Leaf area, competition with grass, and clover cultivar: Key factors to successful overwintering and fast regrowth of white clover (Trifolium repens L.) in spring

The greater sensitivity of white clover (Trifolium repens L.) to low temper ature compared with perennial ryegrass (Lolium perenne L.) is a major probl em in sustaining the relative contributions to yield of the two species in mixed swards. The objectives of this investigation were to examine the dyna mics of leaf development of two white clover cultivars, AberHerald and Gras slands Huia. under field conditions, and to determine the significance of l eaf area in winter, and of competition by perennial ryegrass. for the overw intering and regrowth of white clover in spring. Undefoliated white clover plants developed 3.8-6.6 new leaves between late autumn and early spring, a nd stolon dry matter and total non-structural carbohydrates (TNC) content i ncreased by 262 and by 68 % respectively. In contrast, white clover plants that were defoliated frequently during the winter showed a 28 % decrease in stolon dry matter and an 82 % decrease in the content of TNC. Frequent def oliation in winter caused severe reduction in the rates of emergence of nod es (by 60 %) and of buds (by 67 %), and the rate of death of nodes and buds increased by a factor of 10, leading to small stolon systems of individual plants in spring. Competition had similar, but weaker, effects to those or winter defoliation, presumably caused by shading of white clover leaves. A berHerald had a higher cumulative leaf emergence (by 22 %). a higher mean l eaf number (by 23 %), a higher stolon DM (by 36 %) and a higher TNC content per plant (by 115 %) than Grasslands Huia. Results demonstrate the crucial positive role of leaf area during winter, the negative effect of grass com petition, and the importance of the clover cultivar, for the overwintering and subsequent spring regrowth of white clover. (C) 2001 Annals of Botany C ompany.