MACRONUTRIENT DEFICIENCIES AND INTERACTIONS IN FOUR CULTIVARS OF BREADFRUIT (ARTOCARPUS ALTILIS) GROWN IN HAWAIʻI

Abstract

Breadfruit, referred to as ‘ulu in ‘ōlelo Hawai’i, is a large, perennial tree in the mulberry family. Breadfruit was traditionally an important staple crop in the Pacific, especially in Hawaiʻi where it is a historically significant and culturally appropriate crop. Breadfruit continues to be promoted as a key crop to increasing food security and sustainability throughout the tropics and subtropics. In Hawai‘i, substantial growth in appreciation and industry of breadfruit has occurred over the past two decades. Although the demand for breadfruit has significantly increased, constraints to its supply include limited research on agronomic management for optimal productivity. The nutritional aspects of breadfruit have seldom been studied, as it remains a neglected and underutilized tree crop. An understanding of the adequate macronutrient demand, especially during the critical seedling production phase, is fundamental to the large-scale production of healthy saplings and trees. The following study is a principal examination into macronutrient deficiencies and interactions in breadfruit saplings and trees, providing initial insight into their nutrient use and demand. Nutrient deficiency experiments using seedlings are often used to estimate optimal nutritional management through evaluating the effects of nutrient deficiencies on seedling morphology and physiology, and to determine visual symptoms as an auxiliary too to assess nutrient stress. Knowledge of nutrient deficiency symptoms has great practical importance for the identification and resolution of nutritional problems. An induced macronutrient deficiency experiment was implemented in a controlled setting, and compared with results in the field. This study demonstrated the importance of nitrogen and magnesium in plant growth and performance, such that the removal of these macronutrients severely impaired the growth and development of breadfruit saplings. The omission of nitrogen and magnesium resulted in slowed or reduced growth in biomass accumulation, leaf area, chlorophyll content, root length and surface area. Plant quality index is considered a comprehensive assessment of sapling quality, in which the removal of nitrogen resulted in the lowest plant quality index for breadfruit saplings. Moreover, breadfruit saplings showed classic, characteristic deficiency symptoms compared to other fruit trees. The removal of a macronutrient showed an influence on other foliar nutrients, and these interactions were easily detected using the DRIS computational procedures. DRIS, a bivariate diagnostic tool, examines the interrelationships between nutrients, while simultaneously identifying nutrient imbalances and ranking them in order of most deficient to excessive. The DRIS approach provides a more comprehensive perspective of nutrient management, acknowledging the complexity of nutrient demands and requirements in different environmental settings, plant age, cultivar variations, and dilution effects with increased biomass accumulation. Maintaining balance within a plant-soil ecosystem is essential for optimizing agricultural practices, including integrated nutrient management which involves a comprehensive understanding of plant nutrient requirements. Although breadfruit nutrition is not well-understood, the purpose of this thesis was to expand the theoretical understanding of nutrient requirements in breadfruit, particularly in saplings. Optimizing nutrient management in the early growth phases of tree crops will ensure successful production well into the future, locally and across the globe.