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  • Author or Editor: Billy G. Cumbie x
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Grafting failure of Chinese chestnut (Castanea mollissima Blume) trees is common. Some researchers have suggested that secondary phloem fibers in chestnut rootstock stems may interfere with graft union formation. Others suggest that high rootstock moisture content may reduce grafting success. This study was conducted to ascertain the precise arrangement of secondary phloem fibers in Chinese chestnut seedling rootstocks and to determine if they are associated with grafting failure. We also investigated the effects of moisture content of rootstocks and container growing medium on grafting success. Nodes on Chinese chestnut seedlings had 2/5 phyllotaxy with five alternate buds present in two revolutions around the stem. Vascular cambium in 1-year-old rootstock stems was arranged in a regular five-lobed cylinder in cross-sections. Primary phloem fibers were present in a discontinuous ring and secondary phloem fibers were usually clustered outside the cambium between the lobes. Grafts were successful when scion buds were placed on the rootstock disrupting secondary phloem fibers with cambia of the budded tissues aligned. Containerized rootstocks at 56% and 62% growing medium moisture content before grafting had 25% graft union success rates, whereas non-irrigated plants at 48% moisture had a 75% success rate.

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Differential thermal analysis (DTA) experiments, viability tests, and anatomical studies were conducted to investigate the biophysics of freezing in ‘Darrow’ blackberry (Rubus spp.) buds at selected stages of development from Nov. 1985 through Sept. 1986. As many as four low-temperature exotherms (LTEs) associated with the crystallization of supercooled water were detected in DTA experiments on buds collected 16 Nov. 1985. Anatomical observations revealed seven to nine floral initials present at that date. On 16 Jan. 1986, buds were morphologically similar to those examined in November, with four to 10 LTEs per bud. By 2 Mar., the size of the floral initials increased and distinct floral parts were evident. One to 10 LTEs were observed per bud at this date. Five to seven floral initials were observed in buds collected on 23 Sept. 1986, but floral parts were not evident. DTA experiments conducted in September revealed one or no LTE per bud. The median LTEs for November, January, March, and September buds were −20.5°, −28.0°, −22.0°, and −16.5°C, respectively. T50 values calculated from viability tests were within 4.5° of the median LTEs at all test dates. These results indicate that injury to the entire floral region is associated with a single freezing event when ‘Darrow’ blackberry buds are at an early stage of development. However, as floral differentiation progresses, individual primordia supercool, freeze, and are injured independently.

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