The ability of ryegrass (Lolium perenne L.) to take up
and utilize
aspartic acid (Asp) and serine (Ser), and the effect
of colonization of the roots by the arbuscular mycorrhizal (AM) fungus
Glomus fasciculatum (Thax. sensu Gerd.)
were studied. The seedlings were grown under controlled conditions
in a series of micro-lysimeters. All plants
were fed with a nutrient solution containing either nitrate, Asp or Ser
as the
sole N source. After 49 d, they were
supplied with 15N labelled nitrate, Asp or Ser for 1 h and harvested.
AM colonization increased the growth and total N content of the plants
in
all cases. Similarly, the amount of
Asp or Ser taken up was higher in AM than in control plants. There were
no
differences in biomass production
between the nitrate and Ser-fed plants. However uptake rates were lower
for
Ser than for nitrate. Growth of the
Asp-fed plants was significantly less than the other two treatments, and
uptake of
15N-Asp was lower than uptake of 15N-Ser. Analysis
of
15N incorporation into the amino acids extracted from the roots
suggests the hydrolysis of
Ser followed by re-assimilation of the resulting ammonia via the
GS-GOGAT cycle. There were no differences
in the patterns of accumulation of amino acids in the root-zone of control
and AM-ryegrass. The implication of
these results for the pathway of nitrogen transfer between plants is discussed.