for lichen conservation.

Different patterns of the photosynthetic apparatus recovery during early rehydration following drought stress in two types of intergeneric hybrid of triticale

The study aimed to reveal the activity of the photosynthetic apparatus during early rehydration in double haploid (DH) lines of triticale with different types of flag leaf rolling in response to soil drought. Two DH lines, showing full (Type O) and limited (Type V) flag leaf rolling, were investigated. The measurements of chlorophyll fluorescence, analysis of red/far-red fluorescence excitation spectra, as well as detection of selected PSII proteins (PsbA/D1, PsbS) and transmembrane cytochrome complex b6f (PetC/Rieske protein) were performed. The measurements were carried out on the last day of drought and after 1 h, 3 h, and 5 h of rehydration. The physiological condition of flag leaves was studied based on measurements of water relations, the progress of leaf senescence, histochemical detection of reactive oxygen species, and the content of sugars and phenolic compounds. In Type O, as compared with Type V, a further decline in the activity of the photosynthetic apparatus was observed during the post-drought rehydration period. This was accompanied by the accumulation of reactive oxygen species and more advanced senescence of flag leaves. Fluorescence excitation spectra revealed disorders in photosynthetic light conversion in Type O flag leaves during drought and rehydration over the entire range of excitation wavelengths corresponding to UVC, UVB, and UVA radiation. Type V flag leaves developed an effective UV protection mechanism in the form of increased accumulation of cell wall-bound phenolics, which efficiently absorbed UV radiation. Both types differed in the level of accumulation of PsbA, PsbS, and PetC proteins during drought and early rehydration. Soil drought caused a similar reduction in grain yield in both types. However, in Type O, the largest share in the grain yield was ascribed to the ears of lateral stems, while in Type V to the ears of the main stems. Our research has shown that the limited rolling of flag leaves under drought stress corresponds to the effective photoprotection of the photosynthetic apparatus in the first hours of rehydration.

Intraspecific plasticity in hydraulic and stomatal regulation under drought is linked to aridity at the seed source in a wild pear species

Adaptations of fruit trees to future climate are a current research priority due to the rapid increase in air temperature and changes in precipitation patterns. This is aimed at securing sustainable food production for our growing populations. Key physiological traits in trees conferring drought tolerance are resistance to embolism and stomatal control over water loss. Recently, we have shown in the field that a native wild pear species performs better under drought than two cultivated pear species. A comparative greenhouse study was conducted to investigate traits associated with drought tolerance in four ecotypes of a wild pear species (Pyrus syriaca Boiss), compared with a wild pear species (Pyrus betulifolia Bunge) commonly used as a pear rootstock. Seed sources were collected from semi-arid, sub-humid and humid sites across northern Israel. Measurements of water relations, leaf physiology, hydraulic conductivity and percent loss of hydraulic conductivity (PLC) were conducted under standard irrigation, drought and recovery from drought. The four P. syriaca ecotypes maintained significantly higher leaf gas exchange values and water-use efficiency and had lower PLC than the rootstock species under prolonged drought as well as during recovery. Across the four ecotypes, stomatal closure occurred at stem water potential (Ψ) around -3.5MPa; however, Ψ at 50% PLC ranged from -4.1MPa in the humid ecotype to -5.2MPa in one of the semi-arid ecotypes, rendering the latter with a higher hydraulic safety margin (the Ψ difference between stomatal closure and 50% PLC). Divergence of the ecotypes in xylem vulnerability to embolism closely matched the mean annual precipitation at their seed sources. Thus, selection of pear ecotypes from populations in semi-arid sites may be better than the currently used plant material for preparing our cultivated species for hotter and drier future climate.

Stem water potential-based regulated deficit irrigation scheduling for olive table trees

Regulated deficit irrigation (RDI) involves water stress management in different phenological periods throughout the season. Research in olive trees (oil production) suggested RDI during pit hardening based in pre-dawn and midday stem water potential (SWP) thresholds. However, the previous thresholds may not be extrapolated to table olive because fruit size, a very important feature in the table olive yield quality, is very sensitive to water stress. RDI in table olive deserve further research to determine the optimal water potential thresholds and the duration of the RDI periods for the specificity of the crop (low crop load to promote high fruit size). The aim of this work was to study different RDI schedules during pit hardening, considering different levels and durations of water stress. The experiment was performed in the 2015, 2016 and 2017 seasons, in a commercial mature table olive orchard (cv. Manzanilla) in Dos Hermanas (Seville, Spain). Control treatments were based on midday SWP measurement in order to optimize the water status with values around −1.4 MPa. Two RDI treatments were applied during pit hardening, dated (according to the changes in longitudinal fruit growth) from mid-June to the last week of August) to maintain water potential values around −2 MPa (RDI-1) and −3.5 MPa (RDI-3). Another RDI treatment (RDI-2) received irrigation to maintain values around −3.5 MPa but the recovery was performed at early July in order to obtain different durations of water stress. Irrigation strategies were evaluated with water relations measurements (soil moisture, gas exchange), fruit and shoot growth and quality and quantity yield indicators. Yield was not significantly affected in any of the RDI treatments with an ANOVA analysis. However, fruit drop estimated as the percentage of fruit lost only in the period of water deficit was related with water stress parameters (SWP and stress integral, IS). In addition, the relationship between fruits size and these latter parameters were significant and change according to yield level. Irrigation treatments did not affect next season yield because shoot growth and number of inflorescence at the beginning of each season were not different. RDI effect changed according to yield level, mainly in relation with fruit size. Data suggest that yield levels up to 12 t ha−1 were possible to manage RDI without affecting fruit size or reducing commercial quality.

The seasonal regulation of gas exchange and water relations of field grown macadamia

Macadamia F. Muell is a recently domesticated nut crop characterized by a certain amount of drought tolerance. Whilst stomatal regulation of gas exchange and water relations has been documented in macadamia, there are no reports on non-stomatal limitations to photosynthesis and if changes occur over a season in relation to different phenological stages. It was hypothesised that in order for macadamias to be adapted to a seasonally dry native environment the trees would be characterised by strong stomatal control over gas exchange, indicating an isohydric water use strategy that is related to a hydraulic limtation within the tree. However, due to high assimilate demand during nut filling, the level of stomatal control would vary between fruiting and non-fruiting phenological stages. Gas exchange and water relations measurements were made for 18 months on irrigated mature macadamia trees (cv. HAES 695, ‘Beaumont’) in a subtropical region of South Africa. Results confirmed that macadamias had relatively low light-saturated net CO2 assimilation rates (Amax) (8.34 ± 1.21 μmol CO2 m−2 s-1). The low Amax values resulted from relatively high stomatal and non-stomatal limitations and decreasing stomatal conductance (gs) in response to increasing leaf-to-air vapour pressure deficit (VPDleaf) above ca. 2 kPa. Strict stomatal control and nearly constant midday leaf water potential (ψleaf = -1.16 ± 0.43 MPa) confirmed the predominantly isohydric nature of the crop, which seemed to be a result of low hydraulic conductance in the stem to leaf pathway. Significant differences in leaf gas exchange capacity were, however, observed between the fruiting and non-fruiting phenological stages. The presence of fruit resulted in significantly higher Amax (10.27 ± 2.23 μmol CO2 m−2 s−1) compared to non-fruiting periods (Amax = 6.58 ± 2.00 μmol CO2 m−2 s−1). Increases in Amax were mediated by increased rates of electron transport (Jmax) and triose phosphate use (TPU). Fruiting stages were also characterized by variable responses of gs to increases in VPDleaf, which indicated varying degrees of isohydricity. This study reaffirms that macadamias are inherently adapted to seasonally dry environments, characterized by strict stomatal control, yet under humid conditions or in the presence of developing fruit, macadamias are proposed to deviate from a purely isohydric water management strategy.

Exploring the link between more negative osmotic potential and ryegrass summer performance

This paper outlines recent research studying within-population variation in selected New Zealand perennial ryegrass cultivars, for traits related to tolerance of summer moisture deficit. Two clonal replicates of 220 genotypes from ‘Grasslands Nui’ (Nui, n=50), ‘Grasslands Samson’ (Samson, n=80), and ‘Trojan’ (n=90) were exposed to 1 month of moisture deficit challenge, with plant water relations measurements performed to evaluate putative drought-response mechanisms. Water use of individual genotypes ranged from <100 to >1000 g water/g DM indicating large within-population variation for this trait. Mean water use efficiency (WUE) was for Nui, Samson, and Trojan, respectively, 424±16, 412±10, and 319±9 g water/g DW (P<0.001), suggesting that commercial plant breeding may have indirectly reduced water use in modern cultivars without specific focus on water relations. Principal component analysis indicated more negative osmotic potential may contribute to reduced water use while maintaining yield under water deficit, giving a potential focus for future breeding selection targeting summer water deficit tolerance.

High apoplastic solute concentrations in leaves alter water relations of the halophytic shrub, Sarcobatus vermiculatus

Predawn plant water potential (Psi(w)) is used to estimate soil moisture available to plants because plants are expected to equilibrate with the root-zone Psi(w). Although this equilibrium assumption provides the basis for interpreting many physiological and ecological parameters, much work suggests predawn plant Psi(w) is often more negative than root-zone soil Psi(w). For many halophytes even when soils are well-watered and night-time shoot and root water loss eliminated, predawn disequilibrium (PDD) between leaf and soil Psi(w) can exceed 0.5 MPa. A model halophyte, Sarcobatus vermiculatus, was used to test the predictions that low predawn solute potential (Psi(s)) in the leaf apoplast is a major mechanism driving PDD and that low Psi(s) is due to high Na+ and K+ concentrations in the leaf apoplast. Measurements of leaf cell turgor (Psi(p)) and solute potential (Psi(s)) of plants grown under a range of soil salinities demonstrated that predawn symplast Psi(w) was 1.7 to 2.1 MPa more negative than predawn xylem Psi(w), indicating a significant negative apoplastic Psi(s). Measurements on isolated apoplastic fluid indicated that Na+ concentrations in the leaf apoplast ranged from 80 to 230 mM, depending on salinity, while apoplastic K+ remained around 50 mM. The water relations measurements suggest that without a low apoplastic Psi(s), predawn Psi(p) may reach pressures that could cause cell damage. It is proposed that low predawn apoplastic Psi(s) may be an efficient way to regulate Psi(p) in plants that accumulate high concentrations of osmotica or when plants are subject to fluctuating patterns of soil water availability.

THE EFFECTS OF PROGRESSIVE ROOT REMOVAL PRIOR TO PLANTING ON SHOOT AND ROOT GROWTH OF BETULA PENDULA Roth

Summary Root loss during transplanting of bare rooted nursery stock is thought to limit water uptake during establishment. An initial investigation into the effects on growth under well watered conditions of progressive removal of root material from dormant trees of Betula pendula Roth, was set up in spring 1994. Trees with roots ≤1 mm removed (40% removal by mass) and with roots ≤ 5 mm removed (84% removal by mass) were compared to controls. Some limited measurements of water relations were also made. Shoot and root growth were increasingly limited by progressive root removal. Root regrowth was apparently proportional to the amount of old root material left behind after transplanting. In addition, the most severe treatment (roots ≤ 5 mm were removed) limited root growth in favour of shoot growth. Direct measurements of water relations proved to be difficult and inconclusive, however, increases in stem diameter, an indirect measurement of changes in water relations, indicated that progressive root removal subtly increased water stress in the trees. Alternative hypotheses for the observed of changes in growth based on limitations in sugar physiology and hormone imbalance are also discussed.

Plant ecophysiology and forest response to global change

There are many ways of studying forest responses to global change. Most current national and international programs focus on net gas exchange of the terrestrial biosphere and are typically interdisciplinary, multi-scale projects. Key objectives of these programs are surprisingly similar to those of classical plant ecophysiology studies, i.e., to explore functional relationships of plant or plant community responses to environmental change. Thus, common research questions that link plant ecophysiology to ecosystem functioning can be identified for both research communities, promising complementarity and synergism for joint research projects. Although some well-established ecophysiological relationships, such as light responses or stomatal limitations of photosynthetic gas exchange, are currently employed in many ecosystem-scale net flux studies for gap-filling or modeling, only 14% (n = 27) of all eddy covariance flux studies in forests (n = 196; published between 1992 and April 2002) include plant ecophysiological measurements (n = 24) or biomass and growth estimates (n = 8). Generally, emphasis is on CO2 exchange measurements at various scales (foliage, shoots, branches; n = 14) and water relations measurements (n = 11). These measurements do not fully support the typical parameterization of stand and regional models, which often need information on canopy architecture and nitrogen nutrition. By means of a complementary research approach, valuable information can be acquired that is unobtainable by means of a single approach. This additional information is important for the identification of underlying biotic and environmental drivers, for the regulation of net ecosystem fluxes and their partitioning, and the independent validation of measured net ecosystem fluxes. Thus, combining micrometeorology and ecophysiology at flux sites is strongly recommended for ecosystem functioning studies.

Effect of salinity on water relations and growth of wheat genotypes with contrasting sodium uptake.

Four wheat genotypes with contrasting degrees of Na+ exclusion were selected to see if low Na+ uptake had an adverse effect on water relations or growth rates during exposure to saline conditions. Plants were grown in supported hydroponics with and without 150 mM NaCl, and sampled for measurements of water relations, biomass, stomatal conductance, and ion accumulation. After 4 weeks exposure to salt, biomass was reduced in all genotypes to a similar extent (about 50%), with the effect of salinity on relative growth rate confined largely to the first week. There was little difference between genotypes in the effect of salinity on water relations, as indicated by their relative water content and estimated turgor. Osmotic adjustment occurred in all genotypes, with one of the low-Na+ genotypes having the greatest adjustment. In the low-Na+ genotypes, osmotic adjustment depended on higher K+ and high organic solute accumulation. Stomatal conductance of all genotypes was reduced by saline conditions, but the reduction was greater in the low-Na+ genotypes. These genotypes also showed a larger fall in the value of carbon isotope discrimination measured in expanding leaves, indicating a greater transpiration efficiency when exposed to saline conditions. Chlorophyll fluorescence measurements failed to indicate damage to photochemical pathways in either high- or low-Na+ genotypes. These data indicate that selecting lines with low Na+ accumulation for the purpose of improving salt tolerance is unlikely to introduce limitations for osmotic adjustment.

Water Relations, Forage Production, and Photosynthesis in Tall Fescue Divergently Selected for Carbon Isotope Discrimination

ABSTRACTCarbon isotope discrimination (Δ) has been correlated with the ratio of dry matter production to transpiration (water‐use efficiency, WUE) in C3 plants and is potentially useful for breeding crops with improved WUE. Therefore, an assessment of the selection response of Δ and its relationship to plant water status and forage production is needed. Divergent selection for high Δ (low WUE expected) and low Δ (high WUE expected) was completed for two cycles from a ‘Kentucky 31’ tall fescue base (Festuca arundinacea Schreb.) population (Co ). Water relations, forage production, and Δ were evaluated in Co and the selected populations in irrigated and dryland field environments in 1995 and 1996. Average realized heritability for Δ was 0.49, suggesting that Δ could be successfully manipulated in a breeding program. In 1995, leaf pressure potential (turgor) was higher in the populations selected for low Δ, but in 1996, no differences in water relations measurements were observed. High‐Δ populations always had lower forage production than observed in Co, but the low‐Δ populations never produced more than the Co population. In greenhouse‐grown plants, high‐Δ populations had higher internal substomatal [CO2] than Co, linking Δ with mechanisms that cause lower WUE. However, the internal [CO2] of the low‐Δ populations and Co did not differ, suggesting that selection for low Δ may not have increased WUE as expected. The results show that Δ is a heritable trait in tall fescue, but an absence of increased production in populations selected for low Δ may limit its utility in tall fescue breeding programs.

Changes in water relations of Eucalyptus nitens nursery stock during and after lifting and transplanting

Field symptoms of transplanting shock in Eucalyptus nitens open rooted (1/2 + 1/2) transplants suggest water stress. To identify stages in the development of symptoms, changes in water relations were monitored from undisturbed seedlings growing in a commercial nursery through lifting, transplanting into pots and re-establishment of normal water relations. Hydraulic resistance measured as leaf specific resistivity increased with each step in the transplanting process. Resistivity after lifting was double that measured on undisturbed plants in the nursery and subsequently doubled with each of the two successive stages, root trimming and planting out. Resistivity fell after the initial rise, returning to the values recorded in the nursery in two stages. An initial rapid decrease, which was not accompanied by any measurable root growth, was followed by a seven day period without a change in resistivity before a further gradual decrease as root volume increased with new root growth. All water relations measurements were taken on trees which had not suffered any permanent leaf damage as a result of transplanting. For a small group of trees which did suffer damage there was a significant relationship between root volume at planting and the retained healthy leaf area. Before shoot growth had recommenced the final ratio of turgid leaf area to initial root volume was comparable with plants immediately after lifting and before trimming. The results are discussed in terms of the changes in predawn leaf water potential, leaf water potential and the leaf damage or defoliation symptoms occuring in plantations.

Water relations, gas exchange and morphological development of fall- and spring-planted yellow cypress stecklings

Yellow cypress ( Chamaecyparis nootkatensis (D.Don) Spach) stecklings (stock derived from rooted cuttings) were fall-(September 1990) and spring-planted (April, 1991) on a coastal reforestation site in British Columbia to determine the influence of planting season and various nursery cultural treatments on steckling establishment, growth and survival. Gas exchange and water relations measurements were made in the first year after planting, with morphological development monitored during the first and second year. A randomized complete block design (6 blocks) was used and influence of planting season was analyzed as an independent factor from the nursery cultural treatments. Nursery cultural treatment means within each plant-time population were similar ( t-test, α = 0.05), with no effect on steckling physiology during the first growing season. Moisture stress nursery cultural treatments produced smaller stecklings (e.g. shoot height, diameter, root and shoot dry weights) than well-watered treatments, but morphological differences disappeared after the first year. Water relation parameters indicated that fall-planted stock had greater turgor maintenance than spring-planted stock throughout the first growing season (e.g. fall ≥ spring in predawn and minimum shoot water potentials, total and utilized turgor; fall ≥ spring in osmotic potential at saturation and turgor loss point, and relative water content at turgor loss point). Spring-planted stecklings had greater maximum net photosynthesis under optimum conditions in a controlled environment, but had lower net photosynthesis under low air temperature conditions in the field, compared with fall-planted stecklings. Fall-planted stecklings had greater stomatal conductance under all measured field climatic conditions, except moderate conditions when both populations had similar stomatal conductance. Shortly after fall planting, extreme environmental conditions (i.e. soil surface temperatures of 42°C, evaporative demand > 4 kPa and steckling minimum shoot water potentials of − 1.61 ± 0.04 MPa) resulted in high fall-planted steckling mortality. As a result, survival rates measured 8 weeks after spring planting were 82.9% and 99.8% for fall- and spring-planted stecklings, respectively. Thereafter, spring planting, mortality was low and similar between the two plant-time populations. Fall-planted stecklings had greater root development early in the first growing season, but by season end, spring-planted stecklings had greater root development. At the end of the second growing season, both populations were similar in shoot development, though spring-planted stecklings had greater root development.

Water relations of germination in the recalcitrant seeds of Quercus robur L.

AbstractQuercus robur L. fruits are desiccation sensitive and shed from the plant at high moisture content (c. 48%). Water relations measurements were taken at intervals during the germination of prematurely-harvested immature fruits and naturally-shed mature fruits. In fruits from both harvests the pericarp and seed coat delayed germination by providing physical barriers to emergence of the radicle and by restricting the rate of imbibition. Although immature fruits could germinate in the absence of water they germinated more rapidly in the presence of an external water supply. The fully-mature fruit required a supply of external water, but germination was more rapid than in immature fruits. Substantial physiological changes, resulting in the accumulation of solutes, and increased axis size and extensibility occurred before splitting of the pericarp and subsequent germination of immature fruits. Such changes did not occur in fully-mature fruits, but fruits, from both harvests imbibed water and splitting of the pericarp appeared to result from increased embryo size and tissue pressure. Splitting was minimal prior to germination in immature fruits without an external water supply. It is suggested that the resulting reduction in Ψp when the pericarp splits was greater in the axis than in the cotyledons because of its greater capacity for expansion. This would create a Ψw gradient driving water flow to the axis from the cotyledons allowing continued radicle growth for germination.

Water Relations of Juniperus virginiana and Andropogon Gerardii in an Unburned Tallgrass Prairie Watershed

Water Relations of Juniperus virginiana and Andropogon Gerardii in an Unburned Tallgrass Prairie Watershed

Drought resistance of mycorrhizal pepper plants independent of leaf P concentration ‐ response in gas exchange and water relations

Pepper (Capsicum annuum L.) plants with and without the VA‐mycorrhizal fungus Ghmus deserticola Trappe. Bloss and Menge (VAM and NVAM. respectively), were drought acclimated by four drought cycles (DA) or kept well watered (NDA). All plants were then subjected to an additional drought followed by a 3‐day irrigation recovery period. Measurements of water relations, gas exchange and carbohydrates were made at selected intervals throughout the drought cycles and recovery. To equalize growth and avoid higher P in VAM plants. NVAM plants received higher P fertilization. Consequently, similar transpirational surface and shoot mass were achieved in all treatments, but NVAM had a higher tissue P concentration than VAM plants. Plants that were either VAM or DA, but especially the VAM‐DA plants, tended to be high in net photosynthetic flux (A), A per unit of tissue P concentration (A/P), stomatal conductance (g) or leaf turgor (Ψp) during high environmental stress or recovery from stress. During this time, NVAM‐NDA plants had low A. A/P and leaf chlorophyll, but high soluble carbohydrate concentrations in their leaves. All VAM and DA plants had some osmotic adjustment compared to the NVAM‐NDA plants, but VAM‐DA plants had the most. Osmotic adjustment was not due to accumulation of soluble carbohydrate. The high turgor, A and g in the VAM‐DA plants during and following environmental stress indicated superior drought resistance of these plants; however, osmotic adjustment was only apparent during recovery and cannot account for the observed drought resistance during environmental stress. Drought resistance of VAM‐DA plants was not attributable to high leaf P concentration or confounded by differences in plant transpirational surface.

Sources of Variation in Laboratory Measurements of Water Relations of Reptilian Eggs and Embryos

We performed an experiment on the water relations of eggs and embryos of common snapping turtles (Chelydra serpentina) to evaluate several sources for the random variation that is inherent in laboratory investigations using environmental chambers to provide a uniform environment or to apply specific treatments. Eggs were incubated on wet and dry substrata in an experiment that was replicated within and between environmental chambers set at the same apparent temperature (26°C). Clutch of origin was the identifiable factor accounting for most of the random variation in data for length of incubation and for mass and carapace length of hatchlings, while effects of box and environmental chamber were minor. Only in the case of data for net change in mass of eggs over the course of incubation did box and environmental chamber contribute importantly to random variation. Nonetheless, treatment effects overwhelmed the several sources for random variation, with eggs incubating on wet media (-150 kPa) absorbing more water than eggs on dry media (-950 kPa) and with embryos developing longer and attaining larger size before hatching in wet environments than in dry ones. Experiments focusing on length of incubation and on attributes of hatchlings will benefit more from considering clutch of origin than from replicating by box or environmental chamber. However, studies considering patterns of net water exchange may need to replicate fully by clutch, box, and environmental chamber to achieve the necessary sensitivity in statistical tests and to avoid critical biases resulting from pseudoreplication, especially in those instances in which treatment effects are relatively small.

WATER RELATIONS IN SWEETPOTATO

An experiment was conducted to determine the rate and frequency of irrigation needed for optimum yield in sweetpotato (Ipomoea batatas (L.)Lam). A line source irrigation system was used to provide continuously increasing amounts of water at each irrigation. The physiological responses of sweetpotato to water application were measured. There was an increase in leaf water potential with increasing rates of irrigation. Leaf diffusive resistance decreased as total water rate increased to 76% of pan evaporation (Epan) and then increased with higher rates of irrigation. Marketable yields increased as total water rate increased to 76% of Epan and then decreased rapidly with higher irrigation rates. Water relations measurements indicated that reduction in yield with higher amounts of water application was due to low soil oxygen content.

Photosynthesis, water relations and mortality in Abies lasiocarpa seedlings during natural establishment.

Few studies have investigated the ecophysiology of natural seedling establishment in forest trees not associated with anthropogenic disturbance. Photosynthesis and water relations measurements were made on one- through four-year-old seedlings of Abies lasiocarpa Nutt. (subalpine fir) establishing naturally in an understory environment. First (current)-year seedlings generally had only cotyledons, whereas most second-year seedlings had both cotyledons and primary leaves. Mortality was high (> 60%) in first-year seedlings with the greatest mortality (> 90%) measured at the more open, sun-exposed sites within the understory. Seedling mortality was negligible after the first year of growth at shaded microsites and after the second year of growth at sunny microsites. Photosynthetic CO(2) uptake at light saturation was considerably lower in first-year than in fourth-year seedlings (0.6 micromol m(-2) s(-1) versus 1.7 micromol m(-2) s(-1)) and occurred at lower solar irradiance (240 micromol m(-2) s(-1) versus 600 micromol m(-2) s(-1) of photosynthetically active radiation). Differences in photosynthetic capacity were due to differences in both stomatal and non-stomatal limitations to CO(2) uptake. Carbon dioxide assimilation in first- and second-year seedlings was 28 and 29%, respectively, of the mean value measured for fourth-year seedlings. Although first-year seedlings had low transpiration rates, their water use efficiency (photosynthesis/transpiration) was less than half that of fourth-year seedlings and their water potentials were lower than those of all other age classes (0.46 mol CO(2) mmol(-1) H(2)O and -3.0 MPa, respectively). The stomatal limitation to CO(2) uptake was approximately 21% in first- and second-year seedlings, and increased to 39% in fourth-year seedlings. Intercellular CO(2) concentrations were greater in first- and second-year seedlings (255 and 250 microl l(-1), respectively) than in third- and fourth-year seedlings (203 and 186 microl l(-1), respectively). Thus, abrupt increases in water status and photosynthetic capacity after the first or second year of growth appear crucial for survival to maturity. Moreover, differences in temperature and water relations according to microsite may be major factors determining seedling establishment and, thus, the distributional and successional patterns observed for adult trees of Abies lasiocarpa.

Seasonal changes in shoot water relations of Picea rubens at two high elevation sites in the Smoky Mountains.

Seasonal changes in water relations of current-year shoots of red spruce (Picea rubens Sarg.) were examined in relation to climatic conditions in trees growing at elevations of 1720 and 1935 m on Clingman's Dome, Tennessee, USA, where increment core data have shown that red spruce decline increases with elevation. Relative height growth of trees at 1720 m was 68% greater than in trees at 1935 m. Following two weeks in July with only traces of precipitation, trees at both sites showed decreased saturated osmotic potentials. The magnitude of the reduction was greater in trees at the high elevation site than in trees at the low elevation site. However, during August and September, shoot water relations of trees at both sites were similar. Precipitation patterns and water relations measurements suggested that, at both sites, trees experienced water stress only briefly during the growing season and to a degree that could not account for the lower growth rates of trees at the high elevation site. During the period of cold hardening in October and November, trees at the low elevation site exhibited saturated osmotic potentials that were lower by 0.2 MPa and solute accumulation (osmol kg(dw) (-1)) that was 48% greater than in trees at the high elevation site.