Functional role of plant water fluxes in nutrient acquisition
Doctoral Thesis
2014
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University of Cape Town
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Transpiration is inevitable during photosynthesis; however, it may also function to cool leaves, transport nutrients and drive nutrient acquisition via mass-flow. In addition to transpiration, plants water fluxes occur through hydraulic redistribution (HR). I hypothesized that an important function of plant water fluxes is to drive mass-flow nutrient acquisition, with flux rates positively correlated with nutrient limitation but not deficiency. To test whether nutrient availability regulates mass-flow, Phaseolus vulgaris was grown with N placed at one of six distances behind a root-impenetrable mesh whilst control plants intercepted the N-source. In plants forced to acquire N through mass-flow transpiration rates were 2.9-fold higher and P and K accumulation was greater compared to control plants. The contribution of nocturnal transpiration and HR to nutrient acquisition was assessed by supplying Aspalathus linearis (N-fixer) with no fertilizer or Na¹⁵NO₃ and CaP/FePO₄ either above or below-ground with varying rates of below-ground irrigation. ²H₂O was used to trace HR. HR by A. linearis accounted for the bulk of surface soil moisture at dawn and responded positively to surface fertilization. In contrast, plants supplied below-ground fertilizer exhibited both HR and nocturnal transpiration with increased ¹⁵N and P acquisition. Finally, to establish whether clay fraction moderates mass-flow P availability, Triticum aestivum was grown with 0, 1, 5 or 10% (w/w) clay combined with either Ca-P, Fe-P or inositol-P. Transpiration and nutrient accumulation were monitored. Plants acquired P through massflow and diffusion. The acquisition of N and P increased from 0 to 5% clay (w/w) due to enhanced moisture retention, but further additions (10%) reduced P-availability (Inositol-P > Fe-P > Ca-P). Overall, this thesis explored and confirmed the relatively novel idea that nocturnal and diurnal transpiration by plants are not merely the consequence of stomatal opening for CO₂ acquisition. Rather nocturnal and diurnal transpiration are regulated by nutritional requirements and serves as a driving force for nutrient transport to roots. Likewise, hydraulic redistribution serves to draw water from deep and wet soil layers to the upper layers, which serves as a means to enable uptake of nutrients from the rich, but often dry, upper soil. Plants may thus be opportunistic in their water uptake, taking it up when it is available in order to improve the acquisition of nutrients through mass-flow delivery. Plants in low nutrient substrates elevated their water fluxes for mass-flow nutrient acquisition. Consequently, plants growing in mesic climates with low clay soils are likely to display greater dependence on mass-flow nutrient acquisition. This might vary between C₃ and C₄ plants, which differ in WUE. Plants may also increase mass-flow nutrient acquisition during inter-specific competition thus reducing investment in root proliferation for nutrient interception. Plants growing in elevated atmospheric [CO₂] with suppressed transpiration could show limited mass-flow nutrient acquisition.
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Matimati, I. 2014. Functional role of plant water fluxes in nutrient acquisition. University of Cape Town.