Drought, salinity and forest fires are adverse abiotic factors responsible for the failure of plant regeneration in the Mediterranean regions. The objective of this study was to evaluate Pinus pinea seed germination in response to heat, osmotic and salinity stress. Separate experiments were conducted, using polyethylene glycol (PEG) as an osmotic agent to reproduce drought stress, and sodium chloride (NaCl) and diluted seawater as salinizing agents to simulate salinity stress. Different temperatures were used to reproduce the effects of different fire regimes. Seed germination and the activities of the main enzymes involved in Pinus seed reserve utilization (glyoxylate cycle enzymes) decreased with increasing PEG, NaCl and seawater concentrations. Seawater appears to be the least toxic salinizing agent on Pinus pinea seed germination. The present study indicates that the germination of Pinus pinea seeds is influenced by the concentrations and even more by the nature and interactions of the ions present in the solutions. As regard heat treatments, the maximum germination percentage (80%) was obtained at 80°C and short exposure time (3 min). Increasing exposure time and temperature led to the germination percentage progressively decreasing.
Changes in soil properties related to natural regeneration of silver fir (Abies alba, Mill.) in small (185 m 2 ) and medium (410 m 2 ) gaps, in the Calabria Apennines (Southern Italy), were studied 2 years after gap creation. The organic matter within the medium gaps decayed more rapidly than those in the small gaps. Among the microenvironmental variables and soil properties, soil temperature was most strongly positively related to organic matter decomposition rates. Medium gaps had high soil temperature and photosynthetically active radiation (PAR) transmittance, and low soil moisture. Within medium gaps, we found a great amount of phenolic compounds and a low content of organic matter and humic acids. These results indicate that in the medium gaps mineralization of organic matter was more important than humification. In contrast, in small gaps, the great amount of organic matter, the high content of humic acid and the values of humification index suggested that in these gaps the humification process of organic matter prevailed. Within small gaps, we observed more silver fir regeneration than medium gaps. Difference in organic matter content, particularly in the amount of phenolic compounds, may account for differences observed in natural regeneration of silver fir between small and medium gaps.
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