Xianchong Wan

Two poplar varieties with different resistance to sulfur dioxide were subjected to different concentrations of SO₂ fumigation treatment. Young and mature leaves of Purui poplar (resistant) vs. 74/76 poplar (susceptible) were used to measure the changes in the activity of enzymes and metabolite content. Among the five key enzymes involved in sulfur metabolism and sulfur metabolites, APR, SO enzyme, GSH, and sulfate content have the greatest impact on young leaves of Purui, followed by 74/76 young leaves. The results show that for both Purui and 74/76 poplar, young leaves have stronger sulfur metabolism ability than mature leaves, indicating that young leaves have stronger SO₂ resistance. Purui has stronger sulfur metabolism ability than 74/76 poplar, especially reflected in their young leaves. The comparison between young and mature leaves, as well as the comparison between resistant and susceptible varieties, mutually confirms that sulfur metabolism in leaves is an important mechanism for sulfur dioxide resistance.

Background Frost is a common environmental stress for temperate plants. Xylem embolism occurs in many overwintering plants due to freeze-thaw cycles, so coping with freeze-thaw-induced embolisms is essential for the survival of temperate plants. Methods This study was conducted on Phyllostachys propinqua McClure, a woody bamboo species that was grown under natural frost conditions to explore its responses to winter embolisms. From autumn to the following spring, the following measurements were recorded: predawn branch and leaf embolism, branch and leaf relative water content (RWC), root pressure and soil temperature, xylem sap osmotic potential, branch and leaf electrolyte leakage (EL), branch nonstructural carbohydrate (NSC) content and leaf net photosynthetic rate. Results P. propinqua had a mean vessel diameter of 68.95 ±1.27 µm but did not suffer severe winter embolism, peaking around 60% in winter (January), with a distinct reduction in March when root pressure returned. Leaves had a more severe winter embolism, up to 90%. Leaf RWC was much lower in winter, and leaf EL was significantly higher than branch EL in all seasons. Root pressure remained until November when soil temperature reached 9 °C, then appeared again in March when soil temperatures increased from −6 °C (January) to 11 °C. Xylem sap osmotic potential decreased from autumn to winter, reaching a minimum in March, and then increasing again. Soluble sugar (SS) concentration increased throughout the winter, peaked in March, and then decreased. Conclusions These results suggest that (1) there is a hydraulic segmentation between the stem and leaf, which could prevent stem water loss and further embolization in winter; (2) maintenance of root pressure in early winter played an important role in reducing the effect of freeze-thaw cycles on the winter embolism; (3) the physiological process that resulted in a decrease in xylem sap osmotic potential and tissue water content, and an accumulation of SS associated with cold acclimation also aided in reducing the extent of freeze-thaw-induced embolism. All these strategies could be helpful for the maintenance of xylem hydraulic function of this bamboo species during winter.

In this study, two poplar varieties with different resistance to sulfur dioxide were selected for a comparative experiment. SO₂ fumigation to the poplars was carried out under controlled conditions to determine the variation in the activity of enzymes, the content of metabolites and the expression of enzymes genes in pathways of sulfur metabolism in plants. The results showed that the activity of enzymes and the content of sulfur metabolites were closely related to the response of the poplars to SO₂ stress. Populus × euramericana cv. 'Purui' had two ways of detoxification: oxidation detoxification, oxidizing sulfite (SO₃²⁻) to sulfate SO₄²⁻ by sulfite oxidase; reductive detoxification, SO₃²⁻ being reduced to S²⁻ by sulfite reductase (SiR). Moreover, OASTL and SAT activity, and levels of cysteine (CYS) and glutathione (GSH) also increased in P. × euramericana cv. 'Purui' in response to SO₂ fumigation, and the gene expression encoding Glutathione S-transferases (GST), and some enzymes in cysteine and methionine metabolism was up-regulated. For Populus × euramericana cv. '74/76' with weaker resistance to sulfur dioxide, it only detoxified by increasing the activity of SiR, and but down-regulated the expression of gene encoding 3'-phosphoadenosine 5'-phosphosulfate synthase (PAPSS), which could affect the consumption of sulfite in the exposure to SO₂. Thus, the SO₂-resistant difference of the two poplar varieties is mainly attributed to variation in activity of the enzymes and content of their metabolites in pathways of sulfur metabolism, and gene expression of some enzymes in cysteine and methionine metabolism also plays a role in the resistant difference.

Background: Despite the importance of root hydraulics, there is little research on the in situ dynamic responses of embolism formation and embolism repair of roots distributed in different soil depths in response to different water regimes.Results: The vessel diameter, hydraulic conductivity, and vulnerability to cavitation were in the order of deep root > shallow root > branch. The midday PLC of shallow root was the highest in the dry season, while the midday PLC of deep root slightly higher than that of branch with no significant difference in the two seasons. The capacity of embolism repair of roots was significantly greater than that of branch both in dry season and wet season. The xylem pressure was in the order of deep roots > shallow root > branch, and it was negative in most of the time for the latter two in the dry season, but positive for both of the roots during the observation period in the wet season. The NSC and starch content in roots were significantly higher than those in branches, especially in the dry season. In contrast, roots had lower content of soluble sugar.Conclusions: The relatively stable water condition in soil, especially in the deep layers, is favorable for the development of larger-diameter vessels in root xylem, however it cannot prevent the root from forming embolism. The mechanism of embolism repair may be different in different parts of plants. Deep roots mainly depend on root pressure to refill the embolized vessels, while branches mainly depend on starch hydrolysis to soluble sugars to do the work, with shallow roots shifted between the two mechanisms in different moisture regimes. There is theoretically an obvious trade-off between conducting efficiency and safety over deep roots, shallow roots and branches. But in natural conditions, roots do not necessarily suffer more severe embolism than branches, maybe due to their root pressure-driven embolism repair and relatively good water conditions.

How the mortality and growth of tree species vary with the iso-anisohydric continuum and xylem vulnerability is still being debated. We conducted a precipitation reduction experiment to create a mild drought condition in a forest in the Baotianman Mountains, China, a sub-humid region. Three main sub-canopy tree species in this region were examined. After rainfall reduction, Lindera obtusiloba showed severe dieback, but two other co-occurring species did not show dieback. The water potential at stomatal closure of Dendrobenthamia japonica, L. obtusiloba and Sorbus alnifolia was −1.70, −2.54 and −3.41 MPa, respectively, whereas the water potential at 88 % loss in hydraulic conductivity of the three species was −2.31, −2.11 and −7.01 MPa, respectively. Taken together, near-anisohydric L. obtusiloba with vulnerable xylem was highly susceptible to drought dieback. Anisohydric S. alnifolia had the most negative minimum water potential, and its xylem was the most resistant to cavitation. Isohydric D. japonica conserved water by rapidly closing its stomata. Ultimately, the hydraulic safety margin (HSM) of L. obtusiloba was the smallest among the three species, especially in precipitation-reduced plots. In terms of the stomatal safety margin (SSM), L. obtusiloba was negative, while S. alnifolia and D. japonica were positive. Of the two species without dieback, rainfall reduction decreased growth of D. japonica, but did not influence growth of S. Alnifolia; meanwhile, rainfall reduction led to a decrease of non-structural carbohydrates (NSCs) in D. japonica, but an increase in S. alnifolia. It is concluded that HSM as well as SSM allow interpreting the sensitivity of the three sub-canopy species to drought. The drought-induced dieback of L. obtusiloba is determined by the interaction of stomatal behaviour and xylem vulnerability, and the species could be sensitive to climate change-caused drought although still in sub-humid areas. The isohydric/anisohydric degree is associated with NSCs status and growth of plants.

To explore the interactive effects of wind and light on growth and morphogenesis, poplar saplings were subjected to two different levels of wind and light. Morphogenesis, growth, water relations and gas exchange were examined in a factorial experiment. The results showed that wind reduced both shoot height and root growth but did not affect shoot diameter or root/shoot ratio, while light promoted both shoot and root growth, and the root/shoot ratio. However, light and wind consistently increased stem taper. Wind and light reduced leaf water potential and increased stem xylem cavitation at midday. Lateral shade decreased root and stem hydraulic conductance, while wind reduced stem hydraulic conductance without influencing root hydraulic conductance. Wind and light reduced the vulnerability of poplar saplings to xylem cavitation. Lateral shade strongly reduced the CO ₂ assimilation rate and stomatal conductance. Wind reduced the assimilation rate at midday under full sunlight. In summary, both light and wind increased stem taper, although the two environmental factors had opposite effects on the growth of poplar saplings. Lateral shading reduced the growth of shoots and roots of poplar saplings by restricting assimilation, and light promoted greater allocation of carbon to roots than to shoots, and to a lower rather than upper portion of the stem, leading to increased root/shoot ratio and stem taper. At the sapling stage, wind reduced shoot height and root growth primarily by interrupting water balance, and also induced thigmomorphogenic responses, such as increased taper. Finally, lateral shading did not suppress the thigmomorphological responses to wind.

Hydroponic-grown seedlings of aspen ( Populus tremuloidesMichx.) were used to investigate how low root temperatures (5°C) affect stomatal conductance and water relations. An isohydric manner of the...

Two broadleaf evergreen canopy species (Schima superbaandEngelhardia roxburghiana) with different phenologies in a subtropical region of southern China were used to determine the influence of leaf phenology on the impact of an insect pest attack.S.superbaregenerates its leaves in February, whileE.roxburghianaregenerates its leaves in May. The mothThalassodes quadrariaattacked the two broadleaf evergreen species in March to April, and the newly produced leaves were removed forS.superbabut not forE.roxburghiana. The young trees were artificially defoliated to imitate an insect pest attack during March 2014. Nonstructural carbohydrate (NSC) and growth measurements and a retrospective analysis based on the radial growth of mature trees were conducted in January 2015. The results showed that NSC concentrations decreased inS.superbaduring canopy rebuilding, and the subsequent defoliation severely inhibited leaf and shoot growth, prevented NSC restoration in roots and stem xylem, and caused high mortality. The insect outbreaks reduced the radial growth ofS.superba. In contrast,E.roxburghianaexperienced less growth retardation, lower mortality, and normal radial growth. Thus, taking phenology-dependent variation in NSCs into consideration, defoliation and insect pest outbreaks more negatively impactedS.superbathanE.roxburghiana.