A set of lettuce cultivars, differing widely in nitrate accumulation in the shoot, was grown at different concentrations of nitrate in a nutrient solution and harvested at several plant ages. The results show that lettuce roots accumulate nitrate, and that nitrate content of roots is closely correlated with nitrate content of shoots. Genotypical differences in nitrate content were larger in shoots than in roots. Plant age had only a small effect on nitrate content. Nitrate content in the nutrient medium had no effect on shoot nitrate content, which indicates that nitrate uptake and transport to the shoot is regulated in such a way that the need for nitrate is exactly met. No evidence was found that roots play a regulating role in the process of nitrate accumulation under low-light conditions. A negative relationship between shoot nitrate content and shoot dry matter content and between shoot nitrate content and shoot organic-N content was found, but these relationships do not seem to be causal because one genotype combined a low nitrate content with a low shoot dry matter content and a low organic-N content. It is concluded that the cultivars tested probably differ in several physiological traits that influence the process of nitrate accumulation. More detailed research is needed to explain the large genotypical variation. Attention can then be focussed primarily on the characteristics of the shoot. © 1988.
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