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Water use efficiency in plants
- 1. Water Use Efficiency By R. Gowthami M.Sc (Ag) Plant Physiology
- 2. CONTENT •WUE – definitions •Approaches i. Gravimetric ii. Gas Exchange iii. Irrigation aspects iv. Carbon isotope discrimination •Higher WUE ? •Application •Examples
- 3. WATER USE EFFICIENCY • Amount of water used per unit of plant material produced. DEFINITIONS Agronomists define it in terms of the units of water used per unit of dry matter produced, often using total water lost by both evaporation and transpiration (Teare et al., 1973) Dry matter or crop yield in kg Water used in evapotranspiration in m3 Physiologists are more likely to discuss it in terms of photosynthesis, expressed as milligrams of CO2 per gram of water or even as moles of CO2 per mole of water (Fischer and Turner, 1978). Net CO2 uptake in mg or gm H2O loss(transpired) in gm or kg WUE = WUE =
- 4. Water Use Efficiency Can Be Defined In A Variety Of Different Ways. LEAF LEVEL: On a single leaf basis, instantaneous WUE is more strictly defined, as the current net CO2 assimilation rate, divided by the current transpiration rate. WHOLE PLANT LEVEL: WUE is defined as the ratio of the biomass produced to total water used. STAND OR CROP LEVEL: WUE is defined as the ratio of the biomass produced to total water inputs to the whole ecosystem (yield/total water input).
- 5. Approaches • Gravimetric methods • Gas Exchange (Instantaneous WUE) • Carbon Isotope Discrimination
- 6. Gravimetric methods • Measuring WUE by taking difference between the control and treatment. Here the control is without plants i.e. soil alone and treatment is with plant and soil. • In control only evaporation takes place and in treatment both evaporation and transpiration occurs. • The amount of water loss is more in treatment than in control. • It is a tedious method.
- 7. Gas Exchange (Instantaneous WUE) Both A (CO2 assimilation rate/photosynthesis rate) and E (transpiration rate) are the product of two factors: • stomatal conductance (g) for either CO2 (gc) or water vapour (gw) and • The concentration difference of either CO2 (ca and ci) or water vapour (wa and wi) between the air outside and inside the leaf. • Stomatal coductance is a numerical measure of the rate of passage of either water vapour or CO2 through the stomata or small pores of the plant.
- 8. • However, since CO2 enters the leaf via the same path by which H2O exits the leaf (i.e., the stomata and boundary layer), it is possible to remove the conductance term from the equation altogether. To a first approximation, gw = 1.6 × gc. Therefore, the equation above can be rewritten as: • Since the leaf is using CO2, the value of ci depends on both the stomatal conductance and the photosynthetic capacity of the leaf.
- 9. • High Δ values resulting from high Ci/Ca reflect higher CO2 assimilation rate to transpiration ratio (Farquhar et al., 1989), i.e., lower TE.
- 10. • Ci inversely proportional to WUE • Lesser the value of Ci i.e., minimum stomatal conductance(gs) and maximum mesophyll conductance(gm) • Stomatal conductance is the measure of either exit of water or CO2 entry through the stomata. • Mesophyll conductance is the level of CO2 inside the chloroplast where photosynthesis take place to fix atmospheric CO2
- 11. • Water use efficiency also increases with increase in crop water supply up to a certain point. • Water supply has also been observed to increase fertilizer use efficiency by increasing the availability of applied nutrients, and water and nutrients exhibit interactions in respect of yield and yield components. • The irrigation system perspective of water use efficiency depends upon the water accounting where, Losses occur at each stage as water moves from the reservoir (storage losses) Conveyed and delivered at the farm gate (conveyance losses) Applied to the farm (distribution losses) Stored in the soil (application losses) and Finally consumed by the crops (crop management losses) for crop production.
- 12. Measurements Based On Irrigation Aspects • Depending upon the area of interest, it is possible to measure the Field water use efficiency Crop water use efficiency Water conveyance efficiency Application efficiency WUE (kg/ha-cm or q/ha-cm or kg/ha-mm) 1. Crop Water Use Efficiency /Consumptive Water Use Efficiency: CWUE = Y/CU 2. Field Water Use Efficiency: FWUE = Y/WR Where, G + E + T + D = WR; (G + E + T) = CU Growth (G) Direct evaporation from the soil surface (E) Transpiration (T) Deep percolation loss (D)
- 13. 3. Water conveyance efficiency: Ec=Wf/Wd x 100 Wf= Water delivered at the field supply channel Wd= Water diverted from the source. 4. Water application efficiency : Ea = Ws/Wf x 100 Ws= Water stored in the root zone of the plants Wf= Water delivered to the field
- 14. WUE And Carbon Isotope Discrimination • The efficacy of the use of 13C as an indicator of WUE, has been firmly established under both greenhouse and field conditions. • CO2 in air naturally contains two stable isotopes of carbon, 12C and 13C, in an approximate of 99% to 1 %. • The theoretical model describing fractionation during photosynthesis was developed by Farquhar et uf. ( 1989). ∆= a + (b-a)ci/ca Where ∆ represents the total difference in isotopic composition between photosynthetically fixed carbon in the atmosphere a and b are the fractionation constants associated with diffusion versus carboxylation and have values of 4.4 and 27‰(per mill), respectively
- 15. IRMS • The carbon isotopic ratio (R=13C/12C) of the samples (Rsample) and standard (Rstandard) was determined using an Isotope Ratio Mass Spectrometer (GD 150, MAT, Germany). R values were converted to δ13C (in ‰ or per mil) using the relationship: δ13C (‰) = [Rsample/Rstandard-1] x 1000. • The standard is the CO2 obtained from a limestone from Pee Dee Belmenite “PDB” formation in South Carolina, USA. The δ13C values were converted to carbon isotope discrimination (Δ) values using the relationship established by Farquhar et al., (1989): Δ (‰) = (δ13Ca - δ13Cp) /(1 - δ13Cp/1000) , • where a and p represent air and plant, respectively.
- 16. 13C deflects more than 12C
- 17. WUE and carbon isotope discrimination are negatively related i.e. lesser the discrimination more will be the WUE
- 18. • The difference in ratio (13C/12C) between C3 and C4 is correlated with isotopic fractionation present between the ribulose biphosphate carboxlase (RuBP) activity in C3 plants and phosphoenolpyruvate (PEP) carboxylase activity in C4 plants. • RuBP discriminates more against 13C than PEP (Christeller et al., 1976). • This indicates that why C4 plants are more efficient than C3 plants.
- 19. When the WUE will be higher? • Stomatal conductance (and photosynthesis) are lowest. • The most important of these factors is stomatal conductance. As stomata open, both AN and E increase. • Ambient water vapour concentration (wa) increases. • Leaf temperature (and therefore wi) decreases. • Increasing the ambient CO2 concentration (ca). • Decrease in ci will also increase WUE. However, it is important to remember that a decrease in ci can result from two distinct mechanisms: A decrease in stomatal conductance or An increase in leaf photosynthetic response to internal CO2.
- 20. A decrease in stomatal conductance & an increase in WUE
- 21. Application A study in Groundnut genotypes by Farquhar et.al. • The relationship between WUE and Carbon isotope discrimination was useful in identifying the groundnut genotypes. • WUE ranged from 1.81 to 3.15g/kg which was negatively correlated with Δ which ranged from 19.1 to 21.8%. • Variation in WUE arose mainly from genotypic differences in Total dry matter production rather than differences in water use. • Finally it is concluded that a strong negative relationship existed between WUE and SLA (cm/g) and between Δ ad SLA, indicating that genotypes with thicker leaves had greater WUE.
- 22. Genetic Control of Water Use Efficiency and Leaf Carbon Isotope Discrimination in Sunflower (Helianthus annuus L.) Subjected to Two Drought Scenarios by Afifuddin Latif Adiredjo et.al. • A population of 148 recombinant inbred lines (RILs) o sunflower derived from a cross between XRQ and PSC8 lines was studied to identify quantitative trait loci (QTL) controlling WUE and CID, and to compare QTL associated with these traits in different drought scenarios. • They conducted greenhouse experiments in 2011 and 2012 by using 100 balances which provided a daily measurement of water transpired, and they determined WUE, CID, biomass and cumulative water transpired by plants. • Wide phenotypic variability, significant genotypic effects, and significant negative correlations between WUE and CID were observed in both experiments.
- 23. • C3 crops, such as wheat and barley, are less water-efficient than C4 crops, such as maize and sugarcane. • The most water-efficient crops are the CAM (Crassulacean acid metabolism) crops such as cactus and pineapple (xerophytes). • WUE : CAM> C4> C3
- 24. REFERENCE • Sharma,B.,Molden,D.,Cook,S. Water use efficiency in agriculture: Measurement, current situation and trends. • Boutraa,T., Akhkha,A., Abdulkhaliq A. Al-Shoaibi Ali and Alhejeli,M. (2010) Effect of water stress on growth and water use efficiency (WUE) of some wheat cultivars (Triticum durum) grown in Saudi Arabia. • GUO Shi-wei, ZHOU Yi, SONG Na, SHEN Qi-rong (2006) Some Physiological Processes Related to Water Use Efficiency of Higher Plants. • Nemali,K.S.,(2008) Physiological Responses to Different Substrate Water Contents: Screening for High Water-use Efficiency in Bedding Plants. • Grossnickle,S.C.(2005) Variation in gas exchange and water use efficiency patterns among populations of western redcedar.
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