Effect of perennial drought environments on Citrus tangerina Hort. cv. "Chuanju" growth, nutrient absorption and physiology

The objective of this paper was to study the specifics of Citrus tangerina Hort. cv. "Chuanju" in terms of growth, nutrient absorption and related physiological responses to perennial drought environments using annual irrigations as the control. The main aim of the study was to reveal any adverse impacts of perennial drought on citrus. The phenological phases, new shoot/fruit growth conditions, leaf life spans, and N, P, K, Ca, Mg, Fe, Cu, Zn and B contents of spring-shoot leaves/fruits were measured in four successive years. The relationships among physiological diseases, droughts and nutrient uptakes were researched into along with morphological characteristics, organizational structures and drought-resistance physiology. Also the relationships between physiological response and the adaptability to perennial drought were discussed. The results showed that under perennial drought environments, growth, flowering and fruit-dropping periods clearly delayed while fruit maturity period advanced. Leaf life span shortened; shoots became shorter, thinner and the amounts of shoots and leaves dropped while leaf withering worsened. The degree of leaf leathery increased and so leaves became smaller and thicker. Deficiency symptoms caused by elements such as N, Zn, Fe, B, etc became very distinct. Fresh weight, dry-matter weight, water content, seed amount, juice content and N, P, K, Ca, Mg, Fe, Cu, Zn and B contents in spring shoot leaves obviously dropped along with those of N, P, Ca, Fe and B in fruits. The degrees of fruit gummosis and juice sac granulation worsened such that fruit quality fell drastically. Leaf palisade tissues thickened while spongy tissues thinned out. CTR (tissue tense ration) and stomata density increased as stomata and SR (tissue soothing ratio) got smaller. The contents of proline, soluble sugar and water saturation deficiency in leaves increased. Dehydration rate, relative water content and leaf water potential dwindled. Then while Pn and Tr clearly dropped, WUE drastically improved in summer. The study showed that C. tangerina Hort. cv. "Chuanju" had strong ability to adapt to perennial droughts. Under perennial drought environments, C. tangerina Hort. cv. "Chuanju" adopted specific adjustments to resist adverse environments. Such adjustments involved in growth regularities, plant/leaf shapes and life spans, leaf thicknesses and structures, stomata densities and sizes, leaf water saturation deficiencies, dehydration rates and relative water contents, leaf water potentials, proline/soluble sugar contents, Pn/Tr. Perennial droughts were the main causes of canopy shoot deaths, slow fruit developments, seed abortions, seedcase gummoses, juice sac granulations and decreased element contents of fruits/spring shoot leaves.