Arboriculture & Urban Forestry 35(2): March 2009 levels in leaves and roots. Although leaf glucose levels decreased in September, they were still higher than glucose levels in trunk and twigs. To the contrary, species such as pecan (Carya illinoin- ensis (Wangenh) K. Koch) or ponderosa pine (Pinus ponderosa Laws.) showed constant concentration of glucose during the growing season in different tissues of the tree (Grulke et al. 2001; Kim and Wetzstein 2005). Foliar glucose levels decreased in July and September, possibly due to the developmental stage of leaves (Figure 1). The glucose fraction in roots reached the highest con- centration in the fall which may likely indicate a high transloca- tion of sugars to roots for storage purposes. Root glucose levels were lower in the March–July period, which corresponds to the time when the roots had the highest levels of starch (Figure 2). The decrease in glucose content in leaves during fall can be a result of the high translocation and replacement of carbohydrates from source to sinks or storing organs (McLaughlin et al. 1980). 65 indispensable role during the initiation of vegetative growth in the spring. The reduction of starch levels observed in the summer is like- ly a consequence of growth and high night temperatures and is as- sociated with the hydrolyzation of stored carbohydrates for growth and maintenance (Kaipiainen and Sofronova 2003; Pallardy 2008). Even when the interaction of type of tissue and time of year was significant, the annual mean carbohydrate concentrations varied among different tissues in trees (data not shown). Glucose levels in leaves were about double than those in twig, root, and trunk tissues. For starch, an inverse pattern was found where root and trunk tissues had more than three times the starch content than foliar tissues. This overall concentration pattern corrobo- rates source-sink mechanisms for carbohydrate partitioning de- scribed in other studies (Kaipaiaien and Sofronova 2003; Kaelke and Dawson 2005; Taiz and Zeiger 2006). Dean (2001) indi- cated that root allocation can be affected by stem competition because stems precede roots on the chain of carbohydrate sinks. For future studies aimed at monitoring carbohydrate levels Figure 2. Starch concentrations (mg/g DW of glucose) measured in live oak samples collected at four different dates. Bars repre- sent the SE of the mean. Glucose concentrations in leaves might increase in winter because live oaks maintain live leaves throughout the winter as a response to environmental factors. Ludovici et al. (2002) also found a two-to-three-fold increase in winter glucose concentra- tions when compared to summer levels in needles of loblolly pine (Pinus taeda L). The high levels of glucose in leaves during the winter assessment can be caused as a response to the low temper- atures [below 0 ºC (32 ºF)] that occurred before the sample col- lection. Conversion of starch to sugar is a physiological manifes- tation of cold hardiness in trees (Levitt 1980; Nguyen et al. 1990). A significant interaction (P < 0.001) was also found for starch among time of year and tissues sampled. The highest starch concentrations were measured in roots and trunks dur- ing spring and winter (Figure 2), thus confirming their role in facilitating carbohydrate storage (Allen et al. 2005). Similar re- sults were found for trunks and roots of sessile oak which had higher quantities of carbohydrate reserves in autumn than in late summer (Barbaroux et al. 2003). Trunk tissues showed a depletion of starch from spring to the summer (Figure 2). Gan- sert and Sprick (1998) identified starch disappearance during the summer in beech. Low concentrations of starch were also found during the spring in Scots pine (Domisch et al. 2002). Starch concentrations found in roots and trunk (Figure 2) em- phasize the importance of these two organs as carbohydrate res- ervoirs during the dormant season. These storage tissues have an in trees, additional factors must be considered. For example, twigs exhibited less variation compared with other tissues ana- lyzed and were easy to collect any time of year without causing considerable severe injury to the trees. Leaves are only avail- able during the growing season and it requires being careful to obtain a homogenous sample during the dormant season. In the case of sampling roots and trunks, the tree sustains more dam- age that may disrupt physiological functions. In addition, fine root sampling can be complicated due to the partial loss of the sample during the washing process (Kaipiainen and Sofronova 2003). Changes depending on species have also been reported for carbohydrate partitioning (Newell et al. 2002). Based on the results of this study, trunk, root, twig, or leaf tissue, and can be used to monitor glucose and starch levels throughout the year; however, the seasonal variation in different parts of the tree and the different species should be taken into consideration. LITERATURE CITED Abod, S.A. and A.D. Webster. 1991. Carbohydrates and their effects on growth and establishment of Tilia and Betula: I. Seasonal changes in soluble and insoluble carbohydrates. Journal of Horticulture Science 66:235–246. Allen, M.T., P. Prusinkiewicz, and T.M. Dejong. 2005. Using L-systems for modeling source-sink interactions, architecture and physiology of growing trees: the L-PEACH model. New Phytologist 166:869–880. Barbaroux C., N. Breda, and E. Dufrene. 2003. Distribution of above- ground and below-ground carbohydrate reserves in adult trees of two contrasting broad-leaved species (Quercus petrea and Fagus sylvati- ca). New Phytologist 157:605–615. Dean, T.J. 2001. Potential effect of stand structure on belowground al- location. Forestry Science. 47:69–76. DeLucia, E.H., T.W. Sipe, J. Herrick, and H. Maherali. 1998. Sapling biomass allocation and growth in the understory of a deciduous hard- wood forest. American Journal of Botany 85:955–963. Domisch, T., L. Finer, and T. Lehto. 2002. Growth, carbohydrate and nutrient allocation of Scots pine seedlings after exposure to simulated low soil temperature in spring. Plant Soil 246:75–86. Gansert, D. and W. Sprick. 1998. Storage and mobilization of nonstruc- tural carbohydrates and biomass development of beech seedlings (Fa- gus sylvatica L.) under different light regimes. Trees 12:247–257. Gilman, E.F. and D.G. Watson. 1994. Quercus virginiana: southern live oak. U.S. Dept. Agriculture Forest Service Fact Sheet. ST-564. ©2009 International Society of Arboriculture
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