A great number of varieties are successfully grown in the lower desert of Arizona. New varieties are released or removed from commercial production each year, making it impractical to list them in this publication. The variety a grower may use is based both upon physiological considerations and personal preferences for a particular variety. All varieties are bred for early vigor, size, earliness and uniformity of maturity, shape, texture, pest resistance and so forth. For more specific information regarding varieties in your area, consult your local county Cooperative Extension Office or a seed dealer. For planting purposes, the lettuce season is typically divided into three categories, early (late-August-September 30; known as the 'Empire' slot), mid (October 1-25; known as the 'Winterhaven' slot), and late (October 25-December 10; known as the 'Van Guard' slot).
The primary physiological considerations for selecting a variety are seed germination temperature, length of growing season, and day length sensitivity. Depending on variety, germination is inhibited by temperatures of 77 to 95° F. The prevention of germination due to high temperatures is known as thermoinhibition. A thermoinhibited lettuce seed will germinate when returned to more suitable temperatures. However, continued exposure to high temperatures may induce a secondary dormancy, called thermodormancy. A thermodormant seed will not germinate even when returned to non-inhibiting temperatures. Germination of lettuce may be promoted by light and inhibited by dark; this is known as photodormancy. Specifically, far-red light inhibits germination while red light (abundant sunlight) promotes germination. The leaf-lettuce varieties are often photodormant.
Both thermodormancy and photodormancy may be alleviated by priming. Priming, also know as osmopriming or osmoconditioning, is a controlled hydration process in which the amount of water seeds imbibe is controlled by an osmoticum, typically polyethylene glycol. During priming the seed imbibes enough water to start many of the physiological processes associated with germination, but not enough to cause the radicle to protrude, or the seed to germinate. In addition to alleviating thermo- and photo-dormancy, priming shortens the germination time and synchronizes germination, thus leading to more uniform stands.
Although only certain varieties are primed, virtually all lettuce seed is pelletized (Fig. 2). During this process an inert material, usually diatomaceous earth, is coated around individual lettuce seeds. This allows singularization of the seed during planting. Although necessary, both the priming and pelleting process lead to shortened shelf life, compared to a seed that has not been similarly treated. The cause of this is not currently known, but new technologies are being developed which minimize these negative effects.
Figure 2. Pelletized (right) and raw lettuce seed (left)
Seed deterioration is a naturally occurring phenomenon, but steps may be taken to minimize this process. Seed should be kept at cool temperatures and not exposed to high relative humidity. A good rule of thumb is the 50/50 rule, which states that the combination of temperature (°F) and humidity should not exceed 100. Seed exposed to temperatures above 104° F for as little as two days will have demonstrably lower vigor. The amount of time seed is allowed to remain in the back of pickups, on loading docks, or on a dry seed bed, should be minimized, as it will contribute to accelerated seed deterioration resulting in lowered vigor, and eventually low overall germination.