Μελέτη της διαφοροποίησης οφθαλμών και της ανθοφορίας στη ροδιά. Επίδραση της αζωτούχου λίπανσης στην καρποφορία της

Abstract

The cultivation of the pomegranate has steadily been increasing in Greece since 2005. Research has greatly contributed to highlighting the benefits of the pomegranate in human health and increased consumer interest. However, many problems have emerged from increased and intensified cultivation. The main such problems are the lack of planting material, the choice of appropriate variety, plant protection, marketing, nutrition-fertilization, fruit growing problems and a lack of basic knowledge such as flower bud differentiation and fruiting. For these reasons, the first part of this thesis studied the timing of flower bud initiation, the various stages of differentiation and flowering, flower fertility, the percentage of fruit setting and productivity in two pomegranate varieties: Wonderful and Acco, and the Persephone clone. The experiments were carried out at the AUA orchard in two consecutive years (2015 and 2016). The results showed that all dormant buds had only primordial vegetative tissue, despite small differences in shape and size which were not always visible to the naked eye. Vegetative growth of shoots markedly preceded flowering. Flower bud initiation coincided with bud swelling and breaking, and was complete in the early stages of growth of flowering buds. However, flower bud initiation and differentiation continued during shoot development and more flowers appeared on the current shoots (short or long ones). The study showed there to be seven distinct stages of differentiation–flowering, from the appearance of the first flowering primordial sepals to fruit setting. The stages were distinguished according to the appearance of different floral parts and changes in their color and size. The timing and mode of differentiation was found to differ from those of most deciduous and evergreen fruit trees, since it is completed in one growth season (period). Flowers were borne on very small stems with few leaves or without leaves, mainly on spur shoots or on top of small current shoots derived from lateral buds on one-year-old shoots or on the top of long current shoots. Two types of flowers were found: Hermaphrodite, which are fertile (long-styled); and male unfertile (short-styled). The onset of flowering on spur shoots preceded that on long current shoots. The flowering period from the appearance of the first floral primordia to full bloom was about 1 month. No significant differences were found between Wonderful, Acco and Persephone in the onset of vegetative growth, flowering, and bud differentiation. The percentage of fertile flowers and fruit setting differed significantly between the three, with those for Persephone being significantly lower than the two varieties. The percentage of fertile flowers and fruit setting differed between the two years of the study and indeed within the same variety. The fruit growth curve followed a simple sigmoidal curve with a linear growth trend and with small differences between the three genotypes. The second chapter studied the activity of sucrose phosphate synthase (SPS) and UDP-glucose pyrophosphorylase (UGPase), which are key enzymes for sucrose synthesis, concentrations of major soluble sugars, and expression of APETALA2 transcriptional gene, found to be responsible for floral induction of certain plant species. The study was conducted on the Persephone genotype in 2016, and measurements were made on vegetative and reproductive shoot apices, in ovaries of closed non-pollinated flowers, and in dormant buds. The results showed that dormant buds had a much higher concentration of transported sugars, predominantly mannitol, but lower concentrations of non-transported sugars than the other organs of the pomegranate trees. The very high concentration of mannitol in dormant buds indicates that the main stored sugar in the buds of pomegranate is mannitol. Vegetative shoot apices had higher SPS and UGPase activity than did the ovary and reproductive shoot apices. Dormant buds had much lower UGPase activity than the other studied tissues, but SPS values were slightly lower. APETALA2 gene expression was found to be elevated in reproductive shoot apices and non-pollinated ovaries, indicating its possible involvement in transition from the vegetative to the reproductive phase of the pomegranate (stages of flower bud initiation and differentiation). Its lack of expression in dormant buds raises the question of whether flower bud induction has actually begun at this stage. Future research based on the expression of APETALA2 gene and other floral induction genes may define the time of flower bud induction in the pomegranate. Nitrogen is a component of proteins, amino acids, nucleic acids and enzymes and is therefore the main element of living cells. It increases plant productivity and optimizes the quality of plant products. For these reasons, the effect of different doses of nitrogen fertilization on the quantitative and qualitative characteristics of fruit and on the nutritional status of the pomegranate cv. Wonderful was studied in the second part. The experiments were carried out on a commercial orchard in Geraki Lakonia for two consecutive years (2014 and 2015). The following six nitrogen treatments were tested: Control (84 g N/tree), +50 g/tree, +100 g/tree, +200 g/tree, +300 g/tree and +500 g/tree. The results showed that there was no significant effect of the different doses of nitrogen on the nutritional status, nor on nitrogen concentration in pomegranate leaves. The different doses of nitrogen affected some of the characteristics of the fruit. In 2014, the highest nitrogen dose (+500 g/plant) increased productivity, fruit and aril weight, and juice content compared to lower nitrogen treatments. However, this effect was not maintained in 2015. There was no significant effect of nitrogen on total soluble solids, pH, total acidity, total phenolic compounds, total antioxidant capacity, ascorbic acid, total and individual sugars for either of the two years.