Carbon Cost of the Fungal Symbiont Relative to Net Leaf P Accumulation in a Split-Root VA Mycorrhizal Symbiosis

Abstract

Translocation of '4C-photosynthates to mycorrhizal (+ +), half mycorrhizal (0+), and nonmycorrhizal (00) split-root systems was compared to P accumulation in leaves of the host plant. Carrizo citrange seedlings (Poncirus trifoliata [L.] Raf. x Citrus sinensis [L.] Osbeck) were inoculated with the vesicular-arbuscular mycorrhizal fungus Glomus intraradices Schenck and Smith. Plants were exposed to 14 CO2 for 10 minutes and ambient air for 2 hours. Three to 4% of recently labeled photosynthate was allocated to metabolism of the mycorrhiza in each inoculated root half independent of shoot P concentration, growth response, and whether one or both root halves were colonized. Nonmycorrhizal roots respired more of the label translocated to them than did mycorrhizal roots. Label recovered in the potting medium due to exudation or transport into extraradical hyphae was 5 to 6 times greater for (+ +) versus (00) plants. In low nutrient media, roots of (0+ ) and ( + + ) plants transported more P to leaves per root weight than roots of (00) plants. However, when C translocated to roots utilized for respiration, exudation, etc., as well as growth is considered, (00) plant roots were at least as efficient at P uptake (benefit) per C utilized (cost) as (0+) and (+ +) plants. Root systems of ( + + ) plants did not supply more P to leaves than (0+ ) plants in higher nutrient media, yet they still allocated twice the 14C-photosynthate to the mycorrhiza as did (0+) root systems. This indicates there is an optimal level of mycorrhizal colonization above which the plant receives no enhanced P uptake yet continues to partition photosynthates to metabolism of the mycorrhiza.ranged from 4 to 17% of fixed C (5, 12,13,17,22 (12). One can describe differences in root physiology without compounding effects of different shoots by inoculating one half-root system with a mycorrhizal fungus. In such a system, mycorrhizal root halves of sour orange and Carrizo citrange seedlings accumulated 3 to 5% more of total '4C-photosynthate than the paired nonmycorrhizal half-root system (12).The following experiments were conducted to detail C partitioning in below ground fractions of split-root Carrizo citrange and to quantify C cost relative to net P acquisition in leaves. The first portion of this objective was accomplished through the development of a method of wet acid digestion and oxidation of soil organic matter. Below ground respiration and partitioning within roots also were measured. Thus, all components of below ground partitioning of carbon have been quantified in the present study.Vesicular-arbuscular mycorrhizal (VAM)4 fungi have been shown to increase growth and P uptake in a wide variety of forest and fruit trees (14,19,20) as well as agronomic crops (9,10,16). These fungi are assumed to be obligate symbionts because they have not been cultured axenically. As obligate symbionts they require organic compounds from their hosts, and indeed, the transfer of 14C-labeled compounds from host plant to VAM fungus has been noted (6,13

Field evidence for the carbon cost of citrus mycorrhizas

“…Besides promoting P acquisition, mycorrhizas affect below-ground C expenditure of citrus (Douds, Johnson & Koch, 1988 ;Eissenstat et al, 1993 ;Peng et al, 1993). Without a compensatory increase in shoot C assimilation of citrus seedlings, G. intraradices FL208 reduced  compared with nonmycorrhizal plants of equivalent P status, consistent with an increase in below-ground C expenditure .…”

Phosphorus nutrition‐mediated effects of arbuscular mycorrhiza on leaf morphology and carbon allocation in perennial ryegrass

“…consume 4 -20% of total photosynthates (Douds et al, 1988;Jakobsen & Rosendahl, 1990). Although the present study did not attempt to construct a C balance of the symbiosis, it nonetheless indicates that, for a given P-capture level (RPUR), a determined C gain (RGR) was made irrespective of the presence of AMF.…”

Effects of drought on non‐mycorrhizal and mycorrhizal maize: changes in the pools of non‐structural carbohydrates, in the activities of invertase and trehalase, and in the pools of amino acids and imino acids

“…2) (mycorrhiza idrought interaction terms, P l 0n07). Based on isotope-labelling studies it has been previously suggested that mycorrhizal roots represent stronger sinks for photosynthates than non-mycorrhizal roots (Douds, Johnson & Koch, 1988). Thus, the lack of hexose accumulation in the leaves of mycorrhizal plants upon drought might be due to a lower availability of photosynthates for storage in these tissues.…”