Brassica genotypes differ in growth, phosphorus uptake and rhizosphere properties under P-limiting conditions

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Abstract

Compared to other crops, Brassicas are generally considered to grow well in soils with low P availability, however, little is known about genotypic differences within Brassicas in this respect. To assess the role of rhizosphere properties in growth and P uptake by Brassicas, three Brassica genotypes (mustard, Brassica juncea cv Chinese greens and canola, Brassica napus cvs Drum and Outback) were grown in an acidic soil with low P availability at two treatments of added P: 25 and 100 mg P kg−1 as FePO4 (P25 and P100). The plants were harvested at the 6-leaf stage, at flowering and at maturity. Shoot and root dry weight (dry weight) and root length increased with time and were lower in P25 than in P100. In P25, shoot dry weight was lowest in Outback and highest in Chinese greens. In the P100 treatment, Chinese greens had a higher shoot dry weight than the two canola cultivars. Chinese greens had a lower root dry weight and root length at flowering and maturity than the canola genotypes in both P treatments. Irrespective of P treatment, shoot P concentration was lower in Chinese greens than in the two canola genotypes. Specific P uptake (μg P m−1 root length) decreased with time. In P25, Chinese greens had the lowest specific P uptake at the 6-leaf stage but it was higher than in the two canola genotypes at flowering and maturity. In P100, Outback had the lowest specific P uptake. Available P in the rhizosphere (resin P) decreased over time with the greatest decrease from the 6-leaf stage to flowering. In P25, resin P in the rhizosphere was greatest in Chinese greens at the 6-leaf stage and flowering and smallest in Outback at flowering. Microbial P and acid phosphatase activity changed little over time, were not affected by P treatment and there were only small differences between the genotypes. The rhizosphere microbial community composition [assessed by fatty acid methyl ester (FAME) analysis] of Outback and Chinese greens differed from that of the other two genotypes at the 6-leaf stage and flowering, respectively. At maturity, all three genotypes had distinct microbial communities. Plant traits such as production of high biomass at low shoot P concentrations as well as the capacity to maintain high P availability in the rhizosphere by P mobilisation can explain the observed differences in plant growth and P uptake among the Brassica genotypes.

Introduction

Plants and microorganisms can only take up P from the soil solution. Despite high total soil concentrations of P, its concentration in the soil solution is very low (frequently less than 1 μM; Barber, 1995) compared to the requirement of plants and soil organisms. Poorly available inorganic P forms include Ca phosphates, Fe/Al phosphates, and P adsorbed onto Fe/Al oxides and organic matter. Soluble P fertilisers applied to soil rapidly become unavailable (fixed) due to adsorption and formation of poorly soluble P compounds. Organic P can account for up to 80% of the total P in soils (Schachtman et al., 1998). Organic P only becomes available after mineralisation by phosphatase enzymes released by roots and microorganisms. Another important P pool in soil is the microbial biomass, which can represent between 1% and more than 10% of total soil P (Richardson, 2001).

Under low P availability, plants increase the soil volume exploited by increasing root length, root/shoot ratio and/or root hair length (Caradus, 1981; Föhse et al., 1988). The symbiosis with mycorrhizal fungi can also increase the soil volume exploited and therefore increase plant P uptake (Smith and Read, 1997). Phosphorus solubility can be increased by excretion of organic acid anions into the rhizosphere and/or changing the rhizosphere pH (Gerke and Meyer, 1995; Imas et al., 1997). Phosphatases released by plant roots can mineralise organic P (Tarafdar and Claassen, 1988).

Growth at low P supply relative to growth at high P supply provides a measure of P efficiency (Rengel, 1999; Osborne and Rengel, 2002). Mechanisms such as those mentioned above confer P acquisition efficiency to plants, i.e. the capacity to take up greater amounts of P from the soil than P-inefficient plants. Plants can also exhibit P utilisation efficiency, i.e. producing a large amount of dry weight per unit of P taken up (Rengel, 1999). In a study with a large number of wheat, triticale and rye genotypes, no genotype was efficient in both P uptake and P utilisation (Osborne and Rengel, 2002).

Microorganisms can increase P availability through solubilisation of poorly available phosphates by lowering the pH or excreting organic acid anions (Banik and Dey, 1983a; Whitelaw et al., 1999). They also release phosphatases to mineralise organic P (Tarafdar and Claassen, 1988).

In the rhizosphere, plant and microbial solubilisation and mineralisation processes occur simultaneously. Plants and microorganisms compete for the mobilised P. It has been hypothesised that an active microbial biomass with a high turnover rate can rapidly take up added P, but may also represent a slow sustained source of available P through decomposition of dead microbial cells (Seeling and Zasoski, 1993; Oberson et al., 2001). In agreement with these hypotheses, we showed in a previous study with three Poaceae genotypes (Marschner et al., 2005, Marschner et al., 2006) that plant growth and P uptake were positively correlated with microbial P concentration in the rhizosphere.

Plant genotypes may differ in microbial community composition in the rhizosphere (Rouatt and Katznelson, 1961; Grayston et al., 1998; Marilley and Aragno, 1999; Garcia et al., 2001; Soederberg et al., 2002) which is thought to be due to differences in the amount and composition of root exudates. As the capacity to solubilise or mineralise poorly available P differs among microbial species (Banik and Dey, 1983a, Banik and Dey, 1983b) and they may also differ in internal P requirement, microbial community composition in the rhizosphere could affect P availability to plants.

Canola is an attractive alternative to cereals because of a good market price compared to cereals. Although nowadays usually grown in fertile soils, it is likely that canola will extend into areas where soil P availability is low. Brassicas are considered to be P efficient compared to other crops (Föhse et al., 1988). This is thought to be due to the finely branched root system and long root hairs of Brassicas. Additionally, they can mobilise P by release of organic acid anions (Hoffland et al., 1989; Hoffland, 1992). But little is known about genotypic differences among Brassicas in the capacity to grow in soils with low P availability. Recently, Greenwood et al. (2005) showed that Brassica oleracea genotypes differed in growth and P uptake at low and high P availability. However, the rhizosphere properties of Brassicas such as available P and microbial P and how these relate to P uptake by the plants have not been studied.

The aim of this study was to compare growth, P uptake and rhizosphere properties of three Brassica genotypes (two canolas, one mustard) in a soil with low P availability. The results obtained in the present study were compared with those of the previous study on Poaceae conducted in the same soil (Marschner et al., 2005, Marschner et al., 2006).

Section snippets

Soil preparation

A loam soil (0–10 cm) was collected from Mount Bold (38.11°S 138.69°E) and sieved to 2 mm. The soil properties are as follows: 23% clay, 24% silt, 53% sand, pH (1:5, soil and water) 5.0, 41 g organic C kg−1, 2.6 mg NH4+ kg−1, 26 mg NO3- kg−1, 306 mg total P kg−1 and 2.1 or 19 mg P kg−1 available P, determined as resin or Colwell P, respectively. Large differences in available P determined as resin or Colwell P were also found in a previous study that included various acidic, neutral and acidic soils

Plant growth

Shoot dry weight was lower in P25 than in P100 at all growth stages (Table 1) and it was higher in Chinese greens than in the two canola cultivars. Shoot dry weight in P25 as proportion of that in P100 was greatest for Chinese greens (60% at 6-leaf to 53% at flowering), moderate for Drum (43% at 6-leaf to 50% at flowering) and smallest for Outback (33% at 6-leaf to 29% at flowering) at the first two growth stages, but was higher for Drum (58%) than Chinese greens (48%) and Outback (41%) at

Discussion

This study showed that Brassica genotypes differ in their capacity to utilise iron phosphate as P source under P-limiting conditions. The canola genotype Outback achieved the lowest growth and the mustard Chinese greens the highest growth when P availability was low. At higher P availability, Chinese greens also had the greatest shoot dry weight but there were no differences in growth of the two canola genotypes. Chinese greens was characterised by lower P concentrations in the shoots than the

Conclusions

The present study showed that Brassica genotypes differ in their capacity to grow in P-limiting conditions. This highlights the importance of a large screening of Brassica genotypes if cropping of canola is to be extended into areas with low P availability. Better growth was due to the capacity to maintain high growth rates at low shoot P concentrations and mobilisation of P in the rhizosphere. Plant P uptake was correlated with root length and P availability in the rhizosphere, particularly in

Acknowledgements

This study was supported by a Discovery grant from the Australian Research Council.

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