Gas Exchanges and Accumulation of Osmolites in Declined Persian Oak Stands in Ilam Province (Case study: Gchan and Sheshdar Forest Area)

To investigate the effect of physiological factors of photosynthetic properties and accumulation of smolites on declined Persian oak trees (Quercus brantii Lindl.), two forest stands were selected in Gachan and Sheshdar from Ilam province, Iran.

The trees were classified into 4 groups in 4 replicates based on the severity of crown dieback (healthy, weak = 5–33%; moderate = 34–66%, and severe > 66%). The physiological activities such as photosynthesis, transpiration, respiration, concentration of carbon dioxide in the stomata, mesophilic conduction, water use efficiency, leaf temperature, soluble sugars and free amino acids were measured from leaves in the middle and outer part of the crown. The random sampling was carried out in the dominant slope of the site. The tree DBH ranged from 30-40 cm. One-way ANOVA and LSD tests were used to compare the different treatments and all statistical calculations were prepared in SAS software.

In severe dieback trees from Gachan, the photosynthesis rate (1.06 μmol CO2 m-2s-1,) was associated with a decrease in mesophilic conduction (0.0021 molm2 s-1), transpiration (3.82 mmol H2O m-2s-1), water use efficiency (0.27 μmol CO2 molH2O-1) and an increase in 9intercellular carbon dioxide concentration (488.15 μmol CO2 mol-2) and leaf temperature (36.05 Cᵒ). The results of correlation test indicated the significant role of stomatal and non-stomatal factors to reduce photosynthesis rate. Furthermore, no increases were detected in soluble sugars (0.009μg gr-1FW) and free amino acids (0.012 μg gr-1FW) as compatible solutions to osmotic regulation. In site Sheshdar, photosynthetic parameters did not show significant differences between different crown dieback severities, while they were associated with increased accumulation of soluble sugars in diebacked oak trees. In addition, no significant correlation was observed between photosynthesis and stomatal factors indicating the ability of this species to keep open stomata under severe drought stress.

In general, our results show the variation in photosynthetic capacities of diebacked oak trees to resist the drought stress and/or to escape it in the two sites.