The Effects of some Carbohydrates on the Growth and Organization of Citrus Ovular Callus

Abstract

Summary It has recently been found that long culture periods and resultant reduced rates of growth are associated with increased organization in this callus. This suggested that a depletion of some component in the medium may have been directly or indirectly responsible. The effects of glucose, galactose, fructose, sucrose, lactose, maltose, sorbose, and starch on callus growth and organization were investigated. All carbohydrates were initially added to the basal nutrient medium at a rate of 0.15 M which was previously determined to be the optimum concentration for sucrose supplementation. Sucrose promoted callus growth to the greatest extent followed by glucose, fructose, lactose, galactose, starch, maltose and sorbose in this order. The callus showed a definite preference for sucrose over its constituent monosaccharid es. All the carbon sources except sorbose were relatively effective at promoting growth. It is probable that some hydrolysis of the polysaccharides occurred during autoclaving but the protracted growth of callus indicates the presence of hydrolytic enzymes. A marked increase in organization into embryos and pseudobulbils was brought about by lactose and maltose. The high proportion of embryos (with hypocotyl/root axes) relative to pseudobulbils (lacking such axes) was particularly noticeable in cultures on lactose and to a lesser degree in those on maltose. The reason for the enhanced organization is obscure, especially since the component hexoses were ineffective on their own. Callus in which organization was taking place comprised cells which contained an abundance of lipid droplets and starch granules which were associated with atypical chloroplasts. It is probable that this starch accumulation resulted from photosynthesis since it only occurred in the green callus. Chloroplasts invariably contained large amounts of starch and in most cases lacked well-defined grana. In all cultures other than those on lactose and maltose, chloroplasts were limited to differentiated organs. There were also fewer starch granules and lipid globules in cells of these cultures. Despite the slower growth of callus on lactose and maltose, this is unlikely to account for the enhanced organization as even slower rates of growth were obtained with some other carbohydrates which supported no organization. It seems likely that maltose and more particularly lactose, encouraged the formation of chlorophyll which then leads to starch accumulation and organization. The entire modus operandi of these two carbohydrates on organization requires further investigation.