Genotype variation in the root system of Betula pendula roth
30 clones of Betula pendula were produced using micro-propagation techniques and screened for root system variation. Shoot variation was assessed and the root systems divided into 3 root types; fine, woody, and non-woody. Dry weights, lengths, and various ratios were used in the analysis. There were differences in all the characteristics measured. Root system size could not usefully be predicted from shoot size. Four clones with contrasting root systems were chosen for further experiments. A sequential harvest experiment showed that all the characteristics changed with time, and that the clones grew and developed at different rates. Root distribution and development differed between the clones. A water status experiment indicated differences in rooting depth distribution which could be related to drought avoidance, but no growth differences were found. Two different nitrogen levels were used to show that root system differences had little effect on nitrogen uptake. One clone exhibited growth patterns which suggested that it would grow well in areas of chronic nutrient deficiency. The clone responses to phosphorus deficiency and mycorrhizal infection were also tested. One clone again showed growth patterns resembling those found in areas of chronic nutrient deficiency. Another clone responded poorly to mycorrhizal growth at high phosphorus concentrations. Ranking comparisons to the ranks found during the sequential harvest experiment showed that size was more important than age in determining development. Clone 14 seemed to be less plastic than the other clones, and shoot height and structural root production were controlled more by genotype than environment. The methods used, and some useful characteristics, were discussed in the context of tree selection programmes. Many suggestions for future work are given. These suggestions work towards the ultimate aim of the project to select root system types specifically suited to different environments.