The Adaptation to Freezing Tolerance of Hydrated Lettuce Seeds: Effects of Regional Climate and of Seed Characteristics

Abstract

BACKGROUND: With global warming, soil seed banks at high altitudes face dual challenges, excessive water absorption and thinner snow cover that increase underground temperature. A better understanding of freezing tolerance of hydrated seeds provides insights for conservation in natural soil seed banks. OBJECTIVE: To understand the adaptation mechanisms of seed freezing tolerance under various climates, in relation to cooling rate and seed size. MATERIALS AND METHODS: Twelve ecotypes of lettuce (Lactuca sativa) seeds were collected from different geographical locations around the world. Seeds were fully hydrated and tested for their freezing tolerance using programmed cooling methods. RESULTS: The size of seeds from different climate regions varied, and was correlated with the freezing tolerance of the hydrated seeds (P< 0.05). Larger seeds showed poorer freezing tolerance. The local climates of maternal plants were also well correlated to seed freezing tolerance ( P< 0.05), especially under slow cooling conditions. The seeds collected in regions with high spring rainfall exhibited greater freezing tolerance. CONCLUSION: Freezing tolerance of hydrated seeds is affected by the climate of maternal plants and by seed size. Our data revealed the existence of an adaptation mechanism of freezing tolerance among various ecotypes of lettuce seeds.