Pressure-induced Water Fluxes Through Root Systems of Sour Orange Seedlings

Abstract

Abstract Studies were conducted to compare responses of pressure- and transpiration-induced water fluxes (Jv) through root systems of sour orange (Citrus aurantium L.) (SO) seedlings and to quantify the apoplastic component of fluxes in the both systems. Roots excised from water-cultured SO seedlings were sealed in a nutrient solution-filled pressure chamber and pressurized at 0.5 MPa with air, N2, or CO2. Changing the pressurizing gas from air to N2 or CO2 reduced Jv, but the response to CO2 was more pronounced than to N2. The fluxes recovered when N2 and CO2 were in turn replaced by air. Transpiration of water-culture-grown intact SO seedlings responded similarly to changes in gasses bubbled through the nutrient solution. The Jv in excised root systems was decreased at elevated salt concentration in the root medium, a phenomenon commonly observed in intact plants. Root conductance taken from the slopes of the linear portion of pressure/flux curves was lower at low than high salt levels. The apoplastic water fluxes through pressurized roots and intact SO seedlings was quantified using trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate. The increased proportion of the apoplastic component in pressure-, compared to transpiration-induced, fluxes indicates that root pressurization opens one or more new avenues for water transport that are not operative in a transpiring seedling. Steady-state flow rates through the roots pressurized at 0.5 MPa were attained within ≈3 hr.