Ecophysiology and Nutrition of Cocoa
Cocoa trees are directly impacted by immediate environment on how the crop perform during each stage of its development. Cocoa is susceptible to suffer from environmental stresses among which drought stress. Multi-sided mechanism implies water deficit and dry air in the environ of the crop induces several different genetic and physiological responses. Thus, there is a need to assess the relationship involved in the process of growing under drought stress.
Agricultural commodities such as cocoa in West Africa are affected by climate change. Being that these crops are of major importance to the economy of these countries and livelihoods of people this poses a major problem (CDKN 2014). Yet, in spite of its importance, little is currently known about the effects of climate change on cocoa according to (Medina and Laliberte 2017). Concerns about high temperatures impacts on cocoa through the increase of evapo-transpiration associated with decreasing rainfall have been raised. Currently, water availability is already the most limiting factor for cocoa growth and production (Anim-Kwapong and Frimpong 2004; Läderach et al. 2013; Schroth et al., 2016). As a result, Läderach, (2011) concluded that climate suitability for cocoa would decrease dramatically in some regions in Western Africa.
There are several agronomic strategies that can be employed to make cocoa production more drought resistant e.g. increased soil organic matter etc (Abulai et al. 2017). Success of these strategies hinges in our knowledge of the physiological mechanisms that determine drought tolerance of cocoa trees, and how their operation is determined by external conditions. At plant level, the impact of drought is both direct reduction in growth as well as the disruption of physiological processes (Fahad et al., 2017). Drought also affects the absorption and accumulation of nutrients (Silva et al., 2017) which may have further negative effects on growth and functioning through nutrient limitation effects, and as discussed further below which in the case of e.g. potassium may affect a plant’s to ability to deal with drought (Wei et al. 2013).
When dealing with effects of limited water availability in crops, potassium plays a particular role in contributing to the survival of crop plants under stressed environmental conditions. Potassium plays an integral role in plant-water relationships, and is involved in numerous physiological functions (Arquero et al., 2006; Tsonev et al., 2011; Oddo et al., 2011). Under water deficit conditions, the application of potassium enhances leaf waterpotential, osmotic potential, osmotic homeostasis (Levi et al., 2011).
In the case of cocoa, potassium is a potential mitigating agent of the effect of drought in the crop orchards and that could lead to drought resilience adaptability (Medina and Laliberte, 2017). Cocoa pods are very rich in potassium and large amounts of K are exported from the field during harvest. This emphasizes the need for proper potassium nutrition in the crop. This requirement is often not met due to adverse soil and plant factors resulting in yield reduction, and this may inhibit the ability of the crop to tolerate drought. The interactions between potassium nutrition and environmental stress on several aspects of cocoa physiology is not fully understood. Therefore, it seems quite important to assess the effects of potassium application on adaptive responses of cocoa at the physiological level and water relations.
Water shortage importantly limits cocoa growth and production in West Africa, and this effect is expected to increase under climate change. Water shortage results from both seasonality and irregularity in rainfall pattern and from occasional strong drought events. Soil fertilization with potassium is suggested to improve nutrition as well as a measure to mitigate drought-induced reduction of cocoa production. Application of potassium is suggested to improve physiological responses of crops to drought, as it is involved in stomata closure, water uptake by the roots and water transport within the tree. However, there is a considerable knowledge gap on cocoa responses to water deficit and how these responses are modified by potassium. This PhD project explores the interactive effects of water-stress and potassium availability on cocoa physiology and performance at different levels of integration. It first explores these effects at the leaf level on stomatal conductance associated with photosynthesis and transpiration. From there, it will examine sap flow fluctuation which are indicative of transpiration at the whole plant level. Measurements will be done during both the wet and dry season. Then it explores the impacts of these factors on cocoa production. The new insights on drought and potassium effects are then used to validate a crop model for the study sites. This research is of great significance to provide perspectives for an effective management of cocoa under water stress. It will provide a better understanding on how physiological systems are regulated and stress tolerance is improved while maintaining cocoa yield and quality.
Results reveals a significant effect of genotype, drought treatment and interactive effect of genotype x drought on leaf water potential at predawn (PLWP) and midday (MLWP). Drought treatment significantly decreases leaf water potential. No significant effect of potassium and no interaction between potassium x drought was found. Genotype M positively differs from CI03, and have a better ability to
increase leaf water potential (Figure 1). Drought affected leaf gas exchange by significantly reducing stomatal conductance (Gs) in both genotypes evaluated. Results reveal a significant effect of genotypes and drought treatment on stomatal conductance (Gs) Potassium treatment did not affect stomata conductance neither its interaction with irrigation. No significant interactive effect was found between the genotypes and drought treatment or genotypes and potassium treatments. Drought effect was consistent among genotypes however stomatal conductance activity varied strongly between treatments and tend to be lower in CI03.
Figure 1 Effects of withholding irrigation, potassium fertilization, and genotype on leaf water and stomatal conductance for two cocoa genotypes. Genotypes refer to one clonal variety CI03 and one hybrid variety M. A- Predawn leaf water potential (pLWP) and midday leaf water potential (mLWP), B- Stomatal conductance. K- means without potassium application, K+ with potassium application, Irri means irrigated treatment and No-Irrig withholded irrigation. Values represent means ± S.E.