Cosmetic Stay-green Trait in Snap Bean and the Event Cascade That Reduces Seed Germination and Emergence

Abstract

The persistent color (pc) trait in snap bean (Phaseolus vulgaris L.) is a member of the stay-green gene family and falls into the cosmetic subclass. Cosmetic stay-green variants remain green but lose photosynthetic competence during senescence. It is an economically useful trait in snap bean as a result of its effects on pod quality. The trait produces a dark-green, uniform appearance of fresh pods, but has other pleiotropic effects, including a light-green seed color, bleached-white cotyledons on emergence, and foliage and pods that remain green even while senescing. One additional pleiotropic effect is reduced field germination and emergence compared with white- and colored-seeded genotypes. Nevertheless, with the aid of seed-applied fungicides, pc types occupy ≈40% of commercial snap bean acreage in the United States. This research project was aimed at understanding why and how germination and emergence is affected in pc beans. The effect is thought to be related to soil-borne pathogens because fungicide treatment of pc seeds increases germination and emergence rates to levels comparable to treated white- and colored-seeded genotypes. For our experiments, we increased seeds of 45 experimental lines and commercial cultivars (25 of which were pc) under uniform growing conditions. Initial experiments documented that, in the laboratory, all seeds analyzed in a tetrazolium test had high viability. Furthermore, untreated seeds of pc and non-pc types germinated in the laboratory showed no difference in germinability, whereas in the field, germination of pc types was reduced significantly. In addition, pc types showed substantially greater infection rates of seeds and seedlings, with the main pathogen being Fusarium oxysporum Schl. f. sp. phaseoli Kendrick & Snyder. Water uptake by green pc seeds was significantly more rapid than white and colored seeds. Measurements of electrical conductivity revealed that pc types had greater solute leakage than other seed types. When seed anatomic structure was examined, pc types had a significantly thinner testa, especially the osteosclereid layer. The reduction in germination and emergence appears to begin with a thinner, more fragile testa showing increased cracking that may happen during seed harvest and conditioning (but certainly does happen during imbibition), allowing more rapid water uptake during germination that leads to testa rupture. Increased and rapid solute diffusion into the surrounding spermosphere stimulates and attracts pathogens to colonize the seeds before seedlings can become established. Seed handling and conditioning processes before planting could be modified to improve field emergence and stand establishment. Selection for thicker testa may also mitigate some of the damage observed during germination of pc cultivars.