Plant Water Uptake at the Single Plant Scale: Experiment Vs

Plant water uptake at the single plant scale: experiment vs. model David Matthew Deery Graduate Certificate in Mathematics, Charles Sturt University Bachelor of Applied Science Agriculture Honours, Charles Sturt University Bachelor of Applied Science Agriculture, University of Melbourne A thesis submitted for the degree of Doctor of Philosophy at Charles Sturt University August, 2008 Abstract The purpose of this study is to determine what is limiting the extraction by wheat roots of seemingly available water in the subsoil. The agronomic experience is that subsoil water may contribute significantly to final yield, but that the ability of the roots to extract the water varies from crop to crop in ways that are not well understood. The literature revealed several hypotheses as to why the extraction is incomplete. This study has focused on explicitly testing one of these hypotheses: that the soil is the main resistance to the extraction of water by the plant roots, owing to a combination of low root length density (unit length of root per unit volume of soil), low soil water diffusivity at low soil water content. To test this hypothesis wheat plants were grown in undisturbed and repacked field soil comprising two common soil types used for cropping, namely undisturbed and repacked clay-loam from south-eastern Australia and repacked sand from Western Australia. The plants were kept in a controlled environment where they were challenged with a range of evaporative demands, first rising and then falling, and the transpiration rate, E, and the null measurement of the xylem water potential, B, were measured non-destructively and continuously. The experimental measurements were compared to the output of a mathematical model that solves the radial diffusion equation for the flow of water to a single plant root, assumed to represent all roots. An important function for modelling the flow of water to a plant root is the soil water diffusivity in the range from 100 to 1500 kPa suction. A method was developed for measuring the soil water diffusivity in this range, that can also be used on undisturbed soil. For all soil types, during the rising phase of E, B as a function of E, B(E), was linear at low to moderate E, with a constant slope that represented the hydraulic resistance of the plant. However at high E, B accelerated with E. For the repacked clay-loam and the repacked sand, during the falling phase of E, B(E) was essentially linear over the ii whole range of E. This was in contrast to the undisturbed clay-loam, where B often curved downwards with decreasing E. For the repacked clay-loam and the repacked sand, the model could match the data during the rising phase of E, if it was assumed that only 10% of the roots were taking up water and that the soil water diffusivity was constant and low. However it could not match the data during the falling phase of E, unless it was assumed that there had been a significant rise in the hydraulic resistance of the plant, or perhaps more likely, that an additional, yet constant, interfacial resistance had developed when E was high and B was rapidly increasing. For the sand, the postulated interfacial resistance was about the same size as that within the plant and, for the clay-loam, the postulated interfacial resistance was about a quarter of that within the plant. That the slope of B(E) during the falling phase of E, for the repacked clay-loam and the repacked sand, was essentially constant suggests that the radial flow of water through the soil generated only minor gradients in soil suction and therefore that neither low soil water diffusivity nor low root length density was inhibiting the extraction of water from the soil by the plant roots. For the undisturbed clay-loam soil, the radial-flow model did not agree with the experimental data even when various combinations of soil water diffusivity and root length density were tried. This disagreement may have been due to a skewed distribution of roots in the cores, for few if any roots were seen at the base of any core. However, even when the roots were assumed to be confined to the top 50 or 75% of the total soil volume the model and experimental data did not agree. This work provides evidence that the flow of water to the plant roots, as encapsulated in the radial-flow model, is not inducing large gradients in suction close to the plant roots growing in the three soil types used in the experiments reported here. The clear disagreement between the experimental data and the model suggests that something else is generating the large hydraulic resistances evident between the soil and the leaves of the plants. iii Acknowledgements Firstly I thank my principal supervisor, John Passioura, who despite retirement status proved anything but and continually challenged me to do better. His vast and extensive technical expertise, guidance, dedication and patience is gratefully acknowledged here. I am grateful for the guidance provided by my university supervisors, Jason Condon and Asitha Katupitiya. I am grateful to the Cooperative Research Centre for Irrigation Futures for awarding me a PhD scholarship, without which this project would not have been possible. I am also grateful for the collegiate nature and network of the Cooperative Research Centre for Irrigation Futures that provided an extra tier of support. I am grateful to the CSIRO Water for Healthy Country flagship, for generously providing a PhD top up scholarship and operating funds. I am also grateful for support from the EH Graham Centre at Charles Sturt University. I thank my many colleagues at CSIRO Plant Industry Crop Adaptation. I have benefited greatly from many stimulating discussions and cherish the friendships that I have gained in this stimulating environment. I must also thank my friends for their unconditional friendship. In particular I am indebted for the support and understanding provided by my partner, Fi. Finally, I am forever indebted to my parents, Kevin and Veronica, and my siblings Rob, Chris, Bridget and Simon for their understanding and support. iv Commonly used nomenclature B null measurement of the xylem water potential equal to the pressure drop across the plant and soil, termed balancing pressure [kPa] B* half distance between biopores [m] D( θ) soil water diffusivity as a function of soil water content [m 2 s-1] D soil water diffusivity [m 2 s-1] Dref (θ) soil water diffusivity at reference temperature DT(θ) soil water diffusivity at temperature T E plant transpiration rate [m 3 s-1], [g s-1] or [ µg s-1], F* flux density of water per unit cross sectional area [m 3 m-2 s-1] F flux of water [m s-1] 3 -1 Fair flow rate of air through the cuvette containing the plant [m s ] g stomatal conductance [mol m-2 s-1] I electrical current [Amperes] i time point when two different calculations of the quantity of water lost from one-dimensional soil profile by evaporation or quantity of water taken up by the root from the cylinder of soil, of unit height, defined by ra and rb, are compared k soil hydraulic conductivity [m s-1] L length of sample [m] -3 Lv root length density (length of root per volume of soil) [cm cm ] * -3 Lv length of root occupied biopore per volume of soil [cm cm ] LA leaf area -1 Mm is the molar mass of water [g mol ] m shape coefficient for soil water retention curve N number of replicate samples used n particular node in finite difference grid and shape coefficient for soil water retention curve O rate of outflow (Passioura D( θ) analysis) patm atmospheric pressure (assumed to be 1013 hPa), Pdiff difference in vapour pressure between ingoing and outgoing air- streams from the cuvette [hPa] v ps saturation pressure of liquid [hPa] water at steam point temperature, Ts pw partial pressure of water vapour [hPa] p0 saturation vapor pressure over liquid water [hPa] at given temperature: Note that ratio of pw to p0 is the relative humidity ∆PPlant pressure drop across the plant [kPa] Q rate of extraction of water from the soil [m3 m-3 s-1] R universal gas constant: 8.31 [J K-1 mol -1] -1 RPlant resistance to flow within the plant [kPa s µg ] r radial distance from the root surface [m] ra radius of the root [m] rb radius of the outer boundary that the root is assumed to have exclusive access [m] SS R sum of the squared residuals svol volume of the pot containing soil and plant roots [m 3] T absolute temperature [K] Tref reference temperature o Ts steam point temperature [373.16 K at one atmosphere = 1013.246 hPa] t time [s] 1 1/2 t 2 square root time [s ] V voltage [volts] 3 -1 Vm molar volume of an ideal gas at the given temperature [m mol ] W quantity of water lost from the one-dimensional soil profile by evaporation [m] or quantity of water taken up by the root from the 3 cylinder of soil, of unit height, defined by ra and rb [m ] and amount of water remaining in the soil (Passioura D( θ) analysis) W0 initial quantity of water in the one-dimensional soil profile [m] or 3 initial quantity of water in the cylinder of soil defined by ra and rb [m ] x spatial position along length of sample [m] z depth [m] α inverse of air-entry value [kPa -1] for soil water retention curve θ soil water content [m 3 m-3] 3 -3 θa soil water content at root surface [m m ] vi 3 -3 θb soil water content at the outer boundary [m m ] 3 -3 θi initial soil water content [m m ] 3 -3 θf final soil water content [m m ] 3 -3 θL soil water content at x = L [m m ] 3 -3 θr residual soil water content [m m ] for soil water retention curve 3 -3 θs saturated soil water content [m m ] for soil

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