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More than just a number

In a longer growing season, Dr. Rick Knowles has found the  greater number of smaller tubers from an older seed lot will equal the overall weight of the fewer, but larger, tubers produced by a younger seed lot, with comparable yields overall. Photo courtesy Dr. Rick Knowles.

There are countless factors at play that influence potato yields from year to year, and growers constantly face the challenge of ensuring they maximize their chosen varieties’ yield potential.

Recent research, however, has illuminated a new way to think about yield – namely, in relation to the seed potatoes’ physiological age, or the functional life expectancy of the potato. According to Rick Knowles, a professor in Washington State University’s department of horticulture, determining seed potatoes’ physiological age prior to planting can help commercial potato growers increase yield potential.

The older the physiological age of a seed potato lot, says Knowles, the more plant emergence and establishment speeds up, the greater the increase in stem numbers and tuber set, and the greater the decrease in apical dominance, or the inhibition of lateral bud development. Ultimately, greater physiological age results in a trend toward smaller-sized tubers.  

There is a direct relationship between the seed lot’s degree of apical dominance – the stem numbers per seedpiece – and the number of tubers, or the tuber set, per hill, says Knowles. The more tubers there are beneath each potato plant, the more competition for resources and the smaller the tubers.

The overall effect of physiological age on yield has a lot to do with the length of the growing season. In longer growing seasons, Knowles explains, moderate differences in physiological age between seed lots of the same cultivar likely won’t have a significant effect on overall yield – but differences in age could dramatically impact tuber size distribution.

“In a long growing season area where the crop can fully complete its annual lifecycle, the greater number of smaller average size tubers from an older seed lot will equal the overall weight of the fewer, but larger, tubers produced by a younger seed lot – and the overall yields will be comparable,” he says.

In shorter growing seasons, by contrast, an older seed lot might produce higher yields than younger seed lots, because more time during short growing seasons is devoted to tuber bulking.

Knowles and his team performed several experiments that proved these results in two ways. First, they produced seed of different physiological ages and compared the resulting effects on growth, development and yield in short and long growing season areas. They also compared the yield potentials of commercial seed lots produced in areas with either short or long growing seasons in side-by-side trials.

Controlling physiological aging
According to Knowles, the age of seed potatoes can be manipulated through the accumulation of heat units. “Allowing seed to accumulate degree days above 4 C base temperature during the maturation period and during storage will advance the physiological age of a seed lot,” he says.

Additionally, growers can control the period of maturation and the amount of time that tubers are exposed to temperatures above 4 C in storage.

“Prolonging the initial wound healing period at the beginning of storage or instituting and/or extending the warm-up period toward the end of storage prior to moving the seed through distribution channels will contribute to advancing the physiological age of a seed lot,” says Knowles.

Respiration is also key to the aging process of seed tubers in storage. In higher temperatures, seed respiration rates increase, and this accelerates aging. Higher temperatures also speed up changes in growth hormones in the buds that allow potatoes to sprout. One of these hormones, auxin, says Knowles, controls the degree of apical dominance in the potatoes, or the number of stems – and when temperatures increase age the number of stems also increases.

“More stems results in greater tuber set and more tubers,” explains Knowles. “The increased numbers of tubers compete for a limited amount of resources that can be supplied by the plant, thus resulting in a smaller average size of tubers.”

Minimizing respiration is therefore important in preserving the quality of seed potatoes in storage, and slowing respiration through cooling slows the rate of aging and prolongs postharvest life.

Knowles believes that greater control over the physiological age of seed potatoes will allow growers to manipulate tuber set and size development, and this will eventually pay off. “The various end-use markets – seed, fresh, process – differ in their tuber size class requirements, which is reflected in contracts,” he says. “Hence, growers can add value to a crop by controlling tuber size distribution to supply greater yields of the more lucrative tuber size classes in relation to market needs.”

Manipulating seed physiological age is not the only technique for controlling tuber size distribution, but it’s a potentially powerful one. Growers should be aware of their seed lot’s physiological age before planting – this way they can make informed decisions about in-row seed spacing. “Consistency in how seed is produced and handled will translate into consistent physiological age in terms of stem numbers from year to year,” says Knowles. This will provide opportunities to control tuber set and size development via other techniques such as spacing or treatments with growth regulators. 

July 3, 2014  By Julienne Isaacs