Breeding for Drought Resistance in Cacao Paul Hadley

Breeding for Drought Resistance in Cacao Paul Hadley University of Reading Second American Cocoa Breeders Meeting, El Salvador, 9-11 September 215 9 September 215 University of Reading 26 www.reading.ac.uk

Introduction Cocoa evolved in the understory of the humid tropic rainforest where water limitation is not a common occurrence. Cocoa trees are therefore very susceptible to water stress. However, in many regions where cocoa is grown today, a dry season is common. They can tolerate a short dry period of about 3 months but prolonged periods of dry conditions can reduce bean yields.

Climate Change According to climate change predictions, changes in precipitation patterns are expected in cocoa growing regions. Drier conditions during certain parts of the year are predicted. Average minimum temperatures are also expected to continue to increase which will increase evaporation from the soil and leaf to air vapour pressure deficit.

Climate Change and breeding for improved productivity in cocoa The availability of water is the main environmental factor affecting cocoa yields. For this reason examining the interaction between increased CO 2 and water limitation on growth in cocoa is an important step towards breeding for improved productivity in cocoa in the face of future changes to climate.

INTERNAL ENVIRONMENTAL MANAGERIAL Leaf area index Canopy architecture Light interception Leaf photosynthesis Light intensity Temperature Water stress Shade CO 2 Fertilisers Canopy photosynthesis Planting density Biomass partitioning Pruning Yield Figure 1: A schematic representation of determinants of yield in cocoa

Physiological effects of water shortage Reductions in photosynthesis, stomatal conductance and transpiration occur in response to water stress. Leaf expansion rate is highly sensitive to water stress in cocoa. A reduction in this parameter due to water stress influences the area over which CO 2 assimilation can occur. As chlorophyll development does not occur until cocoa leaves are fully expanded, water stress reduces further the already low photosynthetic capacity of cocoa.

Table 3. Comparison of studies of photosynthesis in cocoa Internal factor External factor Photosynthetic Reference rate (µmol m -2 s -1 ) Canopy position 2.7 (sun leaves) Murray, 194 1.8 (shade leaves) Leaf age 3.8 Lemée, 1955 Tree vigour 2.5-4.4 Hutcheon, 1977 light 3.9 Raja Harun & Hardwick, 1988a temperature, water stress.8-1.8 Raja Harun & Hardwick, 1988b Varietal light, temperature, water stress 2.-3.9 Balasimha et al., 1991 Leaf age irradiance 2.-6.5 Balasimha et al., 1992 Varietal 6.4-8.4 Yapp & Hadley, 1994 Varietal 2.6-3.7 Galyuon et al., 1996

Physiological effects of water shortage The impact of elevated CO 2 combined with water shortage on growth and photosynthesis has not yet been investigated in cocoa. In other plants, in some cases, some of the negative effects of water stress can be ameliorated by the beneficial effects of elevated CO 2 concentration. Elevated CO 2 reduces stomatal conductance, thereby improving WUE; this response can be further increased during water limitation. However, there are exceptions and species variation in responses to elevated CO 2 and water stress have been shown. An increase in WUE in water stressed cocoa may enable trees to be grown in drier regions or may help maintain growth during dry periods which would be otherwise limiting to plants under current CO 2 concentrations.

Climate Change Effects on Cocoa Five year project Interacting effects of temperature, water availability and carbon dioxide concentration Genetic variation Funded by Cocoa Research (UK)

Water Use Efficiency (A/g s ) Photosynthesis (µmol m -2 s -1 ) 9 P <.1 8 7 6 5 4 3 2 1 38 7 35 3 P <.1 25 2 15 1 5 38 7

Water Use Efficiency (A/g s ) Photosynthesis (µmol m -2 s -1 ) 9 P <.1 8 7 6 5 4 3 2 1 38 7 35 3 25 P <.1 Doubling carbon dioxide levels doubles photosynthetic rates and increases water use efficiency 2 15 1 5 38 7

Stomatal conductance (porometer) (mmol m -2 s -1 ) WUE (A/g s ) (µmol mol -1 ) A max (µmol m -2 s -1 ) 7 A 6 5 4 3 2 1 6 5 G 2 18 16 H 4 14 12 3 1 2 8 6 1 4 2 Ambient CO 2 Elevated CO 2 Ambient CO 2 Elevated CO 2 Photosynthetic parameters calculated from photosynthetic light response curves performed on cocoa seedlings grown at ambient and elevated CO 2 under well-watered and water deficit conditions. The response curves were measured at growth CO 2 concentration for each seedling. N = 1. Error bars show standard error. - Well-watered - Water deficit

Growth under elevated CO 2 and water deficit Ambient CO 2 Elevated CO 2 P Water deficit Wellwatered Wellwatered Water deficit CO 2 Water CO 2 Water Leaf number 35 (± 5) 14 (± 1) 36 (± 3) 21 (± 1).72 <.1 n/a Leaf area (cm 2 ) 6153 (± 2358) 267 (±814) 874 (±746.8) 373 (±331).6 <.1.815 Leaf fresh weight (g) 79.4 (± 9.18) 25.9 (± 4.11) 9.5 (± 6.76) 38.9 (± 4.67).17 <.1.232 Leaf dry weight (g) 22.4 (± 2.89) 8.12 (±1.36) 25.2 (±1.79) 12.1 (± 1.45).21 <.1.25 Specific leaf area (m 2 kg -1 ) 28.3 (± 1.64) 27.63 (± 2.19) 32.6 (± 2.52) 25.96 (±1.51).745.55 n/a Stem diameter increase (%) 48 (± 2.3) 3 (± 2.2) 55.4 (± 3.) 38.7 (± 2.8).4 <.1.812 Plant growth parameters measured after 9 days of growth at ambient and elevated CO2 under well-watered and water deficit treatment. n = 1. Mean (± SE). n/a indicates a non-parametric test was performed so no interaction statistic is available. All leaf data refers only to the leaves which developed during the experimental period.

A max (µmol m -2 s -1 ) Stomatal conductance (mol m -2 s -1 ) Genotypic variation in leaf gas exchange under elevated CO 2 and water deficit 12.12 1.1 8.8 6.6 4.4 2.2 WW WS WW WS WW WS WW WS Ambient CO 2 Elevated CO 2 Ambient CO 2 Elevated CO 2 Photosynthetic parameters calculated from the light response curves of six cocoa genotypes grown under an - Pound 7/B; - SCA 6; - SPEC 54/1. Error bars show standard error. elevated CO 2 and water stress treatment.. WW well-watered; WS water stressed. CL 19/1; - ICS 1; - IMC 47; - Pound 7/B; - SCA 6; - SPEC 54/1. Error bars show standard error.

A max (µmol m -2 s -1 ) Stomatal conductance (mol m -2 s -1 ) Genotypic variation in leaf gas exchange under elevated CO 2 and water deficit 12.12 1.1 8.8 6.6 4 2 WW Ambient CO 2 WS WW Elevated CO 2 WS.4 Photosynthesis increased by 54% under.2 increased CO 2 concentration and declined by 6% under water stress. No WW significant WS difference in WW WS photosynthesis between genotypes. Ambient CO 2 Elevated CO 2 Photosynthetic parameters calculated from the light response curves of six cocoa genotypes grown under an - Pound 7/B; - SCA 6; - SPEC 54/1. Error bars show standard error. elevated CO 2 and water stress treatment.. WW well-watered; WS water stressed. CL 19/1; - ICS 1; - IMC 47; - Pound 7/B; - SCA 6; - SPEC 54/1. Error bars show standard error.

A max (µmol m -2 s -1 ) Stomatal conductance (mol m -2 s -1 ) Genotypic variation in leaf gas exchange under elevated CO 2 and water deficit 12.12 1.1 8 Stomatal 6 conductance declined significantly in water stressed plants. 4 No significant effect of CO 2. Significant differences between genotypes. 2.8.6.4.2 WW WS WW WS WW WS WW WS Ambient CO 2 Elevated CO 2 Ambient CO 2 Elevated CO 2 Photosynthetic parameters calculated from the light response curves of six cocoa genotypes grown under an - Pound 7/B; - SCA 6; - SPEC 54/1. Error bars show standard error. elevated CO 2 and water stress treatment.. WW well-watered; WS water stressed. CL 19/1; - ICS 1; - IMC 47; - Pound 7/B; - SCA 6; - SPEC 54/1. Error bars show standard error.

iwue (A/g s )(µmol mol -1 ) Genotypic variation in water use efficiency under elevated CO 2 and water deficit 45 4 35 3 25 2 15 1 5 Intrinsic water-use efficiency of six genotypes of cocoa grown under elevated CO 2 and water stress treatments. WW well-watered; WS water stressed. CL 19/1; - ICS 1; - IMC 47; - Pound 7/B; - SCA 6; - SPEC 54/1. Error bars show standard error.

Genotypic variation in stomatal density under elevated CO 2 and water deficit 12 stomatal density. mm -2 1 8 6 4 2 Stomatal density of each genotype grown under two different CO 2 and water regimes.. WW well-watered; WS water stressed. - CL 19/1; - ICS 1; - IMC 47; - Pound 7/B; - SCA 6; - SPEC 54/1. Error bars show standard error.

Genotypic variation in stomatal density under elevated CO 2 and water deficit 12 stomatal density. mm -2 1 8 6 4 2 Significant genotypic variation in stomatal density. No significant effect of CO 2. Borderline effect of water stress. Stomatal density of each genotype grown under two different CO 2 and water regimes.. WW well-watered; WS water stressed. - CL 19/1; - ICS 1; - IMC 47; - Pound 7/B; - SCA 6; - SPEC 54/1. Error bars show standard error.

Genotypic variation in stem diameter under elevated CO 2 and water deficit Ambient CO 2 Elevated CO 2 Well-watered Water stressed Well-watered Water stressed CL 19/1 6.57 (± 1.35) 9.85 (± 5) 24.89 (± 4.6) 2.27 (± 8.34) ICS 1 17.41 (± 9.37) 17.39 (± 6.7) 38.1 (± 2.77) 25.32 (± 9.64) IMC 47 3.27 (± 9.12) 17.7 (± 6.57) 21.82 (± 5.12) 27.18 (± 5.82) Pound 7/B 19.39 (± 7.96) 15.59 (± 6.75) 14.6 (± 4.46) 12.7 (± 2.4) SCA 6 8.76 (± 3.58) 12 (± 5.76) 14.15 (± 2.6) 19.3 (± 5.11) SPEC 54/1 21.63 (± 8.21) 15.73 (± 4.94) 28.13 (± 8.38) 24.57 (± 2.63) Stem diameter of trees grown at elevated CO 2 increased by 22% compared to 16% for trees grown under ambient conditions. Magnitude of the response varied between genotypes. Stem diameter increased by a lower amount under water stressed conditions. Again, genotypic variation in response to water stress.

Current work Impact of water stress and elevated CO 2 on yield and yield determinants in cocoa. Physiological characterisation of the International Cocoa Quarantine Centre Germplasm Collection

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