144 The experimental design was completely randomized us- ing three replicates per treatment. The data was analyzed us- ing an augmented factorial structure considering time and the two-way and three-way interactions in the model (Lent- ner and Bishop 1986). Because carbon isotope ratio was de- termined only for some treatments, data were analyzed as a complete randomized design, and when the main factors were significant (P < 0.05), mean comparisons were calculated us- ing Dunnet’s test comparing the treatments with the con- trol. The results were analyzed using the SPSS v.13 software. RESULTS AND DISCUSSION Trunk growth revealed a significant difference (P < 0.05) among carbohydrates (Figure 1), but not for concentrations. This might suggest that either the concentrations were insufficient to affect tree growth or that sugars were used for processes other than growth. Because trees were not under visibly stressful conditions, exogenous carbohydrates may have been used for other functions such as storage, defense, or reproduction (Pallardy 2008). Igle- sias et al. (2003) found that fruit set in Satsuma mandarin [Citrus unshiu (Mak.) Marc., cv. Okitsu] increased by 10% when supple- mented with sucrose. Early studies showed that albino corn (Zea mays L) survived and produced inflorescences with supplemen- tation of sucrose through the cut ends of leaves (Spoehr 1942). Martínez-Trinidad et al.: Carbohydrate Injections for Live Oaks indicates that carbohydrates such as sucrose and glucose can af- fect sugar sensing systems that initiate changes in gene expres- sion, which can cause an effect on plant growth (Koch 1996). Results for root growth did not reveal significant differences (P > 0.05) among type of sugars or concentrations. It seems the effect of injections was greater in the aboveground portions of the tree. However, the determination of root growth was based on sampling a small portion of fine roots (four samples per tree), which might be the reason for the lack of significant differences among the results due to high variability among samples. Previous research with soybean [Glycine max (L.) Merr.] and birch (Betula pendula Roth.) has shown an increase in fine roots as a result of exogenous applications of sucrose which apparently caused suppression in photosynthesis and carbon remobilization in favor of enhancing root development (Abdin et al. 1998; Percival and Fraser 2005). There were no significance differences (P > 0.05) found in net carbon assimilation among different sugars or concentra- tions during the two year period. However, the data showed high variation, which affected the analysis. Also, trunk in- jections were performed during the dormant season with old leaves present before new leaf emergence, which could have reduced the potential effect on photosynthesis. In soy- bean plants, Abdin et al. (1998) found that the supplementa- tion with sucrose by injections suppressed photosynthesis. Glucose content in twigs and starch in roots were signifi- cantly greater in trees receiving the highest concentration of carbohydrate (Figure 2). This result was not unexpected due to the potential for translocation of exogenously applied carbohy- drates upward and/or downward from the injection point (Tattar and Tattar 1999). Prior research showed 14 C sucrose infused into Figure 1. Trunk diameter growth indices (cm/cm) of live oaks in- jected with three different types of sugars (glucose, sucrose, and a 50:50 mixture). Bars indicate ±1 standard error. Different letters between types of sugar indicate significant differences (P < 0.05) using LSD. The results also indicate the 50:50 mixture of glucose and sucrose resulted in a small but significant increase in growth index as compared to sucrose or glucose alone (Figure 1). Su- crose is the main sugar translocated by phloem, while glucose is a simple sugar product of photosynthesis and the base unit of storage carbohydrates (Taiz and Seizer 2006). Trunk injections of more than one type of sugar in live oaks might have an additive effect and help trees to utilize carbohydrates better to increase growth. In other studies, growth was also stimulated in annual plants such as soybean [Glycine max (L.) Merr.] and corn (Z. mays) when they were treated with sucrose injections at 300 g/L (40 oz/gal) (Zhou et al. 1997; Abdin et al. 1998). The amount injected and size of plants might play an important role in the potential effect of carbohydrates injected. In addition, research ©2009 International Society of Arboriculture sorghum [Sorghum bicolor (L.) Moench] via a pulse applica- tion can move upwards through the xylem (Tarpley et al. 1994). Corn plants (Z. mays) formed abundant starch when treated with solutions of glucose or sucrose (Spoehr 1942). Similar results were also found in Satsuma mandarin injected with sucrose, which resulted in increased levels of starch in fine roots (Iglesias et al. 2003). The impact of carbohydrate concentrations in this study was more evident in roots where the greatest concentra- tions [120 g/L (16 oz/gal)] resulted in greater starch levels com- pared to the control (Figure 2b). Exogenous carbohydrates could have been either stored or translocated to the roots (Tattar and Tattar 1999). High carbohydrate concentrations in other organs like roots and fruits have been reported for Satsuma mandarin when sucrose was injected in the trunk (Iglesias et al. 2003). Chlorophyll fluorescence measures the photochemical effi- ciency of photosystem II (Maxwell and Johnson 2000) and is used as a nondestructive diagnostic for plant vitality and stress (Percival and Sheriffs 2002; Percival 2004; Percival and Boyle 2005). In this study, supplementing trees with carbohydrates via trunk injections increased Fv/Fm (Figure 3), which suggests a method to improve live oak vitality. In addition to chlorophyll fluorescence, similar trends were observed in glucose content in twigs and starch content in roots in response to carbohy- drate injection, both used as indicators of tree health (Gregory and Wargo 1985; Wargo et al. 2002; Dobbertin 2005). However, given the increase in trunk diameter by only the sugar mixture treatment, a concomitant response in Fv/Fm and photosynthe- sis can be expected which may have explained, in part, by the resultant increase in trunk diameter. Growth of many temperate trees is dependent on stored labile carbon produced via photo-
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