Root distributions of Australian herbaceous perennial legumes in response to phosphorus placement

Contrasting distribution of enzyme activities in the rhizosphere of European beech and Norway spruce

Recent policies and silvicultural management call for forest regeneration that involve the selection of tree species able to cope with low soil nutrient availability in forest ecosystems. Understanding the impact of different tree species on the rhizosphere processes (e.g., enzyme activities) involved in nutrient mobilisation is critical in selecting suitable species to adapt forests to environmental change. Here, we visualised and investigated the rhizosphere distribution of enzyme activities (cellobiohydrolase, leucine-aminopeptidase, and acid phosphomonoesterase) using zymography. We related the distribution of enzyme activities to the seedling root morphological traits of European beech (Fagus sylvatica) and Norway spruce (Picea abies), the two most cultivated temperate tree species that employ contrasting strategies in soil nutrient acquisition. We found that spruce showed a higher morphological heterogeneity along the roots than beech, resulting in a more robust relationship between rhizoplane-associated enzyme activities and the longitudinal distance from the root apex. The rhizoplane enzyme activities decreased in spruce and increased in beech with the distance from the root apex over a power-law equation. Spruce revealed broader rhizosphere extents of all three enzymes, but only acid phosphomonoesterase activity was higher compared with beech. This latter result was determined by a larger root system found in beech compared with spruce that enhanced cellobiohydrolase and leucine-aminopeptidase activities. The root hair zone and hair lengths were significant variables determining the distribution of enzyme activities in the rhizosphere. Our findings indicate that spruce has a more substantial influence on rhizosphere enzyme production and diffusion than beech, enabling spruce to better mobilise nutrients from organic sources in heterogeneous forest soils.

Mechanisms for tolerance of very high tissue phosphorus concentrations in Ptilotus polystachyus

Study of plants with unusual phosphorus (P) physiology may assist development of more P-efficient crops. Ptilotus polystachyus grows well at high P supply, when shoot P concentrations ([P]) may exceed 40 mg P g(-1) dry matter (DM). We explored the P physiology of P. polystachyus seedlings grown in nutrient solution with 0-5 mM P. In addition, young leaves and roots of soil-grown plants were used for cryo-scanning electron microscopy and X-ray microanalysis. No P-toxicity symptoms were observed, even at 5 mM P in solution. Shoot DM was similar at 0.1 and 1.0 mM P in solution, but was ∼14% lower at 2 and 5 mM P. At 1 mM P, [P] was 36, 18, 14 and 11 mg P g(-1) DM in mature leaves, young leaves, stems and roots, respectively. Leaf potassium, calcium and magnesium concentrations increased with increasing P supply. Leaf epidermal and palisade mesophyll cells had similar [P]. The root epidermis and most cortical cells had senesced, even in young roots. We conclude that preferential accumulation of P in mature leaves, accumulation of balancing cations and uniform distribution of P across leaf cell types allow P. polystachyus to tolerate very high leaf [P].

Root physiological adaptations involved in enhancing P assimilation in mining and non-mining ecotypes of Polygonum hydropiper grown under organic P media

It is important to seek out plant species, high in phosphorus (P) uptake, for phytoremediation of P-enriched environments with a large amount of organic P (Po). P assimilation characteristics and the related mechanisms of Polygonum hydropiper were investigated in hydroponic media containing various concentrations of Po (1-8 mmol L(-1)) supplied as phytate. The mining ecotype (ME) showed significantly higher biomass in both shoots and roots compared to the non-mining ecotype (NME) at 4, 6, and 8 m mol L(-1). Shoot P content of both ecotypes increased up to 4 mmol L(-1) while root P content increased continually up to 8 mmol L(-1) for the ME and up to 6 mmol L(-1) for the NME. Root P content of the ME exceeded 1% dry weight under 6 and 8 mmol L(-1). The ME had significantly higher P accumulation in both shoots and roots compared to the NME supplied with 6 and 8 mmol L(-1). The ME showed higher total root length, specific root length, root surface area, root volume, and displayed significantly greater root length, root surface area, and root volume of lateral roots compared to the NME grown in all Po treatments. Average diameter of lateral roots was 0.17-19 mm for the ME and 0.18-0.21 mm for the NME. Greater acid phosphatase and phytase activities were observed in the ME grown under different levels of Po relative to the NME. This indicated fine root morphology, enhanced acid phosphatase and phytase activities might be adaptations to high Po media. Results from this study establish that the ME of P. hydropiper is capable of assimilating P from Po media and is a potential material for phytoremediation of polluted area with high Po.

Effect of nitrogen, phosphorus and potassium availability on emergence, nodulation and growth of Trifolium medium L. in alkaline soil

Little is known about the effects of nutrient availability on the growth of Trifolium medium in alkaline soil. In 2010, a pot experiment (10 N, P and K fertiliser treatments) with seeding of T. medium into alkaline soil was performed and emergence of seedlings, survival, aboveground and belowground organs were studied. The positive effects of increased nutrient availability on seedling emergence ranged from 5% in the control to 17% in the high P treatment. The lowest mortality was in treatments with P and K supply and the highest in treatments with N supply, due to the sensitivity of young plants to high N availability. The highest values of most measured aboveground plant traits were recorded in treatments with simultaneous application of N, P and K. There were highly positive effects of P supply alone or in combination with N and K on the development of belowground organs. Taproot length ranged from 11.5 in high N to 40.2 cm in P treatment. There was a negative effect of N application on nodulation, especially in N treatments, where growth of T. medium was limited by insufficient P supply. The number of nodules per plant ranged from 0.8 to 4.5 in the high N and P treatments. As demonstrated in this study, T. medium is a potentially suitable legume for alkaline soils. It requires a relatively high P and K supply as well as moderate mineral N supply to achieve its maximum growth potential.

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.

Growth, yield and seed composition of native Australian legumes with potential as grain crops

BACKGROUND

Many Australian native legumes grow in arid and nutrient-poor environments. Yet few Australian herbaceous legumes have been investigated for domestication potential. This study compared growth and reproductive traits, grain yield and seed composition of 17 native Australian legumes with three commercial grain legumes.

RESULTS

Seed yields of seven native legumes were > 40% of Cicer arietnum, with highest seed yields and harvest indices in Glycine sp. (14.4 g per plant, 0.54 g g(-1) ) and Lotus cruentus (10.2 g per plant, 0.65 g g(-1) ). Five native species flowered earlier than field pea (Pisum sativa) (109 days), though many were slower to flower and set seed. Largest seeds were found in Glycine canescens (17 mg), with seed of other native species 14 times smaller than commercial cultivars. Seed composition of many native legumes was similar to commercial cultivars (200-330 g protein kg(-1) dry weight (DW), 130-430 g dietary fibre kg(-1) DW). Two Cullen species had high fat content (>110 g kg(-1) DW) and Trigonella sauvissima had the highest crude protein content (370 g kg(-1) DW).

CONCLUSION

The seed composition and reproductive traits of some wild native Australian legumes suggest they could offer potential as grain crops for soils and environments where the current grain legumes are uneconomic. Further evaluation of genetic diversity, especially for seed size, overall productivity, and reproductive development is needed.

Multiple adaptive responses of Australian native perennial legumes with pasture potential to grow in phosphorus- and moisture-limited environments

BACKGROUND AND AIMS

Many Australian legumes have evolved in low-phosphorus (P) soils and low-rainfall areas. Therefore a study was made of the interaction of soil [P] and water availability on growth, photosynthesis, water-use efficiency (WUE) and P nutrition of two Australian native legumes with pasture potential, Cullen australasicum and C. pallidum, and the widely grown exotic pasture legume, lucerne (Medicago sativa).

METHODS

Plants were grown in a glasshouse at 3, 10 and 30 mg P kg(-1) dry soil for 5 months. At week 10, two drought treatments were imposed, total pot dried (all-dry) and only top soil dried (top-dry), while control pots were maintained at field capacity.

KEY RESULTS

Shoot dry weight produced by lucerne was never higher than that of C. australasicum. For C. pallidum only, shoot dry weight was reduced at 30 mg P kg(-1) dry soil. The small root system of the Cullen species was quite plastic, allowing plants to access P and moisture efficiently. Lucerne always had a higher proportion of its large root system in the top soil layer compared with Cullen species. All species showed decreased photosynthesis, leaf water potential and stomatal conductance when exposed to drought, but the reductions were less for Cullen species, due to tighter stomatal control, and consequently they achieved a higher WUE. All species showed highest rhizosphere carboxylate concentrations in the all-dry treatment. For lucerne only, carboxylates decreased as P supply increased. Citrate was the main carboxylate in the control and top-dry treatments, and malate in the all-dry treatment.

CONCLUSIONS

Multiple adaptive responses of Cullen species and lucerne favoured exploitation of low-P soils under drought. The performance of undomesticated Cullen species, relative to that of lucerne, shows their promise as pasture species for environments such as in south-western Australia where water and P are limiting, especially in view of a predicted drying and warming climate.

Putting the P in Ptilotus: a phosphorus-accumulating herb native to Australia

BACKGROUND AND AIMS

Ptilotus polystachyus (green mulla mulla; ptilotus) is a short-lived perennial herb that occurs widely in Australia in arid and semi-arid regions with nutrient poor soils. As this species shows potential for domestication, its response to addition of phosphorus (P) and nitrogen (N) was compared to a variety of the domesticated exotic perennial pasture herb Cichorium intybus (chicory), 'Puna'.

METHODS

Pots were filled with 3 kg of an extremely nutrient-deficient sterilized field soil that contained 3 mg kg(-1) mineral N and 2 mg kg(-1) bicarbonate-extractable P. The growth and P and N accumulation of ptilotus and chicory in response to seven rates of readily available phosphorus (0-300 mg P pot(-1)) and nitrogen (N) (0-270 mg N pot(-1)) was examined.

KEY RESULTS

Ptilotus grew extremely well under low P conditions: shoot dry weights were 23, 6 and 1.7 times greater than for chicory at the three lowest levels of P addition, 0, 15 and 30 mg P pot(-1), respectively. Ptilotus could not downregulate P uptake. Concentrations of P in shoots approached 4% of dry weight and cryo-scanning electron microscopy and X-ray microanalysis showed 35-196 mM of P in cell vacuoles in a range of tissues from young leaves. Ptilotus had a remarkable tolerance of high P concentrations in shoots. While chicory exhibited symptoms of P toxicity at the highest rate of P addition (300 mg P pot(-1)), no symptoms were present for ptilotus. The two species responded in a similar manner to addition of N.

CONCLUSIONS

In comparison to chicory, ptilotus demonstrated an impressive ability to grow well under conditions of low and high P availability. Further study of the mechanisms of P uptake and tolerance in ptilotus is warranted.

Rooting depth and leaf hydraulic conductance in the xeric tree Haloxyolon ammodendron growing at sites of contrasting soil texture

An experiment was conducted on Haloxylon ammodendron C.A. Mey, a small xeric tree. Soil water content, soil evaporation, leaf water potential, leaf transpiration rate and stomatal conductance were measured at the two sites that contrast in soil texture: sandy and heavy textured, 8 km apart on the southern periphery of Gurbantonggut Desert, Central Asia, during the 2005 and 2006 growing seasons. Leaf specific hydraulic conductance was calculated from the measurements, and root distributions of plants grown at the two sites were quantified by whole-root system excavation. In general, plants grown in sandy soil experienced better water status than in heavy textured soil. Low soil evaporation loss is not the main reason for this better plant water status at sandy site. Plants in sandy soil developed much deeper root systems, larger root surface areas and higher root: leaf surface area ratio than in heavy textured soil, which facilitated plants acquiring more water and surviving the prolonged drought period. Plants growing at light textured sites should have an advantage in acclimatising to the changed water conditions of the future. Plants at the more sandy sites have a larger buffering capacity to excessive variation in ambient conditions.