Bingcheng 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.

Moderate Drought Stress Affected Root Growth and Grain Yield in Old, Modern and Newly Released Cultivars of Winter Wheat

To determine root growth and grain yield of winter wheat (Triticum aestivum L) under moderate drought stress, a nursery experiment and a field trial were conducted with or without water stress using three representative cultivars released in different years: CW134 (old landrace), CH58 (modern cultivar) and CH1 (new release). In the nursery experiment, plants were grown in soil-filled rhizoboxes under moderate drought (MD, 55% of field capacity) or well-watered (WW, 85% of field capacity) conditions. In the field trial, plots were either rainfed (moderate drought stress) or irrigated with 30 mm of water at each of stem elongation, booting and anthesis stages (irrigated). Compared to drought stress, grain yields increased under sufficient water supply in all cultivars, particular the newly released cultivar CH1 with 70% increase in the nursery and 23% in the field. When well-watered (nursery) or irrigated (field), CH1 had the highest grain yields compared to the other two cultivars, but produced similar yield to the modern cultivar (CH58) under water-stressed (nursery) or rainfed (field) conditions. When exposed to drought stress, CW134 had the highest topsoil root dry mass in topsoil but lowest in subsoil among the cultivars at stem elongation, anthesis, and maturity, respectively; while CH1 had the lowest topsoil and highest subsoil root dry mass at respective sampling times. Topsoil root mass and root length density were negatively correlated with grain yield for the two water treatments in nursery experiment. When water was limited, subsoil root mass was positively correlated with thousand kernel weight (TKW). In the field trial, CH1 and CH58 used less water during vegetative growth than CW134, but after anthesis stage, CH1 used more water than the other two cultivars, especially in the soil profile below 100 cm, which was associated with the increased TKW. This study demonstrated that greater root mass and root length density in subsoil layers, with enhanced access to subsoil water after anthesis, contribute to high grain yield when soil water is scarce.

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.

Does root pruning increase yield and water-use efficiency of winter wheat?

Apotanda fieldexperimentwereconductedtoassesswhetherseedingdensityinwinterwheataffectsgrainyield and water-use efficiency when combined with root pruning. Both experiments compared four treatments, namely (i) low (normal)plantdensitywithnorootpruning;(ii)lowplantdensitywithrootpruning;(iii)highplantdensity(25%higherthan low density) with no root pruning; and (iv) high plant density with root pruning. Roots to a depth of 25cm were cut back to keep their length to 13cm and to limit their lateral spread to 13cm from the plant. In the pot experiment, two water regimes were employed from stem elongation: (i) plants maintained at 85% field capacity, and (ii) plants maintained at 55% field capacity by regular watering. Low rainfall inspring at the terminal stage of plant growth served as natural water stress in the field experiment. In the field, the higher plant density induced higher root biomass at all sample depths at anthesis. Root pruningsignificantlydecreasedtherootbiomassintheuppersoillayer(0-40cm)andincreasedtherootbiomassinthedeep soil layer (80-120cm). When water was limited, increasing the seeding density by 25% reduced the grain yield, but with adequatewaterincreasingtheseedingdensityincreasedthegrainyield.Rootpruningincreasedthegrainyield,buttherewas nointeractionbetweenseedingdensityandrootpruningongrainyieldineitherthepotsor field.Rootpruningreducedwater use between stem elongation and anthesis which led to more available soil water in the field between anthesis and maturity, and increased the rate of flag leaf photosynthesis at anthesis, whereas the reverse was true of increasing seeding density. Measurements of chlorophyll fluorescence suggested that initially root pruning induced a stress in the plants, but that the pruned plants rapidly recovered so that by anthesis and during grain filling the level of stress was reduced. Increasing the seeding density increased the number of spikes (fertile tillers), but decreased the grain yield per spike and had no effect on the thousand-kernel weight. In contrast, root pruning reduced the number of spikes in both the field and pots, but increased the grain yield per spike and thousand-kernel weight. Under our experimental conditions, restricting the root size by root pruning increased the yield and water-use efficiency of winter wheat in water-limited environments.

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.

Seasonal Root Biomass and Distribution of Switchgrass and Milk Vetch Intercropping under 2:1 Row Replacement in a Semiarid Region in Northwest China

A root system has the plasticity to adapt to environmental change and species interaction. Root biomass and distribution were studied in three comparable stands in the fifth growth year (2005) in a semiarid region of northwest China: (1) pure switchgrass, (2) pure milk vetch, and (3) mixed switchgrass and milk vetch in 2:1 row replacement. The measurements were carried out three times (April, September, and November) in 2005 during the vegetation period using a soil core method (9 cm). For each species, the vertical distribution of root biomass (RB) was measured in six consecutive layers (0–20, 20–40, 40–60, 60–90, 90–120, and 120–150 cm deep) between rows, between plants, and at the center of the plant, respectively. Results indicated a flexible distribution strategy of switchgrass that tended to increase soil exploitation and space sequestration efficiency in soil layers. Milk vetch followed a more conservative strategy: its roots reached the same soil depth in the pure and mixed stands but showed greater root densities in shallower soil layers in the latter. Under intercropping, RB input and root/shoot ratio in switchgrass were reduced, while milk vetch put more photosynthate into root growth. The shift toward a more superficially distributed system of milk vetch in the mixture with switchgrass together with the high RB and wide vertical and horizontal distribution of switchgrass in the study indicates the greater belowground competitive ability of switchgrass in the mixture.

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.

Diurnal and seasonal variations of soil respiration rate under different row-spacing in a Panicum virgatum L. field on semi-arid Loess Plateau of China

Soil respiration (SR) in crop field is affected by environmental factors, agronomic practices and crop types. To clarify how planting density affects the SR dynamics in switchgrass (Panicum virgatum L.) field on the semi-arid Loess Plateau, this research investigated diurnal and seasonal changes of soil respiration rate (RS) under three different row-spacing treatments (20, 40 and 60 cm) in the fourth growing year of switchgrass. Results showed that RS presented a pronounced seasonality under all row-spacing treatments. The highest daily average RS values appeared in August, while the lowest (P<0.05) were in September for each row-spacing. Diurnal variations of RS exhibited single-peak curves in each month. Daily average RS increased significantly as row-spacing enlarged during May and August but there was no significant difference among row-spacing treatments in September. Soil water storage in the depth of 0–100 cm had no significant difference (P>0.05) among the row-spacing treatments, and similar results were found for soil temperature in 0–15 cm soil depth. Soil respiration temperature sensitivity (Q10) values were 1.0–3.7 during the growing months, which were strongly correlated with air temperature in May and June and the soil temperature at 15 cm depth in August. Higher aboveground biomass production and lower RS in most growth months indicated that 20 cm row-spacing treatment was beneficial for increasing the carbon fixation in switchgrass field. Results also implied that it is necessary to take into account the influence of phenology and root growth of switchgrass on soil respiration for accurately evaluating the carbon cycle in the region.

Diurnal and seasonal variations in photosynthetic characteristics of switchgrass in semiarid region on the Loess Plateau of China

In order to use rationally switchgrass (Panicum virgatum L.) introduced in a large scale in semiarid regions on the Loess Plateau of China, we investigated and compared soil water storage dynamics, diurnal and seasonal changes in leaf photosynthetic characteristics, and biomass production of switchgrass grown under three different row spacing (20, 40, and 60 cm). Results indicated that photosynthetic parameters showed a pronounced seasonality. Diurnal course of net photosynthetic rate (PN) was bimodal, showing obvious midday depression, which was mainly due to stomatal limitation in May and June, by nonstomatal limitation in August, and both stomatal and nonstomatal factors in September. Generally, PN, stomatal conductance, instantaneous water-use efficiency, light-saturated net photosynthetic rate, saturation irradiance, and compensation irradiance increased with increasing row spacing. Plant height, leaf width, and a relative growth rate of biomass accumulation were significantly higher at the row spacing of 60 cm, while 20 cm spacing showed significantly higher aboveground biomass production and the biomass water-use efficiency. All these confirmed that soil water is the key limiting factor influencing switchgrass photosynthesis, and suggested that the wide row plantation (i.e., 60 cm) was more beneficial to switchgrass growth, while narrow spacing was in favor of improving switchgrass productivity and water-use efficiency.

Biomass partition, leaf gas exchange and water relations of alfalfa and milkvetch seedlings in response to soil drying

This study compared physiological and growth responses to water stress of two legume species during the seedling stage. Potted alfalfa (Medicago sativa L. cv. Algonquin) and milkvetch (Astragalus adsurgens Pall. cv. Pengyang earlymaturing vetch) seedlings were grown under well-watered [soil water content (SWC) maintained at 14.92% daily] or water-stressed conditions (drying) for 15 days. Net photosynthetic rate (PN), transpiration rate (E) and stomatal conductance (gs) of both species decreased parabolically. When SWC decreased to 7.2% and 10.3%, gs values for alfalfa and milkvetch were significantly different from those of the respective well-watered plants (p<0.05). When SWC decreased to 6.6% for alfalfa and 6.8% for milkvetch, leaf water potentials (ψL) were significantly different from those of the well-watered plants (p<0.05). Thus the difference between the SWC thresholds for a nonhydraulic root signal (nHRS) and a hydraulic root signal (HRS) were 0.6% and 3.5% for alfalfa and milkvetch, respectively. Milkvetch had a lower gs than alfalfa for a given SWC (p<0.05). Although alfalfa seedlings had a higher dry mass (DM) and root:shoot ratio (R/S) than milkvetch in both treatments (p<0.05), we concluded that milkvetch seedlings had greater drought tolerance than alfalfa.