GLUTAMINE-SYNTHETASE ACTIVITY, AMMONIUM ACCUMULATION AND GROWTH OF CALLUS-CULTURES OF ASPARAGUS-OFFICINALIS L EXPOSED TO HIGH AMMONIUM OR PHOSPHINOTHRICIN

Glutamine synthetase (GS) activity, ammonium accumulation and growth r esponses of callus cultures of Asparagus officinalis L, were investiga ted following 4 weeks exposure to media with added ammonium, and again after a further 4 weeks on a modified basal medium (MBM) containing n o added ammonium. Calli grown on MBM supplemented with 40 or 160 mM am monium for 4 weeks had reduced GS activity, greatly enhanced ammonium content and reduced growth compared with calli exposed to 10 mM added ammonium. When calli were transferred back to MBM, GS activity increas ed, ammonium content decreased, and growth was enhanced. Phosphinothri cin (PPT) was used to endogenously alter the ammonium content of callu s tissue. Exposing calli to 10 or 100 mu M PPT for 4 weeks reduced GS activity, enhanced ammonium accumulation and reduced growth compared w ith calli not exposed to PPT, and markedly enhanced callus glutamine c ontent. Calli exposed to 100 mu M PPT did not regrow when transferred back to basal media (BM) without added PPT for an additional 4 weeks. We also separated the effect of PPT-induced ammonium accumulation from alterations in tissue amino acid concentrations by adding up to 25 mM glutamine in addition to PPT during the initial 4-week treatment peri od. Glutamine supplementation overcame the PPT-induced reduction in gr owth even though GS activity was severely reduced and ammonium accumul ated to high concentrations in calli exposed to 100 mu M PPT. All call i continued to grow vigorously when transferred back to BM for an addi tional 4 weeks. The results demonstrate that ammonium accumulation per se was not lethal to asparagus callus tissue, and suggest other effec ts of using PPT as a selective inhibitor of GS.