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Is shoot growth correlated to leaf protein concentration?

2007; Elsevier BV; Volume: 12; Issue: 12 Linguagem: Inglês

10.1016/j.tplants.2007.09.005

1878-4372

M. Andrews, John A. Raven, Janet I. Sprent, Peter J. Lea,

Legume Nitrogen Fixing Symbiosis

Consistently, the shoot-to-root dry weight ratio (S:R) of herbaceous plants decreases when growth is limited by nitrogen (N) or phosphorus (P) supply. However, the ratio has been reported to decrease, change little or even increase with the reduced growth associated with potassium (K) or magnesium (Mg) deficiency [ 1 Marschner H. et al. Effect of mineral nutritional status on shoot-root partitioning of photoassimilates and cycling of mineral nutrients. J. Exp. Bot. 1996; 47: 1255-1263 Crossref PubMed Google Scholar , 2 Andrews M. et al. Relationships between shoot to root ratio, growth and leaf soluble protein concentration of Pisum sativum, Phaseolus vulgaris and Triticum aestivum under different nutrient deficiencies. Plant Cell Environ. 1999; 22: 949-958 Crossref Scopus (103) Google Scholar , 3 Andrews M. et al. A role for shoot protein in shoot-root dry matter allocation in higher plants. Ann. Bot. (Lond.). 2006; 97: 3-10 Crossref PubMed Scopus (39) Google Scholar ]. Deficiencies of N, P, K and Mg are often associated with increased carbohydrate levels in leaves [ 1 Marschner H. et al. Effect of mineral nutritional status on shoot-root partitioning of photoassimilates and cycling of mineral nutrients. J. Exp. Bot. 1996; 47: 1255-1263 Crossref PubMed Google Scholar ]. Recently, Hermans et al. [ 4 Hermans C. et al. How do plants respond to nutrient shortage by biomass allocation?. Trends Plant Sci. 2006; 11: 610-617 Abstract Full Text Full Text PDF PubMed Scopus (779) Google Scholar ] proposed that the increase in carbohydrate concentration (in particular, sucrose) in the shoots of plants grown on reduced N or P supply is linked mechanistically to the increased partitioning of dry matter to the roots. They also suggested that the reason the increased shoot carbohydrate concentration caused by K and Mg deficiency does not consistently result in decreased S:R is that plants deficient in these nutrients are less able to translocate sucrose to the root via the phloem. We agree that the evidence is strong that root growth is closely linked to sucrose translocation from the shoot, which is dependent on the sucrose concentration in the shoot, but these hypotheses fail to consider two consistent findings in the literature. First, although a greater proportion of total plant dry matter is partitioned to the root on reduced N supply, generally the root is smaller on low than on high N supply [ 2 Andrews M. et al. Relationships between shoot to root ratio, growth and leaf soluble protein concentration of Pisum sativum, Phaseolus vulgaris and Triticum aestivum under different nutrient deficiencies. Plant Cell Environ. 1999; 22: 949-958 Crossref Scopus (103) Google Scholar ]. Root growth is not positively correlated with leaf sucrose concentration across different N supplies. Second, there are many reports of strong positive correlations between S:R and plant/shoot N and/or leaf soluble-protein concentration across nutrient deficiencies [ 2 Andrews M. et al. Relationships between shoot to root ratio, growth and leaf soluble protein concentration of Pisum sativum, Phaseolus vulgaris and Triticum aestivum under different nutrient deficiencies. Plant Cell Environ. 1999; 22: 949-958 Crossref Scopus (103) Google Scholar , 3 Andrews M. et al. A role for shoot protein in shoot-root dry matter allocation in higher plants. Ann. Bot. (Lond.). 2006; 97: 3-10 Crossref PubMed Scopus (39) Google Scholar , 5 Ågren G.I. Franklin O. Root:shoot ratios, optimization and nitrogen productivity. Ann. Bot. (Lond.). 2003; 92: 795-800 Crossref PubMed Scopus (162) Google Scholar ]. The relationship between S:R and leaf soluble-protein concentration has been reported to hold across different nutrient forms of N regardless of the charge (NO3−, NH4+, N2, glutamine), across all macronutrient deficiencies, under water stress and when S:R increases under reduced irradiance or CO2 conditions when sucrose levels are likely to decrease substantially regardless of N availability [ 3 Andrews M. et al. A role for shoot protein in shoot-root dry matter allocation in higher plants. Ann. Bot. (Lond.). 2006; 97: 3-10 Crossref PubMed Scopus (39) Google Scholar , 5 Ågren G.I. Franklin O. Root:shoot ratios, optimization and nitrogen productivity. Ann. Bot. (Lond.). 2003; 92: 795-800 Crossref PubMed Scopus (162) Google Scholar , 6 Andrews M. et al. Environmental effects on dry matter partitioning between shoot and root of crop plants: relations with growth and shoot protein concentration. Ann. Appl. Biol. 2001; 138: 57-68 Crossref Scopus (60) Google Scholar , 7 Andrews M. et al. Extension growth of Impatiens glandulifera at low irradiance: importance of nitrate and potassium accumulation. Ann. Bot. (Lond.). 2005; 95: 641-648 Crossref PubMed Scopus (39) Google Scholar ]. The proposal of Hermans et al. [ 4 Hermans C. et al. How do plants respond to nutrient shortage by biomass allocation?. Trends Plant Sci. 2006; 11: 610-617 Abstract Full Text Full Text PDF PubMed Scopus (779) Google Scholar ] cannot explain the increase in S:R associated with decreased leaf sucrose concentration under reduced irradiance or CO2 supply or an increase in S:R with increased N supply at low irradiance when leaf sucrose concentrations are negligible [ 6 Andrews M. et al. Environmental effects on dry matter partitioning between shoot and root of crop plants: relations with growth and shoot protein concentration. Ann. Appl. Biol. 2001; 138: 57-68 Crossref Scopus (60) Google Scholar , 7 Andrews M. et al. Extension growth of Impatiens glandulifera at low irradiance: importance of nitrate and potassium accumulation. Ann. Bot. (Lond.). 2005; 95: 641-648 Crossref PubMed Scopus (39) Google Scholar ]. The strength of the relationship between S:R and leaf protein concentration across different environmental variables was highlighted in a recent study. When tobacco (Nicotiana tabacum) was supplied with (i) different concentrations of N, P, K, S and Mg, (ii) different N forms (NO3−, glutamine, urea, NH4NO3) or (iii) NO3− under low and high irradiance the S:R was not significantly correlated with plant dry weight, but a linear regression model incorporating leaf soluble-protein concentration could explain 82% of the variation in S:R within and across all treatments [ 3 Andrews M. et al. A role for shoot protein in shoot-root dry matter allocation in higher plants. Ann. Bot. (Lond.). 2006; 97: 3-10 Crossref PubMed Scopus (39) Google Scholar ]. Only the values for the low P treatment fell slightly outside the line, which indicates that, for tobacco, there might be a P-specific effect. However, this P effect was not found with a range of other species [ 2 Andrews M. et al. Relationships between shoot to root ratio, growth and leaf soluble protein concentration of Pisum sativum, Phaseolus vulgaris and Triticum aestivum under different nutrient deficiencies. Plant Cell Environ. 1999; 22: 949-958 Crossref Scopus (103) Google Scholar , 6 Andrews M. et al. Environmental effects on dry matter partitioning between shoot and root of crop plants: relations with growth and shoot protein concentration. Ann. Appl. Biol. 2001; 138: 57-68 Crossref Scopus (60) Google Scholar ]. Also, for Lolium multiflorum under extreme Mg deficiency, S:R was exceptionally high [ 6 Andrews M. et al. Environmental effects on dry matter partitioning between shoot and root of crop plants: relations with growth and shoot protein concentration. Ann. Appl. Biol. 2001; 138: 57-68 Crossref Scopus (60) Google Scholar ]. It is possible that this was in part due to a Mg-specific effect, such as impaired photosynthate export in the phloem [ 4 Hermans C. et al. How do plants respond to nutrient shortage by biomass allocation?. Trends Plant Sci. 2006; 11: 610-617 Abstract Full Text Full Text PDF PubMed Scopus (779) Google Scholar ].