Short-rotation coppice Salix genotypes of differing biomass yields were studied over two growing seasons with the long-term aim of identifying traits definitive of high yield for the breeding of elite energy crops. In the first season, basic leaf and stem traits were measured in six Salix genotypes, to identify morphological characteristics associated with high biomass yields. Thereafter, S. viminalis L. 'L78183' (low yield) and the hybrid genotype S. schwerinii E. Wolf x S. viminalis L. 'Tora' (high yield) were compared. Maximum stem heights and stem diameters increased with biomass yield. 'Tora' produced more sylleptic branches on the leading stems than 'L78183'. Leaf traits differed significantly between the two genotypes: individual leaf area and cell number per leaf was greater in 'Tora', whereas cell area was greater in 'L78183', suggesting that final leaf areas were attained in 'Tora' through the production of many, small cells, and in 'L78183' through fewer, large cells. Leaf extension rates were higher in 'Tora' than 'L78183'. This result was mirrored for leaf production rate. Leaf area index, examined at two coppice stages, was higher in 'L78183' (values of 2.06 and 1.67) than in 'Tora' (maximum value 1.43) which had a very open canopy. Furthermore, A/C-i analysis revealed the low-yielding genotype as the most photosynthetically efficient at the individual leaf level whereas light response curves suggest that 'Tora' utilised light more efficiently. The results presented in this study suggest that leaf extension rate, final leaf size and cell number per leaf may be indicative of yield, and may be useful as selection criteria for potentially high-yielding hybrids for biomass use. (C) 2003 Elsevier Ltd. All rights reserved.