Biomass distribution and nitrate assimilation in response to N supply for Vitis vinifera L. cv. Cabernet Sauvignon on five Vitis rootstock genotypes

Effects of nitrogen (N) supply on biomass distribution as well as N effects on NO₃"assimilation, were examined in two-year-old graftlings of Vitis vinifera L. cv. Cabernet Sauvignon on five rootstocks. Whole-plant biomass in all graftlings more than doubled with increased N supply in solution from 0.25 to 8 mM. Whole plant biomass was also affected by rootstock genotype, but to a lesser extent than by N supply. Biomass allocation to roots declined with increased N supply for all stock-scion combinations, but the magnitude of that response varied with rootstock genotype. Nitrate reductase activity (NRA) in leaves increased with increased N supply for all stock-scion combinations, whereas root NRA increased only up to 1 mM N supply, dropping markedly with additional N. NRA in leaves was one to two orders of magnitude higher than NRA in roots - a difference that increased steadily with increased N supply. By implication, grapevine leaves have a much higher capacity for NO₃^-_- reduction than do grapevine roots, and any contribution by roots to whole-vine NO₃^-_- assimilation declines even further as NO₃^-_- availability increases.