The environmental constraints on cocoa (Theobroma cacao) production in north Australia
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This thesis describes the research undertaken to determine whether cocoa (Theobroma cacao) could be successfully produced in north Australia. Initial observations indicated that high solar radiation, high leaf-to-air vapour pressure differences (LAVPD) and cold over-night chilling temperatures would be the most significant environmental parameters affecting the performance of cocoa in north Australia. Experiments were carried out to assess the impact of supra-optimal and sub-optimal climatic conditions on cocoa performance using (1) cocoa seedlings established under solar irradiance ranging from 25 to 70% transmitted light; and (2) previously established cocoa to assess leaf photosynthetic performance, leaf chlorophyll fluorescence, leaf attributes, stem growth and yield quality and quantity under field conditions. Specific areas of study included: The effect of different shade treatments on cocoa establishment Cocoa seedlings subject to irradiance levels of 25 to 70% transmitted light showed no significant differences in relative growth rates. However, at 70% treatment canopies were smaller compared to the three lower light treatments. Differences in total canopy area were attributed to photoinhibition and results are discussed with reference to leaf carotenoid concentrations and leaf longevity. The effect of different shade treatments and environmental variables on leaf performance Maximum photosynthetic rates in the field ranged from approximately 1 to 9 μmol m⁻²s⁻ˡ. Differences in light saturated photosynthesis were attributed to differing shade treatments, changes in LAVPD (0.3 to 4.9 kPa), low and high leaf temperatures (8 to 42°C), dynamic photoinhibition (ΔF/Fm’ 0.2 to 0.7), leaf nitrogen content (1 .7 to 2.4%) and SLA (15 to 26 m² kg⁻ˡ). The impact of cold over-night temperatures Over-night chilling air temperatures of <10°C were shown to cause substantial reductions in the number of open photosystem II reaction centers (ΔF/Fm’) the following morning irrespective of shade treatment. Reductions in ΔF/ Fm’ were associated with extremely low rates of leaf carbon assimilation which was attributed to sub-optimal leaf thermodynamics and photosynthetic protein degradation. Recovery of photosynthesis took five days following chilling events in the field. Flowering response and bean quality under different light treatments Flowering was significantly reduced by 70% treatment during the first 25 months of growth. However, fruit yields were still the largest at 70% treatment compared to the three lower light treatments. This was thought to be an affect of larger canopy photosynthetic rates at 70% transmitted light. However, cocoa grown at 70% treatment produced smaller beans compared to the lower light treatments. Fat extracted from cocoa beans in the NT produced a softer butter than beans originating in Mossman, Queensland. Leaf gas exchange and growth between north Australian locations Experimental sites in north Queensland offered the most suitable growing conditions for cocoa. This was primarily attributed to low LAVPD throughout the year promoting large annual photosynthetic rates compared to cocoa grown in the Northern Territory and Western Australia.
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