Arbor Age

www.arborage.com Arbor Age / January/February 2014 13 research was showing trees treated with gibberellic acid inhibitors were producing the same amount of en- ergy as untreated trees. As is turns out, gibberellic acid is made from some of the same base compounds as chloro- phyll. Thus, reducing gibberellic acid formation results in an increase in chlorophyll production. This accounts for the trees photosynthesizing at the same rate despite smaller leaves. It also accounts for the darker green appear- ance trees treated with paclobutrazol display. Which leads us to our fi rst "thing you can do with a growth regulator besides regulate growth": greener leaves. One of the uses our tree care com- pany has found for growth regulators, such as paclobutrazol (Cambistat) is for greening up trees suffering from mild chlorosis. Chlorosis is a condi- tion in which the plant cannot prop- erly manufacture chlorophyll. It can be related to micro-nutrient defi ciencies, poor soil habitats, or other factors that can affect a plant's ability to thrive. Although growth regulators have no nutrient value, we have observed trees go from a pale green color to a richer green simply through an application of paclobutrazol. Chlorosis resulting from nutrient defi ciencies can be corrected in two ways: provide the plant with the nutrient (such as when we inject soluble iron into a tree suffering from a lack of iron) or by improving the tree's ability to obtain nutrients from the soil. It appears that growth regulation is doing two things that can help with chlorosis management: increase chlorophyll production resulting from the decrease in gibberellic acid production, as well as help the tree bet- ter mine nutrients from the soil. This leads to our second "thing you can do with a growth regulator besides regulate growth": improve root systems. As mentioned, trees treated with growth regulators produce an equal amount of energy as untreated trees, but where they put that energy differs. Think of energy like funds in your bank accounts, the more you allocate to one account, the less you have to allocate to another. With plants, the less energy they are spending on vegetative growth, the more they have to allocate to other resources, including defense compounds, reproductive structures such as fl owers, carbo- hydrate storage, and fi brous root growth. This last one, root growth, has myriad uses in tree care. Arborists will often employ these treat- ments on trees that will be enduring root damage during an up- coming construction project as a means of pre-stress conditioning. They are also utilized after root loss or damage has occurred to encourage the tree to form new roots. It has been shown they can help trees adapt to poor soil conditions better. However, it should be noted that these treatments will not help roots grow in compacted, cement-like soils. Success rates go up signifi cantly when these treatments are combined with physical remediation such as Air-spade work or vertical mulching. Basically, if you can improve the soil conditions for roots, a growth regulator can encourage roots to grow into it. The forester who noticed growth- regulated trees were looking better come late summer stumbled upon our third "thing you can do with a growth regulator besides regulate growth": in- crease drought tolerance. As mentioned in previous articles on drought and trees, plants respond to drought by utilizing abcisic acid to close their stomata and reduce water loss. It turns out — just like we saw with chlorophyll — that gibberellic acid is made from some of the same base compounds as other plant hormones such as abcisic acid. So, by reducing the amount of gibberellic acid the tree is producing, we are increasing the amount of abcisic acid. This slower- growing tree will be putting more energy into root production than shoot production; and with the increased abcisic acid, it can be more responsive to urban tree stress conditions. These treatments also trigger changes in leaf morphology, such as a thicker waxy cuticle layer and increased trichromes — the leaf hairs that help protect the stomata. Although nothing will help a tree through drought times better than water, growth regulators represent one of a small num- ber of tools arborists have to help trees along when it's dry. Fifty years ago, when these products were fi rst being developed, no one could have envisioned all the ways they would be used to benefi t trees. Growth regulators are a good example of where astute observation, lab and fi eld research, and innovative arborists intersect to forward the science and practice of tree care. New re- search is leading to new uses for growth regulators with managing insects and diseases, as well as extending the service life of urban trees. It's a good reminder to stay observant of your management actions and to always be curious about the results you are seeing, because you never know where arboriculture's next breakthrough will come from. Brandon M. Gallagher Watson is director of communications at Rainbow Treecare Scientifi c Advancements, and is an ISA Certifi ed Arborist (#MN- 4086A). 3 Things You Can Do with a Tree Growth Regulator… Besides Regulate Growth Foliage of Honeylocust control specimen. Foliage of treated Honeylocust. Photos provided by Rainbow Treecare Scientifi c Advancements

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