Plants require large amounts of nitrogen; yet its availability is frequently limited. How effectively plants obtain and process nitrogen is therefore very important. Some of our recent work has examined glutamine synthetase (GS) and nitrate reductase (NR), two enzymes important in nitrogen assimilation, especially with respect to their activity in different plant parts at different developmental stages. Relative activities of GS among organs of Pisum sativum L. change dramatically during various developmental stages; portions of developing pods show particularly high activity. NR activity in Typha latifolia L. is very sensitive to the nitrogen source in the medium, suggesting an ability to adapt to changing nutritional conditions. Responses to varying combinations of ammonia and nitrate suggest that cattails may possess both inducible and constitutive forms of NR.
We have used several species of Lemna (duckweeds, growth conditions of which are easily manipulated in culture) to seek clues as to possible mechanisms involved in the control of flowering. We have found responses to amount and type of nitrogen and supplemental carbon sources and to medium pH, and unpredictable relationships between flowering and vegetative growth under various conditions.
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