COS 165-2 - Take my breath away: seed respiration responses to increased heat stress

Abstract: Globally, vegetation communities are increasingly experiencing hotter than average temperatures, including extreme high heat events and unpredictable rainfall. Little is known about the vulnerability to high temperature stress of the very earliest life history stages of plants: germinating seeds. Seeds are not inherently vulnerable while dormant and dry, however once seeds are imbibed, internal metabolic processes begin and any external changes to the environment, such as heat stress, may cause irreparable damage. Using seed respiration as an indicator of viability and vigour, we sought to understand the extent to which the respiration rate of imbibed seeds would change with an increase in temperature from 20°C to 100°C. To better understand the impact of heat loading on these vulnerable seeds we compared responses to ramped vs pulse temperatures of a common duration. We directly compared pulse temperature with ramp temperature at nine different temperatures. To address these questions, we compared four Acacia species from two contrasting biomes in Western Australia: two from the temperate southwest and two from northwest Pilbara, semi-arid to desert. While this study didn’t directly compare responses between these two biomes, a secondary aim was to determine whether there may be observable differences between these two biomes. Seed respiration was then measured using a Q₂ seed respirometer over the course of 3 to 5 days. We found that the seeds from all four Acacia species had fast respiration rates after exposure to 20°C to 50°C but after seeds were exposed to 60-70°C, respiration rates decreased significantly until the seeds died at 80°C. Ecologically speaking, the temperatures experienced by seeds within the top 5cm of the soil seedbank in the northwest Pilbara can easily reach these 80°C temperatures. With increased temperatures expected as a result of climate change, we anticipate a potential shift in community composition with key species not able to regenerate after an extreme heat stress event leaving potential space for an increase in heat-tolerant species and a reduction of biodiversity.