Root starch reserves are necessary for vigorous re-growth following cutting back in Lotus japonicus

Perenniality and vegetative re-growth vigour represent key agronomic traits in forage legume (Fabaceae) species. The known determinants of perenniality include the conservation of the vegetative meristem during and after the flowering phase, and the separation of flowering from senescence. The ability of the plants to store nutrient resources in perennial organs and remobilize them may also play an important role in the perennial growth habit, and in determining the capacity of the plant to re-grow following grazing or from one season to the next. To examine the importance of stored starch, we examined the vegetative re-growth vigour following cutting back of a unique collection of Lotus japonicus mutants impaired in their ability to synthesize or degrade starch. Our results establish that starch stored in the roots is important for re-growth vigour in Lotus japonicus. We extended this analysis to a collection of Lotus (trefoil) species and two ecotypes of Lotus japonicus displaying a large variation in their carbohydrate resource allocation. There was a positive correlation between root starch content and re-growth vigour in these natural variants, and a good general correlation between high re-growth vigour and the perennial life-form. We discuss the relationship between perenniality and the availability of root carbohydrates for re-growth.