NITROGEN-FERTILIZATION OF WHITE RIESLING GRAPES IN WASHINGTON - NITROGEN SEASONAL EFFECTS ON BUD COLD-HARDINESS AND CARBOHYDRATE RESERVES

A commercial vineyard of White Riesling was used to study the influence of 0, 56, 112, and 224 kg N/ha on bud cold hardiness and carbohydrate reserves of bud and cane tissues over a three-year period. High N had significant effects on cold hardiness of buds, levels of soluble sugars and starches extracted from one-year-old bud or cane samples, only on a limited number of dates during the three years of the study. There was a positive relationship between bud hardiness and soluble sugars from bud or cane samples that was highly significant. Regression analysis of yearly data indicated there was no relationship between nitrogen level and either soluble sugars or starch in buds or canes; There was a positive relationship between air temperature and bud low temperature exotherms during the sampling period of each year, and an inverse relationship between air temperature and soluble sugar levels in both buds and canes. These data show the predominant role of air temperature on bud and presumably vine cold hardiness and the attendant changes in carbohydrate reserves. This study further demonstrates that under otherwise good management practices of pruning, cropload, irrigation, and rootstock selection, there should be little concern regarding a detrimental influence of nitrogen applied before harvest on cold hardiness or carbohydrate reserves of grapevines.