Weizhou Xu

Biomass allocation, relative competitive ability and water use efficiency of two dominant species in semiarid Loess Plateau under water stress.

A better understanding of the growth and interspecific competition of native dominant species under water stress should aid in prediction of succession in plant communities. In addition, such research would guide the selection of appropriate conservation and agricultural utilization of plants in semiarid environments that have not been very well characterized. Biomass production and allocation, relative competitive ability and water use efficiency of one C(4) herbaceous grass (Bothriochloa ischaemum) and one C(3) leguminous subshrub (Lespedeza davurica), both important species from the semiarid Loess Plateau of China, were investigated in a pot-cultivation experiment. The experiment was conducted using a replacement series design in which B. ischaemum and L. davurica were grown with twelve plants per pot, in seven combinations of the two species (12:0, 10:2, 8:4, 6:6, 4:8, 2:10, and 0:12). Three levels of water treatments included sufficient water supply (HW), moderate water stress (MW) and severe water stress (LW). These treatments were applied after seedling establishment and remained until the end of the experiment. Biomass production and its partitioning, and transpiration water use efficiency (TWUE) were determined at the end of the experiment. Interspecific competitive indices (competitive ratio (CR), aggressiveness (A) and relative yield total (RYT)) were calculated from the dry weight for shoots, roots and total biomass. Water stress decreased biomass production of both species in monoculture and mixture. The growth of L. davurica was restrained in their mixtures for each water treatment. L. davurica had significantly (P<0.05) greater root:shoot allocation than B. ischaemum for each water treatment and proportion within the replacement series. Aggressiveness (A) values for B. ischaemum with respect to L. davurica were negative only at the proportions of B. ischaemum to L. davurica being 8:4 and 10:2 in LW treatment. B. ischaemum had a significantly (P<0.05) higher CR value under each water treatment, and water stress considerably reduced its relative CR while increased that of L. davurica. RYT values of the two species indicated some degree of resource complimentarity under both water sufficient and deficit conditions. The results suggest that it is advantageous for growing the two species together to maximize biomass production, and the suggested ratio was 10:2 of B. ischaemum to L. davurica because of significantly higher (P<0.05) RYT and TWUE under low water availability condition.

Biomass production and relative competitiveness of a C 3 legume and a C 4 grass co-dominant in the semiarid Loess Plateau of China

Bothriochloa ischaemum L. and Lespedeza davurica (Laxm.) Schindl. are two co-dominant species of great importance in reducing soil and water loss and maintaining the distinctive natural scenery of the semiarid Loess Plateau of China. Our aim was to determine the growth and interspecific competition between these species under water stress to facilitate the prediction of community succession and guide the selection of appropriate methods of conservation and use in the area. A pot experiment was designed to investigate the effects of water stress and competition on biomass production and allocation, relative competitive ability and water use efficiency of the two species. Bothriochloa ischaemum (a C4 perennial herbaceous grass) was planted in the same pot with L. davurica (a C3 perennial leguminous subshrub) at density ratios of 12:0, 10:2, 8:4, 6:6, 4:8, 2:10, and 0:12. The response of the species to their mutual presence at the different ratios was evaluated at three levels of soil moisture (80% ± 5% field capacity, FC (HW), 60% ± 5% FC (MW) and 40% ± 5% FC (LW)). Indices of aggressivity (A), competitive ratio (CR) and relative yield totals (RYTs) were calculated from the dry shoot, root and total weight data. Water stress decreased the biomass production by both species whether in monoculture or mixture, but B. ischaemum was more sensitive to water deficit. Across moisture levels, the growth of L. davurica was more adversely affected by mixed planting. Bothriochloa ischaemum had significantly (P < 0.05) smaller root:shoot ratios than L. davurica and the root mass of both species tended to increase relative to shoot mass as soil water deficit increased. The aggressivity (A), competitive ratio (CR) and relative yield totals (RYTs) of B. ischaemum were positive across treatments. Bothriochloa ischaemum had much higher CR under each water treatment, but water stress considerably reduced its relative CR while increasing that of L. davurica. The RYT values of the two species indicated some degree of resource complimentarity under both water sufficient and deficit conditions. Our results suggest that it is advantageous to grow the two species together to maximize biomass production. We recommend a mixture ratio of 8:4 of B. ischaemum to L. davurica because it gave significantly higher RYT and transpiration water use efficiency under deficit water conditions.

Effects of water stress and fertilization on leaf gas exchange and photosynthetic light-response curves of Bothriochloa ischaemum L.

Bothriochloa ischaemum L. is an important species in many temperate regions, but information about the interactive effects of water stress and fertilization on its photosynthetic characteristics was inadequate. A pot experiment was conducted to investigate the effects of three water [80% (HW), 40% (MW), and 20% (LW) of field capacity (FC)] and four fertilization regimes [nitrogen (N), phosphorus (P), nitrogen with phosphorus (NP), and no fertilization] on leaf photosynthesis. Leaf gas exchange and photosynthetic light-response curves were measured at the flowering phase of B. ischaemum. Water stress decreased not only the leaf gas-exchange parameters, such as net photosynthetic rate (PN), stomatal conductance (gs), transpiration rate (E), and water-use efficiency (WUE) of B. ischaemum, but also downregulated PN-photosynthetically active radiation (PAR) curve parameters, such as light-saturated net photosynthetic rate (PNmax), apparent quantum efficiency (AQE), and light compensation point (LCP). Fertilization (N, P, and NP) enhanced the daily mean PN values and PNmax under the HW regime. Addition of N (either alone or with P) improved the photosynthetic capacity of B. ischaemum under the MW and LW regimes by increasing PN, PNmax, and AQE and reducing dark respiration rate and LCP, but the addition of P alone did not significantly improve the photosynthetic performance. Decline in PN under each fertilization regime occurred during the day and it was caused mainly by nonstomatal limitation. Our results indicated that water was the primary limiting factor for photosynthesis in B. ischaemum, and that appropriate levels of N fertilization improved its potential photosynthetic capacity under water-deficit conditions.

Biomass production, relative competitive ability and water use efficiency of two dominant species in semiarid Loess Plateau under different water supply and fertilization treatments

Water stress and nutrient deficiency are considered to be the main environmental factors limiting plant growth and species interaction in semiarid regions. However, less is known about the interactive effects of soil water, nitrogen and phosphorus on native species growth and relative competitive ability. A replacement series design method was used with 12 mixed plants of Bothriochloa ischaemum and Lespedeza davurica grown in a pot experiment under three water regimes and four fertility treatments. Intercropping systems were assessed on the basis of indices such as biomass production and allocation, relative competitive ability, aggressivity, relative yield total and water use efficiency (WUE). Water stress decreased significantly the total biomass production for each species, either in monoculture or in mixtures. N, P, or NP application can significantly improve biomass production of the two species in their mixtures. There was no obvious change trend in root/shoot ratio of B. ischaemum or L. davurica in different mixture proportions. Relative yield total (RYT) values ranged from 0.98 to 1.39. Aggressivity values of B. ischaemum to L. davurica were positive in all water regimes and fertilizations, implying that B. ischaemum was the dominant species. Relative competition intensity values of B. ischaemum (i.e., RCIB) were less than zero, while greater than zero for L. davurica (i.e., RCIL), indicating that the effects of intraspecific competition with L. davurica were stronger for B. ischaemum, and the opposite for L. davurica. WUE increased gradually as the proportion of B. ischaemum increased in mixtures, and a 10:2 B. ischaemum:L. davurica mixture proportion had significantly higher WUE. Results suggest that it is advantageous to grow the two species together to maximize biomass production and the recommended mixture ratio was 10:2 of B. ischaemum to L. davurica because it gave higher RYT and significantly higher WUE under conditions of water deficit.

Morphological changes in roots of Bothriochloa ischaemum intercropped with Lespedeza davurica following phosphorus application and water stress

Root morphological characteristics are important parameters for the evaluation of plant adaptation to stress environment. In this study, a pot experiment was conducted to investigate changes in the root morphology of Bothriochloa ischaemum intercropped with Lespedeza davurica under three soil water regimes and two phosphorus (P) fertilizer treatments. Results showed that root biomass (RB) per B. ischaemum plant decreased as its proportion increased in the mixtures. There were no significant differences in root:shoot ratio (RSR) among the mixture ratios, and P application did have consistent effects on the RSR. Bothriochloa ischaemum tended to have smaller root surface area (RSA), root average diameter (RAD), specific root length (SRL), and specific root area (SRA) under water stress conditions. There was a negative linear relationship between RB and RSA under each water and P treatment. Negative and positive linear relationships were found between RB and TRL (total root length), TRL and RSA, respectively, except under severe water stress without P application. P application decreased the RAD and increased the SRA and SRL of B. ischaemum under water stress. All these suggest two apparent response mechanisms for B. ischaemum under water stress and P application: an increase in length of small diameter roots and decrease in root weight density.

Accumulation of N and P in the Legume Lespedeza davurica in Controlled Mixtures with the Grass Bothriochloa ischaemum under Varying Water and Fertilization Conditions

Water and fertilizers affect the nitrogen (N) and phosphorus (P) acquisition and allocation among organs in dominant species in natural vegetation on the semiarid Loess Plateau. This study aimed to clarify the N and P accumulation and N:P ratio at organ and plant level of a local legume species mixed with a grass species under varying water and fertilizer supplies, and thus to fully understand the requirements and balance of nutrient elements in response to growth conditions change of native species. The N and P concentration in the organ (leaf, stem, and root) and plant level of Lespedeza davurica (C3 legume), were examined when intercropped with Bothriochloa ischaemum (C4 grass). The two species were grown outdoors in pots under 80, 60, and 40% of soil water field capacity (FC), -NP, +N, +P, and +NP supply and the grass:legume mixture ratios of 2:10, 4:8, 6:6, 8:4, 10:2, and 12:0. The three set of treatments were under a randomized complete block design. Intercropping with B. ischaemum did not affect N concentrations in leaf, stem and root of L. davurica, but reduced P concentration in each organ under P fertilization. Only leaf N concentration in L. davurica showed decreasing trend as soil water content decreased under all fertilization and mixture proportion treatments. Stems had the lowest, while roots had the highest N and P concentration. As the mixture proportion of L. davurica decreased under P fertilization, P concentration in leaf and root also decreased. The N concentration in L. davurica at the whole plant level was 11.1–17.2%. P fertilization improved P concentration, while decreased N:P ratio in L. davurica. The N:P ratios were less than 14.0 under +P and +NP treatments. Our results implied that exogenous N and P fertilizer application may change the N:P stoichiometry and influence the balance between nutrients and organs of native dominant species in natural grassland, and P element should be paid more attention when considering rehabilitating degraded grassland via fertilization application in semiarid Loess Plateau region.

Overexpression of alfalfa Orange gene in tobacco enhances carotenoid accumulation and tolerance to multiple abiotic stresses

The multifunctional Orange (Or) protein plays crucial roles in carotenoid homeostasis, photosynthesis stabilization, and antioxidant activity in plants under various abiotic stress conditions. The Or gene has been cloned in several crops but not in alfalfa (Medicago sativa L.). Alfalfa is widely cultivated across the world; however, its cultivation is largely limited by various abiotic stresses, including drought. In this study, we isolated the Or gene from alfalfa (MsOr) cv. Xinjiang Daye. The amino acid sequence of the deduced MsOr protein revealed that the protein contained two trans-membrane domains and a DnaJ cysteine-rich zinc finger domain, and showed a high level of similarity with the Or protein of other plants species. The MsOr protein was localized in leaf chloroplasts of tobacco. The expression of MsOr was the highest in mature leaves and was significantly induced by abiotic stresses, especially drought. To perform functional analysis of the MsOr gene, we overexpressed MsOr gene in tobacco (Nicotiana benthamiana). Compared with wild-type (WT) plants, transgenic tobacco lines showed higher carotenoid accumulation and increased tolerance to various abiotic stresses, including drought, heat, salt, and methyl viologen-mediated oxidative stress. Additionally, contents of hydrogen peroxide and malondialdehyde were lower in the transgenic lines than in WT plants, suggesting superior membrane stability and antioxidant capacity of TOR lines under multiple abiotic stresses. These results indicate the MsOr gene as a potential target for the development of alfalfa cultivars with enhanced carotenoid content and tolerance to multiple environmental stresses.

Soil Moisture Availability at Early Growth Stages Strongly Affected Root Growth of Bothriochloa ischaemum When Mixed With Lespedeza davurica

Rainfall is the main resource of soil moisture in the semiarid areas, and the altered rainfall pattern would greatly affect plant growth and development. Root morphological traits are critical for plant adaptation to changeable soil moisture. This study aimed to clarify how root morphological traits of Bothriochloa ischaemum (a C4 herbaceous species) and Lespedeza davurica (a C3 leguminous species) in response to variable soil moisture in their mixtures. The two species were co-cultivated in pots at seven mixture ratios under three soil water regimes [80% (HW), 60% (MW), and 40% (LW) of soil moisture field capacity (FC)]. At the jointing, flowering, and filling stages of B. ischaemum, the LW and MW treatments were rewatered to MW or HW, respectively. At the end of growth season, root morphological traits of two species were evaluated. Results showed that the root morphological response of B. ischaemum was more sensitive than that of L. davurica under rewatering. The total root length (TRL) and root surface area (RSA) of both species increased as their mixture ratio decreased, which suggested that mixed plantation of the two species would be beneficial for their own root growth. Among all treatments, the increase of root biomass (RB), TRL, and RSA reached the highest levels when soil water content increased from 40 to 80% FC at the jointing stage. Our results implied that species-specific response in root morphological traits to alternated rainfall pattern would greatly affect community structure, and large rainfall occurring at early growth stages would greatly increase their root growth in the semiarid environments.