Breeding and selection strategies in juglans regia l. For timber production

Laburpena

The objective of the studies undertaken in this thesis has been to establish the bases for a breeding programme for J. regia L. for forestry purposes. The first aim set was to study the variability of the common walnut tree in the Iberian Peninsula, the second consisted of discovering its productive potential and the third comprised attempts to adapt the selection strategies to the peculiarities of the species, establishing the instruments and most appropriate methodologies for the objectives being pursued. The first two aims are dealt with in chapters I and II. Eleven demes, populations of common walnut tree from different climatic and orographic areas, were located in order to know and understand the characteristics of the walnut tree material which grows naturally in the Iberian Peninsula. Through the use of neutral markers (isoenzymes with 8 variable loci), it was possible to determine that genetic variability was similar to that of other European populations and that intrapopulation variability was much higher than interpopulation variability. The information arising from this work has resulted in proposals for an initial reasoned ex situ conservation strategy for J. regia: the maintenance in germplasm banks of an extensive number of seedlings from the same provenance. Parallel to this, studies were conducted of the vegetative performance and forestal growth habit of different progenies of J. regia, pre-selected in different European countries for their potential interest as genotypes for timber producers, comparing them with the commonly used commercial hybrid progenies of Juglans. The results over a 6-year period from two different stations in Catalonia enabled the conclusion that J. regia material clearly suffered from poor growth habit for forestry purposes, a consequence no doubt of the years of selection for fruit production that this species has been subjected to. In the situation as it stands presently, they cannot compete with the hybrid progenies. The possibility of implementing different selection methodologies and strategies for the species is dealt with in chapters III y IV, with the following specific aims: a) establishment of the characters of interest for use in early nursery selection and of the systematic selection to be followed for the selection of progenies from #plus# trees, chosen by phenotype; and b) search for early indicators of characters relevant for selection, in this specific case in relation to WUE (water use efficiency). As a result of this work, it has been possible to choose as early nursery selection characters, with a view to eliminating the inferior progenies, root collar diameter at the end of the first growing period and survival of the plant. A selection methodology was also established based on a multivariate mixed model analysis of the variance which allowed joint retrospective assessments to be made of different years of evaluation, thereby enabling the recovery of progenies which had not initially stood out. The use of carbon isotope composition (d13C) seems to be a method which provides very interesting information about WUE in J. regia. The results from tests on five-year-old progenies show the close relationship between the climate of the area of origin of the genotype and the WUE. This test also enabled assessment of the evolution, over these first years, of genetic variability relating to different adaptive characters and calculation of their heritability. Adaptation to surroundings of the different genotypes indicates the presence of genetic variability in drought resistance and suggests a long-term presence of the J. regia species in the Iberian Peninsula. This fact backs up the studies which consider that the expansion of the walnut tree in the Iberian Peninsula came about from a pre-glacial stronghold of the species as opposed to those who attribute its expansion to the Greeks and Romans.