Water use efficiency in agriculture

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Theodora Benson
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1 Water use efficiency in agriculture Bill Davies The Lancaster Environment Centre, UK

2 Summary Introduction and definitions Impacts of stomata, environment and leaf metabolism on WUE Estimating WUE and modifications through breeding Impact of agronomy on WUE

3 Global agriculture now accounts for 70% of the amount of water used on Earth at a time when global population continues to spiral

4 Environmental Limitation Area affected (%) Drought 25.3 Shallowness 19.6 Cold 16.5 Wet 15.7 Alkaline soils 2.9 Saline or no soil 4.5 Other 3.4 None 12.1 After Boyer, (1982) Crop losses due to water loss out weigh crop losses due to any other causes

5 After Boyer, (1982) There is a large different between record and average yields suggested a large amount of variation due to the environment

6 Some definitions of WUE CO 2 assimilated/water lost harvest index or total biomass/applied water harvest index or total biomass/water transpired harvest index or total biomass/water uptake harvest index or total biomass/water available WUE can be defined either in terms of an instantaneous measurement of the efficiency of carbon gain for water loss; or an integral of this efficiency over time (as a ratio of water use to biomass or yield accumulation). Important to define terms precisely.

7 Y d /E l ( 10 3 ) C3 plants: Cereals Other Poaceae Alfalfa Pulses Sugar beet Native plants C4 plants: Cereals Other Poaceae Other C After Briggs & Shantz (1914) A landmark study was carried out by Briggs and Shantz (1914) in Akron, Colorado, leading to the development of the Akron series.

8 6 5 Yield (t ha-1) Water use (mm) There is a clear relationship between the amount of water transpired and yield across a diverse range of crop species water loss is an inescapable trade-off for carbon gain.

9 Instantaneous WUE is defined as the ratio of CO 2 assimilation into the photosynthetic biochemistry (A) to water lost, via transpiration through the stomata (T).

10 A and T are regulated by stomatal conductance (g s ) and the respective concentration gradients of water and CO 2 between the inside (w i and c i ) and the outside of the leaf (w a and c a ).

11 lower c i The driving force for CO 2 uptake and water loss are independent. Therefore increasing c a (the external concentration of CO 2 ) will increase instantaneous WUE, as the driving force for water loss will remain unchanged, while that for CO 2 uptake will increase.

12 A plant can lower c i and achieve a lower c i /c a by closing its stomata (to limit CO 2 diffusion into the leaf); increasing photosynthetic capacity; or both.

reduction in g s will generally result in a reduction in

13 The relationship between water loss and stomatal conductance is effectively linear meaning that a reduction in g s will generally result in a reduction in water loss, while partial stomatal closure does not always reduce carbon gain

15 But, there are issues when plants grow in communities

16 And an issue with changing leaf temperature. Stomata close, reduce transpiration which causes the leaf to heat up and this drives transpiration harder

17 This difference in water use efficiency between C3 and C4 species is a result of the increased driving force for CO 2 uptake generate by C4 biochemistry and tissue structure

18 This difference in water use efficiency between C3 and C4 species is a result of the increased driving force for CO 2 uptake generate by C4 biochemistry and tissue structure

19 Consequently, C4 species can achieve comparable assimilation rates at lower stomatal conductances and lower c i, significantly enhancing their WUE.

20 Traditional breeding programmes have tried to introduce C4 characteristics into C3 plants, but with little success. It seems tat biotechnology may achieve some increase in WUE when C4 traits are introduced into C3 plants

21 CAM (species that show Crassulacean acid metabolism) plants have enhanced WUE (due to temporal shifts in CO2 fixation), but their low rate of biomass accumulation makes introduction of CAM traits into crop species undesirable.

22 Carbon isotope discrimination has been used to assess the genetic variability in the driving force for CO 2 uptake. Two stable isotopes of carbon are found in molecular CO 2 ( 13 C and 12 C) with a ratio of 1:99 in atmospheric air. Plant tissues contain considerably less 13 C. They are said to discriminate against it. This is a function of its larger molecular mass, which slows its rate of diffusion. A lower c i /c a results in decreased discrimination ( 13 C) against 13 C as the driving force for CO 2 diffusion is increased. Consequently, carbon isotope signatures demonstrating low 13 C are diagnostic of a CO 2 fixation environment with a relatively low c i /c a.

24 Water Use Efficiency (g DM Kg -1 H 2 0) C discrimination (10 3 X ) There is a strict relationship between the degree of 13 C and WUE in crop species, such that tissue with low 13 C exhibits, enhanced instantaneous WUE. In other words low 13 C is indicative of a low Ci/Ca (Page 2450 of Condon et al. (2004) JXB 55: 2449).

25 Decreased 13 C could however be the result of decreased stomatal conductance or high mesophyll photosynthetic capacity (or both). Page 2449 of Condon et al. (2004) JXB 55: 2449

27 HIGH MESOPHYLL ACTIVITY

28 PARTIAL STOMATAL CLOSURE

29 Does the theory translate into practice?

30 1 Relative 13 C L. esculentum F 1 hydrid L. pennellii WUE (g DM Kg -1 H 2 O) What is clear is that WUE is heritable you can breed it into and out of crops, using conventional breeding strategies.

31 Rebetzke, G.J., Condon, A.G., Richards, R.A. and Farquhar, G.D. (2002) Selection for reduced carbon isotope discrimination increases aerial biomass and grain yield of rainfed bread wheat. Crop Science 42,

This has recently led to the commercial production of new wheat varieties known as Drysdale and Rees with high WUE, quality and level of disease resistance.

32 This has recently led to the commercial production of new wheat varieties known as Drysdale and Rees with high WUE, quality and level of disease resistance. Greatest yield advantages are realised in locations with low yields due to drought? See JXB p Condon et al. (2004). Concerns over use of carbon isotope discrimination (e.g. slower growth rates associated with increased photosynthetic efficiency). Probable combination of small decrease in conductance and small increase in photosynthetic capacity, neither of which has a major individual negative effect

33 When water evaporates from the sub-stomatal cavity, the leaf becomes enriched with H 2 18 O, due to its slower rate of diffusion. As the driving force for water loss increases (due to increased stomatal aperture), this enrichment decreases.

34 The ability of tissues to demonstrate discrimination between water molecules containing the two stable isotopes of oxygen ( 18 O and 16 O) may also be of use.

35 Consequently dual screening for low 13 C and low 18 O would identify highly WUE individuals which have exhibited high levels of transpiration (i.e. enhanced WUE is more likely to be the result of enhanced mesophyll photosynthetic capacity).

37 A gene for WUE? Recently the Farquhar group reported the identification of ERECTA, the first gene that regulates transpiration efficiency. ERECTA and other members of the gene family are involved in the control of guard cell density and mesophyll structure. Masle, Gilmour and Farquhar 2005, Nature 436, Shpak et al., 2005, Science 309,

38 WUE and Agronomy The major agronomic way of increasing transpiration efficiency is to maximize the growth of crops during periods of low vapour-pressure deficits. Thus in Mediterranean-type climates autumn sowing rather than spring sowing has a major influence on transpiration efficiency as a greater proportion of the autumn-sown crop's life occurs during the period of low vapour-pressure deficits in winter (Fischer, 1981 ; Singh et al., 1997 ; Richards et al., 2002 ).

39 Seasonal evaporation from soil (mm) Fractional area of shaded soil at flowering (%) Rapid leaf development in annuals has also been suggested to contribute to the efficient use of soil water. By establishing maximal leaf area quickly, evaporation from the soil is minimised and stored soil water is conserved.

40 The relationship between transpiration efficiency of wheat and pan evaporation for various months in the southern hemisphere (left of the line) and the northern hemisphere (right of the line) (adapted from Fischer, 1981, and Richards et al Turner, N. C. J. Exp. Bot : ; doi: /jxb/erh154 Copyright restrictions may apply.

41 Changes with time in decadal wheat yields from 1860 to 2000 with explanations for the trends (from Angus, 2001, with permission from CSIRO Publishing) Turner, N. C. J. Exp. Bot : ; doi: /jxb/erh154 Copyright restrictions may apply.

References. 1 Introduction

References. 1 Introduction 1 Introduction 3 tion, conservation of soil water may result in greater soil evaporation, especially if the top soil layers remain wetter, and the full benefit of sustained plant physiological activity