Functional roles of ammonium (NH4+) and nitrate (NO3–) in regulation of day- and night-time transpiration in Phaseolus vulgaris
Mandilakhe Naku A , Learnmore Kambizi A and Ignatious Matimati
A B C
A Department of Horticultural Sciences, Faculty of Applied Sciences, Cape Peninsula University of Technology, PO Box 1906, Bellville 7535, South Africa.
B Office MB5-20N, Department of Horticulture, Durban University of Technology, ML Sultan Campus, Durban 4000, South Africa.
C Corresponding author. Email: ignatiousmatimati@gmail.com
Functional Plant Biology 46(9) 806-815 https://doi.org/10.1071/FP17179
Submitted: 23 June 2017 Accepted: 18 April 2019 Published: 20 May 2019
Abstract
Nitrogen may regulate transpiration and consequently the delivery of nutrients to root surfaces. It remains unclear whether both NO3– and NH4+ regulate transpiration and nutrient acquisition. In this study we investigated the functional role of NO3– or NH4+ in regulating both day- and night-time transpiration for potential ‘mass-flow’ delivery of nutrients. Phaseolus vulgaris L. plants were grown in troughs with a poly vinyl chloride (PVC) ‘root-barrier’ with a 25-µm mesh window designed to create an N-availability gradient by restricting roots from intercepting a slow-release NO3– or NH4+ fertiliser. ‘Root-barrier’ plants had their fertiliser placed at one of four distances behind the mesh from which nutrient acquisition was by diffusion or mass-flow. Control plants had direct access to fertiliser, termed ‘no root-barrier’ plants. NO3–-fed ‘root-barrier’ plants closest to the N source had 2-fold higher stomatal conductance, 2.6-fold higher transpiration, 1.8-fold higher night-time stomatal conductance, and 1.5-fold higher night-time transpiration than NO3–-fed ‘no-barrier’ plants, despite having comparable photosynthetic rates and biomass. Day- and night-time transpiration of NO3–-fed ‘root-barrier’ plants, however, was downregulated with further distance from the N source. All NH4+-fed plants displayed ammoniacal toxicity symptoms. NH4+-fed ‘no root-barrier’ plants had higher root biomass, 2-fold higher stomatal conductance and photosynthetic rate, 1.7-fold higher transpiration but had the same night-time stomatal conductance and transpiration compared with NH4+-fed ‘root-barrier’ plants closest to the N source. Unlike with NO3–-fed ‘root-barrier’ plants, NH4+-fed ‘root-barrier’ plants continuously increased their water fluxes with distance of N source. Thus, under N-limited conditions plants may be opportunistic in their water uptake, transpiring more when the water is available, in order to acquire nutrients through mass-flow. NH4+-fed plants prone to ammoniacal toxicity allocated their biomass towards the roots at non-limiting N levels, which may be linked to their extensive root system and enhanced photosynthetic rate. Thus, root hydraulic conductivity in response to NH4+-N requires further scrutiny, given that previous studies indicated that NH4+ may not alter the expression of root aquaporins or root hydraulic conductance.
Additional keywords: ammonium, daytime transpiration, mass-flow, night-time transpiration, nitrate, stomatal conductance.
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