Diurnal Variation in Water Potential and Xylem and Phloem Transport. Martin P.N. Gent Connecticut Agricultural Experiment Station, New Haven CT

Transcription

1 Diurnal Variation in Water Potential and Xylem and Phloem Transport Martin P.N. Gent Connecticut Agricultural Experiment Station, New Haven CT

2 Nitrate Sugar. Schematic of short distance diffusion of water and transport of metabolites among compartments in one tissue. Long distance transport Apoplast Nitrate Cytoplast Sugar Nitrate + Sugar Growth Water Water Xylem Short-distance diffusion among compartments Active transport across membranes Phloem

3 Short distance transport Tissues are composed of compartments: cytoplast, xylem,, and apoplast. Water and solute flux between cellular compartments within one tissue or organ is a rapid process, governed by diffusion and/or active transport across membranes permeable to water. This equilibrates water potential among the compartments. All transfers of water and solutes to and from xylem,, or cytoplast, are through the apoplast. Water flux between two compartments is proportional to the water potential gradient times a volumetric conductance, L 12 in L h -1 MPa -1. Flux 12 = L 12 * ( Ψ 1 - Ψ 2 )

4 Water potential in xylem and apoplast Ψ Total = Ψ Pressure + Ψ Osmotic + Ψ N(mole fraction) + Ψ Gravity We assume Ψ G is negligible for annual crop plants. Xylem and apoplast are compartments not bounded by a membrane and these have a low solute concentration. Ψ is determined primarily by the mole fraction of water, N. We assume a linear relation between water content and matric potential of water in plant cell wall material below 100% water content, Ψ N = (N 1) * 10 Mpa = 10 * (V V 0 ) / V 0 Volume, V, is the state variable in this model, and V 0 is the volume of water at saturation. Ψ T for each compartment is defined by the volume of water contained therein.

5 Active transport Cytoplast and are bounded by semi-permeable membranes with active transport specific for nitrate and sugar. Active transport is governed by concentrations in the cytoplast [cyt] and in the apoplast [apo] according to V net = V max * [apo] / ([apo] +K m ) Leak * [cyt] Where V net is net transport from apoplast to cytoplast with the parameters, V max maximum velocity, K m apparent affinity, and Leak apparent conductivity for leakage across the cytoplast membrane.

6 Water potential in cytoplast and For cytoplast and, Ψ is the sum of osmotic and turgor contributions. Ψ decreases with solute concentration; we assume a linear relationship with π = 2.4 MPa L mole -1. Ψ O = - π S / V where S is amount of solutes. The osmotic effect lowers Ψ and draws water into a compartment, which applies force or turgor pressure on the cell wall. Expansion results in turgor linearly related to volume by a modulus of elasticity, ε = 10 MPa. Ψ P = ε (V V 0 ) / V 0 Cytoplast and have positive turgor with Ψ N = 0, thus Ψ Total = Ψ P (V, V 0 ) + Ψ O (S, V) = ε (V V 0 ) / V 0 - π S / V

7 Schematic of transport in for model programmed in VENSIM Volume apoplast Flow air Potential air Potential apoplast Volume cytoplast Potential cytoplast Solutes cytoplast Volume Volume xylem Potential xylem Potential Sugar Photosynthesis Flow xylapo Flow phlapo Flow apocyt Nitrate cytoplast Sugar cytoplast Nitrate apoplast Sugar apoplast Nitrate xylem Nitrate xylapo Nitrate apocyt Sugar phlapo Sugar apocyt NRA Metabolism Growth V0 Max growth <Flow stem> <Flow xylem stem> <Sugar stem> <Nitrate xylem stem> [Nitrate] xylem [Nitrate] cyt [Nitrate] apo [Sugar] cyt [Sugar] apo [Sugar] phl

8 Model plant is, stem and root We simplify the plant to three tissues, root, stem, and. Conductance for solution-root and -air are chosen to give reasonable values for rates of transpiration and tissue water potentials. We assume volume proportions among various compartments; apoplast, cytoplast, xylem, and, are the same for root, stem, and. Areas and volumes of tissues and compartments were chosen to be appropriate for rapidly growing seedlings.

9 Transpiration Represents apoplast Schematic of long-distance transport of Leaf pressure potential Leaf turgor water. Stem pressure potential Flux in xylem Cyt Flux in Stem turgor Root pressure potential Root turgor Cyt Root uptake Solution water potential Soil

10 Long distance transport Water moves in xylem and between organs or tissues of a plant as a linear function of hydrostatic pressure difference times a volumetric conductance. Xylem flux 12 = L xylem * ( Ψ P (xylem,1) - Ψ P (xylem, 2) ) Phloem flux 12 = L * ( Ψ P (, 1) - Ψ P (, 2) )

11 Transport of solutes in The concentration of solutes in the must be higher in than root to counteract differences in tissue water potential that would drive water flow from root to. Photosynthesis is the source of sugars that appear in the cytoplasm in the. Growth consumes sugar and nitrate in the cytoplasm of all organs. This production and consumption results in a sugar concentration gradient that induces the flux of water in from to root. This flux transports sugars.

12 Transport of solutes in xylem In the light, there is rapid flux of water in xylem, due to transpiration from leaves. Nitrate is entrained in this flux of water and transported to leaves. The concentration of nitrate in the xylem is very similar to that in the apoplast.

13 Volume xylem root Volume Xylem Volume root Volume Nitrate xylem root Nitrate xylem Sugar root Sugar <Flow xylapo root> <Flow xylapo > <Flow phlapo root> <Flow phlapo > <Nitrate xylapo root> <Nitrate xylapo > <Sugar phlapo root> <Sugar phlapo > Volume stem Volume Xylem stem Sugar stem Nitrate Xylem stem <Flow phlapo stem> <Flow xylapo stem> <Sugar phlapo stem> <Nitrate xylapo stem> Nitrate xylem stem Flow xylem stem Flow stem Sugar stem Nconc xyl root Sconc phl root Nconc xyl stem Sconc phl stem Nconc xylem Sconc phl Nitrate xylem rootstem Flow xylem rootstem Sugar rootstem Flow rootstem Schematic of long distance transport for model programmed in VENSIM

14 Results The solute concentration and flow rate in are affected by the diurnal variation in water potential. Unless hydraulic conductance is very low (less than 1/100 that in xylem), diurnal changes in water potential cause a reversal of flow in after dawn. One parameter for transport of sugars to must differ compared to cytoplasm to maintain about 1 molar sugar in, which is sufficient for effective longdistance transport. A Leak about 10 fold less from than cytoplast can predict this behavior.

15 Results Increasing transpiration, or the to air conductance, increases xylem flow and decreases water potential in all tissues, more so in leaves than in roots. Increasing transpiration enhances movement of nitrate from root to leaves. This increases the nitrate concentration in leaves far more than it decreases the concentration in cytoplasm and xylem of roots.

16 Xylem flow, ml Effect of transpiration on flow in xylem 500 Dark period Cond=0.018 Cond= Cond= hr

17 Cytoplast nitrate, molar Effect of transpiration on nitrate in xylem Dark period Root Cond=0.018 Cond= Cond= Leaf hr

18 Cytoplast nitrate, molar Effect of soil nitrate on cytoplast nitrate 0.05 Dark period 0.04 Root 0.03 Nitrate=0.025 Nitrate= Nitrate= Leaf hr

19 Results Drying the soil and decreasing the contact between roots and soil solution, or the soil to root conductance, lowered root water potential far more than did increasing transpiration. Water flux in the xylem and were affected only slightly. Decreasing soil-root conductance increased the solute concentration in root. Increasing the transpiration rate decreased the solute concentration in root.

20 Phloem flow, ml/hr Effect of photosynthesis on flow 5.0 Dark period PS=0.012 PS= PS= hr

21 Phloem sugar, molar Effect of photosynthesis on sugar PS=0.012 PS= PS=0.024 Dark period Leaf 1.5 Root hr

22 Cytoplast conc, molar Effect of photosynthesis on cytoplast sugar 0.20 Dark period Leaf PS=0.012 PS= PS= Root hr

23 Results Parameters must differ for nitrate transport from soil to roots, compared to internal transport, in order to accumulate nitrate in root apoplast or xylem. Effective transport of nitrate to leaves required uptake from soil directly to cytoplast, and then leakage to apoplast. Low soil nitrate, or slow transpiration, depleted nitrate in leaves. Slow photosynthesis lowered sugars in all parts, but the concentrations in root cytoplast and were most depleted.

24 Drawbacks to current model. The model does not include nitrate reductase explicitly. This is related to light and photosynthesis in leaves of many plants, and not to growth per se. Similarly, there is no explicit recognition of amino acids, and transport of amino acid from leaves to roots. Plant cells can store many metabolites in the vacuole. This process is not included in the current model. It may buffer synthesis and use of metabolites for growth.

25 Modeling details The linear relations defined in model development were programmed as finite difference equations using a visual dynamic simulation modeling tool (VENSIM Professional version 5.6, Ventana Systems, Cambridge MA) and iterated using a 7 second time step. The model was used to examine the effect of diurnal variation in transpiration, and effects of -air conductance (transpiration rate) and soil-water conductance, on movement of water in xylem and, and on water content and water potential of various tissues in an idealized plant.

26 . Parameters used to model water flux and water potential expressed as per liter tissue Transport V max K m Leak For all tissues mole/(l-hr) mole/l 1/hr Nitrate internal Nitrate soil-root Sugar cytoplast Sugar Conductance for water L hr -1 Mpa -1 Apoplast 2.0 Xylem 5.0 Phloem 0.05 Soil-root 10.0 Leaf-air 0.24

27 Parameters used to model water flux and water potential in one liter or one kg fresh weight of whole plant. Photosynthesis Transpiration Max growth () mole C 8 /(m 2 -hr) = 15 umol C/(m 2 -s) 0.24 L/(m 2 -hr) = 4 mmol H 2 O/(m 2 -s) mole NC 8 O 8 H 12 /hr DM fraction () mole NC 8 O 8 H 12 /L Leaf area ratio 2.8 m 2 /L Volumes (Liter) Leaf Stem Root Apoplast Cytoplast Phloem Xylem Total Volume

28 Recent papers that provide justification and parameter values for the model. Bunce J A How do hydraulics limit stomatal conductance at high water vapor pressure deficits? Plant Cell Env. 29; Franks P J Higher rates of gas exchange are associated with higher hydrodynamic pressure gradients. Plant Cell Env 29: Lambers H, F S Chapin III, T L Pons Plant Physiological Ecology. Springer Verlag, NY. pp Lenz T A, I J Wright, M Westoby Interrelations among pressure volume curve traits across species and water availability gradients. Physiol. Plant. 127: Nobel P S Biophysical plant physiology and ecology. W H Freeman Co, San Fransicsco. Proseus T E, J K E Ortega, J S Boyer Separating growth from elastic deformation during cell enlargment. Plant Physiol. 119: Sack L, N M Holbrook Leaf Hydraulics. Ann. Rev. Plant Biology 57: Steppe K, D J W DePauw, R Lemeur, P A VanRolleghem A mathematical model linking tree sap flow dynamics to daily stem diameter fluctuations and radial stem growth. Tree Physiology 26: Thompson M V, N M Holbrook Scaling transport: water potential equilibrium and osmoregulatory flow. Plant Cell Env. 26: Windt C W, F J Vergeldt, P AdriedeJager, H vanas MRI of long distance water transport: a comparison of the and xylem flow characteristics and dynamics in poplar, castor bean, tomato, and tobacco. Plant Cell Env. 29: