No abstract

Suaeda monoica Forssk. ex J. F. Gmel was found to possess the C4 pathway of photosynthesis. The succulent leaves of Suaeda lack a green bundle sheath formation but have a layer of chlorenchyma, containing large and centripetally arranged chloroplasts, which surrounds the water tissue. We suggest that the proximity of a chlorenchymatous cell layer to the vascular bundles is not necessary for the operation of the C4 pathway.Since the discovery of the C4 carbon fixation pathway (7,12), an increasing number of plants possessing this pathway have been recognized. It was found that in addition to the primary fixation of CO2 into 4-carbon acids, C4 plants possess certain other characteristics, i.e. a low CO2 compensation point (5), low discrimination against 'IC (3,17), and a specific anatomical leaf structure (9,13,14,20). The latter, frequently called the Kranz anatomy, consists of a layer of large cells which usually envelope the vascular bundles and which are densely packed with chloroplasts. This bundle sheath formation is surrounded by chlorenchymatous mesophyll cells. The existence of such differentiated chlorenchyma, with the inner layer attached to the vascular bundles, is commonly believed to be necessary for the operation of the C4 carbon fixation pathway.The characterization of C4 plants by the above mentioned features has been generally recognized. Since it was shown that the balance between C3 and C4 carboxylation could be altered by factors such as salinization, ontogeny, and leaf age (10,11,16), it seemed that the correlation between the appearance of a chlorenchymatous bundle sheath layer and the C4 carbon fixation pathway was not of a basic nature (4). The possibility of finding a C4 plant which lacks these typical structural features was therefore very likely. v. MATERIALS AND METHODS

Young leaves of salt‐depleted Aeluropus litoralis Parl. plants show CO2 fixation by the C3‐carbon fixation pathway. No detectable activity of phosphoenol pyruvate (PEP) carboxylase was found. When A. litoralis plants were exposed to a NaCl solution, the leaves showed a high activity of PEP carboxylase as well as a significant CO2 fixation by the C4‐pathway. — Also in Zea mays L. and Chloris gayana Kunth., the presence of NaCl in the medium influences the balance between phosphoenol pyruvate carboxylase and ribulose‐1,5‐diphosphate carboxylase.

The nitrogenous compounds N-methyl-L-proline (MP), trans-4-hydroxy-N-methyl-L-proline (MHP) and trigonelline (T), which undergo stress-induced accumulation in some Australian plants, were analysed and compared with proline (P) and glycinebetaine (B) for possible protective roles. The activity of pyruvate kinase (PK), prepared from Zea mays leaves and rabbit muscle, was unaffected even in the presence of 750 mM of the proline analogues. Thus, MP and MHP, like P and B, have the properties to act in vivo as compatible osmotica. T was not as compatible, decreasing enzyme activity 20% at 0.5 M. Like P and B, however, MP, MHP and T all also exhibited protective properties. They increased, in vitro, the thermal stability of PK from both plant and animal sources, and they protected PK (Zea mays) from salt inhibition at two substrate levels. The effect of salt on PK (Zea mays) was substrate dependent; at low phosphoenolpyruvate (PEP) levels, salt inhibited the enzyme activity, while salt effects were less severe in the presence of higher substrate levels. In the presence of high NaCl concentrations, the protective effects of high substrate levels and the compatible solutes seem to be additive. The Km (PEP) value of the plant PK increased in the presence of salt but the effect was ameliorated by the compatible solute MHP.

The effects of NaCl and other salts, in vivo and in vitro, on the activity of phosphoenolpyruvate carboxylase from the coastal C3 halophyte Cakile maritima Scop, were investigated. Plants grown with 100 mM NaCl in their growth medium yielded some 30% higher rates of phosphoenolpyruvate carboxylase activity than did salt‐depleted plants. Activity of the enzyme was stimulated when NaCl was added to the reaction mixture in concentrations of up to 200 mM. The magnitude of this in vitro stimulation was similar for plants grown in the presence or absence of NaCl. The effect seems to be caused by chloride rather than by sodium ions.