Selection for crude protein content in Phalaris tuberosa L. III. Physiological relationships between crude protein content and seedling vigour.

Seedling growth of half-sib families from two lines of P. tuberosa previously selected for high and low herbage nitrogen contents respectively was examined in a glasshouse during the autumn and winter. Seedlings from the high nitrogen line were smaller than those of the low nitrogen line but had similar whole-plant relative growth rates. Differences in the rate of nitrogen absorption by the roots (h), and in the rate of seedling weight increase per unit of absorbed nitrogen (EN), accounted for most of the variation in herbage nitrogen content between families. The heights of seedlings were closely correlated with the weights of seeds from which they originated and with EN, and when these two characters were held constant the partial correlation between nitrogen content and seedling weight was negligible (r = -0.05). Among 10 clones differing markedly in herbage nitrogen content in the field, seed weight was closely correlated with the photosynthetic area above the flag leaf node available to each seed, particularly that contributed by the spikelet (r = 0.82***). Two alternative interpretations of this result are discussed. Relationships between nitrogen content, spikelet number, and the areas of photosynthesizing organs above the flag leaf node suggest that seed weight could be maintained during selection for increased nitrogen content by concurrently selecting either for large seeds, or for large flag leaves and a reduced number of spikelets per head. Even so, some loss of actual or potentially attainable seedling vigour seems almost inevitable because of the dependence of nitrogen content on IN and EN and because EN is an important determinant of vigour.