A tissue analysis technique was used to evaluate nitrogen and phosphorus supplies in natural waters for the growth of angiosperm aquatic plants. Tissue content of nitrogen and phosphorus was employed as an index of element availability in lakes from which plants were collected.This required establishment in the laboratory of the critical level for each element, that is, the minimum tissue content associated with maximum growth.To establish critical levels, a system was developed for culturing algae-free plants in a synthetic nutrient medium.The critical nitrogen content for the several species studied was approximately 1.3%; the comparable phosphorus value was 0.13%. The nitrogen and phosphorus contents of 13 species of aquatic plants collected during the summer from nine lakes were compared with the critical concentrations.In nine samples obtained mostly during periods of heaviest plant growth, the phosphorus content was at or below the critical level; in no sample was the nitrogen content less than the critical concentration.The results indicated that in all but one of the lakes, phosphorus supply was more likely to limit higher aquatic plant growth than was nitrogen.The primary importance of this study is as an initial step in the development of a satisfactory technique for evaluating nutrient supplies in natural waters.

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SummaryCritical factors in the selection of appropriate screening procedures to detect different phenotypic responses to nutrient-deficiency stress are discussed. Various morphological, anatomical, and physiological plant factors responsible for adaptations to nutrient deficiency, particularly low-P stress, are reviewed. Also, the relative effectiveness of various screening culture techniques for detecting phenotypic efficiencies based on specific plant features are considered.The relative ineffectiveness of liquid culture media in detecting plant factors critical in P acquisition from low-P natural environments is recognized, and a culture medium that is effective under these conditions is described. P adsorbed onto alumina, after mixing with coarse sand, serves as a P source in nutrient cultures. Buffered P concentrations approximating soil solution concentrations are maintained in this system, and P availability at the root surface seems diffusion-limited. With this system, significant differences in the growth of tomato strains under P stress were detected.The desirability of screening phenotypes at the same degree of depression from maximum yield (equivalent deficiency stress) is discussed. The need for evaluations at equivalent stress is associated with the capacities of plants in general to respond to deficiency stress with morphological and physiological changes that may not be under genetic control, for example an increase in root:shoot ratio. Additional capacity to adjust the same plant factors often are characteristic of specific phenotypes. The relative growth of the same tomato strains under equivalent and non-equivalent P-deficiency stress is compared. Significant strain differences were observed under both conditions. However, the relative responses among strains for several efficiency parameters were very different under the two types of stress.

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