Chloride and carbonate salinity tolerance in Mimusops zeyheri seedlings during summer and winter shoot flushes

Shoot flushes alternate with root flushes and the evergreen red milkwood (Mimusops zeyheri Sond.) fruit tree has winter (May–July) and summer (October-December) shoot flushes in southern hemisphere. Fruit of this plant contain high vitamin C and the tree is being researched and bred for arid inland southern Africa regions. Climate change predictions suggested that by 2030 the regions would experience high temperatures (>45°C), recurrent floods and repeated droughts, which are associated with increased salinity challenges. The objective of this study was to determine the degree of salt tolerance in M. zeyheri seedlings to chloride and carbonate salinity during summer and winter shoot flushes under microplot conditions. Treatments, comprising untreated control, NaCl, KCl, CaCl₂ and Na₂CO₃ each at 1.0 molar (M) solutions, were applied weekly for eight weeks. During both seasons, CaCl₂ and Na₂CO₃ salinity had significant effects on soil pH and EC. During summer, salt types increased (106%–253%) dry shoot mass and reduced (53%–79%) root/shoot ratio. During winter, Na₂CO₃ salinity increased (114%) dry shoot mass, but the effects were not different to those induced by NaCl and CaCl₂ salinity. All other plant growth variables were not affected by salt type. Salt type had significant effects on assimilation of selected nutrient elements in leaf tissues of M. zeyheri seedlings during winter. Sodium chloride did not have significant effects on nutrient elements, whereas KCl, CaCl₂ and Na₂CO₃ significantly affected selected macronutrient elements and Zn. In conclusion, effects of chloride and carbonate salinity on M. zeyheri seedlings were both season- and salt type-specific, with seedlings displaying some degree of salt tolerance to chloride and carbonate salinity.