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White lupin (Lupinus albus L.) develops proteoid roots when grown in phosphorus (P)-deficient conditions. These short, lateral, densely clustered roots are adapted to increase P availability. Previous studies from our laboratory have shown proteoid roots have higher rates of non-photosynthetic carbon fixation than normal roots and altered metabolism to support organic acid exudation, which serves to solubilize P in the rhizosphere. The present work indicates that proteoid roots possess additional adaptations for increasing P availability and possibly for conserving P in the plant. Roots from P-deficient (-P) plants had significantly greater acid phosphatase activity in both root extracts and root exudates than comparable samples from P-sufficient (+P) plants beginning 10 d after emergence. The increase in activity in -P plants was most pronounced in the proteoid regions. In contrast, no induction of phytase activity was found in -P plants compared to +P plants. The number of proteoid roots present was not affected by the source of phosphorus supplied, whether organic or inorganic forms. Adding molybdate to the roots increased the number of proteoid roots in plants supplied with organic P, but not inorganic P. Increased acid phosphatase activity was detected in root exudates in the presence of organic P sources. Nativepolyacrylamide gel electrophoresis demonstrated that under P-deficient conditions, a unique isoform of acid phosphatase was induced between 10 and 12 d after emergence. This isoform was found not only within the root, but it comprised the major form exuded from proteoid roots of -P plants. The fact that exudation of proteoid-root-specific acid phosphatase coincides with proteoid root development and increased exudation of organic acids indicates that white lupin has several coordinated adaptive strategies to P-deficient conditions. Key-words: Lupinus albus; acid phosphatase; exudates; phosphorus deficiency; phytase; proteoid roots; white lupin INTRODUCTIONWhite lupin is a nitrogen-fixing legume that can acquire soil P that is unavailable to other plants, despite its lack of mycorrhizal symbioses (Avio, Sbrana & Giovannetti 1990;Gardner, Parberry & Barber 1982b;Trinick 1977). Instead, its adaptation to P deficiency is the development of proteoid or cluster root morphology, which is characteristic of members of the Proteaceae (for review, Dinkelaker, Hengelar & Marschner 1995). Proteoid roots are short, lateral, densely clustered tertiary roots that are initiated under P-deficient conditions. In white lupin, these proteoid regions display altered carbon metabolism, in which nonphotosynthetic CO 2 fixation supplies up to 30% of the carbon necessary for the production of the large quantities of organic acids exuded (Johnson, Allan & Vance 1994;Johnson et al. 1996a). Organic acids such as citrate, excreted by the proteoid roots of white lupin, are thought to increase P availability by mobilizing sparingly soluble Fe, Al, and Ca phosphates (Dinkelaker, Romheld & Marschner 1989;Gardner, Parberry &...

Production of native and hybridized varieties of Populus has received considerable interest in temperate regions as an alternative to agricultural crops and an additional wood source, while acting as a potential carbon (C) sink to offset emissions of fossil fuel-based greenhouse gases. Research of root system dynamics in Populus species is expanding, however, our understanding of the nature and role of fine roots (FR) is incomplete. The study objective, therefore, was to review the literature regarding FR production, mortality and longevity in Populus, and evaluate the magnitude and significance of the FR fraction to C sequestration. FRs, conventionally defined as less than 2 mm in diameter and responsible for water and nutrient uptake, are an essential component of the tree. Populus FRs are relatively short-lived, with reported lifespans ranging from 30 to 300 days, depending on root diameter, tree species and age, and soil environmental factors. Standing FR biomass fluctuates throughout the growing season. Fine root production generally peaks in mid-summer, and ranges between 1.0 and 5.0 mg ha À1 yr À1 , while FR mortality has less seasonal amplitude. Production and mortality dynamics in Populus are highly plastic in response to soil environmental conditions, and although opposing conclusions have been proposed, research suggests soil moisture and nitrogen to be most important. Results from the literature indicate annual FR turnover to the soil C pool may be small (0.2-1.6 mg C ha À1 yr À1 ), but substantial in maintaining or enhancing C levels in natural and managed stands of Populus.

Pulses play a significant role in nitrogen cycling as they fix atmospheric N 2 through symbiosis. However, it is unknown whether there are differences in the ability of biological nitrogen fixation (BNF) among pulse species and individual cultivars. Here, we quantified the BNF ability of selected pulse cultivars and determined the effect on crop yield. A total of 25 species-cultivar combinations of chickpea (Cicer arietinum L.), dry bean (Phaseolus vulgaris L.), faba bean (Vicia faba L.), field pea (Pisum sativum L.), and lentil (Lens culinaris Medik.) were tested in 2008-2010. Pulses had a higher BNF in the wetter 2010 season, and a lower BNF in the drier 2009 season. In 2010, faba bean and chickpea had the highest BNF at 106 kg N ha −1 , followed by lentil, field pea, and dry bean at 87, 69, and 12 kg N ha −1 , respectively. Across years, field pea had the most stable BNF ability, fixing 55 kg N ha −1 with an average seed yield of 2418 kg ha −1. There are large differences in BNF and yield among cultivars within a species and the magnitude of the difference varied with years. Large genetic variability in BNF and yield suggest the possibility that pulse cultivars with a higher N 2-fixing ability and seed yield can be developed through selection of the N 2-fixing trait.

Topography influences the distribution of water and N, thereby exerting an indirect control on N2‐fixing activity of legumes. A study was conducted to assess the (i) variability of N2 fixation in pea (Pisum sativum L.) in a rolling field, and (ii) degree of landscape‐scale control on N2 fixation. A 90 by 100 m sampling grid, with 10‐m spacings, was established in the field. Each sampling point was classified as either footslope or shoulder. The percentage of N derived from the atmosphere (% Ndfa) was estimated using natural 15N abundance and the A‐value approaches. Spring soil water content and inorganic N were most concentrated in the footslopes, whereas mean estimates of % Ndfa showed inverted spatial distribution patterns. At flowering, natural 15N abundance estimates of % Ndfa did not differ between shoulders and footslopes. In contrast, A‐value mean estimates of % Ndfa were 84% for shoulders and 92% for footslopes. The two approaches gave similar mean estimates of % Ndfa at maturity, with values of approximately 72 and 84% for footslopes and shoulders. A random spatial pattern existed for total aboveground N accumulation, indicating that pea was able to adjust its N2‐fixing activity according to the available soil N. Despite similar spatial patterns for % Ndfa estimated using the two approaches, the correlation between the A value and natural 15N abundance approaches was poor (r = 0.213 at flowering and r = 0.377 at maturity). The poor correlation suggests that N2 fixation by pea was partially controlled at the landscape scale, whereas strong micro‐scale controls may have existed that ultimately regulated N2 fixation.