INFECTIVITY OF ARBUSCULAR MYCORRHIZAL FUNGI AFTER WETTING AND DRYING

In a mediterranean environment, a hot and dry summer is followed by a cool and rainy winter, the growing season. Arbuscular mycorrhizal (AM) fungi survive in dry soil in the summer and are able to colonize newl y emerged plants in winter. However, late summer and autumn rains are frequently followed by periods of drought, resulting in the wetting an d drying of the soil before the onset of regular winter rains. The res ults of three experiments investigating the effect of wetting and dryi ng of soil on the subsequent infectivity of different AM fungi and the ir individual propagules are presented. In a first experiment, pot-cul ture inoculum of Acaulospora laevis Gerd. & Trappe, Glomus invermaium Hall, or fine endophytes, each containing a mixture of propagule forms , was mixed into pots of steam-sterilized soil with low phosphorus con tent. Pots were treated with a wetting and drying cycle where the soil was watered to field capacity for three consecutive days and then lef t to dry to a water content of less than 1.5% g g(-1). Infectivity was assessed in a subsequent growth cycle planted with clover. In a secon d experiment, the infectivity of spores of A. laevis, Glomus monosporu m Gerd. & Trappe or Scutellospora calospora (Nicol & Gerd.) Walkers & Sanders, and of dried mycorrhizal root fragments of G. invermaium or S . calospora was assessed after a wetting and drying cycle in which the soil was watered to field capacity for seven consecutive days before drying. In a third experiment, the infectivity of the extraradical hyp hae of G. invermaium after wetting and drying was assessed. AM fungi a nd their individual propagules responded differently to wetting and dr ying. The infectivity of pot-culture inoculum of A. laevis, G. inverma ium and fine endophytes was increased, decreased and unaffected, respe ctively, by wetting and drying. The infectivity of spores of A. laevis and G. monosporum was increased by wetting and drying but the infecti vity of spores of S. calospora was not affected. Infectivity of mycorr hizal root fragments of G. invermaium and S. calospora was decreased a nd increased, respectively, by wetting and drying. Finally, the infect ivity of the extraradical hyphae of G. invermaium was eliminated by a wetting and drying cycle. These results indicate that the development and function of mycorrhizas after late summer and early autumn rains m ay be limited by the occurrence and predominance of propagules of diff erent AM fungi.