Nine seedlots of the widely planted southern and central European native tree species Acer pseudoplatanus L. were collected along a north–south gradient spanning 21° of latitude in Europe. We investigated how the heat sum during seed development influences seed maturity as assessed by physical, physiological and biochemical traits. Using principal component analysis we found predictable and consistent patterns in all traits, which correlated with heat sum. For example, compared with fruits from their native range (Italy and France, heat sum >3000°C d), fruits from the coldest location (Scotland; heat sum of 1873°C d) were shorter (c. 30 v. 42 mm), germinated over a narrower temperature range (5–20 v. 5–35°C) and had smaller embryos (28 v. > 70 mg) with a higher water content (c. 63 v. 48%), less negative solute potentials (c. –2.4 v. –4.1 MPa) and were more desiccation sensitive (critical water potential of –20.2 v. –55.4 to –60.7 MPa). The observed level of desiccation-tolerance for the French and Italian seedlots is more consistent with the intermediate category than the previous classification of A. pseudoplatanus as recalcitrant. Our results demonstrate that a lower heat sum causes fruits from northern Europe to be dispersed before maximum potential seed quality is achieved.

Soil mechanical impedance (MI) and matric potential can both cause reduction in the root growth rate, modify rooting pattern and root diameter. Cotton seedlings are sensitive to the soil physical environment, particularly during early stages of growth. Soil matric potential and MI effect on root biomass, axial root length and diameter, and the number and length of lateral roots in soil packed to penetration resistances (PR) of 0.1, 1.0, 1.1 and 1.2 MPa (megaPascal = 10 6 Pascal), each at three matric potentials of -10, -100 and -500 kPa (kiloPascal = 10 3 Pascal), were determined. Total root lengths were reduced by 29, 50 and 53% at impedance of 1.0, 1.1 and 1.2 MPa, respectively, as compared to the control, whereas MI of 1.2 MPa resulted in 60% reduction in axial root length. A similar increase in diameter was caused by increasing mechanical impedance, while decreasing matric potential had little effect. Roots that were water stressed did not change their diameter but had a shorter axis and longer lateral length. In contrast, the impeded roots (PR = 1.0, 1.1 and 1.2 MPa) had both a shorter axis and a smaller total length, but had increased diameter. These results not only illustrate the plasticity of root response to stress but also demonstrate how the response differs between different types of stresses.

26 Topic: Research Notes Key Author: Matthew I. Daws Correspondence Address: Seed Conservation Department Royal Botanic Gardens, Kew Wakehurst Place Ardingly, West Sussex RH17 6TN UK E-mail: m.daws@rbgkew.org.uk Authors: M.I. Daws, D. Orr, D.F.R.P. Burslem and C.E. Mullins Effect of high temperature on chalazal plug removal and germination in Apeiba tibourbou Aubl. Pages: 221-225 We tested the effects of a two minute wet heat treatment at a range of temperatures on germination of Apeiba tibourbou seeds. Exposure to >50°C for two minutes resulted in an increase in germination from c. 0 to 80% and resulted in the removal of the chalazal cap. Physical removal of the chalazal cap or mechanical scarification of the seed coat allowed water uptake and subsequent germination. Our results indicate that A. tibourbou seeds exhibit physical dormancy that can be overcome with hot water treatment, chalazal cap removal or mechanical scarification of the testa.

Summary Soil problems for agricultural production depend on the climate so many countries experience problems which we do not generally have in the UK. However, our climate is projected to change as a result of ‘global warming’. Hotter, dryer summers will become more common and the frequency and severity of droughts will increase but other effects of climate change on agriculture are more speculative. In this paper soil– specific problems that affect crop establishment, root growth, the persistence of the benefits of cultivations, and soil erosion are discussed from a worldwide perspective together with their solutions, which are related to current and changing UK conditions.

Summary We investigated the relationship between seed mass and the suitability of microsites for germination for five small‐seeded (< 2·4 mg fresh mass) photoblastic neotropical pioneer trees. We determined the germination response of these species to the ratio of red to far‐red irradiance (r:fr) and compared it to r:fr measured under varying conditions of canopy openness, litter and soil thickness. At a constant temperature the germination percentage of each of the species increased sharply with increasing r:fr above a species‐specific threshold r:fr. The smallest‐seeded species, Miconia argentea, had the lowest r:fr threshold for germination (0·12) while the larger‐seeded Cecropia species and Solanum hayesii possessed higher values (0·21–0·27). The largest seeded species, Solanum hayesii, also showed a positive germination response to a 10 °C temperature fluctuation, which was independent of the response to r:fr. The mean r:fr at ground level declined with decreasing gap size but not sufficiently to suppress the germination of these five species. However, a covering of one litter leaf in direct sunlight reduced the r:fr to between 0·18 and 0·83 depending on the species and leaf wetness. The top‐soil at our study site was aggregated and irradiance was transmitted or reflected between soil aggregates with little change in r:fr. Light did not penetrate the aggregates. We suggest that photoblastic germination in neotropical pioneers has evolved to inhibit germination in response to conditions most likely to alter within the life of an individual seed (i.e. superficial burial by leaf litter or incorporation into the surface soil), rather than to fine scale variation in canopy openness. Among photoblastic species, the pattern of response to r:fr suggests that smaller‐seeded species would germinate in a broader range of microsites than larger‐seeded species. This lower degree of discrimination may be associated with their higher risk of mortality and therefore more limited persistence in the soil seed bank.

Development of a visual method of discriminating between crop seedlings and weeds is an important and necessary step towards the automation of non-chemical weed control systems in agriculture, and towards the reduction in chemical use through spot spraying. Two methods were applied to recognise carrot (Daucus carota L.) seedlings from those of ryegrass (Lolium perenne) and Fat Hen (Chenopodium album) using digital imaging. The first method involved the use of a simple morphological characteristic measurement of leaf shape (perimeter2/area), which had varying effectiveness (between 52 and 74%) in discriminating between the two types of plant, with the variation dependent on plant size. The second involved a self-organising neural network more biologically plausible than many commonly used NN methods. While the latter did not give results as good as those required for commercial purposes, it showed that a neural network-based methodology exists which allows the system to learn and discriminate between species to an accuracy exceeding 75% without predefined plant descriptions being necessary.

Slugs are major pests of oilseed rape that are poorly controlled by conventional bait pellets. A series of laboratory experiments investigated the potential of seed-dressings to control slug damage in this crop. Four compounds: metaldehyde, methiocarb, cinnamamide and 3,5-dimethoxycinnamic acid (DMCA) were tested at a range of doses for phytotoxicity and ability to reduce damage by Deroceras reticulatum (Müller). Metaldehyde and methiocarb were not phytotoxic at any doses, whereas all doses of cinnamamide and DMCA were. All compounds reduced slug damage, but metaldehyde and methiocarb consistently performed better than cinnamamide and DMCA. Metaldehyde and methiocarb seed-dressings were compared with baited pellets containing the same active ingredients at recommended field doses. The seed-dressings protected plants from damage by D reticulatum and Arion subfuscus (Draparnaud) as well as, or better than, baited pellets. We therefore recommend that metaldehyde and methiocarb should be field-tested as seed dressings to control slugs in oilseed rape.

Summary Species‐specific responses to the range of microsites resulting from canopy gap formation may contribute to coexistence in tropical forests. We investigated the effects of four factors affected by canopy gap formation (red : far‐red light, soil nitrate concentrations, soil temperature fluctuations and soil water potential) on the germination response of four pioneer Piper species (P. dilatatum, P. hispidum, P. marginatum and P. peltatum) that are typically found in canopy gaps and clearings. All four Piper species required light for maximum germination. However, the ratio of red : far‐red light (R : FR) resulting in maximum germination varied between species. Piper peltatum will germinate in simulated understorey light conditions; P. dilatatum and P. hispidum require conditions typically found in small to medium gaps; while P. marginatum requires the conditions appropriate to large gaps. Only P. marginatum was affected by nitrate concentration: elevated concentrations increased the germination rate. This suggests that this species could detect canopy gaps using a combination of high R : FR and elevated soil nitrate concentrations. The germination rate of P. marginatum was least sensitive to low water potentials and high daytime temperatures characteristic of large gaps. Piper peltatum was most sensitive to these treatments, while P. dilatatum and P. hispidum were intermediate in response. A principal components analysis of the ratios of germination in understorey to large gap conditions, for four variables, generated a significant axis that explained 88·5% of the variance in germination response between species. Differential species distribution along this axis, based on species‐specific responses, may allow germination to occur in the most suitable microsite for onward growth of the seedling and contribute to species coexistence by reducing interspecific competition.

Germination provides many potentially unrecognized sources of variation in the regeneration niche. In this study we relate germination requirements and seed size for 16 species of pioneer trees to microclimatic conditions present in gaps in semi-deciduous rain forest in Panama. We found that, whereas increased duration of direct irradiance can be an effective indicator of the presence of a canopy gap across all scales of canopy openness, diel fluctuations in soil temperature effectively discriminate both understory sites and small gaps (25 m(2)) from larger gaps. Germination response was significantly related to seed size. Small-seeded species (seed mass <2 mg) showed significantly greater germination in response to irradiance of 22.3 mumol.m(-2).s(-1) than in complete darkness. Their germination was unaffected by an increasing magnitude of diel temperature fluctuation up to a species-specific threshold, above which it declined. Large-seeded species (seed mass >2 mg) germinated equally in light and darkness (with one exception) and either showed a positive germination response to an increasing magnitude of temperature fluctuation (four species) or no significant response (four species). The maximum seed burial depth from which seedlings could emerge successfully was strongly positively associated with seed mass. We conclude that photoblastic germination of tropical pioneer trees results in small-seeded species germinating in gaps only when seeds are located in microsites that are suitable for seedling emergence. A positive germination response to increasing temperature fluctuation can stimulate germination of larger-seeded species in larger gaps and when they are buried beneath an opaque soil or litter layer. Therefore, seed size differences constrain the physiological mechanisms of canopy gap detection in tropical pioneer trees and might contribute to observed differences in the distribution of adult plants in relation to canopy gap size.

Mature heather (Calluna vulgaris) and bracken (Pteridium aquilinum) turfs, transplanted from the field, were subjected to factorially combined experimental treatments for three consecutive years. Summer drought had the greatest effect, decreasing photosynthesis, growth and reproductive output in both species, and opening the bracken canopy. The timing of the drought relative to plant development was critical to which species was worst affected; bracken was worst affected by an early drought, heather by a later drought. Both species showed physiological damage during drought but, as predicted on the basis of their functional types, heather showed greater acclimation of water-use efficiency to drought stress. Contrary to expectations based on functional types, heather responded more rapidly than bracken to increased nitrogen supply (50 kg N ha(-1) yr(-1)). Added nitrogen caused both species to start above-ground growth earlier in the spring. For bracken this stimulation was short-lived; added nitrogen might be preferentially allocated to the rhizome and the longer-term consequences of this are unknown. For heather, nitrogen promoted growth and flowering throughout the season. There was no positive effect on the photosynthetic physiology of either species; changes in resource partitioning, and thus photosynthate production at the canopy level, are the most likely mechanism for the increase in heather shoot growth. Warmer temperatures increased heather shoot growth from early spring onwards but did not advance bracken crozier emergence, although frond height and the proportion of fertile fronds were subsequently increased. No significant effects of warming on the photosynthetic physiology of either species were found. Predictions of responses of heather and bracken to environmental change are complicated by the strong interactive effects of unpredictable climatic events such as drought and extreme winter temperatures. When drought was imposed, damage to heather was much greater in plants receiving increased nitrogen supply. Stimulation of growth by nitrogen resulted in a water demand that was unsustainable in drought conditions, leading to wilting, reduced shoot growth and some acclimation of water-use efficiency. Additionally, a very cold winter spell proved most damaging to heather that had been droughted in the previous summer. For bracken, winter damage occurred in plants that had been warmed, with significantly fewer fronds emerging in the next spring and thus canopy photosynthetic potential being reduced. We predict that positively managed heather has the potential to limit the bracken problem in conditions of environmental change, provided that high levels of nitrogen deposition do not coincide with increased drought frequency.

1. Competition for water between Calluna vulgaris (heather) and Pteridium aquilinum (bracken) was studied in conditions of increased temperature, drought and increased nitrogen supply. All these factors increased the intensity of competition for water, with the combination of drought and increased nitrogen having the greatest effect on water use. 2. Both species increased water‐use efficiency in response to increased nitrogen and drought. The effects of temperature were however, equivocal. Calluna had a greater water demand than Pteridium but acclimated to water stress more readily. 3. Calluna was the superior competitor for water; its water‐use efficiency was reduced as a consequence of its roots depleting water from the Pteridium rooting zone. Pteridium, the poorer competitor, increased water‐use efficiency to cope with reduced water availability owing to competition. 4. There was a strong relationship between carbon isotope discrimination (Δ) and instantaneous water‐use efficiency for both species, but discrimination provided a more sensitive measure of seasonal water‐use efficiency. Reconstruction of the plant's history of water‐use efficiency by retrospective measurement of Δ proved a useful technique for Calluna leaves but was inappropriate for Pteridium rhizome.

Aggregate beds of a hardsetting soil were monitored during flood irrigation and subsequent drying for tensile strength (Y), penetration resistance (PR), vertical movement, matric potential, and water content. The soil slaked during flooding but also swelled. During drying there was a brief period of uniaxial vertical shrinkage followed by development of an interlocking network of vertical cracks at 0.1–0.2 m spacing accompanied by isotropic shrinkage. Macroscopic shrinkage ceased at −100 kPa potential, but evidence suggests that internal shrinkage continued beyond this point and created short (less than 10 mm) discontinuous cracks within the soil. As theoretically predicted, Y increased linearly with effective stress (σ′) for 0 < σ′ < 50 kN m−2, after which theory overpredicted. PR also varied linearly with σ′ in this range. It is suggested that hardsetting constitutes a physically distinct type of soil behaviour, and that all hardsetting soils that have been wetted sufficiently fast to undergo structural collapse will follow a similar relation between Y and σ′.

A comparison was made between the physical behaviour of aggregate beds of a structurally unstable, hardsetting soil and a soil of similar texture but more structural stability. The soils were flood-irrigated and then allowed to drain and dry. Tensile strength, penetrometer resistance, vertical movement, matric potential, and water content were monitored. The hardsetting soil slaked during flooding and its tensile strength increased steadily during drying. The more stable soil did not slake and aggregates retained their structural integrity. This soil was still a bed of loose aggregates with zero tensile strength after drying. Penetrometer results did not indicate clearly the contrast in physical behaviour between the two soils, and can only be used to indicate physical impedance to root growth in soils like the hardsetting soil that are structureless. In combination, air porosity and penetrometer resistance indicate that in the hardsetting soil there was only a narrow range of potentials, between −30 and −100 kPa, in which soil physical conditions were unlikely to limit root growth.

SUMMARYPastes containing sand and 0, 5, 20 and 80 g kaolin kg−1 mixture were equilibrated at matric potentials of – 2, – 10, – 100 and – 1000 kN m−2 or allowed to air‐dry. The strength of cylindrical samples was determined in unconfined compression and by the indirect tensile strength test. Measured strengths are explained using the Coulomb‐Mohr theory and the concept of effective stress. At pore water tensions greater than 10 kN m−2 effective stress was the dominant factor in determining compressive and tensile strength. The contrast between this behaviour and that of friable topsoils is discussed.

A comparison was made between the porosities of sands and of mixtures of sand and kaolin paste which had either been allowed to air-dry or had been consolidated under a load of 1.5 MPa. For all the mixtures of sand and kaolin, air-drying resulted in porosities which were from 0.1 to 0.05 (v/v) lower than those produced by consolidation. In contrast, two sands had porosities 0.01 and 0.05 (v/v) higher when air-dried than when consolidated. This demonstrates that some sandy-textured soils may compact significantly on drying.