Anàlisi de la capacitat de regeneració en estadis inicials del desenvolupament en diverses espècies mediterrànies del gènere Quercus

Abstract

This thesis focuses on vegetative regeneration in the first stages of development of Quercus coccifera, Q. humilis, Q. ilex and Q. suber. One goal is to provide a histological description of the ontogeny of the cotyledonary node from embryo to six-month-old seedling for Q. coccifera, Q. humilis and Q. ilex. Starch and nitrogen content were analysed in spring, summer and winter in different parts of the seedling in four Quercus species. The effect of pruning was also analyzed. Furthermore, the acorn biometry, germination capacity and seedling growth in the four species were studied. The results show that Q. suber acorns are, in general, larger, in both length and diameter, and weigh more than the others. In these species, acorn length and diameter determine weight, but there is also a species effect. The germination capacity of Q. coccifera and Q. ilex depends on acorn weight, whereas in Q. humilis and Q. suber, it is independent of weight. This study also demonstrates that acorn weight affects positively the growth of 3 and 6-month-old seedlings, but in 9-month-old seedling there is no relationship between the weight of the seed and the seedling growth. In reference to seedling growth, Q. ilex seedlings show the highest growth rate of all the studied variables. In all four species, the root biomass is greater than the aerial biomass. With regard to vegetative regeneration, this study demonstrates that the seedlings of the four species show a high sprouting capacity when the aerial biomass is removed above the cotyledonary node. However, seedling survival depends on the species and the pruning intervention. Repeated pruning reduces Q. coccifera, Q. humilis and Q. ilex sprout vigour. But, in Q. suber sprout size after the second pruning is greater than it is after the first pruning. After removing the aerial biomass above the cotyledonary node, sprouts develop from cotyledonary node buds. The anatomy of the cotyledonary node is similar for Q. coccifera, Q. humilis and Q. ilex, but it differs from that described for Q. suber. The cotyledonary nodes of the first three species hardly elongate at all and cotyledonary buds only develop in the axils of the cotyledonary petiole. These buds are present at the embryo stage in Q. humilis and Q. suber, but are not visible in Q. coccifera and Q. ilex. At the seedling stage, the cotyledonary buds of Q. coccifera, Q. humilis and Q. ilex multiply, forming bud clusters. The starch analyses show that the root system of Q. coccifera, Q. humilis, Q. ilex and Q. suber contain about 90% of the total plant starch. In the different fragments of the root system (the cotyledonary node, the first 11cm of the root and the rest of the root) the main starch concentration is in the first 11 cm of root which, in the case of Q. suber, corresponds to the cotyledonary node. With regard to the season, in summer the starch levels in the aerial parts and the roots of the plants decrease in Q. coccifera, Q. humilis and Q. ilex, but these levels recover in winter. In contrast, the starch concentrations in Q. suber are at their lowest in winter. Pruning causes a reduction in starch levels, because the starch is mobilised to synthesize the new sprouts. In all four species the main proportion of starch mobilised is from the first 11 cm of root, which, for Q. suber, is the cotyledonary node. So this study confirms that the cotyledonary node in Q. suber is a lignotuber. However, Q. coccifera, Q. humilis, Q. ilex do not have any lignotuber or specialised sprouting structure, although the presence of buds and substance reserves give them a high sprouting capacity. In reference to nitrogen content, the root systems of the four species contain approximately 70% of the total nitrogen. In summer, the root system nitrogen concentration for Q. humilis and Q. suber is lower than it is in winter, whereas in Q. coccifera and Q. ilex the values are similar in the two seasons. Pruning does not cause any decrease in nitrogen levels, probably because the seedlings received a continuous supply of nitrogen as they were irrigated.