† Background Carboxylate-releasing cluster roots of Proteaceae play a key role in acquiring phosphorus (P) from ancient nutrient-impoverished soils in Australia. However, cluster roots are also found in Proteaceae on young, P-rich soils in Chile where they allow P acquisition from soils that strongly sorb P. † Scope Unlike Proteaceae in Australia that tend to proficiently remobilize P from senescent leaves, Chilean Proteaceae produce leaf litter rich in P. Consequently, they may act as ecosystem engineers, providing P for plants without specialized roots to access sorbed P. We propose a similar ecosystem-engineering role for species that release large amounts of carboxylates in other relatively young, strongly P-sorbing substrates, e.g. young acidic volcanic deposits and calcareous dunes. Many of these species also fix atmospheric nitrogen and release nutrient-rich litter, but their role as ecosystem engineers is commonly ascribed only to their diazotrophic nature. † Conclusions We propose that the P-mobilizing capacity of Proteaceae on young soils, which contain an abundance of P, but where P is poorly available, in combination with inefficient nutrient remobilization from senescing leaves allows these species to function as ecosystem engineers. We suggest that diazotrophic species that colonize young soils with strong P-sorption potential should be considered for their positive effect on P availability, as well as their widely accepted role in nitrogen fixation. Their P-mobilizing activity possibly also enhances their nitrogen-fixing capacity. These diazotrophic species may therefore facilitate the establishment and growth of species with less-efficient P-uptake strategies on more-developed soils with low P availability through similar mechanisms. We argue that the significance of cluster roots and high carboxylate exudation in the development of young ecosystems is probably far more important than has been envisaged thus far.

Arabidopsis thaliana (L.) Heynh. has been described as a freezing-tolerant species based on freezing-resistance assays. Nonetheless, this type of experiment does not discriminate between freezing-tolerance and freezing-avoidance mechanisms. The purpose of this paper was to determine which of these two freezing-resistance mechanisms is responsible for freezing resistance in A. thaliana. This was achieved by comparing the thermal properties (ice-nucleation temperature and the freezing temperature) of leaves and the lethal temperature to 10, 50 and 90% of the plants (LT10, LT50, and LT90, respectively). Two wild-type genotypes were used (Columbia and Ler) and their mutants (esk-1 and frs-1, respectively), which differ in their freezing resistance. This study's results indicated that the mutant esk-1, described as a freezing-tolerant species showed freezing tolerance only after a cold-acclimation period. The mutant frs-1, described as freezing sensitive, presented freezing avoidance. Both wild genotypes presented LT50 similar to or higher than the ice-nucleation temperature. Thus, the main freezing-resistance mechanism for A. thaliana is avoidance of freezing by supercooling. No injury of the photosynthetic apparatus was shown by measuring the maximal photochemical efficiency (Fv/Fm) and pigments (chlorophyll and carotenoid) during cold acclimation in all genotypes. During cold acclimation, Columbia and esk-1 increased total soluble carbohydrates in leaves. esk-1 was the only genotype that presented freezing tolerance after cold acclimation. This feature could be related to an increase in sugar accumulation in the apoplast.

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Summary1. Proteaceae species in south-western Australia thrive on phosphorus-impoverished soils, employing a phosphorus-mining strategy involving carboxylate-releasing cluster roots. Some develop symptoms of phosphorus toxicity at slightly elevated soil phosphorus concentrations, due to their low capacity to down-regulate phosphorus uptake. In contrast, Proteaceae species in Chile, e.g. Embothrium coccineum J.R. Forst. & G. Forst., occur on volcanic soils, which contain high levels of total phosphorus, but phosphorus availability is low. 2. We hypothesised that the functioning of cluster roots of E. coccineum differs from that of south-western Australian Proteaceae species, in accordance with the difference in soil phosphorus status. With more phosphorus to be gained from the soil with high levels of total phosphorus, we expect less investment in biomass and more release of carboxylates. Furthermore, we hypothesised that E. coccineum regulates its phosphorus-uptake capacity, avoiding phosphorus toxicity when grown at elevated phosphorus levels. To test these hypotheses, E. coccineum seedlings were grown at a range of phosphorus supplies in nutrient solution. 3. We show that E. coccineum allocated at least five times less biomass to cluster roots that released at least nine times more carboxylates per unit cluster root weight compared with south-western Australian species (e.g. Banksia, Hakea). The highest phosphorus supply caused a growth inhibition and high leaf phosphorus concentration, without symptoms of phosphorus toxicity. We accept our hypotheses on the functioning of cluster roots and the high capacity to reduce the net phosphorus uptake in plants grown at a high-phosphorus supply. 4. This novel combination of traits indicates divergent functioning of Proteaceae species from southern South America, exposed to frequent phosphorus input due to volcanic activity, in contrast with the functioning of south-western Australian Proteaceae species that thrive on severely phosphorus-impoverished soils. These traits could explain the functioning of E. coccineum on soils that are rich in total phosphorus, but with a low concentration of available phosphorus.

Contrary to that found in South African and Australian Proteaceae, cluster roots of E. coccineum do not appear to be driven by soil P, but rather by soil N and leaf Piarea. We suggest that cluster roots are a constitutive and functional trait that allows plants to prevail in poor N soils.

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One of the main factors that favours the formation of cluster roots is a low supply of phosphorus (P). The soils of southern Chile are mainly formed from volcanic ash, characterized by low levels of available P. Embothrium coccineum, a Chilean Proteaceae species produces cluster roots (CR). The factors that control CR formation in Chilean Proteaceae have not been extensively studied. The objective of this work was to assess the effects of P on the growth and cluster-root formation of E. coccineum. Plants were produced from seeds collected at two different locations: Valdivia and Pichicolo both at 39ºS. They were cultured under similar greenhouse conditions, from June to September, watered twice a week using: distilled water (W), full strength Hoagland's nutrient solution (H) or Hoagland without P (H-P). At the end of the experiment, height, total dry biomass, number of cluster roots (CR) per plant, CR /total root weight, were measured. Also acid exudation of CR was assayed using bromocresol purple on sterile agar plates. Treatments significantly affected growth and proportion of CR, the highest growth was observed with H. Under all treatments plants produced a similar number of CR. However, the proportion of CR biomass was higher with W and H-P than with H. Plants under W exhibited the lowest growth and low shoot/root ratio. Acid exudation of CR was not detectable in our experiment. These results are discussed comparing CR formation in low P conditions on Lupinus albus and other Proteaceae species, and the possible role of CR formation in E. coccineum considering its wide geographical distribution.

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