Thidiazuron (TDZ) is among the most active cytokinin-like substances for woody plant tissue culture. It facilitates efficient micropropagation of many recalcitrant woody species. Low concentrations (<1 p~M) can induce greater axillary proliferation than many other cytokinins; however, TDZ may inhibit shoot elongation. In some cases it is necessary to transfer shoots to an elongation medium containing a lower level of TDZ and/or a less active cytokinin. At concentrations higher than 1 ~M, TDZ can stimulate the formation of callus, adventitious shoots or somatic embryos. Subsequent rooting of microshoots may be unaffected or slightly inhibited by prior exposure to TDZ. The main undesirable side effect of T D Z is that cultures of some species occasionally form fasciated shoots. The high cytokinin activity and positive response of woody species to TDZ have established it as among the most active cytokinins for in vitro manipulation of many woody species.

Clonal micropropagation studies with silver maple (Acer saccharinum L.) included experiments with various shoot. explant types, cytokinins, and stock plant maturation levels. These trials led to successful explant establishment, axillary shoot proliferation, rooting of microshoots, and establishment of plantlets in the greenhouse. Overall, the best cytokinin tested was the phenylurea derivative TDZ. Shoot proliferation on juvenile explants was poor with kinetin, 2iP, and BA. Only zeatin at 10 μm was comparable to TDZ. TDZ at 10 nm was optimal for both juvenile and adult nodal explants. Juvenile explants that were held in vitro for 4 months commonly had at least 60 axillary shoots that could be subculture or excised for rooting. Microshoots rooted within 2 weeks. Following rooting, silver maple plantlets could be transplanted into a growing medium and placed directly onto a greenhouse bench. Studies were also conducted on rooting stem cuttings (macropropagation). Single nodes from juvenile plants rooted under intermittent mist, regardless of auxin application; however, shoot-tip cuttings from adult trees rooted best when auxin in ethanol solution was applied. Chemical names used: N- phenyl- N' -1,2,3 -thiadiazol-5-ylurea (thidiazuron, TDZ), N- (2-furanylmethyl)-1H-purin-6-amine (kinetin), isopentenyladenine (2iP), benzyladenine (BA), (E)-2-methyl-4-(1H-purin-6-ylamino)-2-buten-1-ol (zeatin).

Short communicationIn vitro embryonic axis and seedling shoot tip culture of Juglans nigra L.Abstract: Embryonic axes and seedling shoot tips of Juglans nitra L., Black walnut, were cultured in vitro. Significant variation existed among progeny from individual trees for growth of radicles and epicotyls and production of callus and axiUary shoots from embryonic axes. The concentration of 6-benzyladenine influenced the growth of the radicle and epicotyl and production of callus and axillary shoots of axes. Axes generally initiated growth quicker on solidified woody plant medium than on Driver and Kuniyuki's walnut medium, but axillary shoot proliferation and elongation were eventually better on liquid Driver and Kuniyuki's walnut medium than on woody plant medium which required an etiolation treatment for microshoot elongation. The concentration of BA also influenced both callus growth and axillary shoot proliferation from seedling shoot tips. Axillary shoots which formed in Driver and Kuniyuki's walnut medium rooted best in sterile vermiculite following a 15 s dip in 10 mM indole-3-butyric acid. Micropropagated plants are growing in the greenhouse.

During 1987, we selected the six fastest-growing seedlings or clones from each of 15 provenances that represented the natural distribution range of silver maple (Acer saccharinum L.). Shoots from all 90 trees were cut into nodal segments, rooted as cuttings, and maintained as clonal stock plants in the greenhouse. Rooting was generally excellent and more than half of the clones rooted ≥90%. At the same time, explants were obtained from these field-grown trees and many were established in vitro as aseptic cultures by first pretreating with benomyl and rifampicin. Single-node explants from the greenhouse-grown clonal stock plants were also established and multiplied in vitro. There was a significant effect of clone within provenance on all in vitro growth characteristics. All clones proliferated axillary shoots, but not all at the same rates. Although statistically significant, low correlation coefficients indicated that micropropagation results were not good predictors of nursery performance of the populations from which the clones were selected, nor of the climatic conditions at the site of origin of the trees. The micropropagation system reported herein, therefore, should be applicable to a wide variety of silver maple genotypes. Chemical name used: methyl [1-[butylamino)carbonyl]-1H-benzimidazol-2-yl]carbamate(benomyl).