OSMOTIC AND TOXIC ION EFFECTS ON SEEDLING EMERGENCE AND NUTRITION OF CITRUS ROOTSTOCKS

The effects of isosmotic solutions of sodium chloride (NaCl), polyethylene glycol (PEG), and mannitol with or without supplemental calcium (Ca) were compared to separate osmotic effects from toxic ion effects of salts on seed imbibition, seedling emergence, and the early stage of seedling growth of three citrus rootstocks. The rate of seed imbibition was relatively low for sour orange (SO) in PEG treatments which caused a delay in seedling emergence. These results of PEG supported earlier findings which suggested the existence of a critical level of hydration that seeds must reach before germination can proceed. Final percent emergence (FPE) was reduced the most by NaCl and mannitol compared with PEG. Furthermore, Ca addition to mannitol and NaCl solutions improved FPE in Cleopatra mandarin (CM) and trifoliate orange (TO). These results demonstrated that the effects of mannitol and NaCl on FPE went beyond the osmotic effects. Seedling growth was reduced the most by mannitol not only due to its uptake and transport to the leaves and stems where it caused dehydration and visible leaf injury but also to reduction in shoot Ca and potassium (K) concentrations. This study showed that the effects of salts on seed imbibition and time to emergence of the first seedling was primarily osmotic, while their effects on FPE and seedling growth was mainly toxic.