THE INDUCTION OF GROWTH IN EXPLANTED TISSUE OF THE BANANA FRUIT

Abstract

A method of obtaining proliferative growth of explanted fruit tissue from a number of cultivated varieties of bananas at different stages of development has been described.The explants weighing above 70 mg grow more rapidly than smaller pieces. Entire slices of the fruit, or only the axial part or pulp tissue have been cultured.A culture medium, as suggested by White, consisting of mineral salts, sucrose (2%), and vitamins, and solidified with 0.5% agar served as the basal nutrient medium. The chief supplements were various concentrations of casein hydrolysate; coconut milk; 2,4-D; benzothiazole-2-oxyacetic acid (BTOA); coumarin; 2,3,6-trichlorophenylacetic acid and other substituted phenylacetic acids; kinetin; adeninesulphate; and indoleacetic acid (IAA).The growth was recorded chiefly as the increment of fresh weight although dry weight and cell numbers were estimated in one experiment.Contrary to expectation the tissue from mature bananas responded better to the externally supplied growth substances than that from younger fruit. In fact the capacity for cell division did not decrease even in the pulp of the banana fruit as normally harvested for shipment. However, the tissue from postclimacteric fruit is incapable of any recrudescence of growth. Climacteric, therefore, seems to determine the point of no return beyond which the cells will not grow.Some growth occurred on the basal medium, but the best growth was obtained when 2,4-D; 2,3,6-TPA; or BTOA was added. Coconut milk increased the cell number but not markedly the fresh weight, depressed the effect of 2,4-D, but enhanced the response to 2,3,6-TPA. Among the numerous chloro-substituted phenylacetic acids tried, 2,3-; 2,6-; and 2,3,6-TPA were the most active. Adenine sulphate (in combination with IAA) enhanced the division of pulp cells.The ability to respond to the culture treatments is genetically controlled. Tissue from 'Lacatan' and 'Robusta'; 'T23' and 'T12' bananas of the same age and grown under identical conditions show a great deal of variation in their response to the different growth substances.It is the pulp region of the fruit which is most responsive to growth substances. A fluffy callus capable of being subcultured is produced from the pulp. Starch and other ergastic bodies are usually absent in the callus cells. No differentiation into organs has, however, been observed. The funiculi enlarge but do not form callus; the seeds are not affected.The utility of this technique for a further understanding of the problems of growth and senescence of fruits is discussed.When the explants were cultured in liquid media containing 2,4-D (with or without coconut milk) or 2,3,6-TPA in special glass tubes mounted on a wheel and slowly rotated around a horizontal shaft, a number of cells sloughed off into the bathing fluid. These were of diverse shapes and sizes. Aliquots of the liquid containing free cells have been successfully subcultured. However, under complete isolation and constant observation divisions in single cells were not observed.