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Xuan Liu, Paul W. Robinson, Monica A. Madore, Guy W. Witney and Mary Lu Arpaia

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.

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Xuan Liu, Paul W. Robinson, Monica A. Madore, Guy W. Witney and Mary Lu Arpaia

Changes in soluble sugar and starch reserves in avocado (Persea americana Mill. on `Duke 7' rootstock) fruit were followed during growth and development and during low temperature storage and ripening. During the period of rapid fruit size expansion, soluble sugars accounted for most of the increase in fruit tissue biomass (peel: 17% to 22%, flesh: 40% to 44%, seed: 32% to 41% of the dry weight). More than half of the fruit total soluble sugars (TSS) was comprised of the seven carbon (C7) heptose sugar, D-mannoheptulose, and its polyol form, perseitol, with the balance being accounted for by the more common hexose sugars, glucose and fructose. Sugar content in the flesh tissues declined sharply as oil accumulation commenced. TSS declines in the seed were accompanied by a large accumulation of starch (≈30% of the dry weight). During postharvest storage at 1 or 5 °C, TSS in peel and flesh tissues declined slowly over the storage period. Substantial decreases in TSS, and especially in the C7 sugars, was observed in peel and flesh tissues during fruit ripening. These results suggest that the C7 sugars play an important role, not only in metabolic processes associated with fruit development, but also in respiratory processes associated with postharvest physiology and fruit ripening.

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Etaferahu Takele, Jewell L. Meyer, Mary L. Arpaia, David E. Stottlemyer and Guy W. Witney

The effect of integrated applications of various irrigation and fertilization rates on productivity (yield and size) and returns of the `Hass' avocado (Persea americana Mill.) have been analyzed from 1987 to 1991 in western Riverside County. Eighteen treatment combinations comprised of three irrigation levels [80%, 100%, and 120% crop water use (ETc)], three N fertilizer levels (0.16, 0.7, and 1.4 kg/tree per year), and Zn (0 and 0.2 kg/tree per year) were included in the analysis. Using a partial budgeting procedure, returns after costs were calculated for each treatment combination. Costs of treatments, harvesting, hauling, and marketing were subtracted from the value of the crop. The value of the crop was calculated as the sum of crop returns in each size category. Three years of data on the relationship between irrigation and N showed 1) irrigating at 80% ETc would be ineffective even at very high water prices; 2) for groves where 100% ETc is sufficient, its application with either low or medium N would be beneficial; and 3) at higher irrigation (120% ETc), N application should be at or beyond the medium level.