Floral bud-break of 'Sungold' nectarine plants held for 5 chilling durations at chilling temperatures of 10°C plus 2 weeks at 30°C and continuous 10°C 4. Temperaturechill unit model for 'Sungold' nectarine (550 hr) with Utah high chilling model for comparison 5. Average weekly scaffold temperatures of control and sprinkled 'Sungold' and 'Sunlite' nectarine trees 6. Floral and vegetative bud-break on 'Sungold' nectarine trees, 1978-79 7.
Vegetable and citrus production in west‐central Florida is reportedly of considerable eutrophication hazard to local groundwater and surface‐water bodies, including a 33 000‐ha drinking‐water supply reservoir (Lake Manatee) near the coast of the Gulf of Mexico. Nitrate‐N levels were assessed for three vegetable‐production seasons during 1990 and 1991 using a combination of multilevel samplers into the shallow (surficial) aquifer beneath selected vegetable fields and citrus groves, coupled with piezometric wells around each field's periphery to assess direction and rate of groundwater flow. The NO3‐N concentrations beneath vegetable sites showed sizeable spikes (up to 130 mg L−1 NO3‐N), especially early in the season and at season's end, though almost exclusively at <1‐m depth. These concentrations did not persist, however, with the overwhelming majority of samples from most vegetable sites evidencing NO3‐N concentrations below 1 mg L−1. At citrus sites from the same area, NO3‐N concentrations in the surficial water table (located 2–4 m below the soil surface in this case) have evidenced high (20–40 mg NO3‐N L−1) and persistent levels throughout the entire sampling period. This zone of NO‐N enrichment commonly extends 2 to 3 m into the surficial aquifer before NO3‐N concentrations decline to <10 mg L−1 once more. Rates of lateral movement off‐site averaged 20 to 40 m yr−1 for the vegetable sites and 200 to 400 m yr−1 for the more undulating citrus sites. It is postulated that gaseous denitrification losses naturally remediate the periodic spikes in NO3‐N concentrations beneath the vegetable sites because of the high water tables maintained at such locations, whereas denitrification in the deep sands beneath typical citrus sites proceeds much more slowly due to such limitations as a soluble‐C energy source and/or a suitable microbial population. The results have implication with respect to shallow water table vs. deeply rooted crop‐production systems from the same geographic area, wherever such pairings may occur.
Floral bud break of 1-year-old rooted cuttings of ‘Sungold’ nectarine (Prunus persica (L.) Batsch) was observed following chilling at constant and diurnal temperature regimes. Continuous exposure to 10°C was as effective as 7°. Rate of bud break increased as chilling increased up to 750 hours. Floral bud break of plants exposed to 14 days at 30° during the middle of the chilling period was more rapid but failed to reach the level of activity of plants exposed to constant temperatures. A chill unit model developed for ‘Sungold’ nectarine which has a chilling requirement of 550 hours indicated a broader range of effective temperatures and a higher optimum for rest completion as compared to the Utah model and predicted rest completion more accurately than other methods when applied to orchard temperature data.
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