Alleviation of cotyledonal cracking in green beans (Phaseolus vulgaris L.) by calcium seed treatment.

Abstract

Cotyledonal cracking is a physiological disorder of common beans, and rarely, soybeans that occurs as transverse fissures across the cotyledons. The phenomenon is generally referred to as transverse cotyledonal cracking (TVC). Although TVC has been known for decades now, factors contributing to its occurrence, and how the disorder can be alleviated, are still not well understood. The objective of this study was to investigate the effect calcium seed treatment on cotyledonal cracking in green bean (Phaseolus vulgaris L.) seeds. Six cultivars (Imbali, Sodwana, Outeniqua, Elangeni, Tokai and Tongati) were examined for water absorption patterns during a 6-h imbibition in distilled water. Cultivars were categorised according to the rates of water absorption, in the presence of seed coat, and there was a significant correlation between seed size and water absorption rate. To examine seed predisposition to TVC, seeds were imbibed with and without seed coats and TVC was scored every hour for the 6-h duration of imbibition. There was a significant positive correlation between water absorption rate and TVC. Genetic analysis of the cultivars using SDS-PAGE revealed that there are possible differences between the resistant cultivars and sensitive cultivars, with respect to protein patterns. Imbali, one of the small cultivars (-1.5 g seed -1) that imbibed water uniformly, was resistant to cotyledonal cracking compared to the largest cultivar (Sodwana -2.5 g seed -1), which also had a high rate of water absorption. Priming seeds with calcium (CaS04, Ca(N03)2 and CaCl2) osmolarities (0, 1, 10, 50, 100, and 100 mM) increased seed calcium content and reduced susceptibility to TVC. Comparison of priming and seed coating with respect to field emergence, TVC, stand establishment and seed yield showed that coating was better than priming. However, greenhouse studies showed that the effect of priming in the progeny of treated seeds was significantly better than that of coating, with respect to TVC reduction. In both laboratory and field studies, it was clear that applying calcium concentrations greater than 50 mM was not necessary to alleviate TVC and improve seed performance. Seed germination and emergence were reduced at calcium concentrations greater than 50 mM. It is concluded that calcium is effective in controlling TVC under both laboratory and field conditions. The effect of calcium is associated with regulation of imbibition and improvement of seed calcium content. Enhanced seed calcium content likely improved cell wall integrity.