No abstract

SUMMARYLeek plants (Allium porrum) were grown on partially sterilized soil either inoculated (M) or not (NM) with the vesicular-arbuscular mycorrhizal fungus, Glomus mosseae. They were pulse-fed with ^"COj in an apparatus which allowed CO^ subsequently respired either by the shoots or by the roots plus soil to be separately monitored. There were three experiments. In two, plants were harvested 48 h after labelling and in the third after 214 h. At harvest, the distribution of^C between shoot,, root, soil organic matter and root washings was measured. Similar growth curves for M and NM plants were obtained by supplying extra phosphorus to the latter, so that C distributions for both treatments could be compared directly. In all three experiments, about 7 % more of the total fixed C was translocated from shoot to root in M plants compared to NM plants. In the third experiment, this extra translocate could be accounted for by increased root respiration plus increased loss of C to the soil but, despite this drain, M and NM plants had equal rates of C assimilation per unit of leaf area. However, shoots of M plants had a lower content of dry matter and hence higher assimilation rates expressed on a dry matter basis.Increased hydration is suggested as a mechanism whereby leaf area and hence C assimilation increases in mycorrhizal plants and which offsets the effects of the drain imposed by the mycorrhizas.

S U M M A R yplants (Allitmi porruttt L.) were grown in a controlled environn-ient on a mixture (2: 1 w/w) of-y-irradiated (1-0 Mrad) sandy loam and sand, at six concentrations of bicarbonate-soluble pbospborus (P) ranging from 22 to -'44 mg P kg ' (soil basis). Inoculum of tbe \-esicular-arbuscular (V.A) mycorrbizal fungus Glottttis tnosseae (Nicolson & Gerdemann) (jcrden-iann and Trappe was placed (M) or not (NM) in a layer 3 cm below tbe soil surface.At intervals of 10 d, lengtbs of main axes of roots and tbeir lateral branches, and of tbe segments of infection i them, were measured. From these data we calculated tbe mean (barvest interval metbod) rates of linear ion of root tips and of infection fronts for eacb member of root, averaged over tbe wbole root systen-i. Tbe mean delay, d, between a root encountering tbe layer of inoculum and tbe subsec^uent formation of internal infection, was also derived.Addition of P to soil did not affect rates of extension ot roots, but increased tbe initiation of main axes and laterals. Infection segments extended twice as fast in laterals as in main axes. At low rates of addition, P did not affect fungal behaviour but increased the length of root available for colonization.Wben bicarbonate-soluble P exceeded 140 mg kg ', tbe rates of extension of infection fronts in botb n-iain axes and laterals were approximately balved, and d was considerably increased. Tbe density (tbe area ratio of fungal •^o bost tissue in a longitudinal squasb) of tbe byphae and arbuscules respectively, and tbe number of entry points P*^' -unit lengtb of root, were greatly reduced by added P. However, tbe ratios ot nun-ibers of entry points/bypbal density and of arbuscule density/hyplial density were unaltered, and tbe morpbology of tbe fungus was not noticeably affected.It is probable that fortnation of entry points was tbe rate-limiting step for colonization, and that tbis rate was ••educed by added P.

No abstract

SUMMARYThe percentage colonization of roots by arbuscular mycorrhizal fungi was measured in the field in two crops of spring oats {Avena sativa L.) and one of maincrop potatoes {Solanum tuberosum L.), successively grown over a period of 3 yr on the long-term liming experiments at Rothamsted and Woburn. In both crops, all years, both phosphorus treatments and at both sites, percentage colonization was little affected by soil pH from over a range approximately 4-5 to 7-5. There was little effect of pH on crop yields. Up to nine species of coarse endophyte could be identified from spores (> 50 //m diameter) m plots of pH 55 and above. There were markedly fewer spores at (approximately) pH 5-5 and none in the most acid plots.