Testa morphology and permeability of weedy Amaranthus species

The testa (seed coat) plays an important role in the regulation of seed dormancy and germination as well as the exchange of water, gas and solutes from the environment. In this study we investigated seed coat permeability and its relationship with germination kinetics in contrasting populations of three weedy Amaranthus sp: A. palmeri, A. retroflexus, and A. tamariscinus. The microstructure of the testa was observed for two of these species using Scanning Electron Microscopy and a bespoke image analysis solution was developed to quantify the thickness of the testa in different regions of the seed. The permeability of the testa in contrasting populations was assessed using a range of pesticidal compounds with varying physicochemical properties, and germination responses to gibberellic acid and norflurazon were evaluated. Our results revealed both within and between species differences in testa thickness and permeability, however it was challenging to associate these differences with responses to germination stimulants. Burial experiments demonstrated that testa permeability is a dynamic trait which can change over time depending on the solute. This work provides insights for colleagues using germination stimulants or inhibitors to study seed physiology as well as fundamental understanding for developing more effective weed management strategies. Future work could focus on the biochemical and molecular basis of differential testa permeability in seeds.