Premature Leaf Chlorosis in Cucumber Associated with High Starch Accumulation

Premature leaf chlorosis is a physiological disorder observed in winter-grown cucumber plants and is visually distinct from senescence-related chlorosis. Yellow leaves are characterized by high levels of starch and most of the soluble sugars, including glucose, fructose, galactose, galactinol and inositol. Levels of stachyose were found to be similar in healthy and chlorotic leaves. Electron micrographs of yellow leaves showed that plastids were filled with starch and that only traces of the thylakoid membrane structure remained. Starch levels in cucumber leaves followed a diurnal cycle and morning levels were higher following a low-night-temperature treatment (5 °C minimum) as compared with a high-night-temperature treatment (20 °C minimum). These results are consistent with the hypothesis that this form of chlorosis is due to an accumulation of photosynthate in the source leaf and an overaccumulation of starch in the chloroplast. Reduced export from the source leaf in response to low night temperature may be one of the factors responsible for the symptoms.