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Debate Starting Material - the Role of Physiology

Overview of Cacao Physiology and Cacao Climate Resilience

In the context of the CFCE project, Philippe Bastide provided in the downloadable presentation and overview of cacao physiology and cacao climate resilience, citing the available range of production system options, including:

i) Light shade, windbreaks and cover crops;

ii) Temporary shade with banana and Gliricidia;

iii) Full sun and fertigation,

iv) Permanent “natural”  shade.

Studies show that increasing CO2 levels and other climate extremes, including increased rainfall, flooding, heat and drought, are all directly affecting plant functions including cacao trees. However, we still need to demonstrate that harnessing known genetic resources and their physiological behaviour could help cacao trees to face these challenges, and this is the main aim of the CFCE-CC project. Numerous G X E interactions have been studied, but are still not still well understood including: i) temperature, light and flowering,

ii) the determinism and frequency of flushing,

iii) rainfall, growth cycles and losses of fruits/cherelles;

iv) the effects of environment on biomass;

v) photosynthetic rates, levels of light saturation,

vi) susceptibility to vapour pressure deficits (VPD) and variations of humidity, including presence or absence of leaf protective cuticles.  
The presentation re-iterated some important questions on how physiologists might best assess efficiencies of interactions between density, shade, cover crops, water, nutrients (and fertilizers + phytosanitary treatments) in the changing contexts of drought, heat, flooding, and simultaneously considering intra- and inter-tree competition. 

1. Do we really know Theobroma cacao- a species that is essentially under-domesticated with a high bias in favour of vegetative growth vs flowering and fruit set? 
2. Does this tree remain truly productive over the long-term (> 25 years) with a homogeneous behaviour and water and nutrients use?  
3. What is the nature of relations between rootstock and scion? 
4. Can we model cocoa functioning and cropping systems? 
5. Are growers producing wood and biomass or managing a fruit tree? 
6. Can cacao researchers and producers learn from other fruit crops? 

In arguing for new cacao breeding criteria and new cropping models, the presenter stressed that growers aim for a return on their investment in planting productive material. This also includes managing plant architecture and biomass to optimise yield, via selecting good material, planted and pruned to optimal density and with optimal soil, nutrient, water and pest management. However, in tandem with good agronomic management, it is necessary to have access to: i) a wide range of resilient trees well-adapted to environmental variability; ii) trees with better photosynthetic efficiency and also best use of carbon assimilates; iii) trees with better abilities to use soil and climate resources in a context of increasing climate change impact; iv) smaller trees with shorter internodes that are easier to maintain, and v) pest-
tolerant/ resistant germplasm. 

Incorporating Physiological Traits into Breeding Programmes

This topic has been presented by Dr Andrew Daymond from The University of Reading (UoR) and can be downloaded here

Dr Andrew Daymond presented the status quo of research on cacao physiology, suggesting that although cocoa germplasm diversity has been utilised to an extent through the selection of seed garden parents and clones, the actual amount of germplasm utilised for breeding is relatively small. Characterising the germplasm and understanding how different germplasm responds to environmental variables is important in developing varieties more suited to different growing conditions (present and future). 

This work reiterates the importance of physiology related to

i) photosynthetic potential;

ii) canopy light capture, in terms of leaf area index and extinction coefficient,

iii) biomass partitioning in terms of the ratio of assimilates partitioned between beans and vegetative growth, all aspects that can be assessed via physiological traits measurements. 

Environmental factors impacting cocoa physiology include: temperature extremes; water extremes: (drought or flooding), CO2 levels, relative humidity, and light levels. These are influenced by: geographical location/ altitude, season,climate change and husbandry.

Studying GxE interactions should identify material that demonstrate adaptiveness or resilience in changed geographical locations (e.g. different temperature regimes rainfall patterns), management systems (e.g. shade), or future climates (climate change). 

The team at Reading University is developing screening tools to assess climate resilience, particularly against high temperature stress and water deficit stress, and with the potential to apply these in field situations or breeding programmes. Research has illustrated genotypic variability in physiological traits important for yield and that some genotypes are more resilient to abiotic stresses than others, which have potential for selection and breeding of more climate-resilient genotypes.


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