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 phosphorus (P) on root growth and mycorrhizal colonization of Cucumis salivus 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 between 7 and 24 d. Added P had no effect on root or shoot f. wt or on root length up to 14 d. After that time plants given additional P grew well, but those with no added P grew poorly and showed signs of P deficiency. Increase in root length with added P was due both to an increase in the number of root apices produced per unit length of root per unit time and to an increase in the mean rate of extension of those apices. 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 points was similar with and without added P up to 15 d, and reduction in % colonization up to this time could be attributed entirely to reduction in the rate of growth of infection units within the roots. Later, there was a marked increase in the rate of formation of (secondary) entry 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 quantify the development of arbuscules and vesicles in colonized regions of the roots. At all three times, numbers of arbuscules and vesicles were lower with added P. We conclude that at very early stages of colonization P exerts its effect via reduced growth of infection units. After approx. 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.