F E B R U A R Y 2 0 1 9 12 POTATO GROWER For a long time, plant growth regulators have been used to manipulate the growth of plants. You may ask, why do they affect plant growth? The answer is there are many natu- rally occurring growth regulators that signal to the plant various processes, such as sprout- ing or stem elongation or tuber initiation. When we apply some type of plant growth hormone (I will refer to these as plant growth regulators) at extremely low rates we can manipulate plant growth. One of the most common regulators used by potato growers is gibberellic acid, often referred to briefly as GA or Gibb. For example, gibberellic acid is used to encourage more rapid sprouting and increase stem number which in turn adds to tuber count and decreases tuber size. Other plant growth regulators are used to adjust stem elongation or tuber initiation, manipu- late stem number, or influence tuber shape. When applications are too high, this can lead to undesired results such as overgrowth, too many tubers or even plant injury or death. Herbicides such as 2,4-D or dicamba act as a plant growth hormone. These herbicides used at extremely low rates can manipulate plant growth, but at higher doses they can cause plant death. The reason they work as herbicides is because they remain active in the plant much longer than natural hor- mones. The ability to make slight adjustments in potato plants can be valuable for seed, fresh, chip and process growers to increase or decrease stem number, tuber set and tuber size or to elongate tubers for fry yield. However, the use of these compounds has been frustrating for many because of a num- ber of reasons. Inconsistent application rate and varying varietal responses can lead to undesired results. There is not a one size fits all for plant growth regulators in potato pro- duction. All plant growth regulators act dif- ferently within the plant and responses vary based on cultivar or physiological seed age. How can you solve this mystery? One of the first things you may consider doing is start- ing with a consistent application method since these growth regulators are so potent. What we have done in our research studies is apply plant growth regulators as a seed treat- ment. That way, we could apply a specific amount of active ingredient on a ton of seed. In-furrow applications can vary based on nozzle angle, soil moisture, or how much a seed tuber may roll, among other factors at planting. It is not practical to dip large quan- tities of seed, let alone the difficulty and inconsistent in calculating the correct parts per million (ppm). Before you start using plant growth regula- tors, you must ask the question of what are you trying to accomplish and what com- pounds will be help you achieve that goal. To help unravel this mystery, we set up an experiment to test some different commer- cial plant growth regulators to see if we could define a standard seed treatment method to adjust stem and tuber number to improve tuber size profile, and to identify which cul- tivars responded favorably to the rates used. Our treatments consisted of 1) non-treated check, 2) a seed treatment of Stimulate (which contained 0.16 oz cytokinin/ton of seed + 0.09 oz gibberellic acid/ton of seed + 0.09 oz indole-3-butyric acid/ton of seed, 3) a Andy’s Advice: Understanding Plant Growth Regulators By Andy Robinson, Extension Potato Agronomist, NDSU/UMN