Involvement of ethylene and endo-beta-mannanase in lettuce seed germination at high temperature

The lettuce seed endosperm can delay or prevent germination at high temperature by acting as a physical barrier to radicle protrusion. Weakening of the endosperm tissue around the radicle tip is necessary for germination at high temperature. The objective of this research was to investigate the association of ethylene and endo-β-mannanase with lettuce endosperm weakening and germination at high temperature. Several factors that may alter thermotolerance, such as genotype, seed maturation conditions, light, ethylene, priming, and vigor were used to elucidate the involvement of ethylene and endo-β-mannanase in endosperm weakening. Thermotolerant genotypes (‘Everglades’ and PI 251245) had more ethylene evolution and endo-β-mannanase activity prior to radicle protrusion at high temperature (35°C), than thermosensitive genotypes (‘Dark Green Boston’, ‘Valmaine’, and ‘Floricos 83’). ‘Everglades’ and ‘Dark Green Boston’ pollinated and matured under 30/20°C day/night temperatures had more endo-β-mannanase and germination at 35°C compared to those matured at 20/10°C. In thermotolerant ‘Everglades’ germination in the dark lead to reduced ethylene evolution and endo-β-mannanase activity compared to germination in the light (prior to radicle protrusion). The ethylene precursor, 1-aminocyclopropane-1-carboxilic acid, applied during seed germination at 35°C or during priming, lead to increased endo-β-mannanase activity prior to radicle protrusion and circumvented thermoinhibition in ‘Dark Green Boston’. Conversely, the ethylene action inhibitor, silver thiosulphate, applied during seed germination at 35°C or during priming lead to decreased ethylene evolution, endo-β-mannanase activity, and seed germination at high temperature in both ‘Dark Green Boston’ and ‘Everglades’. Endo-β-mannanase activity was detected during priming (without radicle protrusion). Priming led to retained endo-β-mannanase activity and circumvented thermoinhibition of ‘Dark Green Boston’. Aged seeds of ‘Everglades’ had lower ethylene evolution and endo-β-mannanase activity prior to radicle protrusion and germinated at 60% at 35°C, whereas nonaged seeds had more ethylene evolution and endo-β-mannanase activity and germinated 100%. A relationship between ethylene production and endo-β-mannanase activity prior to radicle protrusion suggested that they induced weakening of the endosperm allowing germination of lettuce at high temperature.