Nutritional Response and Shoot/Root Ratio as Factors in the Composition and Yield of Genotypes of PerennialRyegrass, Lolium perenne L.

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Abstract
Three strains of perennial ryegrass known to give progeny widely varying in yield when grown as spaced plants in the field, were grown in solution culture at high and low levels of nutrition. It was found that although the order of yield remained the same as in the field, the actual relative difference in yield between the three strains was very small in solution culture. Of the factors studied which might be responsible for this wide variation in yield between the three strains when grown in the field, the most prominent were root yield and shoot/root ratio. It was concluded that the large root system of the highest yielding strain enabled it to take advantage of the large volume of soil available for exploitation in the field. In nutrient solution root size was of little importance and the difference in yield between the strains was correspondingly small. Data on factors influencing shoot/root ratio show that low nutrition increased root production and that increase in nitrogen nutrition lead to an increased shoot/root ratio, primarily due to enhanced shoot growth. Plants grown without aeration have higher shoot/root ratios than those grown with aeration. Although there was a lower weight of roots in the non-aerated treatments the actual number of roots was very significantly increased, the roots being finer. The number of tillers was greater in aerated than in non-aerated solution, and the difference was most marked with low nitrogen. It was demonstrated that shoot/root ratio varied widely between genotypes, and that parent/progeny correlation for this ratio was high (r = 0.83 r). A concept of shoot/root relationship is suggested, visualizing the shoot/root ratio as primarily representing the net product of the differential distribution of carbohydrates between root and shoot. This distribution is genetically controlled but is subject to modification by external factors such as nutrition. The problem of the relative ‘efficiency’ of different varieties is considered, particularly with respect to nitrogen utilization.