The influence of temperature variations on ultrastructural characteristics at Prunus persica

The purpose of our paper was to analyse structural and ultrastructural changes in pollen grains and foliar mesophyll occurring in early-middle-flowering peach cultivars that were affected by temperature variations from December 2016 to April 2017 at the Research Station for Fruit Growing Constanta. In the analysed period we had warm weather during the winter, followed by cold weather in spring. For the electronomicroscopic technique through transmission, the samples were cut in fragments of 1 mm/1 mm and were prefixed in 0.1 N sodium cacodylate tampons with 2.5% glutaraldehyde. After prefixation, the samples were washed in 0.1 N cacodylate tampon and fixed in solution of 2% osmium tetraoxide. The specimens were washed again in 0.1 N cacodylate tampon to remove the osmium excess and dehydrated in serial baths of alcohol: 30, 50, 70, 90, 95, 100°, 15-30 min each. The first three baths were executed at 4°C and the rest were carried out at room temperature. After the dehydration the samples were kept overnight in a mixture of propylene oxide with epoxidic resins of the type Epon 812 and DMP-30 as a hardening agent in order to introduce the warm resin polymerization. Subsequently the samples were placed in plastic capsules, covered with Epon 812 and then placed for polymerization in sterilizers at 67°C for 46-60 h. The ultrafine sections of 400-600 A were obtained with the aid of a Leica Ultracut-R ultramicrotome. They were placed on metallic grills and double contrasted with uranil acetate and lead citrate and then were observed under the performing electronic microscope Philips CM120. By analysing the pollen grain preparations at the electron microscope we noticed alterations in the sculptural elements of the exine as well as in the apertures; these data, together with the ones related to the viability and the germination capacity obtained by analysis at the optical microscope, have allowed us to establish that the most affected cultivar was ‘Raluca’, then ‘Monica’ and less one was ‘Filip’. At the same time, the analysis of ultrastructural changes at the level of foliar mesophilus indicated the occurrence of plasmolysis, a decrease in the number of chloroplasts at the cellular level, with the reduction of the number of granoparticles per chloroplast. Also, in ‘Raluca’ we noticed a smaller number of mitochondria, a reduction in the development of the rugged endoplasmic reticulum, the number of ribosomes and dytiosomes. Also, in ‘Monica’, we were able to observe a reduction of the number of granular tilacodes in the grana formations at chloroplasts and a very small number of starch grains as a polysaccharide reserve in chloroplast. Thus, we can assume that the microscopic analysis gave us indications confirming that the less resistant to temperature variations was ‘Raluca’; the pollen analysis data were verified by the foliar mesophilus analysis, that indicates certain changes in the efficiency the photosynthesis process and the protein biosynthesis processes.