As tomato plants grow they partition how much calcium goes to leaves and how much to developing fruit. Under rapid growth, or growth under stressful growing conditions, the fruit may get the short end of the stick. Calcium deficiency in the fruit results in blossom end rot. Recent studies show the application of the plant hormone abscisic acid to leaves or roots of young plants can alter calcium partitioning and help relieve the problem.
HortScience, 2014, 49:1397-1402