Caracterización a edad temprana de clones del género Eucalyptus por su resistencia a estreses abióticos para uso en plantaciones comerciales

Resumen

Eucalyptus is the most widely used hardwood genus in commercial plantations worldwide, covering more than 20.1 million hectares. Its growth rates usually exceed 35 m3 ha-1 year-1 in many locations. These plantations can be grown under a great variety of climates, becoming an important source of woody and non-woody products such as fuelwood, charcoal, cellulose pulp and paper, sawn timber and essential oils. However, as the water and cold stress are two major constraining factors of Eucalyptus plantation growth and survival, the selection of genotypes that improve resistance to these stresses would help to improve productivity and to expand plantations. Stomata features, stomatal and cuticular transpiration, xylem anatomical properties, cold tolerance mechanisms, among others, are characteristics to be taken into account in a genetic improvement programme, as well as the clone’s ability to modify them according to growing conditions. To this end, using nursery plants of 1−2 years-old of nine high-productivity Eucalyptus clones, including five E. globulus and four hybrids from different species, from a plant selection and breeding programme, these characteristics were studied along different seasons of the year. In Chapter 1 foliar parameters were evaluated such as stomata size (length and width) and density, specific leaf area, and leaf structure. Xylem parameters in roots and stems were also evaluated, such as the density, diameter and crosssectional area of the xylem vessels, length of the vessel elements and the density of woody rays. Stomata density, stomata size and SLA varied according to the seasons of the year and differed among clones. All xylem parameters evaluated differed among clones. In Chapter 2 various parameters relating to drought resistance and water use were evaluated, such as the total transpiration rate of the plant and cuticular transpiration, osmotic adjustment, xylem hydraulic conductance and vulnerability to cavitation, among others, as well as the clone’s ability to modify them according to growing conditions. The results revealed significant differences among measurement dates and among clones in all parameters evaluated. Relating the parameters measured in the different tests of the two previous chapters, cuticular transpiration, vulnerability to cavitation and parameters derived from p-V curves, daily transpiration in nursery plants and stomatal 4 characteristics, it was observed, in general, although with slight differences, a similar behaviour of the E. globulus clones with each other. This contrasted with regard to the differences shown by the other clones of hybrids, with each other and with those of E. globulus, but only in some of the parameters, since these differences varied from one parameter to another. Finally, Chapter 3 shows the seasonal evolution of the cold tolerance during the cold season, as well as the effect of autumn fertilization using different doses of N and K on this tolerance. The results indicated a scarce seasonal variation in lethal temperature 50 (LT50), in general less than 2 ºC, in a range from -5.0 ºC to -7.0 ºC, which made the differentiation between clones difficult because of the narrow temperature range in which they moved, although it was significant. This also indicates that these tested eucalypts do not have a high degree of hardening stimulated by temperature. The effect of the applied dose of N was significant, as well as the interaction of N and K, although not the main effect of K, therefore, to set the correct dosage of fertilization, it must be taken into consideration the ratio N/K. The leaf N and soluble sugars concentrations did not correlate significantly with cold tolerance (LT50), at least for the concentration range obtained in this study, in which the plants showed no symptoms of nutrient deficiency. All the results obtained, together with previous experiences with this species, allow us to define the morpho-physiological characteristics of each clone that are potentially applicable to the selection of genotypes resistant to drought or frost. So, the success of the plantations, especially in unfavourable areas, could be further increased. However, it also shows that the drought resistance or water saving strategy varies from one clone to another, according to the combination of characteristics brought into play in each case.