Dry heat exposure increases hydrogen peroxide levels and breaks physiological seed coat-imposed dormancy in Mesembryanthemum crystallinum (Aizoaceae) seeds

Abstract

Background and aimsSeeds from many Aizoaceae species are characterized by their ability to survive extremely high temperatures. In addition, physiological seed dormancy has been described for some members of the family. In this study, we investigated whether we could identify Aizoaceae species with physiological seed dormancy that can be broken by heat exposure. Furthermore, we explored the morphological and biochemical mechanisms through which heat may break this dormancy. MethodsFor 24 Aizoaceae species, germination percentages were compared following exposure to heat treatment (103 °C for 17 h) and control conditions (18 °C and 15% RH). Seeds from Mesembryanthemum crystallinum were also exposed to ultra-drying treatment (3.93 and 2.51% MC). Morphology of the outer seed coat was characterized by SEM, whereas permeability of the seeds was assessed through seed mass increases in water, as well as through analysis of dye infiltration using seed sectioning and light microscopy. Seed oil content was determined by TD-NMR, whilst H2O2 levels were quantified using a spectrophotometric DMAB-MBTH protocol. Key resultsThree Aizoaceae species showed an increase in total germination following dry heat treatment. The physiological seed coat-imposed dormancy of Mesembryanthemum crystallinum seeds was found to be broken completely by dry heat exposure, which imposes both high temperature and ultra-drying stress, but not by high temperature or ultra-drying alone. Although no structural changes of the outer seed surface were detected using SEM, imbibition of water increased after dry heat treatment, whilst the seed coats remained impermeable to high-molecular-weight compounds. Analysis of H2O2 levels indicated that exposure to dry heat increased levels of this reactive oxygen species in M. crystallinum seeds relative to control conditions. ConclusionsHere we describe for the first time in a species of the Aizoaceae family that exposure of seeds to dry heat can break physiological seed coat-imposed dormancy completely. Although M. crystallinum seeds became more permeable to water following dry heat treatment, this was not the case for methylene blue dye, suggesting that dry heat does not remove a potential barrier to diffusion of high-molecular-weight compounds. However, the observed elevation in H2O2 levels following dry heat treatment, possibly enabled by enhanced oxygen uptake through increased seed permeability, indicates a potential mechanism by which physiological dormancy is released in M. crystallinum seeds.