Leaf water relations during summer water deficit: differential responses in turgor maintenance and variation in leaf structure among different plant communities in south-western Australia

We measured leaf water relations and leaf structural traits of 20 species from three communities growing along a topographical gradient. Our aim was to assess variation in seasonal responses in leaf water status and leaf tissue physiology between sites and among species in response to summer water deficit. Species from a ridge-top heath community showed the greatest reductions in pre-dawn leaf water potentials (Psi(leaf)) and stomatal conductance during summer; species from a valley-floor woodland and a midslope mallee community showed less reductions in these parameters. Heath species also displayed greater seasonal reduction in turgor-loss point (Psi(TLP)) than species from woodland or mallee communities. In general, species that had larger reductions in Psi(leaf) during summer showed significant shifts in either their osmotic potential at full turgor (Psi(pi 100); osmotic adjustment) or in tissue elasticity (epsilon(max)). Psi(pi 100) and epsilon(max) were negatively correlated, during both spring and summer, suggesting a trade-off between these different mechanisms to cope with water stress. Specific leaf area varied greatly among species, and was significantly correlated with seasonal changes in Psi(TLP) and pre-dawn Psi(leaf). These correlations suggest that leaf structure is a prerequisite for cellular mechanisms to be effective in adjusting to water deficit.     

Chickpea and white lupin rhizosphere carboxylates vary with soil properties and enhance phosphorus uptake

Chickpea and white lupin roots are able to exude large amounts of carboxylates, but the resulting concentrations in the rhizosphere vary widely. We grew chickpea in pots in eleven different Western Australian soils, all with low phosphorus concentrations. While final plant mass varied more than two-fold and phosphorus content almost five-fold, there were only minor changes in root morphological traits that potentially enhance phosphorus uptake (e.g., the proportion of plant mass allocated to roots, or the length of roots per unit root mass). In contrast, the concentration of carboxylates (mainly malonate, citrate and malate, extracted using a 0.2 mM CaCl₂ solution) varied ten-fold (averaging 2.3 mol g^–1 dry rhizosphere soil, approximately equivalent to a soil solution concentration of 23 mM). Plant phosphorus uptake was positively correlated with the concentration of carboxylates in the rhizosphere, and it was consistently higher in soils with a smaller capacity to sorb phosphorus. Phosphorus content was not correlated with bicarbonate-extractable phosphorus or any other single soil trait. These results suggest that exuded carboxylates increased the availability of phosphorus to the plant, however, the factors that affected root exudation rates are not known. When grown in the same six soils, three commonly used Western Australian chickpea cultivars had very similar rhizosphere carboxylate concentrations (extracted using a 0.2 mM CaCl₂ solution), suggesting that there is little genetic variation for this trait in chickpea. Variation in the concentration of carboxylates in the rhizosphere of white lupin did not parallel that of chickpea across the six soils. However, in both species the proportion of citrate decreased and that of malate increased at lower soil pH. We conclude that patterns of variation in root exudates need to be understood to optimise the use of this trait in enhancing crop phosphorus uptake.     

A climate change context for the decline of a foundation tree species in south-western Australia: insights from phylogeography and species distribution modelling

Background and Aims A worldwide increase in tree decline and mortality has been linked to climate change and, where these represent foundation species, this can have important implications for ecosystem functions. This study tests a combined approach of phylogeographic analysis and species distribution modelling to provide a climate change context for an observed decline in crown health and an increase in mortality in Eucalyptus wandoo, an endemic tree of south-western Australia.Methods Phylogeographic analyses were undertaken using restriction fragment length polymorphism analysis of chloroplast DNA in 26 populations across the species distribution. Parsimony analysis of haplotype relationships was conducted, a haplotype network was prepared, and haplotype and nucleotide diversity were calculated. Species distribution modelling was undertaken using Maxent models based on extant species occurrences and projected to climate models of the last glacial maximum (LGM).Key Results A structured pattern of diversity was identified, with the presence of two groups that followed a climatic gradient from mesic to semi-arid regions. Most populations were represented by a single haplotype, but many haplotypes were shared among populations, with some having widespread distributions. A putative refugial area with high haplotype diversity was identified at the centre of the species distribution. Species distribution modelling showed high climatic suitability at the LGM and high climatic stability in the central region where higher genetic diversity was found, and low suitability elsewhere, consistent with a pattern of range contraction.Conclusions Combination of phylogeography and paleo-distribution modelling can provide an evolutionary context for climate-driven tree decline, as both can be used to cross-validate evidence for refugia and contraction under harsh climatic conditions. This approach identified a central refugial area in the test species E. wandoo, with more recent expansion into peripheral areas from where it had contracted at the LGM. This signature of contraction from lower rainfall areas is consistent with current observations of decline on the semi-arid margin of the range, and indicates low capacity to tolerate forecast climatic change. Identification of a paleo-historical context for current tree decline enables conservation interventions to focus on maintaining genetic diversity, which provides the evolutionary potential for adaptation to climate change.     

Plant community diversity relative to human land uses in an Amazon forest colony

We examined numbers of plant species and individuals relative to land use in an agricultural settlement in the Brazilian Amazon. Land uses were forest, cropped after forest, fallows, cropped after fallow, and pasture. These corresponded roughly to farmers' land-use changes over time. Numbers of species and diversity indices were generally highest in forest, but we found quite similar values in older fallows as a result of both survival/re-establishment of forest species and emergence of plants not encountered in forest. The dominant species in fallows, however, were different from those in forest. Lands cropped using slash-and-burn maintained moderate numbers of species–both forest and non-forest. Trends in plant density (individuals per unit ground area) on cropped lands were mainly related to invasion of weedy species (increasing in time after slash-and-burn) and shifts in crop species (replacement of small-sized rice by larger-sized maize and cassava). Useful forest plants (e.g. for construction purposes, food and medicines) decreased with land conversion, although new species also appeared. The fewest species, tree species, and useful plants, and the greatest losses of the forest flora, were encountered in pastures. Consequently, conversion to pasture rather than slash-and-burn agriculture per se led to high biodiversity loss.     

Rhizosphere carboxylate concentrations of chickpea are affected by soil bulk density

We investigated whether carboxylate exudation by chickpea (Cicer arietinum L.) was affected by soil bulk density and if this effect was local or systemic. We hypothesised that concentrations of carboxylates would increase with distance from the root apex due to continuous and constitutive accumulation of carboxylates, and that exudate accumulation would be greater in a compacted soil than in a loose soil. Plants were grown in split-root or single cylinders containing loose (1400 kg m (-3)) or compacted (1800 kg m (-3)) soil. Rhizosphere carboxylate concentrations were measured of whole root systems as well as of sections along the root. The root diameter was greatest of plants grown in the compacted soil; however, root diameters were the same for both root halves in the split-root design, whether they grew in loose soil or in compacted soil. Similarly, carboxylate concentrations tended to be lower for the whole root system in the compacted soil, but were the same for both root halves in the split-root design, irrespective of whether the roots were in loose soil or in compacted soil. These results indicate that both root diameter and carboxylate exudation by roots in chickpea is regulated systemically via a signal from the shoot rather than by local signals in the roots. There was no accumulation of carboxylates with increasing distance from the apex, probably because microbial degradation occurred at similar rates as carboxylate exudation. Malonate, previously suggested as deterrent to microorganisms, is likely only a selective deterrent.     

Triticum aestivum shows a greater biomass response to a supply of aluminium phosphate than Lupinus albus, despite releasing fewer carboxylates into the rhizosphere

The relationship between carboxylate release and the ability of plants to access phosphorus from AlPO4 and to detoxify aluminium was studied by comparing species with a low and high rate of carboxylate release, Triticum aestivum (wheat) and Lupinus albus (white lupin), respectively. Species were supplied with P at 10, 20, 40 or 100 mg P kg-1 sand in the form of sparingly soluble AlPO4 or soluble KH2PO4; control plants did not receive any P. Triticum aestivum was significantly better than L. albus at accessing P from AlPO4, despite accumulating fewer carboxylates in its rhizosphere. Rhizosphere pH of L. albus did not vary with form or level of P supply, while the rhizosphere pH of T. aestivum increased with the level of P supplied. Based on the evidence in the present study, a model is proposed to explain the poor performance of L. albus, whereby the release of carboxylates and associated protons reduces the chelating ability of exuded carboxylates, thus reducing P acquisition and increasing Al toxicity.     

Climate and landscape drivers of tree decline in a Mediterranean ecoregion

Climate change and anthropogenic land use are increasingly affecting the resilience of natural ecosystems. In Mediterranean ecoregions, forests and woodlands have shown progressive declines in health. This study focuses on the decline of an endemic woodland tree species, Eucalyptus wandoo (wandoo), occurring in the biodiversity hotspot of southwest Western Australia. We determined the change in health of wandoo stands between 2002 and 2008 across its geographic and climatic range, and associated this change in health with non‐biotic variables focusing on: (1) fragment metrics; (2) topography; (3) soil characteristics; and (4) climate. Only fragment metrics and climate variables were found to be significantly related to the observed change in health. Stands that were small with high perimeter/area ratios were found to be most sensitive to health declines. Recent increases in autumn temperatures and decreases in annual rainfall were negatively affecting health of wandoo most prominently in the low rainfall zone of its climatic range. Together, these results suggest the onset of range contraction for this ecologically important species, which is likely to be exacerbated by projected future changes in climate. Our results emphasize the importance of establishing monitoring programs to identify changes in health and decline trends early to inform management strategies, particularly in the sensitive Mediterranean ecoregions.     

Plant and microbial strategies to improve the phosphorus efficiency of agriculture

Background

Agricultural production is often limited by low phosphorus (P) availability. In developing countries, which have limited access to P fertiliser, there is a need to develop plants that are more efficient at low soil P. In fertilised and intensive systems, P-efficient plants are required to minimise inefficient use of P-inputs and to reduce potential for loss of P to the environment.

Scope

Three strategies by which plants and microorganisms may improve P-use efficiency are outlined: (i) Root-foraging strategies that improve P acquisition by lowering the critical P requirement of plant growth and allowing agriculture to operate at lower levels of soil P; (ii) P-mining strategies to enhance the desorption, solubilisation or mineralisation of P from sparingly-available sources in soil using root exudates (organic anions, phosphatases), and (iii) improving internal P-utilisation efficiency through the use of plants that yield more per unit of P uptake.

Conclusions

We critically review evidence that more P-efficient plants can be developed by modifying root growth and architecture, through manipulation of root exudates or by managing plant-microbial associations such as arbuscular mycorrhizal fungi and microbial inoculants. Opportunities to develop P-efficient plants through breeding or genetic modification are described and issues that may limit success including potential trade-offs and trait interactions are discussed. Whilst demonstrable progress has been made by selecting plants for root morphological traits, the potential for manipulating root physiological traits or selecting plants for low internal P concentration has yet to be realised.