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- Author or Editor: Richard H. Zimmerman x
Growth, flowering, and fruiting of micropropagated `Jonathan' apple trees (Malus domestica Borkh.) transferred in Spring 1983 to the field from either a nursery, cold storage, or greenhouse were compared. First-year shoot and trunk growth was greatest for trees transplanted from the nursery and least for trees that were held in the greenhouse before being transferred to the field. Trees pruned low (35 cm) at planting time had more terminal shoot growth and less trunk cross-sectional area after the first growing season than those pruned high (90 cm). The effect of preplanting cultural practices on vegetative growth diminished in the 2nd year and disappeared by the end of the 3rd year in the orchard. Flowering began in 1985 and was only slightly affected by preplanting cultural practices and pruning treatments. Fruiting was not affected by the treatments.
The mean length of the juvenile period for seedlings grown in the field varied from slightly more than 3 years for Malus Sargentii Rehd. to nearly 5 years for M. toringoides (Rehd.) Hughes and M. sikkimensis (Hook, f.) Koehne. Growing the seedlings in the greenhouse for varying lengths of time before field planting shortened the juvenile period in M. toringoides, but had no effect on the juvenile period in M. sikkimensis, and lengthened the juvenile period in the other species used. For a given species, those growing conditions which produced the most vigorous growth of the seedlings also had the greatest effect on shortening the juvenile period. None of the species reported on here is as useful for physiological studies as M hupehensis (Pamp.) Rehd.
Seedlings of most woody plants pass through a juvenile stage during which the seedling cannot be induced to flower (147). The delay in flowering caused by a long juvenile phase may last for years and it is a major problem in breeding most tree crops. Although the juvenile phase cannot be eliminated, it can be shortened considerably by breeding, by controlling the environment in which the seedlings are grown, and by various cultural practices. This review considers the various factors which have been reported to affect the length of the juvenile phase.
The age at which flowering began and the stem diameter at various ages were recorded for more than 9,000 pear seedlings planted in orchards at Beltsville. The juvenile period for individual seedlings varied from 2 to 10 years, with a number of seedlings still not flowering after 8 to 10 years. An overall negative correlation of stem diameter with length of the juvenile period was found. Within planting years, this relation was significant only in certain years; within progenies, the relation was significant less than half the time; within a specific cross repeated in different years, the relation varied considerably from year to year. Stem diameter can be successfully used as a preselection criterion for early flowering (short juvenile period) only when these two characteristics are significantly correlated. Since this condition does not exist in most crosses under the growing conditions at Beltsville, stem diameter is not a valid predictor of early flowering.
Sprays of ethyl hydrogen 1-propylphosphonate (EHPP) at concns of 1.2-9.6 g/liter reduced shoot and root growth of tea crabapple (Malus hupehensis (Pamp.) Rehd.) seedlings. Leaves developed after spraying were severely malformed. Shoot growth produced during the second growing season was not reduced, but leaves were malformed on the new shoots, particularly at the highest concns of EHPP.
The change from juvenile to adult phase of seedlings of tea crabapple [(Malus hupenhensis (Pamp.) Rehd.)] is influenced by 2 factors, the height and the age of the plants. Seedlings grown continuously in the greenhouse are unbranched and undergo phase change from juvenile to adult in 7 to 10 months at a height of approximately 2 m. Trees can be in bloom within 1 year from the time of seed germination. In contrast, seedlings grown periodically in the greenhouse or in the field are branched and produce more shoot growth but are shorter and take longer to flower. Greenhouse-grown seedlings can be brought into flower by chilling for 6 weeks or more, by withholding water until the leaves drop, or by treating the buds with cytokinins and gibberellins. Young seedlings cannot be stimulated to flower by grafting onto older seedlings although older seedlings will continue to form flower buds after grafting onto young seedlings still in the juvenile phase.
Micropropagated trees of `Redspur Delicious' apple (Malus ×domestica Borkh.), planted as small, actively growing trees in May 1982, lacked uniformity in tree size, appearance, and flowering by the spring of 1986. Only four of the 18 trees had a typical spur-type growth habit; these four trees had 80% more spurs per meter of shoot, 8 to 10 times as many flowers the first year of flowering and 9.5-fold higher early fruit yields, but were 40% smaller after 14 years in the orchard and had 25% less cumulative fruit yield than the nonspur types. Shoots from the spur-type trees were recultured in 1988 and the resulting trees planted in an orchard in 1990. These latter trees were uniform in appearance and all had typical spur-type growth, with about 30% more spurs per meter of shoot growth than the spur-type trees from which they were propagated. Micropropagating spur-type apples from previously micropropagated trees that have maintained clonal fidelity may overcome the potential problem of clonal variation in orchard planted micropropagated trees.
Small actively growing micropropagated trees of `Redspur Delicious' apple (Malus xdomestica Borkh.) were planted in an orchard at the end of May 1982. By Spring 1986, a lack of uniformity in tree size, appearance, and flowering was obvious. Only four of the 18 trees had a typical spur-type growth habit. These four trees had significantly more spurs per unit of shoot length, flowered sooner, had higher early fruit yields, and remained significantly smaller after 13 years in the orchard, but had significantly less cumulative yield than the nonspur types. Shoots from the spur-type trees were recultured in 1988 and the resulting trees planted in an orchard in 1990. These latter trees were uniform in appearance and all had typical spur-type growth with ≈30% more spurs per meter of shoot growth than the original trees from which they were propagated.
Highbush blueberry is adapted to well-drained sandy soils containing some organic matter, but these are often unavailable in many areas where blueberry production is desired. I tested the concept of using freely available by-products to produce an artificial medium for growing blueberries. In June 1997, 1-year-old tissue-cultured plants of `Bluecrop' and `Sierra' blueberry were planted into 15-L plastic pots filled with soil or soilless medium in a total of 10 treatments. Soils used were Berryland sand (alone) and Manor clay loam (alone or amended with 25% or 50% compost mix 1); soilless media were composed of coal ash amended with 25% municipal biosolid compost (B), 25% leaf compost (L), 25% or 50% compost mix 1 (1 B: 1 L),\ or 25% or 50% compost mix 2 (1 compost mix 1: 1 acid peatmoss). pH of all mixes containing compost was adjusted to ≈4.5 with sulfur. After the first year, plants of both cultivars in Berryland sand had significantly more shoot growth than in any other treatment except for Manor clay loam. The least growth was produced by plants growing in Manor clay loam amended with compost mix 1 and in coal ash amended with unblended compost (B or L). After the second year, plants in the best treatments were 90 to 100 cm tall. More shoot growth was produced by plants in Berryland sand and in coal ash amended with 25% or 50% of compost mix 1, followed by plants in coal ash amended with 50% compost mix 2 or 25% compost B; plants in Manor clay loam, whether or not amended with compost, had the least growth. In 1998, 95% of the plants flowered and most set fruit, but differences among treatments were not significant. `Sierra' plants produced more growth than those of `Bluecrop' in all treatments.
Combining ability for transmission of juvenile period duration was studied in a large pear breeding population. The 92 parents, consisting of cultivars and selections of Pyrus communis L. and its interspecific hybrids with P. pyrifolia (Burm.) Nakai and P. ussuriensis Maxim., as well as genotypes of P. calleryana Decne., were crossed in 298 combinations. General combining ability was highly significant and of much larger magnitude than specific combining ability, indicating that juvenile period length was under predominantly additive genetic control. Selection of parents based on their juvenile period or their combining ability constants is likely to result in significant reduction in mean juvenile period.