Biuret toxicity symptoms in citrus leaves mimics cell senescence rather than nutritional deficiency chlorosis

Permanent chlorosis of leaves on plants fertilized with urea containing high levels of the contaminant biuret has been observed in several crops including citrus. Little has been reported as to the cellular changes that result from such chlorosis. Branches from 'Ruby Red' grapefruit (Citrus paradisi Macfadyn) and 'Hamlin' orange [C. sinensis (L.) Osbeck] were sprayed with urea solutions containing 1.05% biuret. As visible symptoms developed, leaf tissue samples were prepared for transmission electron microscopy. For comparison purposes, leaves from similar trees showing chlorosis from age-related senescence and Zn deficiency were also sampled. The progressive development of chlorosis in biuret-affected leaves was characterized by: the loss of starch, thylakoidal and granal membranes in chloroplasts along with the enlargement and increase in number of plastoglobuli or lipid bodies. The lipid bodies were liberated alone or in association with membrane vesicles to the cytoplasm and vacuoles. The number and volume of the individual chloroplasts became smaller. Concurrent loss of cytoplasmic content and the enlargement of the vacuolar space were also observed in the biuret affected leaf tissue. Similar findings were observed in the cells of senescent leaves. In cells of leaves showing nutritional deficiency, losses in cytoplasmic content and vacuolar enlargement were observed but there was neither complete loss of thylakoidal or granal membranes nor the release of lipids from the plastids. It was concluded that 1) the cytological characteristics of the biuret-affected samples were more similar to age-related senescent samples than to chlorosis from Zn deficiency and 2) that complete loss of the lipid bodies from the chromoplasts to the cytoplasm and vacuole in the biuret-affected samples and in age-related senescence in citrus leaves was responsible for the permanent nature of the chlorosis.