Arbuscular mycorrhizal fungi and potassium fertilizer as plant biostimulants and alternative research for enhancing plants adaptation to drought stress: Opportunities for enhancing drought tolerance in cocoa (Theobroma cacao L.)

ABSTRACT Drought is the most critical abiotic threat to cocoa growth and productivity. The purpose of this review is to provide an overview of the recent research and developments which have contributed to the biostimulant properties of Arbuscular Mycorrhizal Fungi (AMF) and Potassium (K) fertilizer, and suggest the best research strategies for the application of these biostimulants to enhance cocoa growth and adaptation to drought conditions. We identified multiple services provided by AMF and K fertilizers: increase nutrients uptake, activating nitrate reductase, regulating photosynthesis and stomata conductance, improve water use efficiency, root growth etc. These multiple services could be efficiently exploited to enhance drought resilience and improve the survival rate of cocoa. Therefore, there is the need for further studies to assess the effectiveness in using either K or AMF or their combination in building the drought resilience of cocoa at the seedling phase; understand the rates of potassium fertilizers that will improve the physical (e.g. cell wall turgor, roots growth) and biochemical (e.g. Proline, polyamines, enzymatic) characteristics of cocoa seedlings to alleviate water stress. In addition, develop better K recommendations based on soil types, location specific and current cocoa varieties; understand the role of K and or AMF in enhancing drought resilience in cocoa under saline conditions and breeding cocoa genotypes with higher efficiency in K utilization and/or AMF colonization. Eventually, AMF and K can be developed as biostimulants as additional and complementary strategies to be used alongside others to improve cocoa drought resilience.