It is easy to dismiss bryophytes as ‘lower’ plants, mere primitive precursors long since left behind in the evolutionary race, and of only rather esoteric and incidental biological interest. But this is to let oneself be led astray by a simplistic image of a tidy evolutionary tree – an image that served Darwin well a century and a half ago (Desmond & Moore, 1992), but which we should now see as an intricately branched evolutionary bush with innumerable shoots reaching out from all depths to the growing apices that represent the present day. The earliest land plants may indeed have been at a bryophyte level of organization, but modern bryophytes, no less than vascular plants, are the product of some 450 million years’ evolution since that time (Edwards et al., 1998). Raven (1977, 1984) has emphasized the importance of the evolution of supracellular transport systems in the origin of vascular land plants. Bryophytes, on the other hand, evolved desiccation tolerance and represent an alternative strategy of adaptation to life on land, photosynthesizing and growing when water is available, and suspending metabolism when it is not. They are limited by their mode of life, but also liberated : they are prominent on hard substrates such as rock and bark, which are impenetrable to roots and untenable to vascular plants. Bryophytes (in species numbers the second biggest group of green land plants) may be seen as the mobile phones, notebook computers and diverse other rechargeable battery-powered devices of the plant world – not direct competitors for their mains-based equivalents, but a lively and sophisticated complement to them.

Resurrection plants are a perennial fascination to botanists. The transformation, within a few hours (or even minutes) of supplying them with water, from a plant that has all the appearances of being dead – dry, brown, crisp and shrivelled – to a plant that is obviously living and functioning – turgid, green, pliable and growing – suggests the miraculous. Studies of the complex processes of restoration and repair have been made on a number of resurrection plants, and at many levels of organization from molecules through membranes, organelles and cells to the whole plant (Gaff, 1989). The woody South African shrub Myrothamnus is the most extreme example of such plants, in that it has the highest level of organization to be repaired. It is transformed from lifeless-looking sticks with a water content below 5% to a flourishing bush in a day after its roots receive water. Two papers in this issue from Ulrich Zimmermann's group in Würzburg concentrate on the restoration of the functioning hydraulic systems of the stems (see pp. 221–238, and 239–255).

Extended areas of new crops to produce not only food, but also energy and materials, was one of the visions when the EU agricultural policy was reformed in 1992. However, this vision remains unfulfilled. The perennial C4 grass Miscanthus, originating from eastern Asia, was one of the crops that caught major interest as a potential biomass crop due to its high productivity even in cool northern European conditions (Beale & Long, 1995). However, even very large initial programmes on Miscanthus were conducted almost exclusively within one genotype, namely the sterile, triploid, interspecific hybrid M. × giganteus, and ran into significant problems of low first winter survival and prohibitive high costs of vegetative establishment. Thus, despite the many encouraging results on Miscanthus productivity (van der Werf et al., 1993), environmental acceptability (Christian & Riche, 1998) and harvest and storage suitability (Venturi et al., 1998), it is clear that the introduction of a new crop into agriculture is not simple, and that basic ecophysiological understanding is important to support commercial exploitation. The results in this issue from Clifton-Brown & Lewandowski (see pp. 287–294) contribute to this vital, basic understanding, revealing that low frost tolerance of M. × giganteus rhizomes is probably the cause of low winter survival in cool parts of Europe, but that within the genus Miscanthus better frost tolerance exists.

‘The view that nutrient acquisition by most plants growing in natural ecosystems is mediated by mycorrhiza-forming symbiotic fungi is now largely accepted’ (Read, 2000). Is this bold claim really true for the whole suite of mineral nutrients that plants require? The case is strongest for nutrients that are not very mobile in soil, especially when present in growth-limiting amounts, and phosphate (P) is the classic example. Arbuscular mycorrhizas are by far the most widespread mycorrhizal symbioses, and the ability of arbuscular mycorrhizal (AM) fungi to take up soil nutrients such as P and transfer them to the host plant is an area of intense research. However, there is great variation in the extent to which AM plants benefit in measurable terms from the symbiosis under a given set of environmental conditions, and a paper in this issue, by Koide et al., addresses this problem (Koide et al., pp. 163–168). The variability is especially apparent in the field, thus obscuring the possible roles of mycorrhizas in community structure and succession (Fitter, 1985; McGonigle, 1988).

John Innes Centre, Norwich, UK, June 2000

Genomics research involving legumes, an area that is attracting major funding, has two distinct branches – work involving model species, and work involving crops. This meeting aimed to stimulate communication between these two groups. The major research areas covered included leaf, flower and seed development, establishment of symbioses, pathogen interactions and applied aspects (from the conservation of legume ecotypes to products required by the market).

Summary 196

I. INTRODUCTION 196

1. The neglected dimension 196

2. Basic concepts 197

(a) The heuristic notion of vessel-tiers 197

(b) Ohm's law 199

(c) Conductances, resistances and resistivities 199

(d) Lumen and pit resistances 199

(e) The importance of conduit radius and length in conductance 199

II. EVOLUTIONARY TRENDS IN CONDUIT DIMENSIONS 199

1. Nature and origin of xylem conduits 199

2. Increasing hydraulic conductance with increasing diameter and length 200

(a) Evolutionary trends in tracheid dimensions 200

(b) Origin of the vessel 200

3. Functional limitations to increasing vessel length 201

(a) Safety versus efficiency 201

(b) Containment of cavitation and embolism 202

III. MAXIMUM XYLEM TRANSPORT IN THE PRESENCE OF CAVITATION 203

1. Cavitation is linked to the driving force for transport 203

2. Transport models and extreme assumptions about conduit length 203

(a) Unitary cavitation response (n = 1) 203

(b) Infinitely partitioned response (n = ∞) 204

(c) ΔP and cavitation containment 204

IV. INCLUDING VESSEL LENGTH IN A TRANSPORT MODEL 205

1. Framing questions of optimal conduit length 205

2. A numeric model for flow through n conduit tiers 205

(a) Model structure 205

(b) Model solution 206

3. Optimization when f(P) is linear 206

(a) Isolating the effects of n on cavitation containment 206

(b) Optimal conduit tier-length distributions (OCLDs) 207

(c) Abrupt changes in conduit length 208

(d) Optimal frequency of end walls: incorporating Rpit 208

4. Optimization when f(P) is curvilinear 210

V. CONDUIT LENGTH IN MODERN TAXA: IMPLICATIONS FOR TRANSPORT 210

1. Limitations to the concept of conduit tiers 210

(a) Vessel ends are randomly distributed 210

(b) Dispersion around mean length within each ‘tier’ 210

2. Is the xylem optimally partitioned? 211

(a) Optimal number of end walls 211

(b) Conduit length distribution along the pathway 211

3. Hydraulic segmentation 212

(a) Segmentation in hydraulic resistance 212

(b) Segmentation in cavitation vulnerability 212

VI. CONCLUSIONS 212

1. Anatomy 212

2. Modelling flow 213

VII. APPENDIX: ANALYTICAL SOLUTIONS AND PROOFS 213

1. Analytic solution for Qmaxwith a single tier 213

2. The general case for n tiers 214

3. Analytic solution for Qmaxwith two tiers 214

4. Matric flux and n = ∞ 215

5. Rpit, variable pathway resistance and OCLD 215

6. Proof of Eqn 15 describing limited cavitation containment 215

Acknowledgements 216

References 216

Vascular plants have shown a strong evolutionary trend towards increasing length in xylem conduits. Increasing conduit length affects water transport in two opposing ways, creating a compromise that should ultimately define an optimal conduit length. The most obvious effect of increased length is to decrease the sequential number of separate conduits needed to traverse the entire pathway, and thereby to reduce the number of wall-crossings and the hydraulic resistance to flow within the xylem. This is an essential evolutionary pressure towards the development of the vessel, a conduit of multicellular origin whose length is not restricted by developmental constraints. The vessel has been an essential component in all plant lineages, achieving transport tissues with very high specific conductivity. A countering effect, however, arises from the partitioning of the cavitation response, a process whereby individual xylem conduits drain of water and lose conducting capacity. Flow in the xylem is down a gradient of negative pressure, which is necessarily most negative in the distal regions (i.e. near the foliage). Cavitation can be caused directly by negative pressures, and results in a total loss of the hydraulic conductance of the individual conduits within which it occurs. If cavitation is triggered by low pressure experienced only at the very distal end of a long conduit, the conduit nevertheless loses its conducting capacity along its entire length. Pathways composed of long conduits will therefore suffer greater total conductance loss for equivalent pressure gradients, because the effects of cavitation are not effectively restricted to the tissue regions within which the cavitation events are generated. By contrast, short conduits can restrict cavitation to distal regions, leaving trunk and root tissues less seriously affected. The increased total conductance loss of a system made entirely of very long conduits translates into a lower maximum rate of water transport in the xylem. The loss in hydraulic capacity associated with failure to partition the flow pathway fully, and locally contain the effects of cavitation, theoretically reaches a maximum of 50% for the extreme case in which a single set of conduits traverses the entire pathway. Shorter conduits confine individual cavitation events to smaller regions and permit the pathway as a whole to have a more gradual conductance loss in conjunction with the pressure gradient. A compromise exists between (1) minimizing total conductance loss from cavitation via fine partitioning of the pathway with many tiers of short conduits, and (2) reducing total wall resistance via coarse partitioning with a few tiers of long conduits. An analysis is presented of the optimal number of end walls (i.e. mean conduit length relative to total pathway length) to maximize transport capacity. The principle of optimal containment of cavitation also predicts that conduits should not be of equal length in all portions of the pathway. The frequency of end walls should rather be proportional to the magnitude of the water-potential gradient at each point, and conduits should be longest in the basal portion (roots) and progressively shortened as they move up the stems to the foliage. These concepts have implications for our understanding of the contrasting xylem anatomies of roots and shoots, as well as the limits to evolution for increased hydraulic conductance per xylem cross-sectional area. They also indicate that to model the hydraulic behaviour of plants accurately it is necessary to know the conduit length distribution in the water flux pathway associated with species-specific xylem anatomy.

Summary 11

I. INTRODUCTION 12

II. LICHEN BIONT CHARACTERISTICS 12

1. The mycobiont 12

2. The photobiont 13

III. LICHEN GROWTH 13

1. Allocation of resources 13

2. Growth rates and environmental limitations 14

3. Maintaining an optimal energy use efficiency 15

IV. CARBON ACQUISITION 16

1. Water relations 16

(a) Desiccation tolerance 16

(b) Activation upon re-hydration 17

(c) Diffusion of water and CO2 18

2. Photobiont CO2fixation 20

(a) CO2 acquisition modes 20

(b) Significance of the CCM 22

3. Light and nitrogen relations 22

(a) The light-response curve 23

(b) Photosynthetic capacity 25

(c) Coping with high light 27

V. CARBON SINKS AND EXPENDITURES 28

1. Carbon translocation 28

2. Carbon sinks 29

3. Respiration 30

VI. CONCLUDING REMARKS 30

Acknowledgements 31

References 31

Lichens are nutritionally specialized fungi (the mycobiont component) that derive carbon and in some cases nitrogen from algal or cyanobacterial photobionts. The mycobiont and photobiont live together in an integrated thallus, but they lack specific tissue for the transport of metabolites and resources between them. Carbon is acquired through photosynthesis in the photobiont, which is active when the lichen is wet and exposed to light. Lichen photosynthesis is limited primarily by water, light and nitrogen, but can also be constrained by slow diffusion of CO2 within the wet thallus. The assimilated carbon is exported from photobiont to mycobiont, which also predominates in terms of biomass, and apparently regulates the size of the photobiont population. It has therefore generally been assumed that most of the carbon is used for growth and maintenance of the fungal hyphae. However, the extent of photobiont respiration in relation to mycobiont respiration has seldom been quantified; neither do we know the pool sizes of various carbon sinks within lichens. Owing to this lack of fundamental data we do not know whether, or how, carbohydrate resources are regulated to maintain an optimal balance between energy input and expenditures in these symbiotic organisms. This review summarizes data on growth, carbon gain and carbon expenditures in lichens, with particular emphasis on factors determining the photosynthetic capacity of their photobionts. An attempt is made to introduce an economic analysis of lichen growth processes, a view that has often been adopted in studies of higher plants. Areas in which more data are needed for the construction of a model on ‘lichen resource allocation patterns’ are discussed.

The acropetal water refilling kinetics of the dry xylem of branches (up to 80 cm tall) of the resurrection plant Myrothamnus flabellifolia were determined with high temporal resolution by observation of light refraction at the advancing water front and the associated recurving of the folded leaves. To study the effect of gravity on water rise, data were acquired for cut upright, horizontal and inverted branches. Water rise kinetics were also determined with hydrostatic and osmotic pressure as well as at elevated temperatures (up to 100 °C) under laboratory conditions and compared with those obtained with intact (rooted) and cut branches under field conditions. Experiments in which water climbed under its capillary pressure alone, showed that the axial flow occurred only in a very few conducting elements at a much higher rate than in many of the other ones. The onset of transpiration of the unfolded and green leaves did not affect the rise kinetics in the ‘prominent’ conducting elements. Application of pressure apparently increased the number of elements making a major contribution to axial xylem flow. Analysis of these data in terms of capillary-pressure-driven water ascent in leaky capillaries demonstrated that root pressure, not capillary pressure, is the dominant force for rehydration of rooted, dry plants. The main reasons for the failure of capillary forces in xylem refilling were the small, rate-limiting effective radii of the conducting elements for axial water ascent (c. 1 μm compared with radii of the vessels and tracheids of c. 18 μm and 3 μm, respectively) and the very poor wetting of the dry walls. The contact (wetting) angles were of the order of 80 ° and decreased on root or externally applied hydrostatic pressure. This supported our previous assumption that the inner walls of the dry conducting elements are covered with a lipid layer that is removed or disintegrates upon wetting. Consistent with this, potassium chloride and, particularly, sugars exerted an osmotic pressure effect on axial water climbing (reflection coefficients > zero, but small). Although the osmotically active solutes apparently suppressed radial water spread through the tissue to the leaf cells, they reduced the axial water ascent rather than accelerating it as predicted by the theory of capillary-driven water rise in leaky capillaries. Killing cells by heat treatment and removal of the bark, phelloderm, cortex and phloem also resulted in a reduction of the axial rise rate and final height. These observations demonstrated that radial water movement driven by the developing osmotic and turgor pressure in the living cells was important for the removal of the lipid layer from the walls of those conducting elements that were primarily not involved in water rise. There is some evidence from field measurements of the axial temperature gradients along rooted branches that interfacial (Marangoni) streaming facilitated lipid removal (under formation of vesicle-like structures and lipid bodies) upon wetting.

Summary 37

I. INTRODUCTION 37

II. DIGESTION OF PLANTS IN THE RUMEN: OLD AND NEW CONCEPTS 39

III. RUMEN-INDUCED PLANT METABOLISM: CELL DEGRADATION AND DEATH 41

1. Summary of plant cell death processes 41

2. Anaerobic stress and flooding tolerance of plants 42

3. Plant cell responses to elevated temperatures 45

4. Wounding responses/pathogen attachment 45

5. Senescence in the rumen? 47

IV. FUTURE PROSPECTS 50

Acknowledgements 51

References 51

It is generally assumed that breakdown of plant material in the rumen is a process mediated by gut microorganisms. This view arose because of the identification of a pre-gastric fermentation in the rumen, brought about by a large and diverse microbial population. The extensive use of dried and ground feed particles in forage evaluation might have helped to promote this assumption. However, although the assumption might be correct in animals feeding on conserved forage (hay and silage) where the cells of ingested forage are dead, it is possible that with grazed (living) forage, the role played by plant enzymes in the rumen has been overlooked. In a grazing situation, plant cells that remain intact on entering the rumen are not inert, but will respond to the perceived stresses of the rumen environment for as long as they are metabolically viable. Metabolic adjustments could include anaerobic and heat-shock responses that could promote premature senescence, leading to remobilization of cell components, especially proteins. Moreover, contact of plant cells with colonizing microorganisms in the rumen might promote a type of hypersensitive response, in much the same way as it does outside the rumen. After fresh plant material enters the rumen and prior to extensive plant cell-wall degradation, there is often a phase of rapid proteolysis providing N in excess of that required to maintain the rumen microbial population. The inefficient use of this ingested N results in generation of ammonia and urea in exhaled breath and urine, which promotes welfare and environmental pollution concerns. Therefore an important research goal in livestock agriculture is to find ways of decreasing this initial rate of proteolysis in the rumen. This will benefit the farmer financially (through decreased use of feed supplements), but will also benefit the environment, as N pollution can adversely affect pasture diversity and ecology. This review considers the possible responses of plant metabolism to the rumen environment, and how such considerations could alter current thinking in ruminant agriculture.

The axial and radial refilling with water of cut dry branches (up to 80 cm tall) of the resurrection plant Myrothamnus flabellifolia was studied in both acro- and basipetal directions by using 1H-NMR imaging. NMR measurements showed that the conducting elements were not filled simultaneously. Axial water ascent occurred initially only in a cluster of a very few conducting elements. Refilling of the other conducting elements and of the living cells was mainly achieved by radial extraction of water from these initial conducting elements. With time, xylem elements in a few further regions were apparently refilled axially. Radial water spread through the tissue occurred almost linearly with time, but much faster in the acropetal than in the basipetal direction. Application of hydrostatic pressure (up to 16 kPa) produced similar temporal and spatial radial refilling patterns, except that more conducting elements were refilled axially during the first phase of water rise. The addition of raffinose to the water considerably reduced axial and radial spreading rates. The polarity of water climbing was supported by measurements of the water rise in dry branches using the ‘light refraction’ (and, sometimes, the ‘leaf recurving’) method. Basipetal refilling of the xylem conduit exhibited biphasic kinetics; the final rise height did not exceed 20–30 cm. Three-cm-long branch pieces also showed a directionality of water climbing, ruling out the possibility that changes in the conducting area from the base to the apex of the branches were responsible for this effect. The polarity of water ascent was independent of gravity and also did not change when the ambient temperature was raised to c. 40 °C. At external pressures of 50–100 kPa the polarity disappeared, with basipetal and acropetal refill times of the xylem conduit of tall branches becoming comparable. Refilling of branches dried horizontally (with a clinostat) or inverted (in the direction of gravity) showed a pronounced reduction of the acropetal water rise to or below basipetal water climbing level (which was unaffected by this treatment). Unlike water, benzene and acetone climbing showed no polarity. In the case of benzene, the rise kinetics (including the final heights) were comparable with those measured acropetally for water, whereas with acetone the rise height was less. Transmission electron microscopy of dry branches demonstrated that the inner surfaces of the conducting tracheids and vessels were lined with a continuous osmiophilic (lipid) layer, as postulated by the kinetic analysis and light microscopy studies. The thickness of the layer varied between 20 and 80 nm. The parenchymal and intervessel pits as well as numerous tracheid corners contained opaque inclusions, presumably also consisting of lipids. Electron microscopy of rehydrated plants showed that the lipid layer was either thinned or had disintegrated and that numerous vesicle-like structures and lipid bodies were formed (together with various intermediate structural elements). These, many other data and the physical–chemical literature imply that several (radial) driving forces (such as capillary condensation, Marangoni forces, capillary, osmotic and turgor pressure forces) operate when a few conducting elements become axially refilled with water. These forces apparently lead to an avalanche-like radial refilling of most of the conducting elements and living cells, and thus to the removal of the ‘internal cuticle’ and of the hydrophobic inclusions in the pits. The polarity of water movement presumably results from high resistances in the basipetal direction, which are created by local gradients in the thickness of the lipid film as a result of draining under gravity in response to drought. There are striking similarities in morphology and function between the xylem-lining lipid film and the lung surfactant film lining the pulmonary air spaces of mammals.

The gradients in photosynthetic and carbohydrate metabolism which persist within the fully expanded second leaf of barley (Hordeum vulgare) were examined. Although all regions of the leaf blade were green and photosynthetically active, the basal 5 cm, representing approximately 20% of the leaf area, retained some characteristics of sink tissue. The leaf blade distal from the leaf sheath exhibited characteristics typical of source tissue; the activities of sucrolytic enzymes (invertase and sucrose synthase) were relatively low, whilst that of sucrose phosphate synthase was high. These regions of the leaf accumulated sucrose throughout the photoperiod and starch only in the second half of the photoperiod whilst hexose sugars remained low. By contrast the leaf blade proximal to the leaf sheath retained relatively high activities of sucrolytic enzymes (especially soluble, acid invertase) whilst sucrose phosphate synthase activity was low. Glucose, as well as sucrose, accumulated throughout the photoperiod. Although starch accumulated in the second half of the photoperiod, a basal level of starch was present throughout the photoperiod, by contrast with the rest of the leaf. The 14CO2 feeding experiments indicated that a constant amount of photosynthate was partitioned towards starch in this region of the leaf irrespective of irradiance. These findings are interpreted as the base of the leaf blade acting as a localized sink for carbohydrate as a result of sucrose hydrolysis by acid invertase.

Because recalcitrant seeds are not desiccation-tolerant they must be stored moist. Their limited storage potential presents significant practical problems, but the cause of viability loss is not known. It has been suggested that a stress-induced metabolic imbalance can develop during storage that results in free-radical generation and consequent damage. To investigate this hypothesis, the presence of a stable free radical, lipid peroxidation and representative enzymatic and nonenzymatic protection mechanisms against oxidative attack were monitored in nondormant recalcitrant seeds during moist storage. A comparison was made between seeds of a short-lived sub-tropical species (Avicennia marina) and two longer-lived temperate species (Quercus robur and Castanea sativa). As a test of the hypothesis, seeds of both temperate species were held under conditions of elevated temperature and oxygen concentration to develop different rates of respiration during storage. The number of normal seedlings produced from seeds of the two temperate species declined during storage, but viability remained high, so effects of ageing were not confounded with an increasing proportion of dead seeds in the population. Under these conditions, lipid peroxidation changed little over the storage period, although there was evidence of accumulation of a stable free radical in Q. robur axes. However, this response was not affected by storage conditions that elevated respiration rates. In the shorter-lived A. marina seeds viability declined soon after the start of storage, but the significant increase in free radicals shown by EPR measurement was only evident when an increasing percentage of the seed population was no longer viable. Changes in the activity of scavenging enzymes and the concentration of antioxidants were time-dependent and not related to respiration rates. Therefore, in the present work, no consistent evidence was found to show that metabolism-induced free-radical activity was a significant contributing factor to pre-mortem deterioration in moist-stored recalcitrant seeds.

The present study of structural and physiological changes during the development of the cushion moss, Grimmia pulvinata, quantifies the size-dependence of various parameters of water relations such as changes in surface: volume ratio (S/V) or water loss rates, and also measures net CO2 gas exchange in the light and the dark. Larger cushions had lower S/V values than smaller ones and featured lower rates of area-based evapotranspiration, owing to higher boundary-layer resistance, but did not differ in relative water storage capacity (expressed as a percentage of d. wt). In combination, this leads to considerably longer hydration periods in larger cushions. By contrast, CO2 gas-exchange parameters were negatively correlated with size : larger cushions showed significantly lower (mass-based) rates of net photosynthesis and dark respiration. Using these data, we estimated carbon budgets during a drying cycle as a function of cushion size. When including alternations of dark and light periods, the relationship proved to be rather complicated. Depending on the time of hydration, net carbon budgets not only varied quantitatively with size but sometimes took on both positive and negative values depending on cushion size. We conclude that neglecting plant size can lead to unrepeatable or even misleading results in comparative ecophysiological studies, and therefore urge for adequate attention to be paid to size in these studies.

The ability of seeds to germinate and establish seedlings in a predictable manner under a range of conditions has a direct contribution to the economic success of commercial crops, and should therefore be considered in crop improvement. We measured traits associated with seed vigour and pre-emergence seedling growth in a segregating population of 105 doubled haploid Brassica oleracea lines. The germination traits measured were: mean germination times for unstressed germination; germination under water stress or germination after a heat treatment; and conductivity of seed leachate. The seedling growth traits measured were: seed weight; seedling growth rate; and seedling size at the end of the exponential growth phase. There were some correlations, notably among germination traits, and between seed weight and pre-emergence seedling growth. Heritability of the various traits was typically in the 10–15% range, with heritability of conductivity and mean germination time under water stress 25 and 24% respectively. Collectively the results indicate that germination and pre-emergence seedling growth are under separate genetic control. Quantitative trait loci analyses were carried out on all measurements and revealed significant loci on linkage groups O1, O3, O6, O7 and O9. We suggest that genes at these loci are important in determining predictable seed germination and seedling establishment in practice.

Experiments were conducted on Phillyrea latifolia plants grown under a dense overstorey of Pinus pinea (shade plants) or on seashore dunes (sun plants) in a coastal area of Tuscany (42° 46′ N, 10° 53′ E). Total integrated photon flux densities averaged 1.67 and 61.4 m mol m−2 d−1 for shade and sun sites, respectively. A leaf morphological–structural analysis, a qualitative and quantitative analysis of phenylpropanoids of leaf tissue and leaf surface, and a histochemical localization of flavonoids were conducted. The area of sun leaves reached 57% of that of shade leaves, whereas leaf angle (β), sclerophylly index (ratio of leaf d. wt:leaf area), and trichome frequency (trichome number mm−2 ) were markedly greater in leaves exposed to full solar radiation than in leaves acclimated to shade. The total thickness of sun leaves was 78% higher than that of shade leaves, mostly owing to a greater development of both palisade parenchyma and spongy mesophyll. The concentration, but not the composition, of leaf tissue phenylpropanoids varied significantly between sun and shade leaves, with a marked increase in flavonoid glycosides in sun leaves. Flavonoids occurred almost exclusively in the upper epidermal cells of shade leaves. By contrast, flavonoids largely accumulated in the upper and lower epidermis, as well as in the mesophyll tissue of leaves that were acclimated to full sunlight. Flavonoid glycosides were found exclusively in the secretory products of glandular trichomes of P. latifolia leaves exposed to high levels of light; luteolin 7-O- glucoside and quercetin 3-O-rutinoside were the major constituents. By contrast, verbascoside and an unidentified caffeic acid derivative constituted 72% of total phenylpropanoids secreted by glandular trichomes of shade leaves, whereas they were not detected in glandular trichomes of sun leaves. These findings suggest that the light-induced synthesis of flavonoids in glandular trichomes of P. latifolia probably occurs in situ and concomitantly inactivates other branch pathways of the general phenylpropanoid metabolism. This is the first report of the key role of glandular trichomes and of flavonoid glycosides in the integrated mechanisms of acclimation of P. latifolia to excess light.

We have explored leaf-level plastic response to light and nutrients of Quercus ilex and Q. coccifera, two closely related Mediterranean evergreen sclerophylls, in a factorial experiment with seedlings. Leaf phenotypic plasticity, assessed by a relative index (PI = (maximum value - minimum)/maximum) in combination with the significance of the difference among means, was studied in 37 morphological and physiological variables. Light had significant effects on most variables relating to photosynthetic pigments, chlorophyll fluorescence and gas exchange, whereas nutrient treatment had a significant effect in only 10% of the variables. Chlorophyll content was higher in the shade whereas carotenoid content and nonphotochemical quenching increased with light. Nutrient limitations increased the xanthophyll-cycle pool but only at high light intensities, and the same interaction between light and nutrients was observed for lutein. Predawn photochemical efficiency of PSII was not affected by either light or nutrients, although midday photochemical efficiency of PSII was lower at high light intensities. Photosynthetic light compensation point and dark respiration on an area basis decreased with light, but photosynthetic capacity on a dry mass basis and photochemical quenching were higher in low light, which translated into a higher nitrogen use efficiency in the shade. We expected Q. ilex, the species of the widest ecological distribution, to be more plastic than Q. coccifera, but differences were minor: Q. ilex exhibited a significant response to light in 13% more of the variables than Q. coccifera, but mean PI was very similar in the two species. Both species tolerated full sunlight and moderate shade, but exhibited a reduced capacity to enhance photosynthetic utilization of high irradiance. When compared with evergreen shrubs from the tropical rainforest, leaf responsiveness of the two evergreen oaks was low. We suggest that the low leaf-level responsiveness found here is part of a conservative resource use strategy, which seems to be adaptive for evergreen woody plants in Mediterranean-type ecosystems.

Miscanthus, a perennial rhizomatous C4 grass, is a potential biomass crop in Europe, mainly because of its high yield potential and low demand for inputs. However, until recently only a single clone, M. × giganteus, was available for the extensive field trials performed across Europe and this showed poor overwintering in the first year after planting at some locations in Northern Europe. Therefore, field trials with five Miscanthus genotypes, including two acquisitions of Miscanthus × giganteus, one of M. sacchariflorus and two hybrids of M. sinensis were planted in early summer 1997 at four sites, in Sweden, Denmark, England and Germany. The field trials showed that better overwintering of newly established plants at a site was not apparently connected with size or early senescence. An artificial freezing test with rhizomes removed from the field in January 1998 showed that the lethal temperature at which 50% were killed (LT50) for M. × giganteus and M. sacchariflorus genotypes was −3.4 °C. However, LT50 in one of the M. sinensis hybrid genotypes tested was −6.5 °C and this genotype had the highest survival rates in the field in Sweden and Denmark. Although the carbohydrate content of rhizomes, osmotic potential of cell sap and mineral composition were not found to explain differences in frost tolerance adequately, moisture contents correlated with frost hardiness (LT50) in most cases. The results obtained form a basis for identifying suitable Miscanthus genotypes for biomass production in the differing climatic regions of Europe.