The effect of soil strength on the growth of pigeonpea radicles and seedlings

The effect of soil strength on the growth of pigeonpea radicles and seedlings was investigated in cores of three clay soils prepared at different water contents and bulk densities in the laboratory.Radicle elongation directly into soil cores was reduced from 50–70 mm d^-1 at strengths less than 0.5 MPa to 0 mm d^-1 at 3.5–3.7 MPa. The response to soil strength was affected by the water content of the soil, presumably as a result of reduced oxygen availability in wetter soil. This effect was apparent in soils wet to air-filled porosities less than 0.15 m³ m^-3.Radicles were more sensitive to high soil strength (>1.5 MPa) than were seedling roots which encountered the same conditions at 60 mm in the profile. Radicle growth ceased at 3.5 MPa which reduced seedling root growth by only 60%.Despite a 60% reduction in root length in the high strength zone, seedling roots compensated in zones of loose soil above and below the compacted layer, and total root length and shoot growth were unaffected. There was no evidence of a root signal response which results in reduced shoot growth in some species in response to high soil strength.The proliferation of roots in surface layers and the delayed penetration of the root system to depth in compacted soil are likely to expose seedlings to a greater risk of water-deficit in the field, particularly under dryland conditions where plants rely on stored subsoil water for growth.     

Application of hydrotime model to predict early vigour of rapeseed (<Emphasis Type="Italic">Brassica napus</Emphasis> L.) under abiotic stresses

Rapeseed (Brassica napus L.) is important for edible oil production in semi-arid areas. Abiotic stresses are threatening rapeseed production in such areas. This study was conducted to find tolerant genotypes of rapeseed and to determine which traits of crop establishment is related to abiotic stress tolerance. Hydrotime model parameters were determined in a laboratory germination test, and seedling emergence and growth were evaluated in pot experiments under control, drought, salinity, deep sowing, low and high temperatures for 19 rapeseed genotypes. Results indicated that the predicted germination time courses at the various water potentials generally fitted well with the observed germination data. The estimated values of θ _H, ψ_b(50), and σψ_b differed significantly across genotypes. Seedling emergence and growth differed significantly under each environmental condition. PCA showed that genotypes of Hayola 401 and line 285 were the most tolerant to abiotic stresses during crop establishment and seedling growth. The first PC explained 40% of variations, and a correlation was observed between PC1 and ψ_b(50). Correlations among hydrotime model parameters and early seed vigour variables indicated that ψ_b(50) negatively correlated with seedling emergence percentage and rate (day^?1) under all abiotic stresses. It shows that genotypes with more negative values of ψ_b(50) have more seedling emergence percentage and a larger seedling emergence rate (days^?1) under a wide range of environmental conditions. Thus, it can be concluded that, to identify tolerant genotypes of rapesee to abiotic stresses, ψ_b(50) is a good trait and that breeders can focus on reducing ψ_b(50) to increase tolerance of abiotic stresses.     

Diffusive conductance of rice (Oryza sativa) leaves during gradually induced soil water stress

The diffusive conductance (C_s) of rice (Oryza sativa cvs Jaya and Bala) leaves was measured during a soil drying cycle from flooding to decreasing soil water potential (φ_s) in a controlled-environment chamber. Plants were grown continuously under 5 cm submergence up to 69 days after transplanting and thereafter were subjected to gradual soil drying for a period of 17 days in the vegetative growth stage. In both the cultivars, the values of C_s were generally more on adaxial than abaxial leaf surfaces. This response of stomata during the period of soil drying was independent of leaf rolling. Further, the slopes of the curves (C_s, vs φ_s) also did not differ significantly (P= 0·05). The total C_s, of both cultivars during flooding was almost equal (0·60 cm s^-1) but at the end of the soil drying cycle, the values of total C_s, were 0·11 cm s^-l at ψ_s of -1·3 MPa and 0·08 cm s^-1 at ψ_s, of -0·8 MPa in cvs Jaya and Bala, respectively. For total C_s, slopes differed significantly (P = 0·05). A close relationship between total C_s, and ψ_s, in both cultivars (C_s, = 0·58-0·40 ψ_s, for cv. Jaya and C_s= 0·46-0·56 ψ_s, for cv. Bala) indicated that stomata were sensitive to increasing soil water deficit.     

Canola seed germination and seedling emergence from pre-hydrated and re-dried seeds subjected to salt and water stresses at low temperatures

Low soil temperatures and low water potentials reduce and delay the seed germination of canola (Brassica rapa L., B. napus L.) in western Canada. Germination is also very sensitive to the salinity effects of nitrogen fertiliser placed with the seed, especially when the seed bed is relatively dry. The effects of pre-hydration and re-drying treatment on canola (Brassica rapa L. cv. Tobin) seed germination and seedling emergence at 10°C subjected to either a water or salt stress were determined. Low water potentials, induced by polyethylene glycol (PEG 8000), low soil moisture, or high concentrations of salts, reduced both germination and seedling emergence, and increased the time to 50% germination and emergence of seeds at 10°C. At equal osmotic potentials, Na₂SO₄ was less inhibitory on low temperature germination than either NaCl or PEG, suggesting that the sulphate ion partially alleviated the inhibitory effects of low water potential. Solutions of NaCI produced more abnormal seedlings compared to Na₂SO₄, suggesting that NaCl was more toxic than Na₂SO₄ during seedling development. Pre-hydration and re-drying partially overcame the inhibitory effects of both low water potential and salts on seed germination and seedling emergence at 10°C. The seed treatment increased the germination rate in Petri dishes and seedling emergence from a sandy loam soil. Water potentials or soil water contents required to inhibit 50% germination or emergence at 10°C were lower for treated seeds compared to control seeds. Salt concentrations inhibiting 50% emergence were higher for treated seeds than control seeds. Neither treated nor control seeds produced seedlings which emerged if the soil water content was lower than 9% or when the soil was continuously irrigated with salt solutions of 100 mmol kg^-1 of NaCl or 50 mmol kg^-1 of Na₂SO₄. These results suggest that the pre-hydration and re-drying treatment did not lower the base water potentials at which seedling emergence could occur. Abnormal seedlings were observed in both treated and control seeds, particularly if the soil was watered with NaCl solutions; however, the seed treatment reduced the number of abnormal seedlings.     

Water relations and leaf chemistry of Chrysothamnus nauseosus ssp. consimilis (Asteraceae) and Sarcobatus vermiculatus (Chenopodiaceae)

At Mono Lake, California, we investigated field water relations, leaf and xylem chemistry, and gas exchange for two shrub species that commonly co-occur on marginally saline soils, and have similar life histories and rooting patterns. Both species had highest root length densities close to the surface and have large tap roots that probably reach ground water at 3.4-5.0 m on the study site. The species differed greatly in leaf water relations and leaf chemistry. Sarcobatus vermiculatus had a seasonal minimum predawn xylem pressure potential (ψ_pd) of -2.7 MPa and a midday potential (ψ_md) of -4.1 MPa. These were significantly lower than for Chrysothamnus nauseosus, which had a minimum ψ_pd of -1.0 MPa and ψ_md of -2.2 MPa. Sarcobatus had leaf Na of up to 9.1 % and K up to 2.7 % of dry mass, and these were significantly higher than for Chrysothamnus which had seasonal maxima of 0.4% leaf Na and 2.4 % leaf K. The molar ratios of leaf K/Na, Ca/Na, and Mg/Na were substantially lower for Sarcobatus than for Chrysothamnus. Xylem ionic contents indicated that both species excluded some Na at the root, but that Chrysothamnus was excluding much more than Sarcobatus. The higher Na content of Sarcobatus leaves was associated with greater leaf succulence, lower calculated osmotic potential, and lower xylem pressure potentials. Despite large differences in water relations and leaf chemistry, these species maintained similar diurnal patterns and rates of photosynthesis and stomatal conductance to water vapor diffusion. Sarcobatus ψ_pd may not reflect soil moisture availability due to root osmotic and hydraulic properties.     

Water Potential and Component Potentials in Expanded and Unexpanded Leaves of Two Xeric Grasses

Water (ψ), osmotic (ψ_s+ψ_m) and pressure (ψ_p) potentials were measured in three leaf regions of Agropyron dasystachyum and A. smithii grown in the field. Spanner-type thermocouple psychrometers were used to measure ψ and (ψ_m+ψ_m). Absolute water content (AWC) was measured gravimetrically. The ψ and ψ_p were slightly lower in the emerging leaf blade (EBI) than in the last fully emerged leaf blade (FEBI); (ψ_s+ψ_m) and AWC were similar in the two regions. A gradient as large as 0.7 MPa was observed between the EBI and the base of the same emerging leaf (EBs); the latter included the meristematic regions. Although (ψ_s+ψ_m) and ψ_p were generally higher in the EBs, the gradients diminished as the level of stress increased in the shoot. Under moderate water stress the ψ_p of the EBs remained constant relative to the ψ_p in the exposed blades. The large ψ gradient within the growing leaf could have resulted from high resistance imposed by poor vascular development in the intercalary meristem. Ability to maintain a relatively large ψ gradient may be of general significance in buffering the growing region of xeric grass leaves from extreme, short-term fluctuations in water stress that occur in exposed leaf blades.     

Effect of salt and soil water status on transpiration of Salsola kali L.

Abstract Transpiration of Salsola kali L. plants, grown in small pots under controlled environmental conditions, was followed through a drying cycle of the soil. Three different nutrient solutions were used during the preconditioning growth period: control (C), half-strength Hoagland's nutrient solution; C plus 150mol m⁻³ NaCl; and C plus 150mol m⁻³ KCl. Soil water content at saturation at the beginning of the drying cycle was 20% (w/w). Both NaCl and KCl treatments modified the plants' response to changes in soil water status. The control plants transpired twice as much (per unit leaf dry weight) as the salt-treated plants, even when the soil was at maximal water capacity. Transpiration of the control plants remained high, until the soil water content declined to 5%. After that stage the stomata of these plants closed abruptly. Transpiration of the salt-treated plants started decreasing when the soil water content was approximately 16%, and did so gradually until all the available water was depleted. When transpiration was plotted against soil water potential a sharp decline in the transpiration of control plants was observed with the soil water potential decreasing from -0.04 to -1.2MPa. Transpiration of the salt-treated plants decreased gradually over a wide range of soil water potential (−0.8 to −7.0MPa).     

Effects of soil structural discontinuity on root and shoot growth and water use of maize

The role of roots penetrating various undisturbed soil horizons beneath loose layer in water use and shoot growth of maize was evaluated in greenhouse experiment. 18 undisturbed soil columns 20 cm in diameter and 20 cm in height were taken from the depths 30–50 cm and 50–70 cm from a Brown Lowland soil, a Pseudogley and a Brown Andosol (3 columns from each depth and soil). Initial resistance to penetration in undisturbed soil horizons varied from 2.5 to 8.9 MPa while that in the loose layer was 0.01 MPa. The undisturbed horizons had a major effect on vertical arrangement of roots. Root length density in loose layer varied from 96 to 126 km m^-3 while in adjacent stronger top layers of undisturbed horizons from 1.6 to 20.0 km m^-3 with higher values in upper horizons of each soil. For specific root length, the corresponding ranges were 79.4–107.7 m g^-1 (on dry basis) and 38.2–63.7 m g^-1, respectively. Ratios of root dry weight per unit volume of soil between loose and adjacent undisturbed layers were much lower than those of root length density indicating that roots in undisturbed horizons were produced with considerably higher partition of assimilates. Root size in undisturbed horizons relative to total roots was from 1.1 to 38.1% while water use from the horizons was from 54.1 to 74.0%. Total water use and shoot growth were positively correlated with root length in undisturbed soil horizons. There was no correlation between shoot growth and water use from the loose layers.     

氮磷添加与不同栽植密度交互对樟树幼苗土壤化学性质的短期影响

研究氮磷添加对不同密度樟树(Cinnamomum camphora)幼苗土壤化学性质的影响,以期为全球化背景下樟树人工林生态系统的土壤养分管理提供依据。以1年生樟树幼苗为试验材料,选择氯化铵(NH4Cl)作为氮肥模拟大气氮沉降,以二水合磷酸二氢钠(NaH_2PO_4·2H_2O)模拟磷添加。氮磷处理设置CK、施N、施P和施N+P 4个水平,其中N、P和N+P施肥量分别为40 g m~(-2)a~(-1)(NH_4Cl)、20 g m-2a-1(NaH_2PO_4·2H_2O)和40g m~(-2)a~(-1)(NH_4Cl)+20 g m~(-2)a~(-1)(NaH_2PO_4·2H_2O)。种植密度设置4个水平:10、20、40和80株/m~2,试验时间为2017年6月至9月。研究结果表明,在各密度幼苗土壤中,N和N+P处理引起pH值的显著下降,N、P和N+P处理的土壤有机质和碱解N含量的变化规律不明显,P处理的幼苗土壤全P含量上升,P和N+P处理的土壤有效P含量增加,N+P处理的土壤全K含量以及N、P和N+P处理的土壤速效K含量均下降。在10、20和40株/m~2幼苗的土壤中,P处理的土壤全N含量高于N和N+P处理的,而80株/m~2幼苗的土壤全N含量低于其他密度幼苗。随着种植密度的增加,各施肥处理的土壤pH、全P、有效P、全K和速效K含量均呈现上升趋势,而施N和施P处理的土壤有机质呈现下降趋势,各施肥处理的土壤碱解N含量变化规律不明显。施肥和密度处理对樟树幼苗土壤有机质、碱解氮和速效钾含量有显著的交互作用。     

Development of a technique to study seedling emergence in response to moisture deficit in the field —The seed bed environment*

The ability of crops of sorghum (Sorghum bicolor (L.) Moench) to establish in farmers' fields depends largely on its capacity to germinate and emerge under limited soil moisture conditions. Studies on germination under moisture stress have been previously conducted using osmotic media which do not wholly reproduce the conditions of the seed bed in the field. Hence the need for a field screening technique. A line source irrigation system was used to provide five moisture regimes ranging from -0.08 MPa to -0.92 MPa. The drying rate of the soil and the soil temperature depended largely on incident radiation, and the relationship between the moisture content and daily soil temperature and daily radiation was consistant. Total porosity of the seed bed, derived from bulk density measurements and particle density ranged from 43.8% to 45.3%, which would allow sufficient aeration when as in the experiments conducted here, water content was low (> 0.14 g/g). Under these seed bed conditions the pattern of response of emergence to the moisture gradient was linear or curvilinear. Genotypic differences existed for emergence and its response to water level. The field method developed is useful for identifying genotypes able to germinate and emerge under conditions of low seed bed moisture.     

Effects of urine on soil microbial biomass,methanogenesis, nitrification and denitrification in grassland soils

Urine was added under controlled conditions to intact turfs taken from long-term permanent pasture on clay loam and sandy loam soils in South West England. Methane exchanges were small (<+/−0.03 μg CH₄ m^-2 min^-1) and overall absorption equalled or exceeded emission in both soils. On the clay loam, wetting with water or urine increased soil microbial biomass C and N contents by about 20% but there was no specific effect of urine. Urine, however, caused an increase in soil respiration of >50% and the average increase was greater for cow's urine (30.8 mg CO₂ m^-2 min^-1) than for an artificial urine (20.1 mg CO₂ m^-2 min^-1). Emissions of nitric and nitrous oxides following urine application were substantial (on average 0.36 μg NO-N and 29 μg N₂O-N m^-2 min^-1) but short lived (<40 days). The high levels of ammonium found in the urine treated soils (>200 mg NH₄ ⁺-N kg^-1) were nitrified to nitrate over a period of 42 days. Qualitative changes in the soil microbial biomass were evidently not related to biomass size. Relationships between trace gas emissions and soil processes are discussed. ei]Section editor: R Merckx     

Modelling the effect of soil moisture on germination and emergence of wheat and sugar beet with the minimum number of parameters

Soil moisture and temperature, sowing depth and penetration resistance affect the time and percentage of seedling emergence, which are crucial for the simulation of drought‐limited crop production. The aim of this research was to measure the effect of soil water potential on germination and emergence, shoot and root elongation rates (SER and RER) of two different seed/crop types. Sugar beet and durum wheat seeds were sown into two soils (clay and loam), submitted to five matric potentials (?0.01, ?0.1, ?0.2, ?0.4 and ?0.8 MPa) and incubated at constant temperature (25°C) and humidity. Cumulative count analysis was used to estimate parameters of the distribution of germination or emergence times for each box of beet or wheat seeds and to derive estimates for base potentials (ψ_b), hydrothermal times (H) and numbers of viable units. In a second experiment, NaCl solution was used to mimic the soil matric potentials to estimate potential RER and SER. Germination of sugar beet was slightly more sensitive to matric potential than durum wheat (ψ_b of ?1.13 and ?1.23 MPa, respectively). H_(g) was longer for sugar beet than for durum wheat (67 vs 47 MPa °Cd). For emergence ψ_b was similar for both seed types and soils but hydrothermal times (H_(e)) were 40 MPa °Cd higher for sugar beet than for wheat. Emergence was about 20 MPa °Cd earlier in loam than in clay. SER measured in soils were similar for both crops and for durum wheat it agreed with those determined in NaCl solution. RER and SER fell with decreasing osmotic potential to approximately 20% of their maximum values (1.03 mm h^?1 and 0.57 mm h^?1, respectively). Seedling viability decreased with decreasing matric potential and more in clay than in loam soil and more for sugar beet than durum wheat. Seed and soil aggregate size are discussed with respect to the effects of water diffusion and soil–seed contact on germination and emergence modelling.     

The chemical nature of P accumulation in agricultural soils��implications for fertiliser management and design: an Australian perspective

Sand burial, persistent seed bank and soil water content (SWC) are three factors that potentially can affect regeneration in sand dune plant populations. To evaluate the effects of these three factors on population regeneration of Eremosparton songoricum, a rare and endangered legume, we investigated seed germination, seedling emergence and seedling survival in greenhouse and controlled field experiments in different sand dunes microsites. Freshly matured seeds are physically dormant, and the highest germination was only 9.3?±?5.8% at 25/10°C. Seed germination occurred at burial depths from 0 to 10 cm, but the maximum depth from which seedlings emerged was 6 cm; from 1 to 6 cm, the deeper the burial, the lower the percentage of seedling emergence. Only 36.2% of the total soil seed banks occurred at depths of 0?C6 cm. For artificially sown seeds at different dune microsites, mean seedling emergence percentage was 6.8%. Of 150 seedlings that emerged in the field investigation at the study site, only those germinating in flat sandy areas survived, and mean survival percentage was only 2.0%. Thus, the proportion of non-dormant seeds in soil seed banks that developed into seedlings and survived to the end of the growing season was only 0.2%. Minimum SWC for seed germination, seedling emergence and seedling survival was 2.0%. During monitoring of emergent seedlings in the field, low seedling recruitment was at least partly due to the rate of root extension (1.6?±?0.3 cm day^?1) into the sandy soil, which was slower than that of the downward movement of plant-available moisture (2.8?±?0.6 cm day^?1). Thus, population regeneration under natural conditions rarely occurred via sexual reproduction, owing to the limited water resources available for seedling establishment. Rational field seeding practices, including manually scarified or dry stored seeds before sowing, sowing the seeds at right time and suitable place, are suggested for ecological restoration of endangered E. songoricum populations.     

Effects of an induced drought on soil carbon dioxide (CO2) efflux and soil CO2 production in an Eastern Amazonian rainforest, Brazil

In the next few decades, climate of the Amazon basin is expected to change, as a result of deforestation and rising temperatures, which may lead to feedback mechanisms in carbon (C) cycling that are presently unknown. Here, we report how a throughfall exclusion (TFE) experiment affected soil carbon dioxide (CO₂) production in a deeply weathered sandy Oxisol of Caxiuanã (Eastern Amazon). Over the course of 2 years, we measured soil CO₂ efflux and soil CO₂ concentrations, soil temperature and moisture in pits down to 3 m depth. Over a period of 2 years, TFE reduced on average soil CO₂ efflux from 4.3±0.1 μmol CO₂ m⁻² s⁻¹ (control) to 3.2±0.1 μmol CO₂ m⁻² s⁻¹ (TFE). The contribution of the subsoil (below 0.5 m depth) to the total soil CO₂ production was higher in the TFE plot (28%) compared with the control plot (17%), and it did not differ between years. We distinguished three phases of drying after the TFE was started. The first phase was characterized by a translocation of water uptake (and accompanying root activity) to deeper layers and not enough water stress to affect microbial activity and/or total root respiration. During the second phase a reduction in total soil CO₂ efflux in the TFE plot was related to a reduction of soil and litter decomposers activity. The third phase of drying, characterized by a continuing decrease in soil CO₂ production was dominated by a water stress‐induced decrease in total root respiration. Our results contrast to results of a drought experiment on clay Oxisols, which may be related to differences in soil water retention characteristics and depth of rooting zone. These results show that large differences exist in drought sensitivity among Amazonian forest ecosystems, which primarily seem to be affected by the combined effects of texture (affecting water holding capacity) and depth of rooting zone.     

Effects of air-filled soil porosity and aeration on the initiation and growth of secondary roots of maize (Zea mays)

We studied the possibility whether the initiation of secondary roots is regulated by the air-filled porosity in soil, i.e. the availability of oxygen in the soil. Maize plants were grown in long PVC tubes (1 m long and 12 cm diameter) and were unwatered for different numbers of days so that variations of soil water content with depth were achieved on the same date with plants at the same age. The plants were harvested when their root systems were established in the whole soil column and watering had been withheld for 0, 15, 20, 25 days. A decrease of soil water content was significantly correlated with an increase of air-filled porosity in soil. The number of secondary lateral roots from segments of primary adventitious roots increased dramatically when soil water content decreased from field capacity to about 0.05 g water g^-1 dried soil. The total dried mass of roots at different soil depths was also positively correlated with soil air-filled porosity. It was observed that the elongation of the initiated secondary roots responded differently to the variations of soil air-filled porosity. The length of secondary roots increased initially when the soil was dried from field capacity to 0.18 g g^-1 dried soil (water potential at about−0.2 MPa, air-filled porosity 0.26 cm³ cm^-3), but was drastically reduced when the soil was dried further. Obviously elongation of secondary roots was inhibited when soil water potential began to deviate substantially from an optimum value. The present results suggested that the initiation of secondary roots was greatly promoted by the increase of air-filled soil porosity, i.e. availability of oxygen. This conclusion was further verified in a separate experiment where solution-cultured maize seedlings were subjected to different aeration treatments. An obvious increase in secondary root initiation was found in plants which were aerated with normal air (21% O₂) than in plants which were either not aerated or aerated with 5% O₂ air. ei]Section editor: B E Clothier     

Effects of CO2 enrichment and water stress on gas exchange of Liquidambar styraciflua and Pinus taeda seedlings grown under different irradiance levels

Summary The effects of CO₂ enrichment and water stress on gas exchange of Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) seedlings were examined for individuals grown from seed under high (1000 mol·m^-2·s^-1) and low (250 mol·m^-2·s^-1) photosynthetic photon flux density at 350, 675 and 1000 l·l^-1 CO₂. At 8 weeks of age, half the seedlings in each CO₂-irradiance treatment were subjected to a drying cycle which reduced plant water potential to about -2.5 MPa in the most stressed plants, while control plants remained well-watered (water potentials of -0.3 and -0.7 MPa for sweetgum and loblolly pine, respectively). During this stress cycle, whole seedling net photosynthesis, transpiration and stomatal conductance of plants from each CO₂-irradiance-water treatment were measured under respective growth conditions.For both species, water stress effects on gas exchange were greatest under high irradiance conditions. Waterstressed plants had significantly lower photosynthesis rates than well-watered controls throughout most of the drying cycle, with the most severe inhibition occurring for low CO₂, high irradiance-grown sweetgum seedlings. Carbon dioxide enrichment had little effect on gas exchange rates of either water-stressed or well-watered loblolly pine seedlings. In contrast, water stress effects were delayed for sweetgum seedlings grown at elevated CO₂, particularly in the 1000 l·l^-1 CO₂, high irradiance treatment where net photosynthesis, transpiration and conductance of stressed plants were 60, 36 and 33% of respective control values at the end of the drying cycle. Development of internal plant water deficits was slower for stressed sweetgum seedlings grown at elevated CO₂. As a result, these seedlings maintained higher photosynthetic rates over the drying cycle than stressed sweetgum seedlings grown at 350 l·l^-1 CO₂ and stressed loblolly pine seedlings grown at ambient and enriched CO₂ levels. In addition, water-stressed sweetgum seedlings grown at elevated CO₂ exhibited a substantial increase in water use efficiency.The results suggest that with the future increase in atmospheric CO₂ concentration, sweetgum seedlings should tolerate longer exposure to low soil moisture, resulting in greater first year survival of seedlings on drier sites of abandoned fields in the North Carolina piedmont.     

Responses of Quercus ilex from Different Provenances to Experimentally Imposed Water Stress

Responses of Quercus ilex L. seedlings from three different localities in Italy to experimentally imposed drought stress were analysed. Predawn (Ψ_pd) and midday (Ψ_m) leaf water potential of stressed seedlings decreased on an average until −4.0 and −4.2 MPa, respectively, in the severe water stress. At the end of the severe water stress the relative water content (RWC) was 72.5 – 83.6 % and the photosynthetic rates (P_N) near zero. The critical threshold value of Ψ_pd for complete stomatal closure was from −4.0 to −4.5 MPa. The leaf damage after the severe water stress was significantly greater in seedlings originated from the acorns of climax area (45 % total leaf injured area and 40 % fallen leaves) than in the other seedlings (on an average 20.5 % total leaf injured area and 21 % fallen leaves). This revised version was published online in July 2006 with corrections to the Cover Date.     

Seed Germination Ecology of Feather Lovegrass [Eragrostis tenella (L.) Beauv. Ex Roemer & J.A. Schultes]

Feather lovegrass [Eragrostis tenella (L.) Beauv. Ex Roemer & J.A. Schultes] is a C₄ grass weed that has the ability to grow in both lowland and upland conditions. Experiments were conducted in the laboratory and screenhouse to evaluate the effect of environmental factors on germination, emergence, and growth of this weed species. Germination in the light/dark regime was higher at alternating day/night temperatures of 30/20 °C (98%) than at 35/25 °C (83%) or 25/15 °C (62%). Germination was completely inhibited by darkness. The osmotic potential and sodium chloride concentrations required for 50% inhibition of maximum germination were -0.7 MPa and 76 mM, respectively. The highest seedling emergence (69%) was observed from the seeds sown on the soil surface and no seedlings emerged from seeds buried at depths of 0.5 cm or more. The use of residue as mulches significantly reduced the emergence and biomass of feather lovegrass seedlings. A residue amount of 0.5 t ha^-1 was needed to suppress 50% of the maximum seedlings. Because germination was strongly stimulated by light and seedling emergence was the highest for the seeds sown on the soil surface, feather lovegrass is likely to become a problematic weed in zero-till systems. The knowledge gained from this study could help in developing effective and sustainable weed management strategies.     

Properties of peat-based casing soils and their influence on the water relations and growth of the mushroom (Agaricus bisporus)

Different combinations of peat and chalk or lime sources with differing moisture contents were used to determine how specific physical and chemical properties of the casing soil relate to the growth and water relations of the mushroom. The peat types varied in terms of decomposition and extraction method; the lime addition varied in terms of rate and type (chalk or sugar beet lime). During the colonisation of the casing soil before fruiting, the extension growth rate of mushroom mycelium was most closely correlated (negatively) with the volumetric moisture content of the casing soil. Scanning electron microscopy showed that mycelium growing at a lower casing soil matric potential (Ψ_m) had a much finer and branched structure than mycelium growing at a higher Ψ_m. Across all the peat and lime source treatments, a relationship was found between the mean Ψ_m of the casing soil and mushroom yield, with an optimum Ψ_m of -7.9 to -9.4 kPa. Mushrooms are produced in ‘flushes’ at about 8-day intervals and during the development of each flush of mushrooms, there was a significant decrease in casing soil Ψ_m . This decrease (to below -40 kPa) was greatest in the second flush, which was the highest yielding. There were no relationships between mushroom yield and casing soil osmotic potential Ψ_π within the range -93 to -154 kPa or any of the other chemical properties and water and air holding characteristics of the casing soils which were determined. Across different casing soil treatments, mushroom dry matter content was negatively correlated with mushroom yield and positively correlated with mushroom tissue osmotic potential. This revised version was published online in June 2006 with corrections to the Cover Date.     

The impact of mechanical impedance on the emergence of carrot and onion seedlings

The development of a method using wax layers to simulate the effect of strong soil crusts on seedling emergence is described. Wax layers of different strengths were prepared by melting together white soft paraffin and paraffin wax in different proportions. The wax discs were placed above seeds planted in wet, but well-aerated sand in controlled environments. The effect of adding charcoal to the wax discs to prevent transmission of light was also tested. When wax layer penetrometer pressure was 0.2–0.25 MPa, light transmission greatly decreased the final emergence of carrot, but not onion seedlings. Penetrometer pressures above 0.25 MPa greatly decreased emergence through black wax layers in both carrot and onion. The presence of 2–4 mm stones immediately below wax layers (to simulate aggregates) decreased the emergence of onion shoots through wax layers with penetrometer pressures of 0.25 MPa and above. The emergence of carrot and onion seedlings from wax layers was compared to emergence in the field from different soil types and aggregate sizes. In both laboratory and field experiments, carrot gave better emergence than onion when emergence was relatively poor. Results were consistent with the hypothesis that mechanical impedance is a major factor in poor crop emergence in temperate conditions.