PARTIAL RECOVERY IN PLASTIDS OF LEMNA AEQUINOCTIALIS WELWITSCH 1073 TREATED WITH l‐METHIONINE OR l‐CYSTINE

When light‐grown plants of mutant 1073 of Lemna aequinoctialis Welwitsch were grown in a medium containing 31 μl‐cystine, the tubular clusters characteristic of this mutant disappeared in the mature fronds and chloroplasts and mitochondria appeared normal. l‐Methionine (2 μ) effected a similar but less dramatic change. In young fronds, however, 10% of chloroplasts when supplied with l‐cystine, and 50% when supplied with l‐methionine, still contained tubular clusters. When l‐cystine was supplied to young fronds of the wild‐type strain 6746, chloroplasts were normal but contained much larger starch inclusions. Relative growth rates of both strains were improved by addition of l‐cystine or l‐methionine. In mutant fronds they attained values equivalent to wild‐type control. This partial recovery from ultrastructural defects was not matched by a physiological recovery, i.e. plants were still incapable of photosynthesis and ammonium ion uptake, as were mutant plants in control media. Moreover when the partially recovered plants were tested for ammonium ion uptake, in the presence of either l‐methionine or l‐cystine, ammonium ions from plants moved out into the medium during the first 24 h, while increased ammonium uptake was detected with the wild strain.