Xylem Pressure Potential Research Articles

Physiological responses of the xero‐halophyte Salsola drummondii to seasonal alterations of environmental conditions in a salt desert

AbstractGlobal climate changes of rising temperature are threatening food security by causing desertification in many arid and semi‐arid regions. Morpho‐physiological adaptations in xero‐halophytes under extreme conditions such as high temperature, drought, and salinity could provide information for producing more resilient cash crops. This study examined seasonal variations in growth, photosynthesis, antioxidant defense, water, and ion relations of the xero‐halophyte Salsola drummondii in its natural habitat. The resistance of Sa. drummondii to seasonal alterations was related to plant interactions with soil and atmosphere. Leaf succulence, organic weight, photosynthesis, water use efficiency, compatible solutes, and antioxidant enzyme activities correlated well with precipitation, soil moisture, and electrical conductivity. Photosynthesis and stomatal conductance increased during the monsoon period with low vapor pressure deficit and moderate temperature. Sa. drummondii responded to reduced water availability by decreasing leaf succulence, xylem pressure, and osmotic potentials like a typical C4 plant displaying water and energy‐saving seasonal adjustment. C4 mode of photosynthesis, higher antioxidant enzyme activities, and accumulation of compatible solutes could be involved in protection against photo‐oxidative stress. This study highlights the importance of physiological and biochemical adaptations of Sa. drummondii to seasonal alterations in a salt desert and could serve as a model for future crop development in arid‐saline habitats.

A 37‐year experimental study of the effects of structural alterations on a shrub community in the Mojave Desert, California

Abstract In 1977 an experiment was initiated in the Mojave Desert to investigate the relationship between shrub interactions and structure in a community dominated by Ambrosia dumosa and Larrea tridentata. Here, as in much of the Mojave, Larrea were regularly distributed, Ambrosia occurred in aggregations, and the two were randomly distributed relative to each other. Pre‐dawn xylem pressure potentials (PDXPPs) of single Ambrosia or Larrea in centres of 100 m2 circular plots were monitored to assess effects of intraspecific, interspecific and total removals of neighbouring shrubs. Contrary with theory, results over the next 2 years indicated that interspecific interference was more intense than intraspecific interference in both species. These plots were maintained through 2014. Measurements of seedling recruitment from 1980 to 2014, and of PDXPP, above‐ground biomass, and canopy senescence from 2003 to 2014 were conducted. Recruitment of both species was substantial immediately after the removals, but declined to very low levels after 1983. Ambrosia recruited into all Ambrosia and Total‐removal plots, but Larrea recruited only into plots that contained mature Ambrosia. Pre‐dawn xylem pressure potentials of monitored shrubs continued to be enhanced in removal plots for at least 27 years, but this changed from most being due to interspecific removals in both species to intraspecific removals causing most enhancement in Ambrosia and inter‐ and intraspecific removals causing nearly equal enhancements in Larrea. Above‐ground biomasses of monitored Ambrosia and Larrea were 2.1× and 2.8× larger in Total‐removal plots, 1.6× and 1.7× larger in intraspecific removal plots, respectively, and 1.1× larger in interspecific removal plots for both species than those in Control plots, indicating the absence of intraspecific interference had the dominant long‐term effect. Canopy senescence differed between Ambrosia and Larrea in extent, timing and effect of specific removal treatments; it was greatest for both species in Controls, averaging 75% and 34%, respectively. Synthesis. Shrub interactions and their relations to community structure are mechanistically and spatially complex. Differences between short‐term and long‐term responses to removals reveal multi‐tiered, temporally dynamic feedback loops between shrub interactions and community structure driven by demographics, species‐specific root growth, resource competition, communications and territoriality.

Comparing the morphology and physiology of trees planted in containers and in-ground sites

Restricted root volume poses a constraint to the successful establishment of containerised trees. The potential for growth and survival in containers for urban forests is important to ensure a sustainable population of sizeable urban trees given the staggering pace of development. In this study, it was found that the restricted soil environment inside the containers had no effect on the morphometric and physiological characteristics of container trees. A positive relationship was found between xylem pressure potential and Fv/Fm. A stronger correlation between xylem pressure potential and soil water status was observed. This indicated the importance of soil water status for optimal photosynthetic function and overall tree growth. Biomass data indicated that container trees tended to allocate more carbon to leaves, stems and branches while in-ground trees channelled carbon into leaves and roots. The potential for growing trees in containers is a desirable alternative to in-ground planting, providing urban municipalities with the option to save and relocate trees when confronted with development. In doing so, cities can work towards the development of a sustainable population of medium to large urban trees.

Effects of groundwater abstraction on two keystone tree species in an arid savanna national park.

BackgroundIn arid systems with no surface water, deep boreholes in ephemeral river beds provide for humans and animals. With continually increasing infrastructure development for tourism in arid wildlife parks such as the Kgalagadi Transfrontier Park in southern Africa, we ask what effects increased abstraction may have on large trees. Large trees in arid savannas perform essential ecosystem services by providing food, shade, nesting sites and increased nutrients for many other plant and animal species and for this are regarded as keystone species.MethodsWe determine seasonal fluctuations in the water table while also determining the water source for the dominant large tree species in the Auob and Nossob rivers in the Park. We also determine the extent to which these trees are physiologically stressed using leaf δ13C, xylem pressure potentials, specific leaf area and an estimate of canopy death. We do this both upstream and downstream of a low water use borehole in the Auob River and a high water use borehole in the Nossob River.ResultsOur results show that the trees are indeed using deep groundwater in the wet season and that this is the same water used by people. In the dry season, trees in the Auob downstream of the active borehole become detached from the aquifer and use more isotopically enriched soil water. In the Nossob in the dry season, all trees use isotopically enriched soil water, and downstream of the active borehole use stomatal regulation to maintain leaf water potentials. These results suggest that trees in the more heavily utilised Nossob are under more water stress than those trees in the Auob but that trees in both rivers demonstrate physiological adaptation to the changes in available water with smaller heavier leaves, no significant canopy dieback and in the dry season in the Nossob stomatal regulation of leaf water potentials.DiscussionAn increase in abstraction of groundwater particularly at the Nossob borehole may cause an additional draw down of the water table adding to the physiological stress demonstrated in our study. The managers of the Kgalagadi Transfrontier Park have a mandate that includes biodiversity conservation. To fulfil this mandate, upper and lower thresholds for groundwater abstraction that allow for an adequate ecological reserve have to be determined.

Differences in dehydration rate and ability to rehydrate in contrasting needle abscission resistant balsam fir genotypes

Postharvest needle abscission is a major challenge in balsam fir with respect to postharvest Christmas tree quality. It is hypothesized that differences in dehydration rate or ability to rehydrate may contribute to observed differences in needle abscission resistance (NAR). Branches were collected from low and high NAR genotypes and dehydrated for 0, 1, 2, 5, 7, 14, or 21d before being provided water and evaluated for water content, xylem pressure potential, stomatal conductance, and needle abscission. Fresh branches were 57% water content at −0.5MPa and rapidly declined over 21d without water to approximately 28% water content at −7.0MPa. The ability to rehydrate was different between low and high NAR genotypes; low NAR genotypes could only successfully rehydrate within 2d of dehydration (51% water content, −2.8MPa) while high NAR genotypes were able to rehydrate within 7d of dehydration (36% water content, −4.9MPa). There was no significant difference in postharvest abscission of contrasting NAR genotypes when dehydrated for up to 2d, but high NAR genotypes had significantly lower abscission when water was withheld for 5 or 7d. When dehydrated for 14 or 21d, no branches were able to recover and abscission occurred rapidly. It was concluded that low NAR genotypes were unable to recover from a similar water content as high NAR genotypes, which contributes to genotypic differences in postharvest abscission rates.

Desmostachya bipinnata manages photosynthesis and oxidative stress at moderate salinity

Desmostachya bipinnata is a C4 grass distributed in arid-saline lands of the subtropical region. There are many economic usages of this perennial grass ranging from folk-medicine to cattle-fodder. However, information regarding the mechanism of salinity tolerance of this species is rare. This study examines the effects of salinity (0, 200 and 400mM NaCl) on growth, osmotic relations, photosynthesis and antioxidant defense. Salinity caused progressive decrease in fresh weight; however, dry weight decreased only at 400mM NaCl. Leaf sap osmotic potential and xylem pressure potential decreased while quantities of ash, proline and sugars increased in response to salinity increments. Stomatal conductance, photosynthetic CO2 gain and transpiration decreased with increasing salinity, while intercellular CO2 levels remained unaffected. Maximum and actual quantum yield of PS II and electron transport rate were inhibited with enhanced non-photochemical quenching under high salinity. Levels of superoxide dismutase and total phenols increased, those of guaiacol peroxidase and ascorbate peroxidase remained unaltered and catalase increased transiently at 200mM NaCl. Leaf hydrogen peroxide and malondialdehyde content increased with salinity. Our data suggest that D. bipinnata could maintain photosynthesis and manage oxidative stress at moderate salinity but a further increase in salinity compromised both photosynthesis and the antioxidant system.

Vulnerability of low temperature induced needle retention in balsam fir (Abies balsamea L.) to vapor pressure deficits

Low temperature (LT) exposure has been shown to delay postharvest needle abscission and senescence (NAS) in balsam fir and changes in vapor pressure deficit (VPD) are expected to alter these processes. Two and half year-old seedlings were exposed to a LT of 5°C for 15 days while the control (CT) group was maintained at 22°C. Seedlings were then exposed to four different VPD (0.22, 0.87, 1.3 and 1.86 kPa) and observed for relative water content (RWC), xylem pressure potential (XPP), membrane injury index (MII), stem capacitance (SC) and NAS. An interactive effect of LT and VPD was observed in RWC, XPP, MII and NAS. Low VPD (0.22 and 0.87 kPa) resulted in less negative XPP, lower electrolyte leakage, higher SC and ultimately higher NAS than those at high VPD. Maximum NAS was recorded at 0.22 kPa. At 1.86 kPa, LT had 5× lower RWC (13%), 3× more negative XPP (−1.1 MPa), 1.8× higher membrane damage and 35% lower NAS (47 days) than CT. The SC declined with an increase in VPD with no effect of LT. The XPP and RWC of LT seedlings showed a positive relationship with NAS with R2 values of 0.54 and 0.59, respectively. LT offered no benefits to NAS at high VPD environments.

Late Summer Fog Use In The Drought Deciduous Shrub, Artemisia californica (Asteraceae)

Coastal fog affects many California plant species and can be critically important to species that experience periodic drought. Drought-deciduous species in particular rely on water availability to maintain their leaves during the summer. To determine fog water use in drought-deciduous plants, this study manipulated access to fog drip and measured the water relations of the common shrub, Artemisia californica, near Santa Barbara, CA. Measuring the stable isotope ratio of hydrogen and oxygen, this study found that A. californica uses fog water in the late summer months when fog is present. This additional water increased plant water content but had no effect on pre-dawn xylem pressure potential. While climatic variability inhibits reliable fog input to A. californica in Santa Barbara, this species can use fog water opportunistically and benefits from large fog events during the summer drought.

Excreting and non-excreting grasses exhibit different salt resistance strategies.

The combination of traits that makes a plant successful under saline conditions varies with the type of plant and its interaction with the environmental conditions. Knowledge about the contribution of these traits towards salt resistance in grasses has great potential for improving the salt resistance of conventional crops. We attempted to identify differential adaptive response patterns of salt-excreting versus non-excreting grasses. More specifically, we studied the growth, osmotic, ionic and nutrient (carbon/nitrogen) relations of two salt-excreting (Aeluropus lagopoides and Sporobolus tremulus) and two non-excreting (Paspalum paspalodes and Paspalidium geminatum) perennial C4 grasses under non-saline and saline (0, 200 and 400 mM NaCl) conditions. Growth and relative growth rate decreased under saline conditions in the order P. geminatum > S. tremulus = A. lagopoides > P. paspalodes. The root-to-shoot biomass allocation was unaffected in salt-excreting grasses, increased in P. paspalodes but decreased in P. geminatum. Salt-excreting grasses had a higher shoot/root Na(+) ratio than non-excreting grasses. K(+), Ca(2+) and Mg(2+) homoeostasis remained undisturbed among test grasses possibly through improved ion selectivity with rising substrate salinity. Salt-excreting grasses increased leaf succulence, decreased ψs and xylem pressure potential, and accumulated proline and glycinebetaine with increasing salinity. Higher salt resistance of P. paspalodes could be attributed to lower Na(+) uptake, higher nitrogen-use efficiency and higher water-use efficiency among the test species. However, P. geminatum was unable to cope with salt-induced physiological drought. More information is required to adequately document the differential strategies of salt resistance in salt-excreting and non-excreting grasses.

Interactions of sapsuckers andCytosporacanker can facilitate decline of riparian willows

Drought has caused large-scale plant mortality in ecosystems around the globe. Most diebacks have affected upland forest species. In the past two decades, a large-scale decline of riparian willows (Salix L.) has occurred in Rocky Mountain National Park, Colorado. We examined whether climatic or biotic factors drive and maintain the willow community decline. We compared annual growth and dieback of willows inside and outside of 14-year-old ungulate exclosures and measured groundwater depth and predawn xylem pressures of stems as indicators of drought stress. We also performed an aerial photo analysis to determine the temporal dynamics of the decline. Aerial photo analysis indicated willow decline occurred between 2001 and 2005 and was best explained by an increase in moose population and a decrease in peak stream flows. A new mechanism for willow stem dieback was identified, initiated by red-naped sapsucker wounding willow bark. Wounds became infected with fungus that girdled the stem. DNA analyses confirmed Valsa sordida (Cytospora chrysosperma) as the lethal fungus. Captured sapsuckers had V. sordida spores on feet and beaks identifying them as one possible vector of spread. Predawn xylem pressure potentials remained high through the growing season on all study willows regardless of depth to ground water. Our results indicate that additional mechanisms may be involved in tall willow decline.

How weed control and fertilisation influence tree physiological processes and growth at early establishment in an exotic F1 hybrid pine plantation of subtropical Australia

This study investigated how nitrogen (N) nutrition and key physiological processes varied under changed water and nitrogen competition resulting from different weed control and fertilisation treatments in a 2-year-old F1 hybrid (Pinus elliottii Engelm var. elliottii × P. caribaea var. hondurensis Barr. ex Golf.) plantation on a grey podzolic soil type, in Southeast Queensland. The study integrated a range of measures including growth variables (diameter at ground level (DGL), diameter at breast height (DBH) and height (H)), foliar variables (including foliar N concentration, foliar δ13C and δ15N) and physiological variables (including photosynthesis (An), stomatal conductance (gs), transpiration (E), intrinsic water use efficiency (WUEi) (A/gs) and xylem pressure potential (ΨXPP)) to better understand the mechanisms influencing growth under different weed control and fertilisation treatments. Five levels of weed control were applied: standard (routine), luxury, intermediate, mechanical and nil weed control, all with routine fertilisation plus an additional treatment, routine weed control and luxury fertilisation. Relative weed cover was assessed at 0.8, 1.1 and 1.6 years after plantation establishment to monitor the effectiveness of weed control treatments. Soil investigation included soil ammonium (NH4 +-N), nitrate (NO3 −-N), potentially mineralizable N (PMN), gravimetric soil moisture content (MC), hot water extractable organic carbon (HWETC), hot water extractable total N (HWETN), total C, total N, stable C isotope composition (δ13C), stable N isotope composition (δ15N), total P and extractable K. There were significant relationships between foliar N concentrations and relative weed cover and between tree growth and foliar N concentration or foliar δ15N, but initial site preparation practices also increased soil N transformations in the planting rows reducing the observable effects of weed control on foliar δ15N. A positive relationship between foliar N concentration and foliar δ13C or photosynthesis indicated that increased N availability to trees positively influenced non-stomatal limitations to photosynthesis. However, trees with increased foliar N concentrations and photosynthesis were negatively related to xylem pressure potential in the afternoons which enhanced stomatal limitations to photosynthesis and WUEi. Luxury and intermediate weed control and luxury fertilisation positively influenced growth at early establishment by reducing the competition for water and N resources. This influenced fundamental key physiological processes such as the relationships between foliar N concentration, A n, E, gs and ΨXPP. Results also confirmed that time from cultivation is an important factor influencing the effectiveness of using foliar δ15N as an indicator of soil N transformations.

The water relations of two tropical rainforest species (<em>Virola surinamensis</em> and <em>Eperua falcata</em>): Is <em>Virola</em> unusual as previously reported?

The objective of this study was to examine the water relations and hydraulic architecture and vulnerability to cavitation in Virola surinamensis and V. michelii and to compare to similar measurements in Eperua falcata. In several previous reports Virola was seen to have a rather narrow range of xylem pressure potentials (Ψx) near zero in the course of a wet-season day while having water fluxes quite close to Eperua. We tested the hypothesis that the narrow range of Ψx might be consistent with very high hydraulic conductivities of stems, roots and shoots and high vulnerability to cavitation in Virola compared to Eperua. When this hypothesis proved false we concluded that the previous determinations of Ψx might be wrong in Virola due to latex. We re-examined the determination of Ψx in Virola by the pressure chamber technique and compared results to determination of Ψleaf by the thermocouple psychrometer technique and found that the likely range of Ψx are more negative than previously reported. Problems concerning the determination of Ψx in species with latex are discussed.

Biophysical and Hormonal Changes Linked to Postharvest Needle Abscission in Balsam Fir

Efforts are being made to determine significant biophysical and physiological events related to postharvest needle abscission. It is known that initial postharvest average water consumption is 0.2 mL g−1 day−1 (based on dry shoot tissue), but gradually decreases by up to 75 %. It is hypothesized that some degree of water deficit is manifested through changes in several biophysical and hormonal factors. Parameters including needle loss, water use, relative water content, electrical capacitance, membrane injury, and xylem pressure potential were measured once every 5 days on balsam fir branches collected from a clonal orchard. In addition, needles were sampled at the beginning of the experiment and during peak needle abscission which were then subjected to endogenous hormonal analysis. Peak needle abscission occurred within 24 days. During this time water use decreased by 70 %, relative water content decreased by 23 %, capacitance decreased by 64 %, membrane injury increased by 100 %, needle break strength decreased by 50 %, and xylem pressure potential decreased fourfold. Abscisic acid increased by 32-fold and trans-zeatin riboside increased by fourfold during peak abscission compared to fresh branches. Other cytokinins, such as cis-zeatin riboside, isopentenyl adenosine, trans-zeatin-O-glucoside, and dihydrozeatin riboside all doubled during abscission. Finally, there was a 95 % decrease in indole-3-acetic acid. Observed changes in all biophysical parameters, as well as abscisic acid, could be indicative of a possible postharvest water stress or dehydration. It is possible that dehydration-induced changes in biophysical and hormonal factors trigger and/or modulate postharvest needle abscission.

Effects of weed control and fertilization at early establishment on tree nitrogen and water use in an exotic F1 hybrid pine of subtropical Australia

Purpose We investigated the effects of weed control and fertilization at early establishment on foliar stable carbon (δ13C) and nitrogen (N) isotope (δ15N) compositions, foliar N concentration, tree growth and biomass, relative weed cover and other physiological traits in a 2-year old F1 hybrid (Pinus elliottii var. elliottii (Engelm) × Pinus caribaea var. hondurensis (Barr. ex Golf.)) plantation grown on a yellow earth in southeast Queensland of subtropical Australia. Materials and methods Treatments included routine weed control, luxury weed control, intermediate weed control, mechanical weed control, nil weed control, and routine and luxury fertilization in a randomised complete block design. Initial soil nutrition and soil fertility parameters included (hot water extractable organic carbon (C) and total nitrogen (N), total C and N, C/N ratio, labile N pools (nitrate (NO3 −) and ammonium (NH4 +)), extractable potassium (K+)), soil δ15N and δ13C. Relative weed cover, foliar N concentrations, tree growth rate and physiological parameters including photosynthesis, stomatal conductance, photosynthetic nitrogen use efficiency, foliar δ15N and foliar δ13C were also measured at early establishment. Results and discussion Foliar N concentration at 1.25 years was significantly different amongst the weed control treatments and was negatively correlated to the relative weed cover at 1.1 years. Foliar N concentration was also positively correlated to foliar δ15N and foliar δ13C, tree height, height growth rates and tree biomass. Foliar δ15N was negatively correlated to the relative weed cover at 0.8 and 1.1 years. The physiological measurements indicated that luxury fertilization and increasing weed competition on these soils decreased leaf xylem pressure potential (Ψxpp) when compared to the other treatments. Conclusions These results indicate how increasing N resources and weed competition have implications for tree N and water use at establishment in F1 hybrid plantations of southeast Queensland, Australia. These results suggest the desirability of weed control, in the inter-planting row, in the first year to maximise site N and water resources available for seedling growth. It also showed the need to avoid over-fertilisation, which interfered with the balance between available N and water on these soils.

Methods for measuring plant vulnerability to cavitation: a critical review

Xylem cavitation resistance has profound implications for plant physiology and ecology. This process is characterized by a 'vulnerability curve' (VC) showing the variation of the percentage of cavitation as a function of xylem pressure potential. The shape of this VC varies from 'sigmoidal' to 'exponential'. This review provides a panorama of the techniques that have been used to generate such a curve. The techniques differ by (i) the way cavitation is induced (e.g. bench dehydration, centrifugation, or air injection), and (ii) the way cavitation is measured (e.g. percentage loss of conductivity (PLC) or acoustic emission), and a nomenclature is proposed based on these two methods. A survey of the literature of more than 1200 VCs was used to draw statistics on the usage of these methods and on their reliability and validity. Four methods accounted for more than 96% of all curves produced so far: bench dehydration-PLC, centrifugation-PLC, pressure sleeve-PLC, and Cavitron. How the shape of VCs varies across techniques and species xylem anatomy was also analysed. Strikingly, it was found that the vast majority of curves obtained with the reference bench dehydration-PLC method are 'sigmoidal'. 'Exponential' curves were more typical of the three other methods and were remarkably frequent for species having large xylem conduits (ring-porous), leading to a substantial overestimation of the vulnerability of cavitation for this functional group. We suspect that 'exponential' curves may reflect an open-vessel artefact and call for more precautions with the usage of the pressure sleeve and centrifugation techniques.

Pinus pumila Photosynthesis Is Suppressed by Water Stress in a Wind-Exposed Mountain Site

Wind exposure is known to have stressful effects on plant growth, particularly at high altitudes. We studied how environmental factors affected carbon assimilation in Pinus pumila needles at a wind-exposed site. Needle gas exchange rates were determined for detached shoots in the laboratory where the needles were free from field environmental stresses, and also determined for attached shoots in the field under in situ environment. There was no difference in gas exchange characteristics determined in the laboratory between shoots from the wind-exposed and the wind-protected sites, suggesting that wind exposure did not affect the photosynthetic potential. In the field, however, the photosynthetic rate of one-year-old needles (in situ A area ) was significantly lower at the windexposed site than that at the wind-protected site. There was a positive correlation between the in situ Aarea and the xylem pressure potential, suggesting that water deficit caused photosynthetic suppression at the wind-exposed site. The in situ Aarea was lower at the wind-exposed site, even with the same electron transport rate and the same stomatal conductance. These results suggest that CO2 assimilation is suppressed by lower mesophyll CO2 conductance at the wind-exposed site. We conclude that the carbon gain is limited by water stress in wind-exposed regions.

Influence of Humidity and Temperature on Postharvest Needle Abscission in Balsam Fir in the Presence and Absence of Exogenous Ethylene

Ethylene accumulation increases after harvest and culminates in needle abscission in balsam fir [Abies balsamea (L.) Mill.]. We hypothesize that water deficit induces ethylene evolution, thus triggering abscission. The purpose of this research was to investigate the role of temperature and humidity on postharvest needle abscission in the presence and absence of exogenous ethylene and link vapor pressure deficit (VPD) to postharvest needle abscission in balsam fir. In the first experiment, branches were exposed to 30%, 60%, or 90% humidity while maintained at 19.7 °C (VPD of 1.59, 0.91, or 0.23 kPa, respectively); in the second experiment, branches were exposed to 5, 15, or 25 °C (VPD of 0.35, 0.68, or 1.26 kPa, respectively) while maintained at 60% relative humidity. Needle retention duration, average water use, xylem pressure potential relative water content, and ethylene evolution were response variables. Reducing water loss or xylem tension by changing temperature or humidity effectively delayed needle abscission, although the 90% humidity treatment had the most profound effects. In the absence of exogenous ethylene, branches placed in 90% humidity had a fivefold increase in needle retention, 67% decrease in average water use, and had a final xylem pressure potential of –0.09 MPa. There was a near perfect relationship between VPD and needle retention (R2 = 0.99). These findings suggest that increasing xylem tension or decreasing water status may trigger ethylene synthesis and needle abscission. In addition, these findings demonstrate an effective means of controlling postharvest needle abscission by modifying temperature and/or relative humidity.

Groundwater use by native plants in response to changes in precipitation in an intermountain basin

Many arid basins in western North America are likely to experience future changes in precipitation timing and amount. Where shallow water tables occur, plant acquisition of groundwater and soil water may be influenced by growing season precipitation. We conducted a rainfall manipulation experiment to investigate responses of four common native plant species to ambient, increased, and decreased summer monsoon rainfall. We measured plant xylem pressure potentials (Ψ) and stable oxygen isotope signatures (δ18O) to assess effects of altered precipitation on plant water relations and water acquisition patterns. Reduced rainfall decreased Ψ more in the grasses Sporobolus airoides and Distichlis spicata than the more deeply rooted shrubs Sarcobatus vermiculatus and Ericameria nauseosa. E. nauseosa had little response to natural or experimental differences in available soil water. Plant xylem water δ18O indicated that S. airoides and D. spicata are almost entirely dependent on rain-recharged soil water, while E. nauseosa is almost entirely groundwater-dependent. Sarcobatus vermiculatus used groundwater during dry periods, but utilized precipitation from soil layers after large rainfall events. Persistent changes in precipitation patterns could cause shifts in plant community composition that may alter basin-scale groundwater consumption by native plants, affecting water availability for human and ecosystem uses.

COLD ACCLIMATION INFLUENCE POST-HARVEST NEEDLE RETENTION IN ROOT DETACHED BALSAM FIR (ABIES BALSAMEA (L.) MILL.)

This study investigated the influence of cold acclimation on post-harvest needle retention in balsam fir (Abies balsamea (L.) Mill.) and explored certain physiological roles of roots and low temperature. In a controlled environment study, 18 month old seedlings with either detached or intact roots were exposed to a range of temperatures (-20 to 20°C) for different durations and selected physiological responses were measured. Cold acclimation of seedlings with intact roots resulted in 36% less needle loss, 76% lower fresh weight losses and 45% less water consumption when compared to those exposed to cold acclimation with no roots. A temperature of 5°C registered the lowest average daily water use of 0.16 ml/g/day, lowest needle loss of 7.1 mg/g and a positive weight gain (4%) when compared to -20, -5, 0 or 20°C. An acclimation temperature of -20°C reduced leaf chlorophyll content by 9x when compared to 5°C. Xylem pressure potential did not relate to needle loss. We hypothesize that roots play a key role in cold acclimation and needle retention by increasing membrane stability, preserving stomatal functions and maintaining chlorophyll content through yet unknown signals.

Physiological responses of kiwifruit vines (Actinidia chinensis Planch. var. chinensis) to trunk girdling and root pruning

Hydraulic conductance and photosynthesis were measured in kiwifruit (Actinidia sp.) vines to observe responses to the horticultural practices of trunk girdling and root pruning. Little is understood regarding the physiological effects of these practices in kiwifruit. Combined reductions in hydraulic conductance and photosynthesis were expected following root pruning. The primary response to trunk girdling was expected to be photosynthetic. Measurements of transpiration and xylem pressure potentials (Ψ) were used to calculate hydraulic conductance (K). Stomatal conductance (g s) and photosynthesis (A) were measured periodically. Neither treatment affected fruit growth. The response to root pruning was primarily hydraulic with significant reductions in midday Ψ indicating reduced K; reductions in A and g s were small. Girdling resulted in significant reductions in g s and A, with little effect on Ψ or K. Reduced A following girdling was considered to be a consequence of stomatal and biochemical limitation, with biochemical down-regulation following reduced carbohydrate demand.