School of Plant Biology

What are effects of snow fences on interception of snow? Snow is a vital consideration as potential water conservation method during drought. Dense drifted snow melts more slowly and sublimates less; therefore, development of economical methods to intercept windblown snow into dense drifts is of major concern. The project extended over two winters. During the first winter, one meter tall snow fences were used in a field setting to test effects of changing horizontal board thickness on wind patterns and snow drift geometry, and to validate results of small scale models previously tested in a wind tunnel. All snow fence design hypotheses were tested. As board thickness increased, wind speed decreased further upwind, drift apex occurred further upwind, and total volume increased. Snow fences tested in field conditions did create similar relationships between wind patterns and snow drift geometry to those of small scale models tested in a wind tunnel, thus validating results of previous models. The second winter expanded the "Thick Fence" investigations to develop a more economical design, which would still produce short length, high volume drifts. Two designs, an Angles and a 3-Piece fence, decreased material while still generating high levels of turbulence. A cross-sectional analysis was conducted by digging a snow pit at the apex of the drift, which allowed study of grain types and densities throughout the snowpack. The 3-Piece Fence was more economical and retained the drift characteristics of short length, and high volume. The Angles Fence was similar to the Control-Thick Fence, while the 3-Piece Fence was the least expensive, enhanced drift characteristics earlier during winter, and had higher density with more distribution of rounded particles. These results will help scientists and land managers use various snow fence designs to more economically manipulate snow for enhanced water supply for conservation during drought.

What are effects of environmental factors on sublimation of snow pack? There is a need to retain winter's snow pack to improve water supply during drought. Literature research provided information about sublimation, tree types, water supply, and snow pack variables. One intent of this project was to evaluate relationships of air temperature on tree branch resistance, because that affects unloading of intercepted snow from trees. A second intent was to evaluate effects of environmental factors on sublimation of snow. Procedures included two experiments. The first experiment used two conifer trees, a spring scale, and temperature probe to test flexibility of tree branches as temperature fluctuated over a 24-hour period. The second experiment tested effects of air temperature, relative humidity and wind speed on the mass of five ice and snow objects of varying shapes and densities over a 24-hour period. Two of the three branch resistance experiment hypotheses were supported. As temperature dropped, branches' resistance to bending increased, and as temperature increased, branches' resistance to bending decreased. Testing of Hypothesis 2 of the branch bending experiment could not be finalized because temperature did not drop to a low enough level to find maximum resistance. All five of the sublimation experiment hypotheses were supported. As air temperature, wind speed, and object irregularity increased, sublimation increased. Also as relative humidity and object density decreased, sublimation increased. This current research can be used to emphasize the need to capture snow in dense, smooth snow pack wherever additional stored moisture will be needed in the spring. These results will help us improve our ability to retain as much snow for water supply as possible and more accurately predict probable spring water supply, as well as help us understand what is happening "As the Snow Goes!"

What are effects of trees and shrubs on interception of snow? Interception of falling snow by conifer trees and sagebrush shrubs can increase loss of snow through exposure to sublimation, thus decreasing potential water supply. As many drought afflicted areas are predominantly vegetated by conifers and sagebrush, snow interception by these plants in various stages of drought and maturity is noteworthy. The project extended over two winters. During the first winter, conifer branches from two species, Lodgepole Pine (Pinus contorta) and Subalpine Fir (Abies lasiocarpa) were dried to varying wood moisture contents to simulate branch resistance in various drought stages. The force needed to bend the branches at subzero temperatures was measured, as the branch resistance affects snow retention and unloading. Retained intercepted snow is exposed to loss by sublimation, while unloaded snow is contributed to dense ground snowpack. Fir data supported all hypotheses, because as moisture content decreased, branch resistance increased, and sapwood temperature fluctuated less. Yet, the pine had mixed results, supporting differences between the genus's. Drier, more resistant fir trees intercept and expose more snow to sublimation, while pines, more flexible when drier, unload more snow to be retained for water supply. The second winter's research investigated interception of snow by sage (Artemisia tridentata) in various stages of maturity, and the force needed to bend branches at subzero temperatures. All sagebrush hypotheses were supported. As shrub maturity increased, branch resistance to bending increased, and percent of intercepted snow increased. T-tests showed that young sage (5-6 yrs) did not intercept significant amounts of snow; therefore sage kept at this maturity would provide maximum water conservation. These results will help scientists better understand snow interception and retention on conifer branches and sagebrush for better prediction of winter's snow as water supply during drought and better planning of vegetation communities.

What are the effects of snow fence variables on wind patterns, and geometry and volume of snow collected in drifts? This question arose from a need to capture winter's blowing snow to improve water supply during a drought and to protect critical areas from blowing snow. Last year, during the first phase of this project, various powders were tested to find which would closely replicate snow in a small scale drifting model. It was concluded that Cascade Dish Detergent was the best for modeling wind blown snow. In this year's continuation, Cascade powder was used to further evaluate the relationship of wind blown snow and snow fences. Literature research and professional interviews provided information about water supply from drifted snow, wind flow, drifting patterns, wind tunnel modeling, and snow fence variables, but did not describe the effects of varying horizontal board thickness on the wind pattern or the resulting drifts. The intent of this project was to evaluate relationships of fence thickness and porosity to wind direction and speed, and drift geometry and volume. The model testing system included six scaled model snow fences of varying thicknesses and porosities, a wind tunnel, and apparatus for wind direction and wind speed. Each fence was tested three ways: wind direction, wind speed, and drift geometry. Graphs for each fence were developed to show wind direction and speed, drift profile, and a comparison of wind speed to drift profile. Also, graphs were developed to compare total drift volumes and locations of drift apex for all fences. Three of the four hypotheses were supported. As fence porosity decreased and board thickness increased, pattern of wind direction changed, wind speed decreased, and the drift apex occurred further upwind. The volume hypothesis for thickness was supported because increased thickness resulted in increased drift volume. However, the volume hypothesis for porosity was denied, because increased fence porosity resulted in increased drift volume. Based on these results, snow fences can now be designed for specific intents. Other variables that could be tested next include those of terrain, buildings, and wind intensities. The most valuable extension is application of these fence model results to establish full-size fences to collect the snow in desired locations; this will collect more volume of winter wind blown snow to improve water supply in a drought period and keep critical areas free of drifting snow. This project proves potential to "Get the Drift!" into more helpful locations.

Contents Summary306I.The need to use phosphorus efficiently307II.P-use efficiency and P dynamics in a growing crop307III.P pools in plants307IV.Phosphorus pools and growth rates310V.Are crops different from other plants in their P concentration?310VI.Phosphorus use and photosynthesis311VII.Crop development and canopy P distribution312VIII.Internal redistribution of P in a growing vegetative plant313IX.Allocation of P to reproductive structures314X.Constraints to P remobilisation315XI.Do physiological or phylogenetic trade-offs constrain traits that could improve PUE?316XII.Identifying genetic loci associated with PUE316XIII.Conclusions317 Acknowledgements317 References317 Summary306I.The need to use phosphorus efficiently307II.P-use efficiency and P dynamics in a growing crop307III.P pools in plants307IV.Phosphorus pools and growth rates310V.Are crops different from other plants in their P concentration?310VI.Phosphorus use and photosynthesis311VII.Crop development and canopy P distribution312VIII.Internal redistribution of P in a growing vegetative plant313IX.Allocation of P to reproductive structures314X.Constraints to P remobilisation315XI.Do physiological or phylogenetic trade-offs constrain traits that could improve PUE?316XII.Identifying genetic loci associated with PUE316XIII.Conclusions317 Acknowledgements317 References317 Summary306I.The need to use phosphorus efficiently307II.P-use efficiency and P dynamics in a growing crop307III.P pools in plants307IV.Phosphorus pools and growth rates310V.Are crops different from other plants in their P concentration?310VI.Phosphorus use and photosynthesis311VII.Crop development and canopy P distribution312VIII.Internal redistribution of P in a growing vegetative plant313IX.Allocation of P to reproductive structures314X.Constraints to P remobilisation315XI.Do physiological or phylogenetic trade-offs constrain traits that could improve PUE?316XII.Identifying genetic loci associated with PUE316XIII.Conclusions317 Acknowledgements317 References317SummaryLimitation of grain crop productivity by phosphorus (P) is widespread and will probably increase in the future. Enhanced P efficiency can be achieved by improved uptake of phosphate from soil (P-acquisition efficiency) and by improved productivity per unit P taken up (P-use efficiency). This review focuses on improved P-use efficiency, which can be achieved by plants that have overall lower P concentrations, and by optimal distribution and redistribution of P in the plant allowing maximum growth and biomass allocation to harvestable plant parts. Significant decreases in plant P pools may be possible, for example, through reductions of superfluous ribosomal RNA and replacement of phospholipids by sulfolipids and galactolipids. Improvements in P distribution within the plant may be possible by increased remobilization from tissues that no longer need it (e.g. senescing leaves) and reduced partitioning of P to developing grains. Such changes would prolong and enhance the productive use of P in photosynthesis and have nutritional and environmental benefits. Research considering physiological, metabolic, molecular biological, genetic and phylogenetic aspects of P-use efficiency is urgently needed to allow significant progress to be made in our understanding of this complex trait.Limitation of grain crop productivity by phosphorus (P) is widespread and will probably increase in the future. Enhanced P efficiency can be achieved by improved uptake of phosphate from soil (P-acquisition efficiency) and by improved productivity per unit P taken up (P-use efficiency). This review focuses on improved P-use efficiency, which can be achieved by plants that have overall lower P concentrations, and by optimal distribution and redistribution of P in the plant allowing maximum growth and biomass allocation to harvestable plant parts. Significant decreases in plant P pools may be possible, for example, through reductions of superfluous ribosomal RNA and replacement of phospholipids by sulfolipids and galactolipids. Improvements in P distribution within the plant may be possible by increased remobilization from tissues that no longer need it (e.g. senescing leaves) and reduced partitioning of P to developing grains. Such changes would prolong and enhance the productive use of P in photosynthesis and have nutritional and environmental benefits. Research considering physiological, metabolic, molecular biological, genetic and phylogenetic aspects of P-use efficiency is urgently needed to allow significant progress to be made in our understanding of this complex trait.

Aims Strongly coherent sandsheaths that envelop perennial roots of many monocotyledonous species of arid environments have been described for over a century. This study, for the first time, details the roles played by the structural development of the subtending roots in the formation and persistence of the sheaths. † Methods The structural development of root tissues associated with persistent sandsheaths was studied in Lyginia barbata, native to the Western Australian sand plains. Cryo-scanning electron microscopy CSEM, optical microscopy and specific staining methods were applied to fresh, field material. The role of root hairs was clarified by monitoring sheath development in roots separated from the sand profile by fine mesh. † Key Results and Conclusions The formation of the sheaths depends entirely on the numerous living root hairs which extend into the sand and track closely around individual grains enmeshing, by approx. 12 cm from the root tip, a volume of sand more than 14 times that of the subtending root. The longevity of the perennial sheaths depends on the subsequent development of the root hairs and of the epidermis and cortex. Before dying, the root hairs develop cellulosic walls approx. 3 mm thick, incrusted with ferulic acid and lignin, which persist for the life of the sheath. The dead hairs remain in place fused to a persistent platform of sclerified epidermis and outer cortex. The mature cortex comprises this platform, a wide, sclerified inner rim and a lysigenous central region -all dead tissue. We propose that the sandsheath/root hair/epidermis/cortex complex is a structural unit facilitating water and nutrient uptake while the tissues are alive, recycling scarce phosphorus during senescence, and forming, when dead, a persistent essential structure for maintenance of a functional stele in the perennial Lyginia roots.

CITATIONS 74 READS 82 7 authors, including: Some of the authors of this publication are also working on these related projects: Is the distribution of calcium and phosphorus between cell types the key reason why Lupinus species respond differently to soil pH? View project Seagrass rhizospheres; understanding the bottom to manage the top View project

•Periods of dormancy in shallow roots allow perennial monocotyledons to establish deep root systems, but we know little about patterns of xylem maturation, water-transport capacities and associated economies in water use of growing and dormant roots.•Xylem development, anatomy, conductance and in situ cellular [K] and [Cl] were investigated in roots of field-grown Lyginia barbata (Restionaceae) in Mediterranean southwestern Australia. Parallel studies of gas exchange, culm relative water loss and soil water content were conducted.Periods of dormancy in shallow roots allow perennial monocotyledons to establish deep root systems, but we know little about patterns of xylem maturation, water-transport capacities and associated economies in water use of growing and dormant roots.Xylem development, anatomy, conductance and in situ cellular [K] and [Cl] were investigated in roots of field-grown Lyginia barbata (Restionaceae) in Mediterranean southwestern Australia. Parallel studies of gas exchange, culm relative water loss and soil water content were conducted.•Stomatal conductance and photosynthesis decreased during summer drought as soil profiles dried, but rates recovered when dormant roots became active with the onset of wetter conditions. Anatomical studies identified sites of close juxtaposition of phloem and xylem in dormant and growing roots. Ion data and dye tracing showed mature late metaxylem of growing roots was located ≥ 100 mm from the tip, but at only ≤ 10 mm for dormant roots. Dormant roots remained hydrated in dry soils (0.001–0.005 g g−1).•Effective regulation of growth and water-conserving/obtaining properties permits the survival of shallow roots of L. barbata during summer drought and may represent important strategies for establishing deeper perennial root systems in other monocotyledonous plants adapted to seasonally dry habitats.Stomatal conductance and photosynthesis decreased during summer drought as soil profiles dried, but rates recovered when dormant roots became active with the onset of wetter conditions. Anatomical studies identified sites of close juxtaposition of phloem and xylem in dormant and growing roots. Ion data and dye tracing showed mature late metaxylem of growing roots was located ≥ 100 mm from the tip, but at only ≤ 10 mm for dormant roots. Dormant roots remained hydrated in dry soils (0.001–0.005 g g−1).Effective regulation of growth and water-conserving/obtaining properties permits the survival of shallow roots of L. barbata during summer drought and may represent important strategies for establishing deeper perennial root systems in other monocotyledonous plants adapted to seasonally dry habitats.

• Here, we tested the alternation of root summer dormancy and winter growth as a critical survival strategy for a long-lived monocotyledon (Restionaceae) adapted to harsh seasonal extremes of Mediterranean southwest Western Australia.• Measurements of growth and the results of comparative studies of the physiology, water content, metabolites, osmotic adjustments, and proteomics of the dormant and growing perennial roots of Lyginia barbata (Restionaceae) were assessed in field-grown plants.• Formation of dormant roots occurred before the onset of summer extremes. They resumed growth (average 2.3 mm d−1) the following winter to eventually reach depths of 2–4 m. Compared with winter-growing roots, summer dormant roots had decreased respiration and protein concentration and c. 70% water content, sustained by sand-sheaths, osmotic adjustment and presumably hydraulic redistribution. Concentrations of compatible solutes (e.g. sucrose and proline) were significantly greater during dormancy, presumably mitigating the effects of heat and drought. Fifteen root proteins showed differential abundance and were correlated with either winter growth or summer dormancy. None matched currently available libraries.• The specific features of the root dormancy strategy of L. barbata revealed in this study are likely to be important to understanding similar behaviour in roots of many long-lived monocotyledons, including overwintering and oversummering crop species.Here, we tested the alternation of root summer dormancy and winter growth as a critical survival strategy for a long-lived monocotyledon (Restionaceae) adapted to harsh seasonal extremes of Mediterranean southwest Western Australia.Measurements of growth and the results of comparative studies of the physiology, water content, metabolites, osmotic adjustments, and proteomics of the dormant and growing perennial roots of Lyginia barbata (Restionaceae) were assessed in field-grown plants.Formation of dormant roots occurred before the onset of summer extremes. They resumed growth (average 2.3 mm d−1) the following winter to eventually reach depths of 2–4 m. Compared with winter-growing roots, summer dormant roots had decreased respiration and protein concentration and c. 70% water content, sustained by sand-sheaths, osmotic adjustment and presumably hydraulic redistribution. Concentrations of compatible solutes (e.g. sucrose and proline) were significantly greater during dormancy, presumably mitigating the effects of heat and drought. Fifteen root proteins showed differential abundance and were correlated with either winter growth or summer dormancy. None matched currently available libraries.The specific features of the root dormancy strategy of L. barbata revealed in this study are likely to be important to understanding similar behaviour in roots of many long-lived monocotyledons, including overwintering and oversummering crop species.

The influence of phosphorus (P) availability on growth and P uptake was investigated in South African Proteaceae: (1) Protea compacta R.Br., endemic on severely nutrient-impoverished colluvial sands; (2) Protea obtusifolia Bueck ex Meissner; and (3) Leucadendron meridianum I. J. Williams, the latter both endemic on comparatively fertile limestone-derived soils. Plants were grown hydroponically in 1000 L tanks at 0.01, 0.1 or 1.0 µm P for 14 weeks. Biomass accumulation was influenced by P availability, doubling as [P] increased from 0.1 to 1.0 µm. Total biomass was greatest for P. compacta, but L. meridianum and P. obtusifolia had two to four times greater relative biomass accumulation at 0.1 and 1.0 µm[P]. Proteoid root clusters developed at both 0.01 and 0.1 µm[P], but were suppressed at 1.0 µm[P]; this was a 10-fold lower [P] than previously reported to inhibit cluster root formation. Rates of net P uptake at 5 µm P decreased in response to increased P availability from 0.01 to 1.0 µm P. Significant between-species differences in rates of P uptake and capacity to down-regulate P uptake were observed: P. compacta < P. obtusifolia < L. meridianum. The species responses are discussed in terms of adaptation to mosaics in soil P availability and the high beta diversity in the natural habitat.

When grown in nutrient solutions of extremely low [P] (≤1.0 µm), the sedge Schoenus unispiculatus Benth. (Cyperaceae) develops dauciform roots, which are short and carrot shaped, and produce dense numbers of long root hairs. It has been suggested that dauciform roots of monocotyledonous sedges function to acquire P from nutrient-poor, P-fixing soils in a manner similar to that of cluster (proteoid) roots developed by some dicotyledonous species, but without evidence to substantiate this claim. To elucidate the ecophysiological role of dauciform roots, we assessed carboxylate exudation, internal carboxylate and P concentrations and O2 uptake rates during dauciform root development. We showed that O2 consumption was fastest [9 nmol O2 g−1 fresh mass (FM) s−1] and root [P] greatest (0.4 mg P g−1 FM) when dauciform roots were young and rapidly developing. Citrate was the most abundant carboxylate in root tissues at all developmental stages, and was most concentrated (22.2 µmol citrate g−1 FM) in young dauciform roots, decreasing by more than half in mature dauciform roots. Peak citrate-exudation rates (1.7 nmol citrate g−1 FM s−1) occurred from mature dauciform roots, and were approximately an order of magnitude faster than those from roots of species without root clusters, and similar to those of mature proteoid (cluster) roots of Proteaceae. Both developing and mature dauciform roots had the capacity to acidify (but not alkalinize) the rhizosphere. Anatomical studies showed that epidermal cells in dauciform roots were greatly elongated in the transverse plane; epidermal cells of parent roots were unmodified. Although structurally distinct, the physiology of dauciform roots in sedges appears to be analogous to that of proteoid roots of Proteaceae and Fabaceae, and hence, dauciform roots would facilitate access to sorbed P and micronutrients from soils of low fertility.

Background Global phosphorus (P) reserves are being depleted, with half-depletion predicted to occur between 2040 and 2060. Most of the P applied in fertilizers may be sorbed by soil, and not be available for plants lacking specific adaptations. On the severely P-impoverished soils of south-western Australia and the Cape region in South Africa, non-mycorrhizal species exhibit highly effective adaptations to acquire P. A wide range of these non-mycorrhizal species, belonging to two monocotyledonous and eight dicotyledonous families, produce root clusters. Non-mycorrhizal species with root clusters appear to be particularly effective at accessing P when its availability is extremely low.

Grevillea crithmifolia R. Br. is a species of Proteaceae that is resistant to developing P-toxicity symptoms at phosphorus supplies in the root environment that induce P-toxicity symptoms in the closely related Hakea prostrata (Proteaceae). It was discovered previously that development of P-toxicity symptoms in H. prostrata is related to its low capacity to down-regulate net P-uptake rates (i.e. its low plasticity). The plasticity of net P-uptake rates and whole-plant growth responses in G. crithmifolia has now been assessed in two separate experiments: (i) a range of P, from 0 to 200 lmol P d 21 , was supplied to whole root systems; (ii) using a splitroot design, one root half was supplied with 0, 3, 75, or 225 lmol P d 21 , while the other root half invariably received 3 lmol P d 21 . Fresh mass was significantly greater in G. crithmifolia plants that had received a greater daily P supply during the pretreatments, but symptoms of P toxicity were never observed. Clusterroot growth decreased from about half the total root fresh mass when the leaf [P] was lowest (c. 0.1 mg P g 21 DM) to complete suppression of cluster-root growth when leaf [P] was 1-2 mg P g 21 DM. Split-root studies revealed that cluster-root initiation and growth, and net P-uptake rates by roots were regulated systemically, possibly by shoot P concentration. It is concluded that, in response to higher P supply, G. crithmifolia does not develop symptoms of P toxicity because of (i) greater plasticity of its net P-uptake capacity, and (ii) its greater plasticity for allocating P to growth and P storage in roots. This ecologically important difference in plasti-city is most probably related to a slightly higher nutrient availability in the natural habitat of G. crithmifolia when compared with that of H. prostrata.

• The incidence of species that develop specialised ‘dauciform’ lateral roots, which are hypothesised to be important for phosphorus (P) acquisition, is uncertain. We investigated their occurrence in Australian reed, rush and sedge species, grown at low P concentration in nutrient solution, and studied the response of Schoenus unispiculatus (Cyperaceae) to a range of P concentrations.• We assessed the fraction of root biomass invested in dauciform roots, their respiration and net P-uptake rate, and the P status of roots and leaves.• Dauciform-root development occurred only in particular genera of Cyperaceae when grown at low P supply. Increased P supply was associated with increased growth of S. unispiculatus and increased leaf [P]. Dauciform-root growth was reduced by increased P supply, and reduced P uptake co-occurred with the complete suppression of dauciform roots.• The P-induced suppression of dauciform roots in Cyperaceae is similar to that observed for proteoid roots in members of Proteaceae and Lupinus albus. The response of dauciform roots to altered P supply and their absence from root systems of some sedge species are discussed in terms of managed and natural systems.The incidence of species that develop specialised ‘dauciform’ lateral roots, which are hypothesised to be important for phosphorus (P) acquisition, is uncertain. We investigated their occurrence in Australian reed, rush and sedge species, grown at low P concentration in nutrient solution, and studied the response of Schoenus unispiculatus (Cyperaceae) to a range of P concentrations.We assessed the fraction of root biomass invested in dauciform roots, their respiration and net P-uptake rate, and the P status of roots and leaves.Dauciform-root development occurred only in particular genera of Cyperaceae when grown at low P supply. Increased P supply was associated with increased growth of S. unispiculatus and increased leaf [P]. Dauciform-root growth was reduced by increased P supply, and reduced P uptake co-occurred with the complete suppression of dauciform roots.The P-induced suppression of dauciform roots in Cyperaceae is similar to that observed for proteoid roots in members of Proteaceae and Lupinus albus. The response of dauciform roots to altered P supply and their absence from root systems of some sedge species are discussed in terms of managed and natural systems.

Harsh hakea (Hakea prostrata R.Br.) is a member of the Proteaceae family, which is highly represented on the extremely nutrientimpoverished soils in southwest Australia. When phosphorus is limiting, harsh hakea develops proteoid or cluster roots that release carboxylates that mobilize sparingly soluble phosphate in the rhizosphere. To investigate the physiology underlying the synthesis and exudation of carboxylates from cluster roots in Proteaceae, we measured O 2 consumption, CO 2 release, internal carboxylate concentrations and carboxylate exudation, and the abundance of the enzymes phosphoenolpyruvate carboxylase and alternative oxidase (AOX) over a 3-week time course of cluster-root development. Peak rates of citrate and malate exudation were observed from 12-to 13-d-old cluster roots, preceded by a reduction in cluster-root total protein levels and a reduced rate of O 2 consumption. In harsh hakea, phosphoenolpyruvate carboxylase expression was relatively constant in cluster roots, regardless of developmental stage. During cluster-root maturation, however, the expression of AOX protein increased prior to the time when citrate and malate exudation peaked. This increase in AOX protein levels is presumably needed to allow a greater flow of electrons through the mitochondrial electron transport chain in the absence of rapid ATP turnover. Citrate and isocitrate synthesis and accumulation contributed in a major way to the subsequent burst of citrate and malate exudation. Phosphorus accumulated by harsh hakea cluster roots was remobilized during senescence as part of their efficient P cycling strategy for growth on nutrient impoverished soils.

Storage of phosphorus (P) in stem tissue is important in Mediterranean Proteaceae, because proteoid root growth and P uptake is greatest during winter, whereas shoot growth occurs mostly in summer. This has prompted the present investigation of the P distribution amongst roots, stems, and leaves of Hakea prostrata R.Br. (Proteaceae) when grown in nutrient solutions at ten P-supply rates. Glasshouse experiments were carried out during both winter and summer months. For plants grown in the low-P range (0, 0.3, 1.2, 3.0, or 6.0 mmol d ±1 ) the root [P] was > stem and leaf [P]. In contrast, leaf [P] > stem and root [P] for plants grown in the high-P range (6.0, 30, 60, 150, or 300 mmol P d ±1 ). At the highest P-supply rates, the capacity for P storage in stems and roots appears to have been exceeded, and leaf [P] thereafter increased dramatically to approximately 10 mg P g ±1 dry mass. This high leaf [P] was coincident with foliar symptoms of P toxicity which were similar to those described for many other species, including non-Proteaceae. The published values (tissue [P]) at which P toxicity occurs in a range of species are summarized. X-ray microanalysis of frozen, full-hydrated leaves revealed that the [P] in vacuoles of epidermal, palisade and bundlesheath cells were in the mM range when plants were grown at low P-supply, even though very low leaf [P] was measured in bulk leaf samples. At higher P-supply rates, P accumulated in vacuoles of palisade cells which were associated with decreased photosynthetic rates.

The reliability of cryoSEM for visualizing gas embolisms in xylem vessels of intact, functioning roots is examined and discussed. The possibility that these embolisms form as a result of freezing water columns under tension is discounted by a double-freeze experiment. Two regions of the same root, one frozen under tension, the other isolated from the tension by the first freeze, had the same percentage of embolisms, as did also long pieces of root frozen simultaneously along their length. The reliability of energy-dispersive X-ray analysis to measure xylem sap concentration in situ in frozen tissue was established by measurement of KCl standard solution frozen on stubs, and within xylem vessels. Solute heterogeneity within the vessels varied with freezing procedure; deep-freeze > LN 2 > cryopliers > liquid ethane, but only the deep-freeze method gave unsatisfactory estimates of concentration for the standard solution. It is concluded that cryoanalytical SEM is useful for direct observation of gas and liquid-filled compartments, and for solute analyses at depth within intact plant organs.