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The average first flowering date of 385 British plant species has advanced by 4.5 days during the past decade compared with the previous four decades: 16% of species flowered significantly earlier in the 1990s than previously, with an average advancement of 15 days in a decade. Ten species (3%) flowered significantly later in the 1990s than previously. These data reveal the strongest biological signal yet of climatic change. Flowering is especially sensitive to the temperature in the previous month, and spring-flowering species are most responsive. However, large interspecific differences in this response will affect both the structure of plant communities and gene flow between species as climate warms. Annuals are more likely to flower early than congeneric perennials, and insect-pollinated species more than wind-pollinated ones.

The role played by lateral roots and root hairs in promoting plant anchorage, and specifically resistance to vertical uprooting forces has been determined experimentally. Two species were studied, Allium cepa (onion) which has a particularly simple root system and two mutants of Arabidopsis thaliana, one without root hairs (rhd 2‐1) and another with reduced lateral root branching (axr 4‐2). Maximum strength of individual onion roots within a plant increased with plant age. In uprooting tests on onion seedlings, resistance to uprooting could be resolved into a series of events associated with the breakage of individual roots. Peak pulling resistance was explained in a regression model by a combination of a measure of plant size and the extent to which the uprooting resistance of individual roots was additive. This additive effect is termed root co‐operation. A simple model is presented to demonstrate the role played by root co‐operation in uprooting resistance. In similar uprooting tests on Arabidopsis thaliana, the mutant axr 4‐2, with very restricted lateral development, showed a 14% reduction in peak pulling resistance when compared with the wild‐type plants of similar shoot dry weight. The uprooting force trace of axr 4‐2 was different to that of the wild type, and the main axis was a more significant contributor to anchorage than in the wild type. By contrast, the root hair‐deficient mutant rhd 2‐1 showed no difference in peak pulling resistance compared with the wild type, suggesting that root hairs do not normally play a role in uprooting resistance. The results show that lateral roots play an important role in anchorage, and that co‐operation between roots may be the most significant factor.

Mycorrhizal fungi interact with a wide range of other soil organisms, in the root, in the rhizosphere and in the bulk soil. These interactions may be inhibitory or stimulatory; some are clearly competitive, others may be mutualistic. Effects can be seen at all stages of the mycorrhizal fungal life-cycle, from spore population dynamics (prédation, dispersal and germination) through root colonization to external hyphal growth. Two areas that seem likely to be of particular importance to the functioning of the symbiosis are the role of bacteria in promoting mycorrhiza formation and of soil animals in grazing the external mycelium. Mycorrhizal fungi also modify the interactions of plants with other soil organisms, both pathogens, such as root-inhabiting nematodes and fungi, and mutualists, notably nitrogen-fixing bacteria. These interactions are probably important both in natural ecosystems, where pathogens are increasingly recognized as playing controlling roles, and in agricultural systems, where mycorrhizas may be valuable in designing integrated systems of pest control and growth stimulation.

Plantago lanceolata with or without the mycorrhizal fungus Glomus mosseae were grown over a 100 d period under ambient (380 +/- 50 micromol mol(-1)) and elevated (600 +/- 150 micromol mol(-1)) atmospheric CO(2) conditions. To achieve similar growth, non-mycorrhizal plants received phosphorus in solution whereas mycorrhizal plants were supplied with bonemeal. Measures of plant growth, photosynthesis and carbon input to the soil were obtained. Elevated CO(2) stimulated plant growth to the same extent in mycorrhizal and non-mycorrhizal plants, but had no effect on the partitioning of carbon between shoots and roots or on shoot tissue phosphorus concentration. Mycorrhizal colonization was low, but unaffected by CO(2) treatment. Net photosynthesis was stimulated both by mycorrhizal colonization and elevated CO(2), and there was a more than additive effect of the two treatments on net photosynthesis. Colonization by mycorrhizal fungi inhibited acclimation, in terms of net carbon assimilation, of plants to elevated CO(2). (13)C natural abundance techniques were used to measure carbon input into the soil, although the results were not conclusive. Direct measurements of below-ground root biomass showed that elevated CO(2) did stimulated carbon flow below-ground and this was higher in mycorrhizal than non-mycorrhizal plants. For the four treatment combinations, the observed relative differences in amount of below-ground carbon were compared with those expected from the differences in net photosynthesis. A considerable amount of the extra carbon fixed both as a result of mycorrhizal colonization and growth in elevated CO(2) did not reveal itself as increased plant biomass. As there was no evidence for a substantial increase in soil organic matter, most of this extra carbon must have been respired by the mycorrhizal fungus and the roots or by the plants as dark-respiration. The need for detailed studies in this area is emphasized.

We used differences in small subunit ribosomal RNA genes to identify groups of arbuscular mycorrhizal fungi that are active in the colonisation of plant roots growing in arable fields around North Yorkshire, UK. Root samples were collected from four arable fields and four crop species, fungal sequences were amplified from individual plants by the polymerase chain reaction using primers NS31 and AM1. The products were cloned and 303 clones were classified by their restriction pattern with HinfI or RsaI; 72 were subsequently sequenced. Colonisation was dominated by Glomus species with a preponderance of only two sequence types, which are closely related. There is evidence for seasonal variation in colonisation in terms of both level of colonisation and sequence types present. Fungal diversity was much lower than that previously reported for a nearby woodland.

To quantify the involvement of arbuscular mycorrhiza (AM) fungi in the intraspecific transport of carbon (C) between plants we fumigated established Festuca ovina turf for one week with air containing depleted ¹³C. This labelled current assimilate in a section of mycorrhizal or non-mycorrhizal turf. Changes in the 13/12C ratio of adjacent, unfumigated plants, therefore, allowed the movement of C between labelled and unlabelled plants to be estimated. In mycorrhizal turves, 41% of the C exported to the roots from the leaves was transported to neighbouring plants. The most likely explanation of this is was the transport of C via a common hyphal network connecting the roots of different plants. No inter-plant transport of C was detected in non-mycorrhizal turves. There was no evidence that the C left fungal structures and entered the roots of receiver plants. Mycorrhizal colonisation increased carbon transport from leaves to root from 10% of fixed carbon when non-mycorrhizal to 36% in mycorrhizal turves. These results suggest that AM fungi impose a significantly greater C drain on host plants than was previously thought.

1. Plants can respond to nutrient-rich patches by proliferation of roots or by increased rates of ion uptake; such patches are therefore considered to be an important source of nutrients for plant growth. However, little is known about the spatial and temporal heterogeneity of nutrient concentration at scales that are applicable to the development of root systems of herbaceous perennial plants. 2. A woodland site in North Yorkshire (UK) was studied to measure the scale and extent of nutrient heterogeneity. Phosphate, nitrate, ammonium, pH and soil moisture were measured by destructive soil coring over a period of 2 years, and phosphate, nitrate and ammonium were also measured at a smaller scale using Rhizon Soil Solution Samplers. 3. Soil moisture was relatively constant at 0.6 g g(-1) fresh soil weight. Temporal variation was found in all other variables, but the timing of peaks in nutrient concentration was not predictable. 4. Soil coring showed differences in phosphate, ammonium and nitrate concentrations at a scale of over 2 m, which may be too large a scale to affect individual herbaceous perennial plants but might be important for trees. However, the soil solution sampling showed two- to fivefold differences in nitrate and ammonium at scales of 20 cm; the root systems of many herbaceous plants will spread over such distances. 5. Peaks in nutrient concentration that occurred in localized areas lasted no more than 4 weeks. Therefore if these nutrient-rich patches are to be utilized by plants, their roots must respond rapidly. 6. The study showed that localized nutrient-rich areas may form an important source of nutrients for plants in some natural habitats, especially when the general levels of nutrient availability are low. Hence the patchy nature of the soil should be considered in root foraging and population studies.

Plants respond strongly to environmental heterogeneity, particularly below ground, where spectacular root proliferations in nutrient-rich patches may occur. Such 'foraging' responses apparently maximize nutrient uptake and are now prominent in plant ecological theory. Proliferations in nitrogen-rich patches are difficult to explain adaptively, however. The high mobility of soil nitrate should limit the contribution of proliferation to N capture. Many experiments on isolated plants show only a weak relation between proliferation and N uptake. We show that N capture is associated strongly with proliferation during interspecific competition for finite, locally available, mixed N sources, precisely the conditions under which N becomes available to plants on generally infertile soils. This explains why N-induced root proliferation is an important resource-capture mechanism in N-limited plant communities and suggests that increasing proliferation by crop breeding or genetic manipulation will have a limited impact on N capture by well-fertilized monocultures.

An essential component of an understanding of carbon flux is the quantification of movement through the root carbon pool. Although estimates have been made using radiocarbon, the use of minirhizotrons provides a direct measurement of rates of root birth and death. We have measured root demographic parameters under a semi-natural grassland and for wheat. The grassland was studied along a natural altitudinal gradient in northern England, and similar turf from the site was grown in elevated CO₂ in solardomes. Root biomass was enhanced under elevated CO₂. Root birth and death rates were both increased to a similar extent in elevated CO₂, so that the throughput of carbon was greater than in ambient CO₂, but root half-lives were shorter under elevated CO₂ only under a Juncus/Nardus sward on a peaty gley soil, and not under a Festuca turf on a brown earth soil. In a separate experiment, wheat also responded to elevated CO₂ by increased root production, and there was a marked shift towards surface rooting: root development at a depth of 80-85 cm was both reduced and delayed. In conjunction with published results for trees, these data suggest that the impact of elevated CO₂ will be system-dependent, affecting the spatio-temporal pattern of root growth in some ecosystems and the rate of turnover in others. Turrnover is also sensitive to temperature, soil fertility and other environmental variables, all of which are likely to change in tandem with atmospheric CO₂ concentrations. Differences in turnover and time and location of rhizodeposition may have a large effect on rates of carbon cycling.