Studies were conducted to examine changes in soil (Ys) and plant water status during summer in a 16-year old Quercus suber plantation in southern Portugal. . Intensive measurements were conducted on five trees with mean height and DBH of 5.3 m and 11.6 cm, respectively, growing at close proximity to each other. Weather conditions and soil water potential (Ys) at the rhizosphere of each of the trees measured at 0.3 and 1 m soil depth were continuously monitored. Predawn (Ypd) and midday (Ymd) leaf water potentials were determined every month. Soil and plant samples were also collected in June and September from different locations within the study site for d 18 O isotope composition analysis. Pressure-volume (p-v) curves were constructed from plant shoots at different times during the vegetative period to determine osmotic potential at full saturation (P 100 ), water potential at turgor loss point (Y tlp ), relative water content at turgor loss point (R* tlp ) and bulk modulus of elasticity (e). Significant P < 0.05 decline in Ys occurred between May and September, the lowest value recorded being -2.0 MPa. Decline in soil moisture affected tree water status, but decline in leaf water potential varied significantly (P < 0.05) among the trees. At the end of summer drought, lowest Ypd measured was -1.7 MPa while the highest measured during this time was -0.8 MPa. Differences among trees were attributed to differences in rooting depth, as shown by regression analysis of 18 O isotopes. Radial stem growth ceased when Ys within the upper 0.3 m depth approached -1.5 MPa. The upper soil layers contributed approximately 33% of the total tree water requirement, between spring and mid summer when drought was experienced by trees. Deep soil layers however, supplied most of the water required during drought and no growth was recorded during this time. Stressed trees increased solute concentration of their tissues by a Magnitude of 0.7 MPa while bulk tissue elastic modulus increased by about 17 MPa. The study emphasizes the significance of roots as determinants of tree productivity and survival in the Mediterranean ecosystems.

Since 2002, Silver buffaloberry (Shepherdia argentea) has been introduced from North America in order to improve the fragile ecological environment in western China. To elucidate the salt-resistance mechanism of S. argentea, we conducted a test with two-year-old seedlings subjected to 0, 200, 400, and 600 mM NaCl solutions for 30 d. The results showed that significant salt-induced suppression of plant fresh mass (FM) and stem height of S. argentea seedlings occurred only at the highest salinity level (600 mM). Leaf number, plant dry mass (DM), and chlorophyll (Chl) content declined markedly at both 400 and 600 mM. Leaf area (LA) and leaf water potential (Ψ w ) continuously declined with the increase of salinity. There was also a progressive and evident decrease in net photosynthetic rate (P N ), transpiration rate (E), and stomatal conductance (g s ) with the increase of salinity and time. The correlation analysis indicated that P N was positively correlated with g s at all salinity levels while correlated with intercellular CO 2 concentration (C i ) only at moderate salinity levels (<600 mM). Based on the initial slope of the P N /C i curves, the estimated carboxylation efficiency (CE) was strongly inhibited at 600 mM. We confirm that S. argentea is highly tolerant to salinity. Moreover, our results show that at moderate salinity levels, salt-induced inhibition of photosynthesis is mainly attributed to the stomatal efficient closure predetermined by a low water potential in leaves; while at the high salinity levels, the inhibition is mainly due to the suppression of chloroplast capacity to fix CO 2 caused by the serious decline in both CE and Chl contents.Additional key words: growth; net photosynthetic rate; non-stomatal limitations; P N /C i curves; relative water content; stomatal limitations.

How to improve plant tolerance and yield under salt stress is critical for ensuring sufficient food supply since plant survival and agricultural productivity are both affected by salinity. Some evidence has showed that beneficial microorganisms have a high ability to improve plant salt tolerance and increase crop yield. But few studies were involved in effects of halotolerant yeasts on plants under salt stress. In this present research, Meyerozyma guilliermondii, a halotolerant yeast, was inoculated with tomato plants followed by salt treatment of four different NaCl concentrations (0, 100, 200, and 300 mM). Our results showed that inoculation of M. guilliermondii increased the chlorophyll biosynthesis and photosynthetic machinery effectiveness under salt stress, contributing to biomass accumulation. Under salt treatment of 300 mM NaCl, the yeast inoculation significantly increased ascorbate concentrations in leaves, yet showed no effects on levels of glutathione and proline. Antioxidant enzymes were affected differently by the yeast inoculation. It was found that the yeast inoculation increased superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities under 300, 100, and 200 mM NaCl, respectively. Total soluble sugar levels increased in inoculated tomato plant leaves; however, there were no significant differences between different NaCl concentrations. Under 300 mM NaCl, the yeast inoculation significantly decreased H2O2 levels and reduced malondialdehyde levels. All together, our results showed that halotolerant yeast M. guilliermondii inoculation might be a strong candidate for regulating tomato growth under salt stress by increasing ability to scavenge reactive oxygen species and chlorophyll intactness, and by strengthening photosynthetic machinery.