PLANT PHYSIOLOGY , Vol 105, Issue 4 1159-1166, Copyright © 1994 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Nitrogen Source Regulation of Growth and Photosynthesis in Beta vulgaris L
T. K. Raab and N. Terry
Department of Plant Biology, University of California, Berkeley, California 94720
Sugar beets (Beta vulgaris L. cv F58-554H1) were grown hydroponically in a
16-h light, 8-h dark period at a photosynthetic photon flux density of 0.5
mmol m-2 s-1 for 4 weeks in half-Hoagland culture solution containing only
nitrate-nitrogen. Half of the plants were then transferred to half-Hoagland
solution with ammonium-nitrogen (7.35mM), while the other half continued on
7.5 mM nitrate. Growth analysis was carried out by sampling the plants at
3-d intervals over a period of 21 d. Compared to plants supplied with
nitrate, ammonium initially slowed the growth of shoots more than roots.
Ammonium reduced both the area expansion of individual leaves and the
relative water content of these leaves, but increased the amount of dry
matter/area. The increase in specific leaf weight in ammonium-grown leaves
was associated with a doubling of chloroplast volume, as much as a 62% rise
in chlorophyll content, and a 4.3-fold higher accumulation of soluble
protein. Ammonium nutrition substantially decreased the rate of expansion
of photosynthetic (leaf) surface but did not decrease the rate of
photosynthesis per area; in fact, net photosynthetic CO2 exchange rates
were slightly higher than in nitrate plants, due to the build-up in stromal
enzymes of the Calvin cycle, several of which increased in total
extractable activity on a leaf area basis, e.g. ribulose-1,5- biphosphate
carboxylase oxygenase, sedoheptulose-1,7-biphosphatase. Nitrogen source had
no effect on stomatal conductance. Rates of photosynthesis per chlorophyll
were decreased slightly in ammonium-grown leaves, possibly due to an
increased CO2-diffusion resistance associated with the enlarged
chloroplasts.
