Effects of Soil Water Regime and Nitrogen Form on Blossom-end Rot, Yield, Water Relations, and Elemental Composition of Tomato1

Abstract Decreasing soil water potential (minima of −0.3, −2.0 and −6.0 bars) reduced fruit number, set, and mean and total fruit weight of tomato ( Lycopersicon esculentum Mill). Under the 2 wettest soil water regimes, NH 4 -N compared to NO 3 -N fertilization reduced total and mean fruit weights but increased fruit number whereas no differences in these variables were found between N-forms under the driest regime. Incidence and severity of blossom-end rot (BER) were increased by NH 4 nutrition and by decreasing soil water potential (SWP). Decreasing SWP either had no effect or increased leaf Ca, Mg, and K concentrations but decreased fruit concentration of these ions. At any soil water regime, NH 4 fertilization decreased leaf Ca and Mg concentration but generally increased leaf K and fruit Ca, Mg, and K concentrations. While BER incidence and severity did not appear to be related to fruit Ca, Mg, and K concentration, the disorder was associated with increased stylar to calyx fruit-half concentration ratios of these ions. Basal (pre-dawn) leaf xylem pressure potential ( ψ p ) was unaffected by N nutrition but was greater (less negative) under the wettest regime. Compared to plants supplied with NO 3 -N whose minimal and mean light-saturation ψ p values decreased with decreasing SWP, plants given NH 4 -N reached a constant ψ p level regardless of soil water regime. Since leaf diffusive resistance (R L ) values increased with decreasing SWP, but were unaffected by N form, the lower transpiration and transpiration rates under NH 4 -N might be explained by increased non-leaf resistances to water flux and/or by reduced soil-plant water potential gradients.