One lump or two?

Tree loss soon after transplanting is a serious problem for all involved in the landscape industry. Recent reports from the University of Ulster and the University of Washington have found failure rates for tree planting are typically 25 to 50 percent. Such losses can prove to be a heavy financial burden.

One of the main reasons for these high failure rates is that even when accepted nursery practices are followed, less than 5 percent of the actual root system may be moved with a tree. This results in severe water stress following leaf flush in spring because the root system is of insufficient size to support the water demand of the tree crown, a phenomenon commonly known as transplanting shock.

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Although several factors have been associated with transplanting shock, it is now widely believed that survival of newly planted trees is largely dependent on rapid root growth so that the plant regains its ability to absorb water from the soil, replenish water loss and subsequently reduce water stress. A treatment that could be applied to a root system prior to or just after transplanting would be a great benefit to the industry.

Researching sugar

One of the simplest and most promising compounds tested by the Bartlett Tree Laboratories (Charlotte, N.C.) and University of Reading in England, has been low concentrations of sugars.

We know that tree growth occurs in the presence of carbon dioxide, water and oxygen, driven by the power of sunlight. However, note that the end product of photosynthesis is sugar — more accurately, sucrose: the same type of sugar we use to sweeten our coffee. This, then, begs the question: Why not simply feed sugar to trees directly?

Preliminary results have been interesting. For example, supplying sugars to birch root systems following severe pruning (removal of 90 percent of the root system to simulate transplanting shock) increased root metabolism by promoting lateral root branching and root formation (see Table 1, left).

Although sugars gave good results with birch in this instance, they should be interpreted with some degree of caution. At present, it is more than simply mixing a bag of sugar with soil or compost and then planting the tree. If the sugar concentration is too high, this can put the tree under osmotic stress or encourage the buildup of pathogenic fungi in the soil. Further, the sugar concentration that worked well for birch proved detrimental to other species such as beech and red oak.

This, then, leads to the additional question of how exactly do sugars promote root growth? Work by the University of Florida and the Scottish Agricultural College has gone a long way in explaining the answer. Researchers at both locations have found that sugars function not only as food for growth, but also affect the sugar-sensing systems that initiate changes in gene expression. In other words, they help regulate the type of growth that will occur (roots vs. shoots, for example), not just the amount of growth. For example, incubation of root systems in sugar leads to the repression of photosynthetic genes, decreased rates of photosynthesis and enhances root development.

These results clearly demonstrate that sugars enhance not only the genes for root growth, but also the genes involved with defense against fungal, bacterial and insect pests (see Table 2, left). A tree suffering from transplant shock is probably more susceptible to attack from these pests than at any other time during its life. The possibility of sugars having a dual role in altering the growth pattern of trees in favor of root formation and enhancing resistance by treating roots during or immediately after transplanting offers potential for a simple system for reducing transplanting losses and increasing disease resistance. It should be emphasized, however, that although we know that genes for defense are enhanced, the degree of resistance for the whole tree is unknown.

These potential benefits may also account for the carbohydrate- or molasses-/sugar-based fertilizers that have recently emerged in the commercial market. Manufacturers claim these fertilizers work by stimulating vigor of root crops such as leek, potato and carrot, increasing yields by up to 20 percent. Unlike conventional N-P-K fertilizers, these fertilizers contain sugars.

Continuing research sponsored by the International Society of Arboriculture at the Bartlett Tree Research Laboratories is currently investigating the influence of sugars singly and in combination with fertilizers. Ultimately, this work is aimed at determining the optimal sugar type, concentration and mode of application required to maximize root growth without the associated problems of osmotic stress and disease buildup. Some day, tree planters may actually ask, “Would you like sugar with your tree?”

Dr. Glynn Percival is a researcher at the Bartlett Tree Research Laboratory in Reading, England. Dr. E. Thomas Smiley is a researcher at Bartlett Tree Research Laboratory in Charlotte, N.C.

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