THE DEVELOPMENT OF MYCORRHIZAL INFECTION IN CUCUMBER - EFFECTS OF P-SUPPLY ON ROOT-GROWTH, FORMATION OF ENTRY POINTS AND GROWTH OF INFECTION UNITS

This paper describes an experiment in which the effects of soil phosph orus (P) on root growth and mycorrhizal colonization of Cucumis sativu s L. were studied. The aim was to determine the relative contributions of root growth, formation of new infections and growth of infections within the root to the % colonization. Plants were harvested daily bet ween 7 and 24 d. Added P had no effect on root or shoot f. wt or on ro ot length up to 14 d. After that time plants given additional P grew w ell, but those with no added P grew poorly and showed signs of P defic iency. Increase in root length with added P was due both to an increas e in the number of root apices produced per unit length of root per un it time and to an increase in the mean rate of extension of those apic es. Effects of P on the % colonization of the roots by the mycorrhizal fungus Glomus intraradices Schenk and Smith were apparent before any effects on root length were observed. The length of the lag phase was not affected by addition of P. The rate of formation of new entry poin ts was similar with and without added P up to 15 d, and reduction in % colonization up to this time could be attributed entirely to reductio n in the rate of growth of infection units within the roots. Later, th ere was a marked increase in the rate of formation of (secondary) entr y points in the absence of added P, which was not observed when P was added. At 15, 20 and 25 d, additional plants were harvested to quantif y the development of arbuscules and vesicles in colonized regions of t he roots. At all three times, numbers of arbuscules and vesicles were lower with added P. We conclude that at very early stages of colonizat ion P exerts its effect via reduced growth of infection units. After a pprox. 2 wk, an increase in the rate of growth in length of roots and low rates of formation of secondary infections also contribute to the low values of % colonization.