UPTAKE AND TRANSPORT OF CALCIUM AND THE POSSIBLE CAUSES OF BLOSSOM-END ROT IN TOMATO

The possible causes of blossom-end rot (BER) in tomato fruit were investigated by comparing the uptake of calcium by the roots, the distribution of Ca-45 within the fruit and the vascular bundle network in the fruit of susceptible cultivars (Calypso and Spectra) with those of a less susceptible cultivar (Counter) grown in a range of salinities (electrical conductivity of 5, 10 and 15 mS cm-1). The daily calcium uptake rates at 5 mS cm-1, as estimated from the xylem exudation of the decapitated stem stump in young plants of Calypso and old plants of Spectra, were lower than that of Counter. The uptake of Ca-45 by, and the transport to, the distal part of the detached fruit of susceptible cultivars, especially Calypso, were less than in Counter at 10 mS cm-1. The number of vascular bundles in both proximal and distal fruit tissues was similar in all cultivars and was only slightly reduced by salinity. However, the number of bundles containing lignified xylem vessels, as detected by safranin staining, was reduced substantially by salinity, particularly in Calypso. The estimated area of the fruit tissue served by individual xylem bundles in the BER susceptible fruit grown at high salinity was greater than in Counter. The incidence of BER in all trusses was linearly related to the product of average daily irradiance and daily temperature throughout the year. Temperature appears to be the major environmental factor which induces BER, regardless of cultivars and salinity treatment. The most likely causes of BER in susceptible cultivars are the interactions of (a) light and temperature on fruit enlargement, (b) inadequate xylem tissue development in the fruit and (c) competition between leaves and fruit for the available Ca.