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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 &...

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Exposure of variegated coleus (Coleus blumei Benth.) plants to a s a h e root-zone environment (60 IIlM NaCI:12 mM CaCI,) resulted in a significant decline in elongation growth rate over the 30-d experimental period. During the initial 5 to 10 d of exposure, mature source leaves showed strongly diminished rates of photosynthesis, which gradually recovered to close to the control rates by the end of the experiment. In green leaf tissues, starch levels showed the same transient decline and recovery pattern. Low starch levels were accompanied by the appearance of severa1 novel carbohydrates, including high-molecular-weight raffinose family oligosaccharides (RFOs) with a degree of polymerization (DP) of 5 to 8, and an O-methylated inositol (OMI). New enzyme activities, including ga1actan:galactan galactosyltransferase, for the synthesis of high-DP RFOs and myo-inositol 6-Omethyltransferase for 0-methylation of myo-inositol, were induced by salinity stress.Phloem-sap analysis showed that in the stressed condition substantially more sucrose than RFO was exported, as was the OMI. In white sink tissues these phloem sugars were used to synthesize high-DP RFOs but not OMls. In sink tissues ga1actan:galactan galactosyltransferase but not myo-inositol 6-O-methyltransferase was induced by salinity stress. Models reflecting the changes in carbohydrate metabolism in source and sink tissues in response to salinity stress are presented.Salinity is only one of many environmental factors resulting in suboptimal crop growth, yet its impact is perhaps one of the most far-reaching in agronomic terms. One frequently reported effect of salinity stress is a reduction in the shoot/root ratio of the stressed plant (Hanson and Hitz, 1982). This observation has led to the conclusion that carbon allocation is altered by salt stress, a conclusion that is supported by observed changes in photosythetic rate under stress conditions (Everard et al., 1994). The reductions in photosynthesis may be at least partially due to nonstomatal factors, suggesting that chloroplast metabolism, and therefore carbon partitioning processes, are also adversely affected by salinity stress (Everard et al., 1994). However, despite the clear alterations in growth and photosynthetic metabolism that occur in response to salt stress, the exact nature of the changes in carbon metabolism that are in- duced by salt stress have not been elucidated (Munns, 1993).Apart from some recent studies on celery, a mannitoltranslocating species (Everard et al., 1994;Stoop and Pharr, 1994), and iceplant (Vernon and Bohnert, 1992), a cyclitolaccumulating species, most research on salinity stress responses has been performed on species that synthesize and export only the disaccharide SUC. In salt-tolerant species such as celery and iceplant, salt stress has been found to induce synthesis of polyols (linear polyhydric sugar alcohols) and cyclitols (cyclic polyhydric sugar alcohols), at the expense of more common storage carbohydrates such as starch and Suc (Loescher and Everard, 1996). Th...

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