Amino acid content of water-stressed plantlets of Populus tremuloides clones in relation to clonal susceptibility to Hypoxylon mammatum in vitro

Plantlets from five clones of Populus tremuloides were cultured in media at three different osmotic potentials to assess the effects of water stress on amino acid content. Differences in amino acid content were then related to differences in susceptibility (determined in a previous study) of clones to Hypoxylon mammatum. Changes in amino acid concentrations varied among the amino acids. Alanine, arginine, asparagine, glutamine, glutamate, and proline increased generally but not consistently among all clones. Comparisons among the clones for these six amino acids showed significant variations. Proline, which has been shown to accelerate radial growth of H. mammatum more than other amino acids, increased progressively with increased water stress in all clones. Levels of proline among the clones were correlated (r = 0.89) with susceptibility to infection and colonization of the clones by H. mammatum. Alanine and glutamine were also similarly correlated with infection rating. However, one clone, which grew poorly in culture, did not show this relationship. In comparison with the other clones, it had low levels of all amino acids, which did not increase significantly in response to water stress. Yet in the inoculation study, it had the highest infection rating at all stress levels, suggesting that other stress-induced factors are also important. These results suggest that nitrogen status of some aspen clones under water stress may increase their susceptibility to hypoxylon canker.