SUMMARY. A study was made of the osmotic adjustment of tomato plants (varieties Abunda, Alicante, Ace and Edkawy) to salt stress and the role of some putative compatible solutes in protecting against NaCl the activity of glutamine synthetase (GS) and malic enzyme extracted from tomato leaf. In all varieties shoot growth was more affected by NaCl than was root growth. Edkawy grew more than Ace irrespective of salt treatment. However, Edkawy growth was more affected than Ace growth.
Abunda and Alicante achieved complete osmotic adjustment in four weeks whereas Ace and Edkawy did not. Abunda and Alicante grown in saline conditions accumulated more sodium than Ace and Edkawy. Salt did not affect leaf potassium of Ace or Edkawy, whereas it decreased it in Abunda and Alicante. Both chloride and nitrate increased in Alicante leaves whereas phosphate and sulphate decreased.
Citrate and malate decreased in leaves of both Ace and Edkawy. Neither glucose nor fructose content in leaves of Ace was affected. Inositol was the only organic compound analysed which increased in response to salt treatment.
Only one isoform of GS was found in tomato leaf and evidence is given to suggest that this was the chloroplast form. The in vitro activity of GS was not affected when assayed in the presence of 500 mM sorbitol or betaine. Inositol caused a small but significant increase in GS activity whereas proline strongly inhibited it.
The activity of malic enzyme was much more affected by NaCl than was that of GS. Betaine, sorbitol, inositol or proline had no effect on malic enzyme activity, in the presence of 200 mM NaCl. In the absence of NaCl, proline caused a slight but significant inhibition of malic enzyme activity.
The inositol content of protoplasts isolated from mesophyll cells was 0.55 æmol/106 protoplasts. This value is much higher than the content of inositol in the bulk tissue of non-stressed plants.