Carbohydrate allocation patterns in citrus genotypes as affected by phosphorus nutrition, mycorrhizal colonization and mycorrhizal dependency

Abstract

SUMMARYAmong closely related citrus genotypes growing in high phosphorus (P) orchard soils, there is a tendency for less mycorrhizal-dependent (M-dependent) species to have lower rates of root colonization than more M-dependent species. We hypothesized that the less M-dependent the citrus species the more limited is their carbohydrate (CHO) allocation to the M fungus. In a glasshouse study at low and high P supply, lower total incidence of Glomus intraradices FL 208, intensity of vesicles formation, and accumulation of 16: l^j cis fungal fatty acid as measures of root colonization were related to lower mycorrhizal dependency (MD; M plant d. wt/non-mycorrhizal (NM) plant d. wt expressed at low P supply) in five citrus genotypes. At high P supply, when host carbon (C) production was not affected by P nutrition, less M-dependent genotypes had consistently lower starch concentrations in their root and shoot tissues than did more M-dependent genotypes, irrespective of M inoculation. At low P supply, M plants were more heavily colonized by G. intraradices and had lower starch levels than the M plants supplied with additional P. At high P, M plants of the more dependent genotypes allocated more C to starch pools relative to the NM plant than the least dependent genotype. Concentration of sucrose in tissues did not vary consistently with the dependency of citrus genotypes or with M inoculation except in high P NM plants, when less M-dependent genotypes had lower levels of sucrose in their roots than the more dependent species. At high P supply, sucrose concentrations were lower in colonized roots than in NM roots. Across citrus genotypes, sucrose in M fibrous roots decreased relative to that in NM roots with increase in root colonization suggesting that more sucrose was allocated for growth and maintenance of G. intraradices in roots of M-dependent species. The concentration of reducing sugars in root tissues varied in relation to MD of citrus genotypes in the same way as that of starch and sucrose, but was less responsive to M colonization. Responses of total non-structural CHO in tissues indicated that more heavily colonized plants at low P expended more C to acquire P than did M plants of similar biomass and P status grown at high P supply. Total CHO pools increased with MD of citrus genotypes, providing evidence than C allocation patterns in the host affect M colonization.

Plant growth and phosphorus uptake in mycorrhizal rangpur lime seedlings under different levels of phosphorus

“…At the 1,000 mg kg -1 P supply, both +AMF and -AMF plants had the same pattern of P uptake and shoot P concentrations. It is notorious that mycorrhizal citrus have higher P concentrations than control plants (Peng et al, 1993;Melloni et al, 2000) but, in this case, only at 25 and 200 mg kg -1 P. In these cases, shoot P concentrations in +AMF plants were above the sufficiency level of 1.9 g kg -1 for citrus, whereas the -AMF plants remained below the deficiency level of 1 g kg -1 (Graham et al, 1997). The EM may be responsible for more than 75% of the plant P uptake (Li et al, 1991).…”

“…Growth increase in +AMF plants under limiting P availability has been reported as the most common effects of AMF (Melloni & Cardoso, 1999b;Graham, 2000;Melloni et al, 2000), but less common is the plant growth depression under high P supply (Graham, 2000;Jifon et al, 2002). Under high P availability, AMF infection may become a C sink (Graham et al, 1997), which carries the host to a growth depression in relation to -AMF plants. According to Peng et al (1993), C expended by mycorrhizal roots was 37% higher than nonmycorrhizal ones, but total plant dry weight reduction was only 8%.…”

“…Some results point towards a positive correlation between total EM and P availability (Cardoso-Filho et al, 1999;Melloni & Cardoso, 1999b;Melloni et al, 2000), others state the opposite (Miranda & Harris, 1994;. Citrus dependence on AMF decreases as P availability increases and, in some cases, high P availability may result in plant growth depression (Peng et al, 1993;Graham et al, 1997;Melloni & Cardoso, 1999b). The generally accepted hypothesis to explain this negative effect is the endophyte action, as C sink on the host (Graham et al, 1997;Jifon et al, 2002).…”

“…Citrus dependence on AMF decreases as P availability increases and, in some cases, high P availability may result in plant growth depression (Peng et al, 1993;Graham et al, 1997;Melloni & Cardoso, 1999b). The generally accepted hypothesis to explain this negative effect is the endophyte action, as C sink on the host (Graham et al, 1997;Jifon et al, 2002). Indeed, the relative development of the internal and the external mycelium should be in balance, to promote a positive growth response (Abbott et al, 1984).…”

Field evidence for the carbon cost of citrus mycorrhizas

“…This alteration of cost : benefit occurred for trees on citrus genotypes that vary widely in mycorrhizal dependency, colonization rates and C allocation responses to mycorrhizal development under P deficiency and sufficiency (Graham et al, 1991 ;Graham, Duncan & Eissenstat, 1997). Even though mycorrhiza-mediated P uptake in low availability conditions was inhibited by benomyl, young citrus trees did not experience P deficiency on any of the rootstocks despite their wide range in dependency on mycorrhizas for P uptake in low P soils (Menge, Johnson & Platt, 1978 ;Graham & Syvertsen, 1985).…”