Light and shade plants growth analysis

Hypostomatous Picture

Figure 3.8 Transverse sections through leaves of yellow archangel Lamiastrum galeobdolon ssp. montanum (left) and of wood-sorrel Oxalis acetosella (right). Both species are hypostomatous, i.e. have their stomata restricted to the undersides of the leaves. (a), (a0) sun leaves; (b), (b0) shade leaves, whose palisade mesophyll consists of funnel cells (see text for an explanation). (From (right) Packham and Willis, 1977. Journal of Ecology 65; (left) Packham and Willis, 1982. Journal of Ecology 70, Blackwell Publishing.)

Figure 3.8 Transverse sections through leaves of yellow archangel Lamiastrum galeobdolon ssp. montanum (left) and of wood-sorrel Oxalis acetosella (right). Both species are hypostomatous, i.e. have their stomata restricted to the undersides of the leaves. (a), (a0) sun leaves; (b), (b0) shade leaves, whose palisade mesophyll consists of funnel cells (see text for an explanation). (From (right) Packham and Willis, 1977. Journal of Ecology 65; (left) Packham and Willis, 1982. Journal of Ecology 70, Blackwell Publishing.)

shade-intolerant silver maple Acer saccharinum is multilayered while the shade-tolerant sugar maple A. saccharum is a monolayered tree. Multiple layers of leaves works because, as noted above, most leaves need around just 20% of full sunlight to photosynthesize at their maximum rate (this does vary between species and the degree of environmental stress leaves are under; for example photosynthesis is less efficient in water-stressed leaves). This broad figure of 20% is a quite small requirement for leaves, so providing each layer of leaves is open enough to allow more than this past them, then lower layers of leaves can survive. This is also helped by the leaves inside the canopy being shade leaves capable of utilizing light more efficiently than those growing in full sun - the sun leaves (Fig. 3.8, and see below for a discussion of how these leaf types differ). Care is needed when considering light below leaves that only wavelengths of light usable in photosynthesis are considered. Yellow and blue light (peaking at 430 and 662 nm wavelength, respectively) is used by chlorophyll and these frequencies make up the bulk of the photosynthetically active radiation (PAR). Given that the soft green light (around 600 nm) which we appreciate under a

TREE LAYER

Carpinus betulus hornbeam Alnus glutinosa alder + Tilia cordata small-leaved lime Acer pseudoplatanus sycamore Fagus sylvatica beech Quercus robur pedunculate oak Fraxinus excelsior: ash

SHRUB LAYER

Ribes uva-crispa gooseberry Sambucus nigra elder Lonicera xylosteum fly honeysuckle Crataegus sp. hawthorn A Prunus padus bird cherry

* Prunus avium wild cherry

+ Acer campestre field maple Coryllus avellana hazel Humulus lupulus hop a Euonymus europaeus spindle Rubus idaeus raspberry Rubus caesius dewberry Cornus sanguinea dogwood Hedera helix ivy Hex aquifolium holly

HERB LAYER: GREEN IN SPRING

Leucojum vernum spring snowflake a Gagea lutea yellow star-of-bethlehem

* Anemone nemorosa wood anemone

* Adoxa moschatellina moschatel

Ranunculus ficaria lesser celandine Corydalis cava hollowroot Veronica hederifolia ivy-leaved speedwell Arum maculatum lords-and-ladies

* Allium ursinum ramsons

EARLY FLOWERING SUMMER GREEN

Pulmonaria officinalis lungwort a Primula elatior oxlip

Mercurialis perennis dog's mercury

* Ranunculus auricomus goldilocks buttercup

* Paris quadrifolia herb paris

Ranunculus lanuginosus downy buttercup a Polygonatum multiflorum solomon's-seal Geum urbanum wood avens Stellaria nemorum wood stitchwort Geranium robertianum herb-robert

* Vicia sepium bush vetch Phyteuma spicatum spiked rampion

* Melica uniflora wood melick Galium aparine cleavers Silene dioica red campion

* Poa nemoralis wood meadow-grass Aegopodium podagraria ground-elder

* Carex remota : remote sedge Stachys sylvatica hedge woundwort Dactylis glomerata cock's-foot Chaerophyllum temulum rough chervil

jfirmrrk I

Nodose Cock
' J F M A* M J J A'S'O" N D

Leaves of Leaves of previous year current year

Flowering period Period when closed

(cleistogamous) flowers produced

LATE FLOWERING

Lamium maculatum spotted dead-nettle Scrophularia nodosa common figwort Crepis paludosa marsh hawk's-beard Urtica dioica stinging nettle Impatiens noli-tangere touch-me-not balsam

* Campanula trachelium nettle-leaved bellflower

* Campanula latifolia giant bellflower

A Hordelymus europaeus wood barley Filipendula ulmaria meadowsweet

* Festuca gigantia giant fescue

Circaea lutetiana enchanter's-nightshade Brachypodium sylvaticum false brome Galium sylvaticum 'wood bedstraw'

FERNS

Athyrium filix-femina lady-fern Dryopteris filix-mas male-fern Gymnocarpium dryopteris oak fern Dryopteris dilatata broad buckler-fern

WINTERGREEN SPECIES

Hepatica nobilis liverleaf

* Luzula pilosa hairy wood-rush

* Oxalis acetosella wood-sorrel

* Potentilla sterilis barren strawberry

* Viola reichenbachiana early dog-violet Glechoma hederacea ground ivy

* Lamiastrum galeobdolon yellow archangel

* Carex sylvatica wood sedge Stellaria holostea greater stichwort

* Veronica montana wood speedwell

* Galium odoratum woodruff

* Sanicula europaea wood sanicle

* Milium effusum wood millet Deschampsia cespitosa tufted hair grass

II 1 1 1 1

Figure 3.9 Phenological development of different species in the tree, shrub and herb layers of central European damp oak-hornbeam Quercus-Carpinus forests. * implies that the species concerned was one of the 23 shown in this diagram which were listed by Rose (1999) as Ancient Woodland Vascular Plant Indicators (AWVPs) in all four regions of southern Britain (see Section 6.4.1).~ means that the plant is not listed in all four regions. + means these species should not be used as AWVPs in England unless they are well within a wood and do not appear to have been planted. See Sections 5.9 and 6.4.1 for more detail. Data are an average of 3 years. (Modified from Ellenberg, 1988. Vegetation Ecology of Central Europe. Cambridge University Press.)

I CL

Figure 3.9 Phenological development of different species in the tree, shrub and herb layers of central European damp oak-hornbeam Quercus-Carpinus forests. * implies that the species concerned was one of the 23 shown in this diagram which were listed by Rose (1999) as Ancient Woodland Vascular Plant Indicators (AWVPs) in all four regions of southern Britain (see Section 6.4.1).~ means that the plant is not listed in all four regions. + means these species should not be used as AWVPs in England unless they are well within a wood and do not appear to have been planted. See Sections 5.9 and 6.4.1 for more detail. Data are an average of 3 years. (Modified from Ellenberg, 1988. Vegetation Ecology of Central Europe. Cambridge University Press.)

dense canopy is of little use to plants, we may often fail to appreciate just how dark it can be for an understorey plant.

Light is the most generally limiting factor for herbs, shrubs and young trees in woodlands, where many of them show remarkable adaptations to different levels of PAR. In British woodlands, shade evasion is found in such plants as bluebell Hyacinthoides non-scripta, wood anemone Anemone nemorosa and lesser celandine Ranunculus ficaria that have their main period of growth before the tree canopy expands (Fig. 3.9). Truly shade-tolerant species such as enchanter's nightshade Circaea lutetiana frequently have their shade leaves so modified, by increasing the light-catching area and chlorophyll content, that for the same expenditure of weight they achieve much the same photo-synthetic rates as sun leaves receiving considerably higher levels of radiation. Species such as yellow archangel Lamiastrum galeobdolon, which grows along hedgerows as well as in woodlands, possess great phenotypic plasticity (the capacity for marked variation in appearance caused by environmental conditions) and show differences between their sun and shade leaves considerably greater than those in wood-sorrel Oxalis acetosella, a species well adapted to shade but not to sun (Packham and Willis, 1977, 1982). These and many other primarily woodland species, show very marked variation when they develop under different light regimes, tending to become wider and thinner under the heavy shade of a dense woodland canopy. Their sun leaves, which develop under high light intensities, are smaller, thicker and often yellowish-green. Shade leaves in many species commonly have a spongy mesophyll (the main tissue of the leaf) with very large air spaces and palisade cells (the main photosynthetic tissue directly under the top epidermis) which taper downwards from the margin adjoining the upper epidermis and are known as funnel cells (Fig. 3.8). They also have larger epidermal cells, a lower vein density and a thinner cuticle than sun leaves. In addition to filtering out wavelengths usable in photosynthesis, leaves also change the red/far-red (R/FR) ratio of the light by preferentially absorbing red light (red light is approximately 660 nm in wavelength, far-red is 700-1000 nm); the ratio is typically 1.2 in daylight and is reduced to around 0.2 in the green light filtering through leaves. Mitchell and Woodward (1988) demonstrated that yellow archangel responds to these differences in the light quality. When receiving PAR filtered to give a low R/FR ratio, as under a deciduous tree canopy in summer, this species flowers less vigorously and its internodes and petioles are more elongated compared with plants receiving the same amount of unfiltered light. Foraging for PAR amongst ground vegetation is assisted by these responses. The R/FR ratio is also important in the germination of seeds. Short exposures to red light tend to break dormancy while far-red promotes dormancy. This helps ensure that

Oxalis Acetosella Wood

Figure 3.10 Variation in leaflet widths of clones of wood-sorrel Oxalis acetosella from two contrasting sites in Shropshire, UK. Clones N, A and L were from a woodland streamside near Telford; clones LG, LC and LE were from beneath bracken Pteridium aquilinum in a montane pasture on the Long Mynd. Clones were gathered from the wild in late March 1971 and grown in cold frames under three light regimes: under clear polythene only (stippled bars), under light further reduced by a white muslin shade (clear bars); and in light greatly reduced by a black muslin shade (filled bars). The plants were harvested in August 1972. (Redrawn from Packham and Willis, 1977. From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Figure 3.10 Variation in leaflet widths of clones of wood-sorrel Oxalis acetosella from two contrasting sites in Shropshire, UK. Clones N, A and L were from a woodland streamside near Telford; clones LG, LC and LE were from beneath bracken Pteridium aquilinum in a montane pasture on the Long Mynd. Clones were gathered from the wild in late March 1971 and grown in cold frames under three light regimes: under clear polythene only (stippled bars), under light further reduced by a white muslin shade (clear bars); and in light greatly reduced by a black muslin shade (filled bars). The plants were harvested in August 1972. (Redrawn from Packham and Willis, 1977. From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

seeds germinate when they are receiving direct unfiltered sunlight when the seedlings will have most chance of survival.

Figure 3.10 shows variations in leaflet widths of clones of wood-sorrel from two contrasting habitats in Shropshire, mid-England when grown under three different light regimes. The results of this experiment demonstrate both the effects of phenotypic plasticity, with the leaflets of clones from both sites becoming larger as shading increased, and of genetic factors. Leaflet width/ leaflet number relationships between the two sites differed; plants originating from the woodland streamside near Telford having fewer but larger leaves than those from the more exposed montane site, where wood-sorrel grew beneath a cover of bracken Pteridium aquilinum. Individual clones from both sites also differed considerably amongst themselves; note particularly the marked contrast between the Long Mynd clones LG and LC.

Growth analysis (Table 3.1) affords valuable techniques for studying the ways in which plants respond to changes in light level, humidity regime, temperature and other features of the environment that are altered by shading. Apart from cases where shading is so extreme as to result in the production of relatively small, etiolated leaves, chlorophyll content per unit dry weight is usually higher in shade

Table 3.1. Definitions used in growth analysis dW 1

Where LA = total leaf area

LW = total leaf dry weight W = total plant dry weight dW

-= rate of dry weight increase of the whole plant dt

Relative growth rate RGR = ULR + LWR + SLA

Leaf area ratio (LAR) is a morphological index of plant form, the leaf area per unit dry weight of the whole plant.

In contrast, unit leaf rate (ULR), the rate of increase in dry weight of the whole plant per unit leaf area, is a physiological index closely connected with photosynthetic activity. A high LAR together with a low ULR is characteristic of heavily shaded woodland herbs in temperate forests.

Relative leaf growth rate (RLGR) is analogous to RGR and is the rate of increase in leaf area per unit leaf area.

6. Stomatal index

Ground area occupied by plant

Number of stomata per unit area f Number of stomata per unit area + Number of epidermal cells per unit area leaves than in sun leaves. Differences in chlorophyll contents between sun and shade leaves appear much greater when expressed on a dry-weight than a fresh-weight basis, while chlorophyll values per unit area may actually be lower for shade leaves than sun leaves. Shade plants do not always have thin leaves; many rain-forest species such as Cordyline rubra and Lomandra longifolia have thick leaves with unusually large chloroplasts concentrated in the upper palisade, giving them a high chlorophyll content and a low specific leaf area (SLA: see Table 3.1). Stomatal frequency and pore size vary amongst rain-forest plants, but differences between sun and shade species are not significant.

Morphology Figure Ground
Figure 3.11 Schematic diagram of the basic vegetative morphology of ground ivy Glechoma hederacea. (From Hutchings and Slade, 1988. Plants Today 1, Blackwell Publishing.)

Plants and animals both carry out foraging, a process in which an organism searches or grows within its habitat so as to obtain essential resources. The main resource-gathering structures of plants are roots (see Section 2.3) and leaves, whose distribution in two- or three-dimensional space is partly determined by genetic 'growth rules' which influence many features of form. This is particularly well seen in the woodland labiates ground ivy Glechoma hederacea and yellow archangel, where the distance between adjacent nodes on growing stems, the probability of branching at nodes, and the angle between branches are important (Fig. 3.11). In such plants the basic plant units, known as ramets, consist of two horizontal leaf blades (laminas), each attached to a rooted node by an erect petiole. If planted, a parent ramet produces two primary stolons with new ramets at each node, and under favourable conditions a branched clonal system soon develops.

Clonal integration, plasticity and the effects of different nutrient and light levels on foraging in ground ivy are of particular interest, as this plant can be seen as a model for other woodland species of similar growth form and ecology. Slade and Hutchings (1987a,b,c) found that mean numbers of stolon branches, ramets per clone and mean weight per stolon length were all greatest in a highlight, high-nutrient treatment; they were least with low light and high nutrients. The latter plants had the highest percentage dry-weight allocation to petioles (leaf stalks) and the lowest to roots; the relatively low-mass stolons and the few side branches allowed them to forage for light extensively rather than intensively as did plants in the high-light, high-nutrient treatment whose behaviour would tend to keep them within any favourable area encountered naturally. Plants in the high-light, low-nutrient treatment possessed the highest root/shoot ratio of the three treatments; this again is an effective response, one that promotes effective foraging for nutrients.

Hughes (1959), using artificial shading under controlled conditions, showed that the meristematic activities of the sun and shade leaves of small balsam Impatiens parviflora are fundamentally similar. The shade leaves simply expand more; because of this greater expansion the cells are wider so the stomatal frequency is lower, though the stomatal index (Table 3.1) remains the same. Working with the same species, Young (1975) found that leaf weight ratio (LWR) was markedly affected by the rooting medium, but little altered by changes in total daily light. Specific leaf area (SLA) varies with temperature, rooting medium, day length, total daily light and the 'physiological age' of the leaf or plant. Because SLA is, over a substantial range of daily light level, inversely proportional to total daily radiation received while unit leaf rate is directly proportional to it, the net effect of these two relationships is that the relative growth rate (RGR) of small balsam remains approximately constant over the light range concerned.

The root/shoot ratio of woodland plants appears to be related to soil moisture content as well as intensity of radiation, but in shade plants the proportion of plant weight devoted to roots is commonly low. Temperate shade species growing under lower PAR tend to possess high fresh-weight/dry-weight ratios, and SLAs, while RGR is low, and the niches in which many such plants occur are often not particularly favourable physiologically; their presence in them is related to their ability to compete under the conditions concerned. In nature shade plants often occur at light levels well below the optimum for the species. Heavy shading has, however, eliminated competition from sun species of much higher potential RGR.

Unlike shade plants that cannot utilize the additional light, sun plants can continue to increase net photosynthesis until much higher light levels are reached. This higher capacity for carbon dioxide fixation per unit leaf area in sun plants is directly related to the greater amounts of carboxylating enzymes and the greater volume of the leaf per unit leaf area. The proportion of chlorophyll a to chlorophyll b increases in plants grown at high light intensities; this makes sense because chlorophyll a is responsible for capturing the energy from light, so more of the extra energy in high-light environments can be utilized. Even fully grown leaves may partially adapt to a change in average light level by modifying their enzyme systems.

Boardman (1977) reviewed the wide variety of morphological, physiological and biochemical mechanisms involved in the phenotypic plasticity that enables a given genotype to adapt to various light levels. His conclusion was that the light levels to which a genotype can adjust reflect a genetic adaptation to the conditions of its native habitat and that adaptation for great photosynthetic efficiency in strong sunlight precludes high efficiency in dense shade.

Was this article helpful?

0 0
Worm Farming

Worm Farming

Do You Want To Learn More About Green Living That Can Save You Money? Discover How To Create A Worm Farm From Scratch! Recycling has caught on with a more people as the years go by. Well, now theres another way to recycle that may seem unconventional at first, but it can save you money down the road.

Get My Free Ebook


Responses

  • matthias
    Why do the number of stomata change in light and shade?
    7 years ago

Post a comment