Analysis: Sensitivity to Heat

Lettuce Seed Laboratory Analysis is all about TIME and TEMPERATURE

1A, 1B, 2A, and 2B

When we perform laboratory analyses of lettuce seeds, we rate them by various designations. The sample and germination designations are as follows:

• 1A and its corresponding % germ indicates the raw seed germination percentage in 24 hours @ 23 degrees C / 72 degrees F.
• 1B and its corresponding % germ indicates the ‘seed priming indicator’ germination percentage in 24 hours @ 23 degrees C / 72 degrees F.
• 2A and its corresponding % germ indicates the raw seed germination percentage in 24 hours @ 28 degrees C / 83 degrees F.
• 2B and its corresponding % germ indicates the ‘seed priming indicator’ germination percentage in 24 hours @ 28 degrees C / 83 degrees F.

Why 28C and Not Hotter?

Germination temperatures in desert growing areas can often exceed 28 degrees Celsius  (83 degrees Fahrenheit). But we test seeds at 28 degrees C. Why? Because 28 degrees C is the temperature at which thermo inhibition is first induced in most lettuce varieties. In addition to temperature, the seed is further stressed by germination in a light-free environment. The ability of temperature to induce thermo inhibition is intensified in the absence of light. Most seeds are covered with soil after planting, which limits their exposure to light.

We determined these germination protocols through many years of experience correlating laboratory results with field results. Independent research in ensuing years demonstrated that the daily (24hr) wet soil/irrigated average temperatures was 28C. This includes days when maximum daytime temperatures could easily exceed 45 degrees C (113 degrees Fahrenheit).