Relative flov'cr-bud hardiness was established for 4l cultivars of peach by survey of natural bud mortality follo',ving critical January temperatures in the years 1963, 1965 and 1966. Impedance to electric current (lffio cycle/sec) and scion diametel u'ere shown to be-closely related with established l6vels of hardiness, and their relationship is direct and inverse, respectively. Detailed study of impedance alone for reple_-sentative hardv. medium-tendbr and tender cuitil'ars risulted in failure to differentiate becrveen establishcd hardiness groups as a consequence of variability amributable to scion diameter influence. Incorporation of botl-r impedance and diameter measures facilitated the differentiation of nredium-tender and hardy from tender cultivars, but only at periods of maxinum flower-bud hardiness. Thc impedance method was used effectively to establish the periodicity, and pafticularly the minimum and maximum levels, of bud hardiness. Its application in differentiating between hardiness levels in cultir.ars rvhen integraced with scion diameter observations u'as confinled b)' artificial-freezing srudies.The application of these methods is restricted to bearing trees, since neicher impedance nor diameter of non-bearinq trees could be related to the hardiness responses exhibited b)'bearin€i trees of"the same cultivars.

Bearing trees of ‘Loring’, ‘Redhaven’ and ‘Babygold 5’ peach trees (Prunus persica (L.) Batsch) on Siberian C seedlings defoliated earlier than those on the other seedling rootstocks. Early cold acclimation of scions in the fall and scion cold hardiness in mid-winter were enhanced more by Siberian C seedlings than by those of the other seedling rootstocks. Bud survival and fruit set of ‘Redhaven’ and ‘Bablygold 5’ scions were greater on Siberian C seedlings than on any of the other seedling rootstocks following an outdoor cold stress at -23.4°C in January. The cold hardiness of phloem, cambium, and xylem stem tissues were closely correlated with each other in the fall, but were not correlated with cold hardiness of flower buds on the same shoots. Seedlings of Siberian C appeared to enhance early scion dormancy and they increased scion bud hardiness by as much as 4.7° in the fall, and 1.3° in mid-February, compared with those of other rootstocks tested.

An 8-year study was made to assess the performance of Harrow Blood and Siberian C rootstock seedlings in comparison with the commercial seedling rootstocks Rutgers Red Leaf, Veteran, Halford and Bailey. Rootstocks significantly influenced tree size of ‘Loring’, ‘Redhaven’ and ‘Babygold 5’ peach (Prunus persica (L.) Batsch). Siberian C had the largest influence on size control, reducing tree volume by about 20%. Tree height and spread, trunk circumference and trunk cross-sectional areas were also influenced by rootstocks, but annual growth of terminal shoots was not. Rootstocks influenced crotch angle development of ‘Loring’, but had no effect on crotch angle development of ‘Redhaven’ or ‘Babygold 5’. Cropping efficiency of ‘Babygold 5’ was influenced by rootstocks but cropping efficiency of ‘Redhaven’ and ‘Loring’ were not. Yields were significantly influenced by rootstocks. The highest cumulative yields of ‘Loring’ were on Veteran seedlings, the highest of ‘Redhaven’ were on Rutgers Red Leaf and the highest of ‘Babygold 5’ were on Halford. Yields were also a function of tree size with the highest yields being obtained on the largest trees. Trunk circumference and crosssectional area were the only growth measurements that were significantly correlated with the yield of each cultivar. Tree survival was best on Harrow Blood and Siberian C and poorest on Rutgers Red Leaf and Veteran. Tree mortality was associated with winter injury and canker (Leucostoma spp.) infection but not with incompatibility.

Impedance tester and electrode probe. For personal use only.

Leaf abscission in peach is correlated with the release of tryptophan into the cytoplasm following a sudden drop in air temperature below the physiological minimum. The tryptophan accumulates in the variety Elberta and results in increased activity levels of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, and an unidentified neutral growth promoter. The rapid defoliation of the canker resistant variety Elberta was, however, associated more closely with the endogenous levels of tryptophan and 5-hydroxytryptamine. Susceptible Dixired variety, which defoliates slowly, contains lower activity levels of these compounds at the time of leaf abscission, and shows enhanced activity of kynurenic and xanthurenic acids throughout this same period. This suggests that tryptophan does not accumulate in Dixired to cause rapid abscission because of a greater genetic capacity for degradation by way of the kynurenine, hydroxyanthranilic, nicotinic acid pathway common in the metabolism of animals.