Calcium-enhanced phosphorus toxicity in calcifuge and soil-indifferent Proteaceae along the Jurien Bay chronosequence

Physiological and molecular responses in phosphorus-hyperaccumulating Polygonum species to high phosphorus exposure

Phosphorus (P)-hyperaccumulators for phytoextraction from P-polluted areas generally show rapid growth and accumulate large amounts of P without any toxicity symptom, which depends on a range of physiological processes and gene expression patterns that have never been explored. We investigated growth, leaf element concentrations, P fractions, photosynthetic traits, and leaf metabolome and transcriptome response in amphibious P-hyperaccumulators, Polygonum hydropiper and P. lapathifolium, to high-P exposure (5 mmol L-1), with 0.05 mmol L-1 as the control. Under high-P exposure, both species demonstrated good growth, allocating more P to metabolite P and inorganic P (Pi) accompanied by high potassium and calcium. The expression of a cluster of unigenes associated with photosynthesis was maintained or increased in P. lapathifolium, explaining the increase in net photosynthetic rate and the rapid growth under high-P exposure. Metabolites of trehalose metabolism, including trehalose 6-phosphate and trehalose, were sharply increased in both species by the high-P exposure, in line with the enhanced expression of associated unigenes, indicating that trehalose metabolic pathway was closely related to high-P tolerance. These findings elucidated the physiological and molecular responses involved in the photosynthesis and trehalose metabolism in P-hyperaccumulators to high-P exposure, and provides potential regulatory pathways to improve the P-phytoextraction capability.

The plant growth promoting rhizobacterium Achromobacter sp. 5B1, rescues Arabidopsis seedlings from alkaline stress by enhancing root organogenesis and hormonal responses

Soil alkalinity is a critical environmental factor for plant growth and distribution in ecosystems. An alkaline condition (pH > 7) is imposed by the rising concentration of hydroxides and cations, and prevails in semiarid and arid environments, which represent more than 25% of the total arable land of the world. Despite the great pressure exerted by alkalinity for root viability and plant survival, scarce information is available to understand how root microbes contribute to alkaline pH adaptation. Here, we assessed the effects of alkalinity on shoot and root biomass production, chlorophyll content, root growth and branching, lateral root primordia formation, and the expression of CYCB1, TOR kinase, and auxin and cytokinin-inducible trangenes in shoots and roots of Arabidopsis seedlings grown in Petri plates with agar-nutrient medium at pH values of 7.0, 7.5, 8.0, 8.5, and 9.0. The results showed an inverse correlation between the rise of pH and most growth, hormonal and genetic traits analyzed. Noteworthy, root inoculation with Achromobacter sp. 5B1, a beneficial rhizospheric bacterium, with plant growth promoting and salt tolerance features, increased biomass production, restored root growth and branching and enhanced auxin responses in WT seedlings and auxin-related mutants aux1-7 and eir1, indicating that stress adaptation operates independently of canonical auxin transporter proteins. Sequencing of the Achromobacter sp. 5B1 genome unveiled 5244 protein-coding genes, including genes possibly involved in auxin biosynthesis, quorum-sensing regulation and stress adaptation, which may account for its plant growth promotion attributes. These data highlight the critical role of rhizobacteria to increase plant resilience under high soil pH conditions potentially through genes for adaptation to an extreme environment and bacteria-plant communication.

Biomass partitioning and ionomics of Macadamia with high manganese and low phosphorus concentrations

Knowledge of the ionome of plant organs helps us understand a plant's nutritional status. However, the ionome of Macadamia (Proteaceae), which is an important nut-producing tree, remains unknown. We aimed to characterise the allocation of biomass and nutrient-partitioning patterns in three macadamia genotypes. We excavated 15 productive trees (three cultivars at 21years of age; two cultivars at 16years of age) in an orchard. Biomass, nutrient concentrations, and contents of roots, stems, branches, and leaves were analysed. Dry weight of roots, stems, branches and leaves accounted for 14-20%, 19-30%, 36-52%, and 12-18% of total plant weight, respectively. No significant difference was found in the total biomass among the cultivars at the same age. Compared with most crop plants, macadamia had low phosphorus (P) concentrations in all organs (<1gkg-1 ), and low leaf zinc (Zn) concentration (8mgkg-1 ). In contrast, macadamia accumulated large amounts of manganese (Mn), with a 20-fold higher leaf Mn concentration than what is considered sufficient for crop plants. Leaves exhibited the highest nutrient concentrations, except for iron and Zn, which exhibited the highest concentrations in roots. The organ-specific ionomics of Macadamia is characterised by low P and high Mn concentrations, associated with adaptation to P-impoverished habitats.

The Influence of Serpentine Soil on the Early Development of a Non-Serpentine African Thistle, Berkheya radula (Harv.) De Wild

Serpentine soils are rich in heavy metals and poor in nutrients, limiting plant species’ performance and survival. Nevertheless, specificities of such limitations as well as adaptability features required for thriving in serpentine environments are barely known. The Barberton Greenstone Belt in South Africa is an example of an area containing serpentine soil with adapted vegetation. In this study, a pot experiment was performed to compare development features (i.e., germination rates, leaf count, leaf length, biomass and photosynthetic capacity) during the early development of the non-serpentine species Berkheya radula, a genus consisting of known metal hyperaccumulators from serpentine areas in South Africa. B. radula was grown in serpentine soils taken from the Barberton region. B. radula leaves had heavy metals in concentrations that confirmed the species as a phytoextractor. There were trends for enhanced productivity and photosynthesis in the serpentine treatments compared to the control. Leaf count, leaf length, electron transport efficiency (ψ_Eo/(1 − ψ_Eo), density of reaction centers and PI_ABS,total were significantly and positively correlated with at least one of the heavy metals in the leaves. Germination rates were positively influenced by K, whereas biomass and the density of reaction centers were negatively affected by Ca and P, and only Ca, respectively. The heavy metals Zn, Ni and Co were positively correlated with each other, whereas they were negatively correlated with the macronutrients K, Ca and P. The latter correlated positively with each other, confirming higher fertility of the control soil. Our study suggests that B. radula exhibits metallophyte characteristics (i.e., preadapted), despite not naturally occurring on metal-enriched soil, and this provides evidence that the potential for bioaccumulation and phytoremediation is shared between serpentine and non-serpentine species in this genus.

Phosphorus Acquisition and Utilization in Plants

Tremendous progress has been made on molecular aspects of plant phosphorus (P) nutrition, often without heeding information provided by soil scientists, ecophysiologists, and crop physiologists. This review suggests ways to integrate information from different disciplines. When soil P availability is very low, P-mobilizing strategies are more effective than mycorrhizal strategies. Soil parameters largely determine how much P roots can acquire from P-impoverished soil, and kinetic properties of P transporters are less important. Changes in the expression of P transporters avoid P toxicity. Plants vary widely in photosynthetic P-use efficiency, photosynthesis per unit leaf P. The challenge is to discover what the trade-offs are of different patterns of investment in P fractions. Less investment may save P, but are costs incurred? Are these costs acceptable for crops? These questions can be resolved only by the concerted action of scientists working at both molecular and physiological levels, rather than pursuing these problems independently.

Development of Soil Substitutes for the Sustainable Land Reclamation of Coal Mine-Affected Areas

The main purpose of this paper was to outline a novel approach for the use of industrial by-products generated in coal mines and coal-fired power plants as the components for artificial soils. Several coal combustion by-products, coal mine waste and organic waste materials were tested at laboratory scale for use in the reclamation of areas degraded by coal mining activity. The role of artificial soils was the land rehabilitation of the high acidic waste heap. The results revealed that the amounts of organic matter (14.87–25.01%) and nutrients in the soil substitutes were sufficient to support plant growth, i.e., N (0.37–0.51%), P (0.23–0.47%), K (1.78–3.17%), Ca (4.93–8.39%) and Mg (1.16–1.71%). A phytotoxicity test using white mustard (Sinapis alba) seeds under laboratory conditions showed good germination results (56–66%) for three soil substitutes that did not contain fly ash from biomass combustion, compared to the reference soil (84%). The relationships established for the aqueous leachate parameters of soil substitutes vs. the Sinapis alba germination revealed negative correlations with electrical conductivity (r = −0.88), SO42− (r = −0.91) and Cl− (r = −0.70) ions; the two latter ones were responsible for the salinity which hampered the germination process of the soil substitutes. Moreover, quite similar correlations were obtained between the germination of Sinapis alba and the trace elements of the soil substitutes: Fe (r = −0.69), Cd (r = −0.72), Cu (r = −0.80), Pb (r = −0.78) and Zn (r = −0.74). However, negative and significant correlations in aqueous leachates were shown only with Ni concentration (r = −0.73). The relevance of these results for the effect of salinity on germination and the early growth of S. alba was discussed in detail and was confirmed with the Principal Component Analysis (PCA). The study proved that the physicochemical characteristic of recycled wastes exhibited their potential usefulness for the reclamation of affected areas such as mine waste heaps.

Foliar nutrient allocation patterns in Banksia attenuata and Banksia sessilis differing in growth rate and adaptation to low-phosphorus habitats

BACKGROUND AND AIMS

Phosphorus (P) and nitrogen (N) are essential nutrients that frequently limit primary productivity in terrestrial ecosystems. Efficient use of these nutrients is important for plants growing in nutrient-poor environments. Plants generally reduce foliar P concentration in response to low soil P availability. We aimed to assess ecophysiological mechanisms and adaptive strategies for efficient use of P in Banksia attenuata (Proteaceae), naturally occurring on deep sand, and B. sessilis, occurring on shallow sand over laterite or limestone, by comparing the allocation of P among foliar P fractions.

METHODS

We carried out pot experiments with slow-growing B. attenuata, which resprouts after fire, and faster growing opportunistic B. sessilis, which is killed by fire, on substrates with different P availability using a randomized complete block design. We measured leaf P and N concentrations, photosynthesis, leaf mass per area, relative growth rate and P allocated to major biochemical fractions in B. attenuata and B. sessilis.

KEY RESULTS

The two species had similarly low foliar total P concentrations, but distinct patterns of P allocation to P-containing fractions. The foliar total N concentration of B. sessilis was greater than that of B. attenuata on all substrates. The foliar total P and N concentrations in both species decreased with decreasing P availability. The relative growth rate of both species was positively correlated with concentrations of both foliar nucleic acid P and total N, but there was no correlation with other P fractions. Faster growing B. sessilis allocated more P to nucleic acids than B. attenuata did, but other fractions were similar.

CONCLUSIONS

The nutrient allocation patterns in faster growing opportunistic B. sessilis and slower growing B. attenuata revealed different strategies in response to soil P availability which matched their contrasting growth strategy.

Phosphorus toxicity, not deficiency, explains the calcifuge habit of phosphorus-efficient Proteaceae

The calcifuge habit of plants is commonly explained in terms of high soil pH and its effects on nutrient availability, particularly that of phosphorus (P). However, most Proteaceae that occur on nutrient-impoverished soils in south-western Australia are calcifuge, despite their ability to produce cluster-roots, which effectively mobilize soil P and micronutrients. We hypothesize that the mechanism explaining the calcifuge habit in Proteaceae is their sensitivity to P and calcium (Ca), and that soil-indifferent species are less sensitive to the interaction of these nutrients. In this study, we analyzed growth, gas-exchange rate, and chlorophyll fluorescence of two soil-indifferent and four calcifuge Hakea and Banksia (Proteaceae) species from south-western Australia, across a range of P and Ca concentrations in hydroponic solution. We observed Ca-enhanced P toxicity in all analyzed species, but to different extents depending on distribution type and genus. Increasing P supply enhanced plant growth, leaf biomass, and photosynthetic rates of soil-indifferent species in a pattern largely independent of Ca supply. In contrast, positive physiological responses to increasing [P] in calcifuges were either absent or limited to low Ca supply, indicating that calcifuges were far more sensitive to Ca-enhanced P toxicity. In calcifuge Hakeas, we attributed this to higher leaf [P], and in calcifuge Banksias to lower leaf zinc concentration. These differences help to explain these species' contrasting sensitivity to Ca-enhanced P toxicity and account for the exclusion of most Proteaceae from calcareous habitats. We surmise that Ca-enhanced P toxicity is a major factor explaining the calcifuge habit of Proteaceae, and, possibly, other P-sensitive plants.

Calcicole-calcifuge plant strategies limit restoration potential in a regional semi-arid flora

AIM

To examine calcicole and calcifuge plant strategies, as well as nutrient-acquisition strategies, as drivers of the distribution of species in response to edaphic factors, and the degree to which these strategies may act as filters to species establishment in ecological restoration on heavily altered or reconstructed substrates.

LOCATION

An 82,000-ha area within a major mining province in the Mid-West region of Western Australia, harboring vegetation communities ranging from species-poor halophytic scrub on saline flats to dense biodiverse shrubland on the skeletal soils of ancient Banded Ironstone Formations (BIF).

METHODS

Univariate and multivariate analyses were employed to examine how variation in soil chemistry and landscape position (undulating plains, slopes, and BIF crests and ridges) influenced patterns of floristic diversity, calcifuge plant strategies, and nutrient-acquisition strategies in 538 plant species from 830 relevés.

RESULTS

Landscape position was the strongest driver of species richness and vegetation functional composition. Soils became increasingly acidic and P-impoverished along an increasing elevational gradient. Vegetation from different landscape positions was not compositionally dissimilar, but vegetation of BIF crests and ridges was up to twice as biodiverse as vegetation from adjacent lower-relief areas and harbored higher proportions of calcifuge species and species with mycorrhizal associations.

MAIN CONCLUSIONS

Topographic and edaphic complexity of BIF landforms in an otherwise relatively homogenous landscape has likely facilitated species accumulation over long time periods. They represent musea of regional floristic biodiversity, excluding only species that cannot establish or are inferior competitors in heavily weathered, acidic, skeletal, and nutrient-impoverished soils. Plant strategies likely represent a major filter in establishing biodiverse, representative vegetation on postmining landforms in geologically ancient regions.

Incorporating rock in surface covers improves the establishment of native pioneer vegetation on alkaline mine tailings

BACKGROUND AND AIMS

Rates of tailings production and deposition around the world have increased markedly in recent decades, and have grown asynchronously with safe and environmentally suitable solutions for their storage. Tailings are often produced in regions harbouring biodiverse native plant communities adapted to old, highly-weathered soils. The highly-altered edaphic conditions of tailings compared with natural soils in these areas will likely select against many locally endemic plant species, making phytostabilisation, rehabilitation or ecological restoration of these landforms challenging.

METHODS

We established four substrate cover composition treatments on a dry-stacked magnetite tailings storage facility in semi-arid Western Australia, representative of standard industry practices for rehabilitating or restoring post-mining landforms in the region. Plots were seeded with a selection of locally native plant species and monitored for five years to determine whether different substrate cover treatments yielded different edaphic conditions (soil moisture, substrate surface temperature and substrate chemistry) and influenced soil development and the success of native vegetation establishment.

RESULTS

No vegetation established from seeds on unamended tailings with no surface cover, and substrate chemistry changed minimally over five years. In contrast, rock-containing surface covers allowed establishment of up to 11 native plant species from broadcast seeds at densities of ca. 1.5 seedlings m^-2, and up to 3.5 seedlings m^-2 of five native pioneer chenopods from capture of wind-dispersed seeds from surrounding undisturbed native vegetation. Greater vegetation establishment in rock-containing surface covers resulted from increased heterogeneity (e.g., lower maximum soil temperature, greater water capture and retention, surface microtopography facilitating seed capture and retention, more niches for seed germination). Soil development and bio-weathering occurred most rapidly under the canopy of native pioneer plants on rock-containing surface covers, particularly increases in organic carbon, total nitrogen, and organo-bound aluminium and iron.

CONCLUSIONS

Seed germination and seedling survival on tailings were limited by extreme thermal and hydrological conditions and a highly-altered biogeochemical environment. The design of surface cover layers appears crucial to achieving closure outcomes on tailings landforms, and designs should prioritise increasing surface heterogeneity through the incorporation of rock or other structure-improving amendments to assist the establishment of pioneer vegetation.

Role of roots in adaptation of soil-indifferent Proteaceae to calcareous soils in south-western Australia

Very few of the >650 Proteaceae species in south-western Australia cope with the high calcium (Ca) levels in young, calcareous soils (soil indifferent); most are Ca sensitive and occur on nutrient-impoverished, acidic soils (calcifuge). We assessed possible control points for Ca transport across roots of two soil-indifferent (Hakea prostrata and Banksia prionotes) and two calcifuge (H. incrassata and B. menziesii) Proteaceae. Using quantitative X-ray microanalysis, we investigated cell-specific elemental Ca concentrations at two positions behind the apex in relation to development of apoplastic barriers in roots of plants grown in nutrient solution with low or high Ca supply. In H. prostrata, Ca accumulated in outer cortical cells at 20 mm behind the apex, but [Ca] was low in other cell types. In H. incrassata, [Ca] was low in all cells. Accumulation of Ca in roots of H. prostrata corresponded to development of apoplastic barriers in the endodermis. We found similar [Ca] profiles in roots and similar [Ca] in leaves of two contrasting Banksia species. Soil-indifferent Hakea and Banksia species show different strategies to inhabit calcareous soils: H. prostrata intercepts Ca in roots, reducing transport to shoots, whereas B. prionotes allocates Ca to specific leaf cells.

Accumulation of phosphorus and calcium in different cells protects the phosphorus-hyperaccumulator Ptilotus exaltatus from phosphorus toxicity in high-phosphorus soils

Ptilotus exaltatus accumulates phosphorus (P) to > 40 mg g^-1 without toxicity symptoms, while Kennedia prostrata is intolerant of increased P supply. What physiological mechanisms underlie this difference and protect P. exaltatus from P toxicity? Ptilotus exaltatus and K. prostrata were grown in a sandy soil with low-P, high-P and P-pulse treatments. Both species hyperaccumulated P (>20 mg g^-1) under high-P and P-pulse treatments; shoot dry weight was unchanged for P. exaltatus, but decreased by >50% for K. prostrata. Under high-P, in young fully-expanded leaves, both species accumulated P predominantly as inorganic P. However, P. exaltatus preferentially allocated P to mesophyll cells and stored calcium (Ca) as occasional crystals in specific lower mesophyll cells, separate from P, while K. prostrata preferentially allocated P to epidermal and spongy mesophyll cells, but co-located P and Ca in palisade mesophyll cells where granules with high [P] and [Ca] were evident. Mesophyll cellular [P] correlated positively with [potassium] for both species, and negatively with [sulfur] for P. exaltatus. Thus, P. exaltatus tolerated a very high leaf [inorganic P] (17 mg g^-1), associated with P and Ca allocation to different cell types and formation of Ca crystals, thereby avoiding deleterious precipitation of Ca₃(PO₄)₂. It also showed enhanced [potassium] and decreased [sulfur] to balance high cellular [P]. Phosphorus toxicity in K. prostrata arose from co-location of Ca and P in palisade mesophyll cells. This study advances understanding of leaf physiological mechanisms for high P tolerance in a P-hyperaccumulator and indicates P. exaltatus as a promising candidate for P-phytoextraction.

Edaphic niche characterization of four Proteaceae reveals unique calcicole physiology linked to hyper-endemism of Grevillea thelemanniana

Endemism and rarity have long intrigued scientists. We focused on a rare endemic and critically-endangered species in a global biodiversity hotspot, Grevillea thelemanniana (Proteaceae). We carried out plant and soil analyses of four Proteaceae, including G. thelemanniana, and combined these with glasshouse studies. The analyses related to hydrology and plant water relations as well as soil nutrient concentrations and plant nutrition, with an emphasis on sodium (Na) and calcium (Ca). The local hydrology and matching plant traits related to water relations partially accounted for the distribution of the four Proteaceae. What determined the rarity of G. thelemanniana, however, was its accumulation of Ca. Despite much higher total Ca concentrations in the leaves of the rare G. thelemanniana than in the common Proteaceae, very few Ca crystals were detected in epidermal or mesophyll cells. Instead of crystals, G. thelemanniana epidermal cell vacuoles contained exceptionally high concentrations of noncrystalline Ca. Calcium ameliorated the negative effects of Na on the very salt-sensitive G. thelemanniana. Most importantly, G. thelemanniana required high concentrations of Ca to balance a massively accumulated feeding-deterrent carboxylate, trans-aconitate. This is the first example of a calcicole species accumulating and using Ca to balance accumulation of an antimetabolite.

Phosphorus toxicity disrupts Rubisco activation and reactive oxygen species defence systems by phytic acid accumulation in leaves

Phosphorus (P) is an essential mineral nutrient for plants. Nevertheless, excessive P accumulation in leaf mesophyll cells causes necrotic symptoms in land plants; this phenomenon is termed P toxicity. However, the detailed mechanisms underlying P toxicity in plants have not yet been elucidated. This study aimed to investigate the molecular mechanism of P toxicity in rice. We found that under excessive inorganic P (Pi) application, Rubisco activation decreased and photosynthesis was inhibited, leading to lipid peroxidation. Although the defence systems against reactive oxygen species accumulation were activated under excessive Pi application conditions, the Cu/Zn-type superoxide dismutase activities were inhibited. A metabolic analysis revealed that excessive Pi application led to an increase in the cytosolic sugar phosphate concentration and the activation of phytic acid synthesis. These conditions induced mRNA expression of genes that are activated under metal-deficient conditions, although metals did accumulate. These results suggest that P toxicity is triggered by the attenuation of both photosynthesis and metal availability within cells mediated by phytic acid accumulation. Here, we discuss the whole phenomenon of P toxicity, beginning from the accumulation of Pi within cells to death in land plants.

Growth performance and element concentrations reveal the calcicole-calcifuge behavior of three Adiantum species

BACKGROUND

The calcicole or calcifuge behavior of wild plants has been related to element deficiency or toxicity. For fern species, however, knowledge about their adaptive differences and responses to soil environmental changes is virtually absent. In the karst regions of southern China, most Adiantum species favor calcareous soils, but A. flabellulatum prefers acidic soils. Such contrasting preferences for soil types in the same genus are interesting and risky because their preferred soils may "pollute" each other due to extreme precipitation events. We mixed calcareous and acidic soils at 1:1 (v/v) to simulate the "polluted" soils and grew three Adiantum species (the calcicole A. capillus-veneris f. dissectum and A. malesianum and the calcifuge A. flabellulatum) on the calcareous, acidic and mixed soils for 120 d and assessed their growth performance and element concentrations.

RESULTS

The calcareous soil showed the highest pH, Ca, Mg and P concentrations but the lowest K concentration, followed by the mixed soil, and the acidic soil. After 120 d of growth, the calcifuge A. flabellulatum on the calcareous and mixed soils exhibited lower SPAD and relative growth rate (RGR) than those on the acidic soil, and its leaf and root Ca, Mg and Fe concentrations were higher and K was lower on the calcareous soil than on the acidic soil. The calcicole A. capillus-veneris f. dissectum on the calcareous soil had similar leaf element concentrations and RGR with those on the mixed soil, but their leaf Ca, Fe and Al were lower and leaf P and K concentrations, SPAD and RGR were higher than those on the acidic soil. For the calcicole A. malesianum, leaf Ca, Fe and Al were lowest and leaf P and RGR were highest when grown on the mixed soil, intermediated on the calcareous soil, and on the acidic soil. Compared with A. malesianum, A. capillus-veneris f. dissectum had lower leaf Ca, Fe and Al but higher leaf Mg concentration when grown on the same calcareous or mixed soils.

CONCLUSIONS

A. capillus-veneris f. dissectum is a low leaf Ca calcicole species while A. malesianum is an Al accumulating calcicole species. They can effectively take up P and K to leaves and hence can thrive on calcareous soils. In contrast, the calcifuge A. flabellulatum grown on calcareous soils is stunted. Such growth performance may be attributed to the increased leaf Ca and decreased leaf K concentration. If their preferred soils are "polluted", A. flabellulatum can grow worse, A. capillus-veneris f. dissectum can remain almost unaffected while A. malesianum will perform better.

Divergent responses of above- and below-ground chemical defence to nitrogen and phosphorus supply in waratahs (Telopea speciosissima)

Plant nutrition can affect the allocation of resources to plant chemical defences, yet little is known about how phosphorus (P) supply, and relative nitrogen (N) and P supply, affect chemical defences, especially in species with intrinsically conservative nutrient use adapted to P-impoverished soils. Waratah (Telopea speciosissima (Sm.) R.Br.), like other Proteaceae, is adapted nutrient-poor soils. It was identified as having cyanogenic glycosides (CNglycs) throughout the plant. T. speciosissima seedlings were grown for 15 weeks under two N and P concentrations. CNglycs (N-based defence) and nutrients were quantified in above- and below-ground organs; foliar carbon (C)-based phenolics and tannins were also quantified. CNglyc concentrations in roots were on average 51-fold higher than in above-ground tissues and were affected by both N and P supply, whereas foliar CNglyc concentrations only responded to N supply. Leaves had high concentrations of C-based defences, which increased under low N, and were not correlated with N-based defences. Greater root chemical defence against herbivores and pathogens may be important in a non-mycorrhizal species that relies on basal resprouting following disturbance. The differing responses of secondary chemistry in above- and below-ground organs to P and N demonstrate the importance of broadening the predominantly foliar focus of plant defence studies.

Calcium modulates leaf cell-specific phosphorus allocation in Proteaceae from south-western Australia

Over 650 Proteaceae occur in south-western Australia, contributing to the region's exceptionally high biodiversity. Most Proteaceae occur exclusively on severely nutrient-impoverished, acidic soils (calcifuge), whilst only few also occur on young, calcareous soils (soil-indifferent), higher in calcium (Ca) and phosphorus (P). The calcifuge habit of Proteaceae is explained by Ca-enhanced P toxicity, putatively linked to the leaf cell-specific allocation of Ca and P. Separation of these elements is essential to avoid the deleterious precipitation of Ca-phosphate. We used quantitative X-ray microanalysis to determine leaf cell-specific nutrient concentrations of two calcifuge and two soil-indifferent Proteaceae grown in hydroponics at a range of Ca and P concentrations. Calcium enhanced the preferential allocation of P to palisade mesophyll (PM) cells under high P conditions, without a significant change in whole leaf [P]. Calcifuges showed a greater PM [P] compared with soil-indifferent species, corresponding to their greater sensitivity. This study advances our mechanistic understanding of Ca-enhanced P toxicity, supporting the proposed model, and demonstrating its role in the calcifuge distribution of Proteaceae. This furthers our understanding of nutrient interactions at the cellular level and highlights its importance to plant functioning.

Calcium Application Enhances Drought Stress Tolerance in Sugar Beet and Promotes Plant Biomass and Beetroot Sucrose Concentration

Numerous studies have demonstrated the potential of sugar beet to lose the final sugar yield under water limiting regime. Ample evidences have revealed the important role of mineral nutrition in increasing plant tolerance to abiotic stresses. Despite the vital role of calcium (Ca²⁺) in plant growth and development, as well as in stress responses as an intracellular messenger, its role in alleviating drought stress in sugar beet has been rarely addressed. Here, an attempt was undertaken to investigate whether, and to what extent, foliar application of Ca²⁺ confers drought stress tolerance in sugar beet plants exposed to drought stress. To achieve this goal, sugar beet plants, which were grown in a high throughput phenotyping platform, were sprayed with Ca²⁺ and submitted to drought stress. The results showed that foliar application of Ca²⁺ increased the level of magnesium and silicon in the leaves, promoted plant growth, height, and leaf coverage area as well as chlorophyll level. Ca²⁺, in turn, increased the carbohydrate levels in leaves under drought condition and regulated transcriptionally the genes involved in sucrose transport (BvSUC3 and BvTST3). Subsequently, Ca²⁺ enhanced the root biomass and simultaneously led to induction of root (BvSUC3 and BvTST1) sucrose transporters which eventually supported the loading of more sucrose into beetroot under drought stress. Metabolite analysis revealed that the beneficial effect of Ca²⁺ in tolerance to drought induced-oxidative stress is most likely mediated by higher glutathione pools, increased levels of free polyamine putrescine (Put), and lower levels of amino acid gamma-aminobutyric acid (GABA). Taken together, this work demonstrates that foliar application of Ca²⁺ is a promising fertilization strategy to improve mineral nutrition efficiency, sugar metabolism, redox state, and thus, drought stress tolerance.