Mechanism of seed priming in circumventing thermodormancy in lettuce

Lettuce (Lactuca sativa L. cv Minetto) seeds were primed in aerated solutions of 1% K(3)PO(4) or water at 15 degrees C in the dark for various periods of time to determine the manner by which seed priming bypasses thermodormancy. Seeds which were not primed did not germinate at 35 degrees C, whereas those which were primed for 20 h in 1% K(3)PO(4) or distilled H(2)O had up to 86% germination. The rate of water uptake and respiration during priming were similar regardless of soak solution. Cell elongation occurred in both water and 1% K(3)PO(4), 4 to 6 h prior to cell division. Both processes commenced sooner in water than K(3)PO(4). Radicle protrusion (germination) occurred in the priming solution at 21 h in water and 27 h in 1% K(3)PO(4).Respiration, radicle protrusion and cell division consistently occurred sooner in primed (redried) seeds compared to nonprimed seeds when they were imbibed at 25 degrees C. Cell division and elongation commenced after 10 h imbibition in primed (redried) seeds imbibed at 35 degrees C. Neither process occurred in nonprimed seeds. Respiratory rates were higher in both primed and nonprimed seeds imbibed at 35 degrees C compared to those imbibed at 25 degrees C, although radicle protrusion did not occur in nonprimed seeds which were imbibed at 35 degrees C. It is apparent that cell elongation and division are inhibited during high temperature imbibition in nonprimed lettuce seeds. Seed priming appears to lead to the irreversible initiation of cell elongation, thus overcoming thermodormancy.