Nitrogen dynamics and fertilizer use efficiency in leaves of different ages of sugar beet (Beta vulgaris) at variable water regimes

The consequences of nitrogen and water stress on sugar beet yield and quality are well documented in literature, whereas their interaction has been, surprisingly, little investigated. In the present research, the influence of early and late water stress on the dynamics of N uptake in leaves of different ages and on fertilizer efficiency was studied. Sugar beet plants, grown with a ¹⁵N-labelled fertilizer in 20 soil columns (300 L each) under a rain shelter, were subjected to three water regimes: well-watered control (WW); early (S1) and late (S2) stress, in a completely randomized design. Periodical samplings on succeeding leaves showed a decline in nitrogen concentration with ageing, compensated for by a recovery at each new leaf. Fertilizer N was mostly taken up early in the season, as its falling contribution to total leaf nutrient shows (from 35% to 17% in 63 days). The early stress constrained the availability of fertilizer N, whose uptake was later compensated for by younger leaves, after restoration of soil moisture. At harvest, WW yielded about 20% more biomass and sugar than S1 and S2. Nitrogen concentration in stressed plants compensated for biomass losses, and the total amount of nutrient did not vary among treatments, as well as the recovery of fertilizer N (41%) and the share of plant nutrient deriving from fertilizer (16%). It is perceived that sugar beet possesses an ability to buffer the effects of water stress on nitrogen nutrition, thanks to a flexibility concerning timing and the leaves acting as sinks for either a fertilizer-derived or soil-derived nutrient.