'Hass' avocado carbohydrate fluctuations. I. Growth and phenology

Seasonal fluctuations in nonstructural carbohydrates (starch and soluble sugars) were studied in 'Hass' avocado (Persea americana Mill.) trees on 'Duke 7' rootstock over a 2-year period in southern California. On a dry weight basis, total soluble sugar (TSS) concentrations ranged from 33.0 to 236.0 mg.g(-1) dry weight and were high compared to starch concentration (2.0 to 109.0 mg.g(-1) dry weight) in all measured organs (stems,leaves, trunks and roots). The seven carbon (C7) sugars, D-mannoheptulose and perseitol, were the dominant soluble sugars detected. The highest starch and TSS concentrations were found in stem tissues, and in stems, a distinct seasonal fluctuation in starch and TSS concentrations was observed. This coincided with vegetative growth flushes over both sampling years. Stem TSS and starch concentrations increased beginning in autumn, with cessation of shoot growth, until midwinter, possibly due to storage of photosynthate produced during the winter photosynthetic period. TSS peaked in midwinter, while starch increased throughout the winter to a maximum level in early spring. A second peak in stem TSS was observed in midsummer following flowering and spring shoot growth. At this time, stem starch concentration also decreased to the lowest level of the year. This complementary cycling between stem TSS and starch suggests that a conversion of starch to sugars occurs to support vegetative growth and flowering, while sugars produced photosynthetically may be allocated directly to support flowering and fruit production.