Greg Cawthray

Carbon trading for phosphorus gain: The balance between rhizosphere carboxylates and arbuscular mycorrhizal symbiosis in plant phosphorus acquisition

Two key plant adaptations for phosphorus (P) acquisition are carboxylate exudation into the rhizosphere and mycorrhizal symbioses. These target different soil P resources, presumably with different plant carbon costs. We examined the effect of inoculation with arbuscular mycorrhizal fungi (AMF) on amount of rhizosphere carboxylates and plant P uptake for 10 species of low-P adapted Kennedia grown for 23 weeks in low-P sand. Inoculation decreased carboxylates in some species (up to 50%), decreased plant dry weight (21%) and increased plant P content (23%). There was a positive logarithmic relationship between plant P content and the amount of rhizosphere citric acid for inoculated and uninoculated plants. Causality was indicated by experiments using sand where little citric acid was lost from the soil solution over 2 h and citric acid at low concentrations desorbed P into the soil solution. Senesced leaf P concentration was often low and P-resorption efficiencies reached >90%. In conclusion, we propose that mycorrhizally mediated resource partitioning occurred because inoculation reduced rhizosphere carboxylates, but increased plant P uptake. Hence, presumably, the proportion of plant P acquired from strongly sorbed sources decreased with inoculation, while the proportion from labile inorganic P increased. Implications for plant fitness under field conditions now require investigation.

Enhanced rates of herbicide metabolism in low herbicide‐dose selected resistant Lolium rigidum

Lolium rigidum is an obligately cross-pollinated, genetically diverse species and an economically important herbicide resistance-prone weed. Our previous work has demonstrated that recurrent selection of initially susceptible L. rigidum populations with low herbicide rates results in rapid herbicide resistance evolution. Here we report on the mechanisms endowing low-dose-selected diclofop-methyl resistance in L. rigidum. Results showed that resistance was not due to target-site ACCase mutations or overproduction, or differential herbicide leaf uptake and translocation. The in vivo de-esterification of diclofop-methyl into phytotoxic diclofop acid was rapid and similar in resistant versus susceptible populations. However, further metabolism of diclofop acid into non-toxic metabolites was always faster in resistant plants than susceptible plants, resulting in up to 2.6-fold lower level of diclofop acid in resistant plants. This corresponded well with up to twofold higher level of diclofop acid metabolites in resistant plants. The major polar metabolites of diclofop acid chromatographically resembled those of wheat, a naturally tolerant species. Clearly, recurrent selection at reduced herbicide rates selected for non-target-site-based enhanced rates of herbicide metabolism, likely involving cytochrome P450 monooxygenases.

Increased ecological amplitude through heterosis following wide outcrossing in Banksia ilicifolia R.Br. (Proteaceae).

To assess whether wide outcrossing (over 30 km) in the naturally fragmented Banksia ilicifolia R.Br. increases the ecological amplitude of offspring, we performed a comparative greenhouse growth study involving seedlings of three hand‐pollinated progeny classes (self, local outcross, wide outcross) and a range of substrates and stress conditions. Outcrossed seedlings outperformed selfed seedlings, with the magnitude of inbreeding depression as high as 62% for seed germination and 37% for leaf area. Wide outcrossed seedlings outperformed local outcrossed seedlings, especially in non‐native soils, facilitated in part by an improved capacity to overcome soil constraints through greater root carboxylate exudation. Soil type significantly affected seedling growth, and waterlogging and water deficit decreased growth, production of cluster roots, root exudation and total plant P uptake. Our results suggest that the interaction of narrow ecological amplitude and the genetic consequences of small fragmented populations may in part explain the narrow range of local endemics, but that wide outcrossing may provide opportunities for increased genetic variation, increased ecological amplitude and range expansion.

Rhizosphere processes do not explain variation in P acquisition from sparingly soluble forms among Lupinus albus accessions

SevenLupinusalbusL.landraceswereselected,basedontheirgeographicoriginandthesoiltypeandpHatthe site of collection of the seeds, and compared with the cv. Kiev mutant. We hypothesised that those landraces collected from red/yellow acidic sands (pH 5-5.7) would be better at acquiring P from FePO4 or AlPO4 than those selected from brown neutral (pH 7) or fine, calcareous, alkaline sands (pH 9), and that those selected from fine calcareous sands would be more effectiveatacquiringPfromCa5OH(PO4)3.Plantsweregrowninsandandsuppliedwith40mgP/kgastheabovesparingly solubleforms,orassolubleKH2PO4;controlplantsreceivednoP.AllgenotypeswereabletousethesePsources.Variationin using poorly soluble P was not due to differences in rhizosphere carboxylate concentration, cluster-root development, or rhizosphere-extract pH. L. albus landraces with a better ability to use P from different sparingly soluble forms could be exploited to develop cultivars that are more P-acquisition efficient on soils that are low in (P) or highly P-sorbing; however, desirable genotypes cannot simply be selected based on soil type of origin.

Variable response of three Trifolium repens ecotypes to soil flooding by seawater

BACKGROUND AND AIMS Despite concerns about the impact of rising sea levels and storm surge events on coastal ecosystems, there is remarkably little information on the response of terrestrial coastal plant species to seawater inundation. The aim of this study was to elucidate responses of a glycophyte (white clover, Trifolium repens) to short-duration soil flooding by seawater and recovery following leaching of salts. METHODS Using plants cultivated from parent ecotypes collected from a natural soil salinity gradient, the impact of short-duration seawater soil flooding (8 or 24 h) on short-term changes in leaf salt ion and organic solute concentrations was examined, together with longer term impacts on plant growth (stolon elongation) and flowering. KEY RESULTS There was substantial Cl(-) and Na(+) accumulation in leaves, especially for plants subjected to 24 h soil flooding with seawater, but no consistent variation linked to parent plant provenance. Proline and sucrose concentrations also increased in plants following seawater flooding of the soil. Plant growth and flowering were reduced by longer soil immersion times (seawater flooding followed by drainage and freshwater inputs), but plants originating from more saline soil responded less negatively than those from lower salinity soil. CONCLUSIONS The accumulation of proline and sucrose indicates a potential for solute accumulation as a response to the osmotic imbalance caused by salt ions, while variation in growth and flowering responses between ecotypes points to a natural adaptive capacity for tolerance of short-duration seawater soil flooding in T. repens. Consequently, it is suggested that selection for tolerant ecotypes is possible should the predicted increase in frequency of storm surge flooding events occur.

Internal phosphorus concentration modifies the initiation, growth and functioning of cluster roots in Hakea prostrata R. Br.

Hakea prostrata R.Br. plants were grown in nutrient solution in a split-root design, to address the question if the external or the internal P concentration affects initiation, growth and carboxylate exudation of cluster roots in this species. To this end, one root half was exposed to 1 μM P, whereas the other was exposed to either 1, 25 or 75 μM P. The P concentration increased in the shoots of plants exposed to a higher P concentration, but P was not readily translocated from the roots grown at a high P supply to the other root half grown with 1 μM P. In the roots exposed to 1 μM P, cluster root initiation was not suppressed by the high P supply to the other root half. In contrast, in the roots exposed to 1 μM P, cluster root growth and the rate of carboxylate exudation were suppressed by the high P supply to the other root half.