THE EFFECT OF RAIN IN THE DRY-SEASON ON THE FORMATION OF VESICULAR-ARBUSCULAR MYCORRHIZAS IN THE GROWING-SEASON OF ANNUAL CLOVER-BASED PASTURES

In a mediterranean climate with hot dry summers and cool winter growin g seasons, rain in summer may initiate seed germination and seedling e mergence in annual clover-based pastures. If soil moisture becomes lim iting and the seedlings die before the onset of winter rains then a 'f alse break' has occurred. The effect of false breaks on the formation of mycorrhizas in subterranean clover (Trifolium subterraneum L.) in t he winter growing season was investigated. Undisturbed cores were coll ected in the summer from two annual clover-based pastures in Western A ustralia. The cores were treated with one or two false breaks, or left dry. For each false break, 3 centimetres of water were applied to the surface of the core over a 3 day period and then the cores were let d ry in glasshouse conditions. The second false break and the growing se ason were initiated after the complete death of plants that had emerge d in the previous cycle. The growing season was simulated by watering all cores to field capacity and planting to a uniform stand of subterr anean clover. Mycorrhizal colonization in the growing season was decre ased by false breaks with little difference between one and two false breaks. In one soil, the percentage of the root length containing arbu scules (AC %) at 14 d after planting in the growing season was decreas ed from 52 to 33% by false breaks and at 42 d after planting from 61 t o 46%. In the other soil, the AC % at 28 d after planting in the growi ng season was decreased from 72 to 55% by false breaks. Assessment of the morphology of the mycorrhizas indicated that colonization by fine endophytes was decreased but colonization by Acaulospora spp., Glomus spp. and Scutellospora and Gigaspora spp. was not decreased by false b reaks. Shoot phosphorus concentration at 14 d after planting and shoot dry mass at 42 d after planting were decreased by false breaks in onl y one soil.