Effect of Root Temperature on Dry Matter Distribution, Carbohydrate Accumulation, and Acetylene Reduction Activity in Alfalfa and Birdsfoot Trefoil 1

Although birdsfoot trefoil ( Lotus corniculatus L .) is a legume better adapted to poor or marginal soils than is alfalfa ( Medicago saliva L .), reduced carbohydrate storage in roots and increased susceptibility of trefoil to roots and crown diseases in warm climates results in poor persistence. The objective of this study was to examine the effect of high root temperature on carbohydrate content and acetylene reduction activity in alfalfa and trefoil during regrowth. A total of six samplings of each species grown at 16 and 30 C root temperatures were made at flowering and 2 weeks later during regrowth. Dry matter distribution, total nonstuctural carbohydrate (TNC) accumulation, N concentration, and acetylene reduction activity of roots were measured in plants grown in sand with nutrient solution. High root temperature resulted in significantly reduced total plant or root dry weight only in alfalfa at flowering. However, trefoil root TNC was significantly lower at 30 C at both growth stages and was only 4.3% on a dry weight basis 2 weeks after cutting. It appeared that trefoil root carbohydrate concentration is influenced by root temperature as well as air temperature, thus helping explain why trefoil often fails to persist in warm climates. Both total plant N and rate of acetylene reduction at flowering were much greater for alfalfa than for trefoil. Rates of acetylene reduction were significantly reduced by high root temperature only in alfalfa. Total plant N was affected much less than was acetylene reduction activity at 30 C in both species.