Salt Tolerance and Regulation of Enzymes of Starch Synthesis in Cassava (Manihot esculenta Crantz)

The growth rate of cassava plants (Manihot esculenta cv. MAUS7) decreased with increasing concentrations of NaCl from 0 to 75 mM in nutrient solutions supplied regularly in a porous growth medium in a glasshouse. Tuber weight was reduced to one-half between 30 and 50 mM NaCl and there was some burning of apical leaves at 50 and 75 mM NaCl. By comparison with other plants, this cultivar of cassava can be considered to have medium sensitivity to salinity. Na+ and Cl- concentrations increased in all tissues with increasing concentrations of supplied NaCl, except that Na+ remained low in laminae until the 75 mM treatment. K+ levels decreased in tubers. Starch concentrations remained the same in tubers, and K+ stimulated starch bound ADPglucose starch synthase by 1 .5-fold. Leaves and tubers contained activities of ADPglucose pyrophosphorylase and ADPglucose-starch synthase similar to those found in leaves and storage organs of other starch synthesizing plants. ADPglucose pyrophosphorylase from leaves was stimulated 20-fold by 3-phosphoglyceric acid (3PGA) while the enzyme from tubers was almost completely dependent on 3PGA at pH 8.5. The A0.5 values for 3PGA (the concentration required for one-half maximal activation) for the leaf and tuber enzymes at pH 8.5 were 1.31 mM and 7.41 mM respectively. At pH 7.5 the leaf enzyme was stimulated 26-fold and the tuber enzyme was again almost completely dependent on 3PGA. The A0.5 values at pH 7.5 were 1.17 mM and 3.8 mM, respectively. The I0.5 values for PI (concentrations required to cause 50% inhibition) in the presence of 3PGA were 2 mM, 0.25 mM and 0.04 mM for leaf enzyme at pH 8.5 and tuber enzyme at pH 8.5 and 7.7 respectively. The results support the view that it is not possible to generalize about the magnitude of the control of ADPglucose synthesis in leaves as opposed to non-chlorophyllous tissues.

Effect of Sodium Chloride on Expansion Rates and Invertase Activity of Leaves

The rate of expansion and the invertase (�-fructofuranosidase, EC 3.2.1.26) activity of leaves of two relatively salt-sensitive plants, Phaseolus vulgaris and Zea mays, decreased with increasing concentrations of NaCl from 0 to 50 mM in nutrient solutions supplied regularly in a porous growth medium. Sucrose concentrations in the leaves were higher and reducing sugar concentrations were lower at the higher concentrations of NaCl. Growth of leaves, invertase activity, and concentrations of reducing sugar and sucrose changed much less, if at all, with NaCl treatment of Hordeum vulgare, a reasonably salt-tolerant plant. Cellulase activities were not different in bean leaves from plants grown on different NaCl concentrations. The Km values for sucrose for invertase from bean leaves were 5.8, 8.1 and 10.4 mM for plants grown on 0, 20 and 50 mM NaCl respectively. NaCl at concentrations up to 200 mM did not affect the in vitro activity of invertase from barley, bean or maize leaves.