Effect of water stress on the phasic development of annual Medicago species

Abstract

Although annual medics (Medicago spp.) grow and persist in subtropical eastern Australia, their growth in the field is greatly affected by water stress, which can occur at any time after germination. To study this effect in some detail, the phasic development of six annual Medicago cultivars was measured under four water regimes in a glasshouse. Stress was varied by allowing the soil to dehydrate to different degrees before rewetting, the cycles being repeated continuously. Water stress delayed flowering in all medics except M. scutellata, the mean delay being 14 days. Corresponding delays occurred in the onset of pod production. In contrast the later stages of phasic development were accelerated in some species. Both the length of flowering and life span of the M. truncatula cultivars were significantly reduced by water stress. In the case of cv. Jemalong, severe water stress reduced its length of flowering by 53 days and its life span by 27 days. These results suggest that annual medics have no mechanisms for evading dry seasonal conditions by earlier flowering, but once flowering has begun phasic development is accelerated in some species by water stress. The implications of this study for the interpretation and prediction of field behaviour of annual medics are that water stress can affect phasic development mainly through a delay in flowering. However, the effects appear to be small compared with those due to vernalization, photoperiod and temperature.