Cultivating walnuts is only economically efficient when grafted material is planted in orchards. In comparison with the spreading of walnut seedlings, common in many European countries, grafted trees are much more precocious and give twice or even a three times higher yield (Germain, 1997; McGranahan and Leslie, 1991; Szentivanyi, 1990). In several European countries where walnuts are commercially grown, grafted trees were first planted some 30 to 40 years ago. The propagation method, applied in many nurseries, is “omega” grafting by machine performed in the first half of April. One-year-old seedlings, the progeny of selected mother plants from domestic populations of common walnut, are most frequently used as rootstocks. In addition to well-rooted rootstock, the quality of the scion wood strongly influences grafting's success. A short section of a 1-year-old shoot with two dormant buds is used as a scion. Rejuvenated 1-year-old shoots are taken from selected mother plants that are “cut on a head” each year to obtain new vigorous shoots for scions. Rejuvenated shoots usually consist of two growth units. The first one builds up during the spring growth flush and the second during the summer growth flush. Medium vigorous annual shoots of 12 to 18 mm in diameter with short internodes are the most appropriate for the grafting method described. In addition to the diameter, there are another two important characteristics that determine high-quality scion wood: a high number of nodes with visible or latent vegetative buds, useful for budding, and an appropriate ratio between wood and pith, which has to be ≈3 to 4:1. Such shoots are normally developed only on well-structured mother trees, between 3 and 4 m high, with a trunk of 1 m. They have to have strong central axes. Lateral branches that represent “heads,” i.e., the basis for further rejuvenated shoots, have to be spirally arranged around the central axis.
When mother plants are still young and in some years, particularly when the spring is extra rainy and warm, the new shoots grow too vigorously. They become too strong, have a low number of nodes, too long internodes, a smaller number of vegetative buds, and unsuitable wood:pith ratio. Consequently, they are less suitable as a scion wood for grafting, and for the nurserymen, they represent mother material of lower value.
In such years, a particular method is needed for reducing vegetative growth, thus helping the nurseryman to achieve quality rejuvenated shoots for scions.
Over the last decade in deciduous fruit tree production, prohexadione-calcium (ProCa) has been introduced as a plant bioregulator to control vegetative and cropping performance. ProCa, an acylcyclohexadione, is a new plant growth retardant that reduces shoot growth by inhibiting the late stages of gibberellin (GA) biosynthesis, e.g., hydroxylation of GA20 to GA1 (Brown et al., 1997; Rademacher, 1993, 2000). ProCa is easily absorbed by leaves and is translocated acropetally, but basipetal movement is limited (Evans et al., 1997). In the plant tissues, it is rapidly metabolized (Evans et al., 1999) and shows benign toxicological and ecotoxicological properties (Rademacher et al., 1998).
There are many reports dealing with ProCa's influence on vegetative growth in pome fruit trees as well as in stone fruit trees. According to Medjdoub et al. (2004), ProCa has been shown to be an effective inhibitor of vegetative growth in ‘Smoothee Golden Delicious’/M9, in which terminal and lateral annual shoots were inhibited from the beginning of the growing season. In pears, ProCa reduced the shoot growth of both bearing trees in the orchard and nonbearing scions under greenhouse conditions (Costa et al., 2001). As reported by Smit et al. (2005), in some pear cultivars, shoot growth was reduced by up to 50%. In sweet cherry, short-term reductions in terminal shoot elongation caused by ProCa was reported by Elfving et al. (2003). Until now, no investigations have been carried out concerning nut trees or about applying ProCa to moderate the vegetative growth of mother trees, which are severely pruned annually.
This study was designed to evaluate the potential of ProCa for the reduction of shoot growth in mother walnut trees. During two experiments conducted in 2005 and 2006, we tried to reduce vegetative growth of rejuvenated annual shoots and to increase their wood:pith ratio with the aim of improving the quality and uniformity of the scion wood.
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