Senescence, turion development, and turion germination in nitrate- and sulfate-deficient Spirodela polyrhiza. Relationships between nutrient availability and exogenous cytokinins

Senescence, turion development, and turion germination were investigated in NO3-deficient (1/20 Hoagland's nitrate) and SO4-deficient (1/200 Hoagland's sulfate) Spirodela polyrhiza L. (Schleiden) grown in continuous light at 25 °C. These deficiencies resulted in frond senescence and the development of turions. Supplements of nitrate or sulfate reversed parent frond senescence and initiated frond growth in primordia not committed to turion development. Amino acids normally derived from nitrate and sulfate partially reversed the deficiency-induced symptoms of senescence. Turions formed in NO3-deficient media germinated in media containing 1 mM nitrate, glutamate, aspartate, and glycine, respectively, providing dormancy was broken by osmotic treatment. Similarly, turions formed in SO4-deficient media required 1 mM sulfate or methionine for germination. Exogenous isopentenyl adenine and benzyl adenine (5 or 0.5 mg/L) were able to overcome turion dormancy and stimulated germination in the presence or absence of exogenous salts. In addition, these cytokinins prevented turion development in NO3- and SO4-deficient cultures, although their effect on senescence, as judged by chlorophyll levels, was minimal. Analysis of protein-bound amino acids in control and NO3- and SO4-deficient plants showed that deficient fronds contain significantly lower levels especially of the sulfur amino acids. The concentration of free amino acids decreased in NO3-deficient fronds but increased, relative to controls, in SO4 deficiency.