Nitrogen regulation of transpiration controls mass-flow acquisition of nutrients.
Transpiration may enhance mass-flow of nutrients to roots, especially in low-nutrient soils or where the root system is not extensively developed. Previous work suggested that nitrogen (N) may regulate mass-flow of nutrients. Experiments were conducted to determine whether N regulates water fluxes, and whether this regulation has a functional role in controlling the mass-flow of nutrients to roots. Phaseolus vulgaris were grown in troughs designed to create an N availability gradient by restricting roots from intercepting a slow-release N source, which was placed at one of six distances behind a 25 μm mesh from which nutrients could move by diffusion or mass-flow (termed 'mass-flow' treatment). Control plants had the N source supplied directly to their root zone so that N was available through interception, mass-flow, and diffusion (termed 'interception' treatment). 'Mass-flow' plants closest to the N source exhibited 2.9-fold higher transpiration (E), 2.6-fold higher stomatal conductance (gs), 1.2-fold higher intercellular [CO2] (Ci), and 3.4-fold lower water use efficiency than 'interception' plants, despite comparable values of photosynthetic rate (A). E, gs, and Ci first increased and then decreased with increasing distance from the N source to values even lower than those of 'interception' plants. 'Mass-flow' plants accumulated phosphorus and potassium, and had maximum concentrations at 10mm from the N source. Overall, N availability regulated transpiration-driven mass-flow of nutrients from substrate zones that were inaccessible to roots. Thus when water is available, mass-flow may partially substitute for root density in providing access to nutrients without incurring the costs of root extension, although the efficacy of mass-flow also depends on soil nutrient retention and hydraulic properties.
Department of Biological Sciences, University of Cape Town, Private Bag X1, Rondebosch 7701, South Africa.
Ignatious
I
Matimati
G Anthony
GA
Verboom
Michael D
MD
Cramer
D001692:Biological Transport; D018533:Biomass; D002245:Carbon Dioxide / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism / Q000494:pharmacology*; D027805:Phaseolus / Q000254:growth & development / Q000502:physiology*; D010758:Phosphorus / Q000378:metabolism; D018515:Plant Leaves / Q000254:growth & development / Q000502:physiology; D018517:Plant Roots / Q000254:growth & development / Q000502:physiology; D054046:Plant Stomata / Q000254:growth & development / Q000502:physiology; D018526:Plant Transpiration / Q000502:physiology*; D011188:Potassium / Q000378:metabolism; D012987:Soil / Q000737:chemistry; D014867:Water / Q000378:metabolism*
D012987:Soil; D014867:Water; D002245:Carbon Dioxide; D010758:Phosphorus; D009584:Nitrogen; D011188:Potassium
Interception; phosphate; potassium; urea; water flux; water use efficiency.