Osmolyte allocation in response to Tylenchulus semipenetrans infection, stem girdling, and root pruning in citrus

Abstract

Previous studies indicated that Tylenchulus semipenetrans infection reduced concentrations of inorganic osmolytes, (Na+, Cl-, K+), in roots, along with leaf K+ in citrus. However, infection increased leaf Na+ and Cl-, along with carbohydrates in roots. Pruning of roots also increased carbohydrates in intact roots, whereas shoot pruning increased carbohydrates in shoots. Carbohydrates are translocated as reducing sugars, which collectively form organic osmolytes. Because changes in concentrations of osmolytes regulate osmotic potential in plant cells, we hypothesize that increasing concentrations of organic osmolytes in an organ displaces inorganic osmolytes. We measured the osmotic potentials of young citrus trees under nematode infection, stem girdling, and root pruning at two salinity levels. All treatments reduced leaf osmotic potentials at four sampling times. At harvest, 16 days after pruning and girdling treatments, organs with higher carbohydrates had lower inorganic osmolytes and vice versa, regardless of the treatment. Pruning simulated effects of nematode infection, whereas girdling reduced the effects of nematodes. Results suggested that high organic osmolytes in roots displace inorganic osmolytes, thereby avoiding very low osmotic potentials.