Morphological and physiological responses of Heteropogon contortus to drought stress in a dry-hot valley.

BACKGROUND: Heteropogon contortus is a valuable pasture species that is widely used for vegetation restoration in dry-hot valleys of China. However, to date, its morphological and physiological responses to drought, and the underlying mechanisms are not well understood. This study was aimed to investigate the morphological and physiological changes of H. contortus under drought stress during the dry-hot season. Heteropogon contortus was planted in pots and subjected to four levels of soil water treatments: above 85 % (control), 70-75 % (light stress), 55-60 % (moderate stress) or 35-40 % (severe stress) of field capacity. RESULTS: Within the total stress period (0-29 days), H. contortus grew rapidly in the light stress, whereas severe stress had a negative impact on growth. Aboveground biomass decreased together with increasing drought stress, whereas root biomass increased. Consequently, the root/shoot ratio of the severe stress treatment increased by 80 % compared to that of the control treatment. The ratio of bound water/free water (BW/FW) was the most sensitive parameter to drought and showed a value under severe stress that was 152.83 % more than that in the control treatment. Although leaf water potential (LWP) and leaf relative water content (RWC) decreased with progressive water stress, H. contortus managed to maintain a relatively high RWC (nearly 70 %) in the severe stress condition. We also detected a significant reduction (below 0.6) in the ratio of variable fluorescence/maximum fluorescence (Fv/Fm) in the severe stress treatment. CONCLUSIONS: Our results show that H. contortus adapts to drought mainly by avoidance mechanisms, and its morphological and physiological characteristics are inhibited under severe stress, but can recover at a certain time after re-watering. These findings might help limited water resources to be fully used for vegetation management in the studied region.

[Biomass allocations and their response to environmental factors for grass species in an arid-hot valley]. / å¹²çƒ­æ²³è°·è‰æœ¬æ¤ç‰©ç”Ÿç‰©é‡åˆ†é åŠå ¶å¯¹çŽ¯å¢ƒå› å­çš„å“åº”.

The effects of water supply frequency, nutrient addition and clipping on biomass allocations among roots, stems and leaves as well as their allometric scaling relationships for six grass species from an arid-hot valley were investigated. The results showed that the fraction of leaf biomass significantly increased from 25.1% to 31.2% and the faction of stem biomass decreased from 43.7% to 34.2% under clipping treatment. Fertilization significantly decreased the faction of root biomass from 34.0% to 30.8%. Water treatments had no significant effect on biomass allocations. Species identity significantly affected biomass allocations among roots, stems and leaves. Species adapted to infertile soils allocated more biomass into leaves and roots and less into stems. There were significant interactions between species and environmental factors, suggesting that the effects of environmental factors on biomass allocations differed among species. Allometric constants and scaling exponents of leaf-stem for species adapted to infertile soils were greater than those for other species. Allometric constants and scaling exponents of stem-root for species adapted to infertile soils were lower than those for other species. In total, nutrient addition significantly increased allometric constants of leaf-stem and leaf-root while clipping significantly reduced scaling exponents of stem-root. The frequency of water supply had no significant effects on the allometric scaling relationships among different organs. The effects of environmental factors on the allometric scaling relationships between different organs differed among species. The differences in biomass allocations and their responses to environmental factors among different species might affect plant adaptations to environmental changes.

[Characterization of Pollutant Wash-off in the Urban Stormwater].

To understand the pollution character of urban road runoff, the concentrations of TSS, EC, N and P in the ring road runoff of Beijing from June to September 2013 were evaluated, the correlations among pollutants were examined, and the load of TSS, N and P were estimated. Result showed that the small particulates in the range of 1-10 µm consisted of 60% TSS in the road runoff. Totally 89 percent of the nitrogen (N) was dissolved phase state in the road runoff, 80 percent of the phosphorus (P) was particular phase state in the road runoff. Based on the characteristics of correlations between EC, TSS and TN, TP, EC and TSS were the surrogate indexes of pollution degree assessment for the dissolved N and particulate P in the urban road runoff, respectively. Based on our results, the SS, N and P year load per unit area in Beijing ring road runoff were 16 725.69, 1777.91 and 24.23 mg x (m2 x a)(-1), respectively. Our findings described the polutant wash off character in urban road runoff, which provide a scientific basis for management of nonpoint pollution in a city and an alternative method for controlling pollution.