Effects of Pruning and Bark Ringing on Total Nonstructural Carbohydrates in Crabapple

Materials and Methods

One-year-old, dormant, unbranched bare-root seedlings of Sargent crabapple (Malus sargentii),20 - 30 cm tall were used in this research. There were six treatments, all applied priorto planting on June 1 (Expt. 1) and June 8 (Expt. 2). The six treatments included: 1) 50% stem pruning, 2) 50% stem pruning plus bark ringing, 3) 50% root pruning, 4) 50% root pruning plus bark ringing, 5) bark ringing alone and 6) control - no pruning or ringing. All pruning was performed by length, which was defined as 50% of the length from the shoot apex to the root-stem collar and 50% of the length from the root-stem collar to the longest root (shoot and root pruning, respectively). Bark ringing was done directly above the root-stem collar making two circumferential cuts 5 mm apart. Trees were nicked initially until the bark would slip to indicate how deep to cut. A scalpel was used to make a third cut to allow the bark (periderm and phloem) to be peeled away (5).

Seven trees for each treatment were planted in a peat-perlite mixture (1:1, v:v) in one gallon containers. The plants were placed under intermittent mist during the first week to prevent desiccation of the new foliage. Trees were grown in a greenhouse at approximately 27°C day/21°C night and watered as needed. Water soluble fertilizer was applied with each irrigation at 200 mg/liter of 20 N-4.3 P-16.6 K. Plants were harvested on three dates beginning 18 days after all ringing/pruning treatments were applied. Seven trees from each pruning treatment were harvested 18, 28, and 38 days after the experiment was initiated. Afactorial design was utilized (12) and the experiment was repeated approximately one week later. Plants were again removed from cold storage, treated (bark ringed and/or pruned) and planted the same day.

Upon each of the three harvest dates, nonstructural carbohydrates were assayed for from the stems, leaves and roots of the seedlings. All leaves were removed, and shoots severed at the soil line. Roots were removed from the containers and the media washed away. Plant tissues were dried at 80°C for 48 hours. Dried tissue was ground utilizing a Wiley mill with a 40 mesh screen, placed in 25 ml vials, and refrigerated at 3.5°C until assays were performed for total nonstructural carbohydrates. Samples were analyzed within two weeks of tissue collection.

The assay chosen was modified by the procedure outlined by Smith (11). Colimetric procedures were substituted for titration since errors are less likely to occurthan with titration. Tissue samples were placed in solution, brought to a boil, and then cooled to ambient temperature. The tissue was then digested by a 5.0% alpha-amylase solution, derived from the fungus Aspergillus oryzae. Upon digestion and filtering of the solutions, samples were analyzed by use of a Varian 635 U V-visible spectrophotometer. Absorbance was read at 520 nm with a fixed wavelength. Total nonstructural carbohydrates were calculated by using the absorbance of the samples against glucose standards of 25 to 125 mg forming a standard curve. Data are presented as nonstructural carbohydrate as a percent of total tissue dry weig ht. Data were analyzed by an analysis of variance followed by mean separations utilizing Newman-Keuls test at p = 0.05 (12). Coefficients of correlation were determined for each plant tissue assayed overtime to determine carbohydrate gains or losses for total nonstructural carbohydrates.

Results and Discussion

When the seedlings were sampled for total nonstructural carbohydrates priorto treatments and planting, stems and roots were similar in reserve carbohydrate content. This was expected since Malus spp. have no specialized perennial storage organs and equal distribution of reserves often occurs throughout the tree as a result (13).

After 18 days no significant differences were seen in total nonstructural carbohydrates among any of the treatments for leaves, stems or roots (Table 1). Stem and root total nonstructural carbohydrate was similar to initial values. Leaf total nonstructural carbohydrate levels were about 50% of stem or root levels for Sargent crabapple 18 days after planting.

At the second harvest (28 days), leaf carbohydrates showed a slight increase from most treatments over the first harvest, with shoot total nonstructural carbohydrate decreasing slightly from the first harvest (Table 1). Nonstructural carbohydrate in stems steadily decreased with the onset of new growth in trees that were not subjected to bark ringing. However, when bark ringing was performed, stem carbohydrates steadily increased over time. The three bark ringing treatments produced a significant decrease in total nonstructural carbohydrate within the bark-ringed seedlings for root and stems by the second harvest date.

Leaf carbohydrate levels were increased for most treatments at the second harvest date. However, there were no significant differences among the treatments and harvest dates for leaf carbohydrates. Stem carbohydrate levels generally decreased for most treatments at the second harvest date. Stems were being slowly depleted of carbohydrates indicating a movement of carbohydrate from stems into breaking buds. The stem may have been serving as a source of energy for the newly emerging foliage. Root carbohydrate levels for all treatments were lower than for the first harvest date.

At the third harvest, total nonstructural carbohydrates of roots were lower with the treatments which involved bark ringing when compared to those where the phloem was not interrupted (no bark-ringing). Stem total nonstructural carbohydrate was significantly higher when assayed after the third harvest for the bark ringed treatments. Root total nonstructural carbohydrates decreased at the third harvest for the bark-ringed treatments.

Potter et al (6) reported that total nonstructural carbohydrates in prickly pear roots declined in th spring due to upward translocation. Since root carbohydrates steadily decreased in our research, this suggests that new root growth made use of existing root-stored-reserves and photosynthates from non bark-ringed trees. Bark ringing was apparently effective in blocking phloem translocation as revealed by the low carbohydrate levels in roots of plants with bark ringing. Root carbohydrates in plants without bark ringing was much higher than for bark-ringed plants. However, roots were never totally depleted of carbohydrates even after the third and final harvest date.

Pruning treatments utilized in these studies decreased carbohydrate supply to seedling shoots. Yamashita (16) with mulberry stated that pruning enhanced conversion of sucrose to starch in the stem, resulting in a larger accumulation of new carbohydrates. Fishier (4) reported that bark ringing resulted in accumulated carbohydrates above the bark ring, as in our study. Tschaplinski and Blake (14), found shoot decapitation in poplar (Populus maximowiczii x nigra) severely depleted starch reserves over a several week period. The plants were invigorated by pruning and an increase in photosynthetic rate resulted. From our experiments it is unclear if pruning stimulated net photosynthesis. Even though leaf carbohydrates steadily increased, stem carbohydrates equally decreased. Therefore, it is impossible to determine if pruning stimulated photosynthate production and the foliage demands were too high to be met without stem reserves.