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12 Arbor Age / January/February 2014 www.arborage.com PLANT HEALTH CARE PLANT HEALTH CARE On a hot day in late summer, an observant city forester noticed something different between two groups of linden trees planted along opposite sides of the street. The lindens growing along one side were showing the typical symptoms of late-season drought: yellowish-green colored leaves, scorched edges, and an overall "wilty" appearance. Along the opposite side of the street, the linden trees were growing under utility lines and generally looked pretty good. Although the leaves appeared slightly smaller, they were still green — darker green, in fact, than would be expected — and they had no scorched edges. Despite the same diffi cult grow- ing conditions as the trees across the way, why were trees on this side of street thriving while the others suffered? Curious, the forester looked into the history of the site and learned the trees growing under the power lines had been treated with the tree growth regulator paclobutrazol. The product has been widely used in the industry for decades to minimize the rate at which pruned trees would grow back into utility lines. While this would account for the smaller leaves he ob- served, it didn't seem to account for these tree's ability to better withstand the stressful growing environment of this streetscape. This serendipitous observation would eventually lead to scores of research trials over several years that would show tree growth reg- ulators were doing much more in trees than just reducing growth. The study and use of plant growth regulators began with agricultural crops in the 1930s. Broadly defi ned, a growth regulator is simply any chemical used to alter the growth of a plant or a part of the plant. While one could technically argue water and nutrients are chemicals that alter the growth of plants, growth regu- lators more specifi cally work with plant hormones to achieve their result. There are growth regulators that can stimulate accelerated growth by promoting the formation of auxins, growth regulators that can decrease fruit production by affecting the formation of cytokinins, and growth regulators that can ripen fruit by increasing ethylene. The growth regulator mentioned ear- lier, paclobutrazol, decreases vegetative growth by inhibiting the formation of gibberellins, which are the group of hormones responsible for cell elonga- tion. When applied to, say, a tree under a power line that had just been trimmed, these treatments can reduce the length of regrowth the tree would naturally respond with. The tree still produces the same number of cells, only those cells do not stretch out as far. Over a three-year period, treated trees grow at a rate 40 to 70 percent less than the growth rate of the untreated trees. So, how does a product that is known to reduce cell elongation result in trees being seemingly more drought and stress tolerant? That answer would come though research aimed at better understand- ing the secondary effects growth reg- ulation was having on trees. Trees pro- duce their energy by photosynthesis, which takes place in the leaves. Thus, it should follow that smaller leaves should result in lower photosynthe- sis rates and less energy. However, B y B r a n d o n M . G a l l a g h e r W a t s o n 3 Things You Can Do Things You Can Do with a Tree Growth with a Tree Growth Regulator… Regulator… Besides Regulate Growth Besides Regulate Growth Untreated leaf at 3 years. Treated leaf at 3 years.