The annual meeting was held on the weekend of 19-20 October in the Department of Biological Sciences at the University of Exeter, by kind permission of Professor J. Webster. About 50 members attended and on the first day heard a series of six lectures on the general theme of water relations in bryophytes.
Dr D. J. Read: ‘Physiological aspects of drought resistance in bryophytes’.
Factors contributing to the drought resistance of higher plants and bryophytes were compared and contrasted. Small cell size, small vacuoles, as well as high osmotic content and dense cytoplasm were features associated with resistant cells of higher plants. Although any of these factors might contribute to resistance in bryophytes by lessening the protoplasmic contraction which causes cell damage, they cannot fully explain it, either independently or in combination. Differences in protoplasmic tolerance of dehydration must be the basic cause of different levels of desiccation resistance in bryophytes. Membranes of resistant hepatics appear to have a higher phospho-lipid content than those of susceptible forms and this may confer greater protoplast stability under stress.
The importance of the rate of water loss from a shoot in determining survival was discussed. It was suggested that in many cases the speed with which desiccation occurred was more important to subsequent physiological activity than the absolute level of vapour pressure deficit to which shoots were subjected.
Mr J. W. Bates: ‘Sodium uptake and loss in halophytic and glycophytic mosses’.
Total digest data for the halophyte Grimmia maritima collected from seashore habitats suggest that it is a ‘salt-tolerator’ since it contains large amounts of sodium; but most of this seems to be extracellular. The intracellular sodium content of G. maritima from an exposed seashore site (18µg/g air-dry moss) closely resembles that of the glycophyte G. pulvinata from an inland habitat (17µg/g air-dry moss) indicating that the former species is a ‘salt- avoider’.
Treatment with artificial seawater solutions led to a marked net uptake of sodium and net loss of potassium from the intracellular fraction in G. pulvinata, but not in G. maritima. With increased calcium concentration the net uptake of sodium into the intracellular fraction was reduced in G. maritima but increased in G. pulvinata. Calcium may have the effect of decreasing the permeability of the cell membrane to other ions. Sodium uptake may be affected by light, external magnesium concentration and the state of hydration of the moss.
Active uptake and loss of sodium were investigated using metabolic inhibitors. Evidence suggests that gametophytes of G. pulvinata have an active sodium uptake mechanism whereas those of G. maritima have an active sodium efflux mechanism.
Dr G. R. Stewart and Dr J. A. Lee: ‘The effects of water stress on membrane properties’.
A number of different ultrastructural and molecular models for the structure of cell walls were described and an attempt was made to reconcile the known response of bryophytes to variations of water stress with these models.
Mr N. J. Collins: ‘Photosynthetic activity of tundra mosses as affected by water content, temperature and radiation’.
An extensive vegetation dominated by cryptogams has developed at some localities in the maritime Antarctic. In many mosses clear innate markers of seasonal growth are present so that the new growth, and hence net annual production, may be recognized easily. Two contrasting communities have been studied in the field at Signy Island, South Orkney Islands, one dominated by turf-forming mosses and the other by carpet-forming mosses. Net annual dry matter production for turf-forming mosses was 340-660 g/m² (this value does not allow for translocation). Translocation is probably insignificant in carpet-forming species so net annual production can be more accurately stated as 220-890 g/m²
The turf-former Polytrichum alpestre experiences temperatures between -5 and +5°C for 80% of the summer; those for the carpet-former Drepanocladus uncinatus are slightly lower. Radiant flux density levels are extremely low for the period from 1800 hrs to 0559 hrs here.
Grown under a temperature regime similar to that experienced in the field, P. alpestre has a temperature optimum for net photosynthesis of about +5°C, while D. uncinatus has an optimum of +15°C. At higher temperatures the optimum for D. uncinatus remains the same, but in P. alpestre it is +15°C. Field water content appears non-limiting to photosynthesis at some sites since the mosses were consistently at or above the water content necessary to maintain maximal rates of net photosynthesis,
Carbon fixation, computed from the combination of the ‘cold’ response curves for P. alpestre with the micro-climate data, amounts to 315 gC/m² in a 120 day season; when the ‘hot’ curves are used carbon fixation amounts to nearer 1000 gC/m² in a 120 day season. There is reasonable correspondence with values for dry matter production.
With a broad response curve allowing net carbon fixation over most of the temperature regime, D. uncinatus is well adapted to the oceanic climate of Signy Island. With its ability to acclimatise to changed temperatures over a period of at most 15 days, P. alpestre may be able to utilise periods of high temperatures more efficiently.
Mrs G. Morton: ‘The spatial arrangement of chalk grassland bryophytes with respect to their water relations’.
The objective of this work is to propose an ecological basis from which management plans for chalk grassland nature reserves can be devised, taking account of the bryophyte component. The study site at Aston Rowant National Nature Reserve in the Chilterns has a range of aspects under one treatment, and for one aspect a range of management treatments.
Data analysis by an ordination technique showed stand aspect to be the major source of variation in species composition. Solar radiation, amplified by wind incidence, creates the mesic/xeric gradient found. Management had a modifying influence on the effect of aspect. Increasing biomass caused by winter grazing or by lack of grazing modifies the microclimate, allowing the establishment of species typical of more mesic aspects.
The proportion of rainy days governs the growth of Pseudoscleropodium purum suggesting that differences in response to site moisture regime by various species cause the distribution patterns found. Moisture content of moss samples varies considerably with aspect and with the biomass above the moss.
The drought resistance of several species was tested over 90 days – longer than any dry period occurring in the Chilterns. No species died but the production of new growth after 30 days in a mist unit varied considerably between species. Fissidens cristatus, which occurred throughout the site, showed a steady slow production rate throughout the drought. Mnium undulatum, restricted to sheltered northerly aspects, showed great initial vigour which fell sharply to a net loss in production after long droughts.
Species typical of mesic situations have a greater potential growth rate (competitive ability), whereas those of xeric situations can maintain a constant, slow growth rate even after prolonged drought. The distribution of these types is due to modification in local climate by differences in solar radiation and wind caused by aspect, and changes in biomass of the sward.
It may be possible to manipulate grassland microclimate by management so that desired changes in bryophyte constituents can be induced.
Mr T. J. K. Dilks: ‘Ecological aspects of desiccation resistance in bryophytes’.
The ability of 17 species of bryophytes to withstand periods of continuous desiccation at known relative humidities in relation to their geographical and ecological distribution, was discussed. Desiccation tolerance was estimated by the length of time before net assimilation (measured on a Warburg respirometer) became negative after up to 24 hours rehydration, by the chlorophyll-a content and percentage survival after 30 days in a glasshouse under mist, and by the chlorophyll-a content after 7 days at 17°C. The resistance to desiccation was also considered in relation to the minimum water content observed in the field, the length of time taken to dry out after rain and the degree of shading indicated by ‘fish-eye’ photographs. Hookeria lucens was the species most susceptible to low water contents, followed by a group of species of Atlantic distribution, several woodla nd species which grow in the open in the north and west, species of unshaded habitats such as rocks, tree branches and sand dunes, and finally Rhacomitrium and Andreaea species. A series of experiments on intermittent and interrupted desiccation showed that in tolerant or moderately tolerant species (Hylocomium splendens, Rhytidiadelphus loreus, Tortula ruraliformis) short periods (24 hours) of desiccation each week had little effect on subsequent assimilation or growth, and that a moist period of 24 hours resulted in substantially complete recovery from the effects of a preceding dry period of 6 to 11 days.
|Mr N. J. COLLINS:||‘The growth of tundra mosses’.|
|Mr A. C. CRUNDWELL and
Dr H. L. K. WHITEHOUSE:
|‘Tortula amplexa and T. bolanderi in pure culture from European sources’.|
|Mr M. V. FLETCHER:||‘More mosses from New Zealand’.|
|Mrs G. MORTON:||‘Growth of Pseudoscleropodium purum monitored photographically’.|
|Dr M. C. F. PROCTOR:||‘Hemispherical photography as a tool for studying the light climate’.
‘Scanning electron micrographs of bryophytes’.
|Dr H. L. K. WHITEHOUSE:||‘Ditrichum pusillum in arable fields’.|
G. C. S. Clarke
Field Meeting, South Devon
The localities visited on the excursion on 20 October were all in v.-c. 3. The first site was Chudleigh Rocks, which is a short stretch of steep wooded valley on Devonian limestone with oak, ash, Tilia cordata, etc. Leptodon smithii, Metzgeria fruticulosa and Cololejeunea minutissima occurred as epiphytes; Pleurochaete squarrosa, Trichostomum sinuosum, Riccia sorocarpa, Weissia longifolia var. longifolia and Tortula intermedia were on exposed limestone rocks; and Cololejeunea rossettiana, Lejeunea lamacerina var. azorica, Marchesinia mackaii, Isothecium striatulum, Cinclidotus mucronatus and Isopterygium depressum were on damp shaded limestone rocks. The second site was a wooded stretch of the valley of the River Bovey, partly on granite and partly on metamorphosed Culm Measures, part of the site being within the Yarner Wood National Nature Reserve. Bryum flaccidum* was on an old stone bridge, Porella pinnata on large boulders in the river and Solenostoma triste on shaded rocks on the bank. The trees along the west side of the river sported a number of epiphytes such as Hypnum cupressiforme var. mamillatum, Lejeunea ulicina, Radula complanata and Neckera pumila. On the rocky bank alongside the footpath were Bazzania trilobata, Plagiochila spinulosa and Trichocolea tomentella, while Schistostega pennata was found in shady recesses and Heterocladium heteropterum var. heteropterum on boulders. On the east side of the river Hedwigia ciliata was plentiful on rock faces and boulders.
* = New vice-county record.
Our thanks go to Dr M. C. F. Proctor for organising and leading the trip, and the Nature Conservancy Council and the Warden of Yarner Wood for giving us permission to visit the last site.
T. J. K. Dilks