PLANT PHYSIOLOGY , Vol 114, Issue 4 1307-1312, Copyright © 1997 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Leaf-Atmosphere NH3 Exchange in Barley Mutants with Reduced Activities of Glutamine Synthetase
M. Mattsson, R. E. Hausler, R. C. Leegood, P. J. Lea and J. K. Schjoerring
Plant Nutrition Laboratory, Department of Agricultural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark (M.M., J.K.S.)
Mutants of barley (Hordeum vulgare L. cv Maris Mink) with 47 or 66% of the
glutamine synthetase (GS) activity of the wild type were used for studies
of NH3 exchange with the atmosphere. Under normal light and temperature
conditions, tissue NH4+ concentrations were higher in the two mutants
compared with wild-type plants, and this was accompanied by higher NH3
emission from the leaves. The emission of NH3 increased with increasing
leaf temperatures in both wild-type and mutant plants, but the increase was
much more pronounced in the mutants. Similar results were found when the
light intensity (photosynthetic photon flux density) was increased.
Compensation points for NH3 were estimated by exposing intact shoots to 10
nmol NH3 mol-1 air under conditions with increasing temperatures until the
plants started to emit NH3. Referenced to 25[deg]C, the compensation points
were 5.0 nmol mol-1 for wild-type plants, 8.3 nmol mol-1 for 47% GS
mutants, and 11.8 nmol mol-1 for 66% GS mutants. Compensation points for
NH3 in single, nonsenescent leaves were estimated on the basis of
apoplastic pH and NH4+ concentrations. These values were 0.75, 3.46, and
7.72 nmol mol-1 for wild type, 47% GS mutants, and 66% GS mutants,
respectively. The 66% GS mutant always showed higher tissue NH4+
concentrations, NH3 emission rates, and NH3 compensation points compared
with the 47% GS mutant, indicating that NH4+ release was curtailed by some
kind of compensatory mechanism in plants with only 47% GS activity.