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 growing seasons, rain in summer may initiate seed germination and seedling emergence in annual clover-based pastures. If soil moisture becomes limiting and the seedlings die before the onset of winter rains then a 'false break' has occurred. The effect of false breaks on the formation of mycorrhizas in subterranean clover (Trifolium subterraneum L.) in the winter growing season was investigated. Undisturbed cores were collected in the summer from two annual clover-based pastures in Western Australia. 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 dry in glasshouse conditions. The second false break and the growing season were initiated after the complete death of plants that had emerged in the previous cycle. The growing season was simulated by watering all cores to field capacity and planting to a uniform stand of subterranean clover. Mycorrhizal colonization in the growing season was decreased by false breaks with little difference between one and two false breaks. In one soil, the percentage of the root length containing arbuscules (AC %) at 14 d after planting in the growing season was decreased from 52 to 33% by false breaks and at 42 d after planting from 61 to 46%. In the other soil, the AC % at 28 d after planting in the growing 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 breaks. Shoot phosphorus concentration at 14 d after planting and shoot dry mass at 42 d after planting were decreased by false breaks in only one soil.