Morphological and Anatomical Adaptations to Light Conditions

1.1. The shoot as a metameric system

The shoot consists of repeating elements — metamers (or phytomers). Each metamer includes a node (leaf attachment point), an internode (stem segment between nodes) and an axillary bud (primordium of a future lateral shoot) (Beck, 2010). The pattern of internode elongation, leaf orientation at the node and the ability of buds to grow determine the overall appearance of the plant.

Depending on internode length, we distinguish:

• Rosette shoots — internodes extremely shortened, leaves clustered (e.g., in plantain, dandelion, many forest herbs). This form allows leaves to be placed in one ground layer, which is advantageous under light deficiency in the lower part of the phytocoenosis (Serebryakova et al., 2006).
• Elongated shoots — internodes well developed, leaves distributed along the stem (typical for most trees, shrubs and herbs of open habitats).

1.2. Types of branching and their role in forming the light canopy

Shoot branching increases the photosynthetic surface. Two main types of branching are distinguished:

• Monopodial — the main axis (leader) grows indefinitely from the apical bud, and lateral branches arise from its sides. This type forms a pyramidal or columnar crown (e.g., in spruce, fir, poplar). With monopodial branching, the upper layers strongly shade the lower ones, so in shade‑tolerant species the lower branches die early (Strasburger, 1971).
• Sympodial — the apical bud dies early or stops growing, and its function is taken over by one of the lateral branches. This results in a “false” main axis consisting of several successive lateral branches. This is characteristic of many deciduous species (birch, willow, linden) and allows the crown to be more open, letting light penetrate inside (Valladares et al., 2002).

In plants living under the forest canopy (sciophytes and shade‑tolerant plants of the lower layers), so‑called plagiotropic (horizontal) orientation of lateral shoots is often observed. They “spread” sideways to bring leaves out from under the direct shade of upper plants (Liu et al., 2025).

1.3. Leaf mosaic and leaf rotation

One of the most striking examples of adaptation to light is leaf mosaic. The phenomenon consists in leaves being arranged on the shoot so as to minimally shade each other (Strasburger, 1971). This is achieved through:

• Different petiole lengths.
• Different angles of leaf divergence from the stem.
• The ability of petioles to twist (torsion), whereby the leaf blade turns perpendicular to the incident light.

Leaf mosaic is particularly well expressed in plants of the lower layers and in shade‑tolerant shrubs (e.g., linden, hazel, elm). In heliophytes (light‑loving), on the contrary, leaves are often oriented vertically or at an acute angle to the sun’s rays (“eucalypt” type), which avoids overheating at noon (Dörken & Lepetit, 2018).

1.4. Layered structure of phytocoenosis and light differentiation

In a closed plant community (forest, meadow, field), layers are formed. The upper layer receives 100% of solar radiation. In the lower layers, illumination can drop to 1‑5% of full (Valladares et al., 2002). This leads to plants of different layers, and sometimes leaves of the same plant (upper and lower), having different morphology and anatomy (see Section 4).

Thus, shoot architecture is not a random shape but a complex, ecologically determined system that increases the plant’s competitiveness for light. The agronomist should remember that thickening of sowing disrupts leaf mosaic, increases mutual shading, leading to stem elongation, lodging and reduced yield (Ghorbel et al., 2023).

2.1. Heliophytes (light‑loving plants)

Heliophytes are plants of open, well‑lit habitats. They cannot tolerate prolonged shading; their ecological optimum lies in full solar radiation. Otherwise, they are depressed, elongate and do not form generative organs (Khan et al., 2025).

1. Typical habitats: steppes, meadows, deserts, alpine meadows, rock outcrops, as well as sparse forests (edges, burnt areas).

2. Brief morphological and anatomical traits:

• Leaves often small, narrow, sometimes dissected or with a waxy bloom (to reflect excess rays).
• Pubescence (trichomes) developed to scatter light and reduce overheating.
• Shoots often shortened, with rosette or dense leaf arrangement.
• Mechanical tissues and conducting system well developed (adaptation to high transpiration loads).

3. Examples from natural flora: feather grass (Stipa), fescue (Festuca valesiaca), field poppy (Papaver rhoeas), most cacti, many acacia species (Serebryakova et al., 2006; Strasburger, 1971).

4. Examples from agricultural crops: maize (Zea mays), sunflower (Helianthus annuus), sugarcane, cotton, as well as most open‑field vegetable crops (tomato, pepper, cucumber — under optimal agrotechnics).

A feature of heliophytes is their high photosynthetic activity under direct sunlight. However, under shading they quickly reduce productivity, so in agronomy it is important to avoid dense sowings and carry out timely weeding (Ghorbel et al., 2023).

2.2. Sciophytes (shade‑loving plants)

Sciophytes are plants of the lower layers of dense forests, deep gorges, tropical rainforests. They react positively to shading and are depressed under direct sunlight (Novichonok et al., 2018; Valladares et al., 2002). This does not mean they can grow in complete darkness — they need diffuse light, but its intensity must be low.

1. Typical habitats: under the canopy of spruce forests, beech forests, tropical hylaea.

2. Brief morphological and anatomical traits:

• Leaves large, thin, delicate, often dark green.
• Leaf mosaic very well expressed, leaves spread almost horizontally.
• Cuticle thin, stomata located mainly on the lower side.
• Palisade tissue (columnar mesophyll) poorly developed or absent (see Section 4).

3. Examples from natural flora: wood sorrel (Oxalis acetosella), false lily‑of‑the‑valley (Maianthemum bifolium), European wild ginger (Asarum europaeum), many ferns (male fern), mosses, as well as understory species of tropical forests (e.g., some Begonia, Marantaceae) (Serebryakova et al., 2006).

4. Examples of cultivated plants close to sciophytes: Pure sciophytes are almost absent among agricultural plants. However, a number of crops grow better under moderate shade: lettuce, spinach, some herbs (mint, lemon balm). In fruit growing, shade‑tolerant species (fir, yew, beech) are close to sciophytes, but they are rarely used as fruit trees.

It is important not to confuse sciophytes with plants that merely tolerate shade (shade‑tolerant). Sciophytes are never found in open places under natural conditions; their leaves are quickly damaged by ultraviolet radiation and overheating (Novichonok et al., 2018).

2.3. Facultative heliophytes (shade‑tolerant plants)

This is the most extensive and ecologically plastic group. Such plants can grow both in full light and in partial shade, but reach maximum productivity under good illumination. Under shading, they change morphology and anatomy, approaching sciophytes, and in light, they approach heliophytes (Khan et al., 2025; Brenes-Arguedas et al., 2011).

• Morphological and anatomical plasticity: It is this group that exhibits the phenomenon of “sun” and “shade” leaves within the crown of one tree (see Section 5).
• Examples from natural flora: most broad‑leaved trees (common oak, small‑leaved linden, Norway maple), many shrubs (hazel, euonymus), forest herbs (lily‑of‑the‑valley, ground elder).
• Examples of agricultural crops: apple (Malus domestica), pear (Pyrus communis), plum, cherry, raspberry, currant, as well as many forage grasses (clover, alfalfa) and vegetable crops (e.g., broccoli, some cabbage varieties). Most cereal crops (wheat, barley) also show traits of facultative heliophytes — under dense sowing they elongate but can produce acceptable yields within certain limits.

Knowledge of the ecological group has direct practical significance when developing sowing schemes (stand density), shaping fruit tree crowns, selecting crops for compacted planting and for vertical farming (Ghorbel et al., 2023).

3.1. Size and shape of the leaf blade

Generalised scheme of morphological adaptations of plants to different light levels: under high light, compact shoots, small leaves with vertical orientation are formed; under low light, stem elongation, large horizontal leaves and suppressed root system are observed.

The scheme shows the main external traits of plants grown under excessive (sunny) and deficient (shaded) illumination. Differences in shoot height, leaf size and orientation, as well as root development are clearly visible.

The dependence of leaf size on illumination is one of the most noticeable patterns.

• In heliophytes (light‑loving): Leaves are generally small or medium. Reducing leaf area reduces overheating and transpirational water loss under bright light. Often the leaf blade is strongly dissected (pinnate or palmate) or even needle‑shaped (in conifers, some grasses). Such a shape improves heat exchange and prevents “overheating” (Dörken & Lepetit, 2018; Serebryakova et al., 2006).
• In sciophytes (shade‑loving): Leaves are large, entire, with smooth margins. A large blade allows capturing maximum diffuse light penetrating under the forest canopy. However, such leaves are quickly damaged under direct sunlight (Valladares et al., 2002; Novichonok et al., 2018).
• In facultative heliophytes (shade‑tolerant) within the crown: The upper, well‑lit leaves are smaller and often more rigid (“sun form”), while the lower, shaded leaves are larger and thinner (“shade form”). This phenomenon is called heterophylly (leaf dimorphism) within a single plant (Dörken & Lepetit, 2018; Liu et al., 2025).

3.2. Leaf orientation in space

The angle of inclination of the leaf blade regulates the amount of absorbed radiation.

• Heliophytes: Often have a vertical or steeply inclined leaf orientation (e.g., in eucalyptus, many grasses). This position avoids overheating at midday — rays slide across the blade surface rather than falling perpendicularly. Moreover, some heliophytes can turn leaves to follow the sun (diaheliotropism) or, conversely, avoid excess light (paraheliotropism) (Beck, 2010; Evert, 2006).
• Sciophytes and shade leaves: Leaf blades are oriented almost horizontally to maximise interception of overhead diffuse light. At the same time, leaf mosaic reaches perfection — leaves are arranged so as not to shade each other (Strasburger, 1971).
• In some plants (wood sorrel, some clover species), nyctinasty (sleep movements) is observed: in bright light the leaflets fold and droop, at dusk they unfold, which is also a protective reaction against excessive illumination (Serebryakova et al., 2006).

3.3. Pubescence, waxy bloom and other covering structures

The leaf surface may be covered with hairs (trichomes), waxy bloom or scales.

• In heliophytes: Pubescence is often dense (felt‑like), hairs reflect part of the sun’s rays and create a micro‑layer of still air at the leaf surface, reducing transpiration. Waxy bloom (cuticle) gives the leaf a glaucous tinge, increases albedo (reflection) and protects against ultraviolet radiation (Evert, 2006; Khan et al., 2025). Examples: olive tree, sea buckthorn, cacti, many steppe plants.
• In sciophytes: Pubescence absent or very weak, cuticle thin, waxy bloom not expressed. A smooth surface better transmits diffuse light into the leaf (Serebryakova et al., 2006).
• In some heliophytes (e.g., Verbascum — mullein), pubescence is so dense that the leaf appears white or grey — an extreme form of protection against overheating.

3.4. Features of the shoot system (internode length, branching, crown shape)

Shoot architecture is closely related to light conditions (see Section 1).

• In heliophytes under optimal illumination: Shoots are often shortened, compact, with few long internodes (rosette or semi‑rosette forms). However, under shading (e.g., dense sowing), heliophytes exhibit a sharp elongation of internodes — this is etiolation, the attempt to bring leaves to the light. Etiolated shoots are thin, weak, and lodge (Ghorbel et al., 2023).
• In sciophytes: Internodes are generally long, even under low light the plants “stretch” upward. Branching is often monopodial (in trees) or sympodial with one pronounced leader (in herbs) to break through to the light through leaf layers (Strasburger, 1971).
• In plants of open spaces (heliophytes and shade‑tolerant in light): A compact, bushy or rosette form develops with many lateral shoots. The crown often has a pyramidal or columnar shape, ensuring good illumination of all layers (Serebryakova et al., 2006).

3.5. Brief summary table of morphological traits

For ease of comparison, the main morphological traits are summarised in the table.

Thus, the external appearance of a plant is a “mirror” of the light regime of its habitat. For the agronomist, this means that by inspecting crops or an orchard one can quickly assess whether plants receive enough light. The appearance of signs of etiolation (elongation, pallor, lack of pubescence) is a signal for the need for thinning, pruning or supplementary lighting.

In the next section we move on to microscopic (anatomical) adaptations, which underlie differences at the tissue level.

4.1. Epidermis and cuticle

The outer layer of cells (epidermis) and its derivatives play a key role in regulating light absorption and transpiration.

• In the sun leaf: Epidermal cells are small, with strongly thickened outer walls. The cuticle (waxy layer) is thick, often covered with an additional waxy bloom or dense trichomes (hairs). This protects the leaf from overheating and excessive water loss (Evert, 2006; Beck, 2010). In conifers (e.g., Abies alba), the epidermis of sun leaves may be multilayered and contain sclerenchyma (lignified) cells, providing additional rigidity (Dörken & Lepetit, 2018).
• In the shade leaf: Epidermal cells are larger, their walls thin, the cuticle barely visible, waxy bloom and trichomes absent. This structure ensures maximum transmission of diffuse light into the leaf and does not hinder gas exchange (Serebryakova et al., 2006).

4.2. Stomatal apparatus

Stomata are the “valves” regulating gas exchange and transpiration.

• In the sun leaf: Stomatal density (number per mm²) is high (sometimes several hundred). Stomata are often located only on the lower side of the leaf (hypostomatic type). In plants of arid habitats (xerophytic heliophytes), stomata may be sunken into stomatal crypts (pits), reducing water loss. Stomatal sizes are generally smaller than in shade leaves (Evert, 2006; Dörken & Lepetit, 2018).
• In the shade leaf: Stomatal density is low (2–5 times less than in sun leaves), but the stomata themselves are larger. Stomata are often present on both sides of the leaf (amphistomatic type), facilitating gas exchange under low light (Valladares et al., 2002). In some sciophytes, stomata are even raised above the epidermal surface.

4.3. Mesophyll (assimilation parenchyma)

This is the main and most indicative difference between sun and shade leaves.

• In the sun leaf: Palisade (columnar) tissue is well developed. It consists of 2–3 (or more) layers of elongated, tightly packed cells arranged perpendicular to the leaf surface. This structure allows chloroplasts to efficiently capture direct sunlight, while the cells do not overheat due to close contact and good thermal conductivity. Spongy (spongy) tissue is less developed, intercellular spaces small (Strasburger, 1971; Evert, 2006; Dörken & Lepetit, 2018).
• In the shade leaf: Palisade tissue is reduced — consists of one layer of low cells or is absent altogether. Spongy tissue, on the contrary, is strongly developed, with large air‑filled intercellular spaces. This promotes better scattering of light inside the leaf (light “gets tangled” in the system of intercellular spaces) and efficient gas exchange under low illumination (Valladares et al., 2002; Serebryakova et al., 2006). The cells of the spongy parenchyma have an irregular rounded shape and are loosely adjacent to each other.

In some plants (e.g., many monocots, including grasses), the mesophyll may be uniform, without clear differentiation into palisade and spongy tissue (isolateral mesophyll). In such cases, in sun leaves the mesophyll cells are smaller and more densely packed, while in shade leaves they are larger with more developed intercellular spaces (Beck, 2010).

4.4. Vascular system and mechanical tissues

Light conditions also affect the development of veins and supporting tissues.

• In the sun leaf: Vascular bundles (veins) are robust, often surrounded by sclerenchyma fibres (mechanical tissue). Xylem (vessels) has a larger diameter, necessary for intensive water supply under high transpiration. Sclerenchyma is also developed along the leaf margin and around large veins, giving the leaf rigidity (Evert, 2006; Dörken & Lepetit, 2018).
• In the shade leaf: Vascular bundles are smaller, mechanical tissue poorly developed or absent. Xylem consists of narrower vessels (or tracheids). The leaf is soft and easily damaged by mechanical impact (Valladares et al., 2002).

4.5. Chloroplasts (structural level)

Although we omit detailed biochemistry, it is important to note structural differences in chloroplasts visible under an electron microscope.

• In the sun leaf: Chloroplasts are small, numerous. Inside them, the grana system (stacks of thylakoid membranes) is well developed, allowing efficient use of direct light energy (Dörken & Lepetit, 2018).
• In the shade leaf: Chloroplasts are larger, but their number per unit area is smaller. The grana system is less developed, but the number of stromal thylakoids is increased, which is characteristic of adaptation to diffuse light (Beck, 2010; Evert, 2006).

4.6. Summary table of anatomical traits

For clarity, the main differences are summarised in the table.

Thus, the internal structure of the leaf accurately reflects the light regime under which it developed. Even if the plant belongs to the same species (e.g., apple), its leaves may have different anatomy depending on position in the crown. For the agronomist, this means that when shaping fruit tree crowns, when thickening sowings, or when growing seedlings in a greenhouse, the inevitable heterogeneity of the leaf apparatus must be taken into account.

In the next section, we consider how this theoretical knowledge is applied in practice.

5.1. Causes of modifications

The main cause is the uneven distribution of light in the crown. Upper leaves receive direct solar radiation, often in excess. Lower leaves live under diffuse light, and sometimes in deep shade. Since the leaf blade is formed from meristem after the bud has opened, the light conditions at the time of initiation and during leaf growth determine its final structure (Evert, 2006; Beck, 2010).

It is important to emphasise: the differences between sun and shade leaves are not hereditary but modification changes. They are not inherited, but are a manifestation of the reaction norm of the genotype. If a branch with shade leaves is brought out into the light, new leaves on it will form according to the sun type (Valladares et al., 2002).

5.2. Comparative characteristics of sun and shade leaves

The table (summarising previously given data) presents the key differences.

5.3. Significance for the plant

The presence of different leaf types in the crown increases the overall productivity of a tree or shrub:

• Sun leaves act as an “energy station” during high insolation hours, producing a maximum of organic matter.
• Shade leaves use diffuse light penetrating into the lower layers, thus adding additional photosynthetic surface without significantly increasing transpiration.

Without such differentiation, lower leaves would either die from lack of light (if they were of the sun type), or upper leaves would overheat and be damaged (if all leaves were of the shade type) (Valladares et al., 2002; Liu et al., 2025).

5.4. Examples from the plant world

Apple (Malus domestica): Classic object for observation. Upper leaves (on the periphery) are small, hard, with dense pubescence on the underside. Lower leaves (deep in the crown) are large, thin, almost hairless.

Silver fir (Abies alba): In this conifer, sun leaves (on upper, well‑lit branches) are thicker, contain a hypodermis and have higher stomatal density than shade leaves on lower, shaded branches (Dörken & Lepetit, 2018).

Silver birch (Betula pendula): On the same tree one can find leaves with different degrees of dissection — upper leaves more entire, lower leaves with deeper lobes (related to illumination).

Ivy (Hedera helix): Young (shade) leaves on stems creeping along the ground have a lobed shape and thin structure; older (sun) leaves on flowering shoots are entire, leathery, with a thick cuticle.

5.5. Applied value for agronomy and horticulture

• Shaping fruit tree crowns: The goal of pruning is to create a crown in which even the lower leaves receive enough light not to be purely “shade” leaves but to approach “sun” or intermediate types. This increases flower bud initiation and fruit quality.
• Sowing density: With excessive stand density of maize, sunflower or cereals, the lower leaves are in deep shade and develop as shade leaves. Their productivity is extremely low, and respiration may exceed photosynthesis. This leads to premature death of lower leaves and reduced yield (Ghorbel et al., 2023).
• Greenhouse cultivation: The use of shading nets or intercropping (e.g., tomatoes + lettuce) allows managing light conditions so that the main crop forms predominantly sun leaves and the associated crop forms shade leaves (efficient use of space).
• Vertical farms: In multi‑layer cultivation, it must be taken into account that plants on lower tiers will form shade leaves. Therefore, for lower tiers, shade‑tolerant or sciophyte crops (e.g., lettuce, spinach, microgreens) should be selected.

Thus, the phenomenon of leaf modification within a single plant is a vivid proof of the high plasticity of the plant organism. For the agronomist, this means that by managing the light regime (pruning, planting density, shading), he can consciously shape the leaf apparatus of the desired type.

In the next section we consider practical conclusions and their application in agriculture.

6.1. Optimising stand density and sowing patterns

Dense sowings (cereals, row crops, vegetables) lead to early canopy closure and mutual shading of plants. In plants, the shade avoidance syndrome is activated: internodes elongate, leaves become thinner, stomatal density decreases, the proportion of palisade tissue decreases and the proportion of spongy tissue increases (Ghorbel et al., 2023; Liu et al., 2025). As a result, stems become thin and lodge, and leaves become less productive.

Recommendation: For each crop, the optimal stand density must be determined experimentally, such that even the lower leaves receive enough light to form a mixed or sun type. Modern intensive cereal varieties (e.g., wheat, rice) are often bred for “dwarfism” and erect leaves, allowing increased density without lodging.

6.2. Shaping the crown of fruit trees and shrubs

In horticulture, the main task is to create uniform illumination of all parts of the crown. If the crown is dense, shade leaves with low photosynthetic activity form in the interior, and moreover, the initiation of flower buds (fruit formations) and fruit quality (colour, sugar content, vitamin content) deteriorate.

Recommendation: Annual pruning (thinning) of the crown, forming sparse‑tiered, spindle or palmette crowns. Removal of water sprouts, thickening branches and “fat” shoots. Modern intensive orchards use dwarf rootstocks and spindle‑shaped crowns (slender spindle, super spindle), which ensure uniform light penetration to any point of the crown (Khan et al., 2025).

6.3. Selecting crops for multi‑layer cultivation (vertical farms and greenhouses)

In greenhouses and vertical farming, plants of different layers can be placed, selecting them according to light requirements.

Recommendation: The upper layers (most illuminated) are allocated to heliophytes (tomato, pepper, eggplant, cucumber), the middle layers to facultative heliophytes (lettuce, broccoli, culinary herbs), the lower layers to shade‑tolerant or sciophyte crops (spinach, chard, microgreens, mushrooms, some lettuce types). Knowledge of anatomical requirements allows increasing planting density without loss of productivity (Ghorbel et al., 2023).

6.4. Using shading nets and regulating the light regime

In hot regions, excess direct light can cause photoinhibition, leaf and fruit burns (especially in young plants, after transplanting seedlings, or in crops with delicate leaves). The use of shading nets (e.g., with 20‑50% shade) mimics partial shade conditions, which promotes the formation of a mixed leaf type with less load on the photosynthetic apparatus (Khan et al., 2025). Light regulation is also important when growing seedlings in spring greenhouses (supplementary lighting with phytolamps prevents etiolation).

6.5. Breeding for tolerance to density and light stress

In breeding programs, it is important to evaluate not only yield but also morphological and anatomical parameters of the leaf: leaf thickness, stomatal density, development of palisade tissue, leaf blade angle, and ability of leaves to reorient. “Sun leaf” traits correlate with tolerance to density and lodging (Liu et al., 2025).

Examples: In maize breeding for intensive technology, lines with more vertical leaf position (“liguleless” forms) and robust palisade tissue are selected, allowing increased stand density without reducing photosynthesis. In rice, varieties with short straw and erect leaves have been created, resistant to lodging at high seeding rates.

6.6. Practical examples

Rice: In dense sowings of traditional rice varieties, stem elongation and formation of shade leaves are observed, leading to lodging. Modern varieties with the “dwarf” gene (sd1) have shorter internodes and retain sun leaves even at high seeding rates.

Apple: In apple crowns shaped as “slender spindle”, sun leaves constitute the bulk even in the lower part of the crown, fruits are uniformly coloured, and yield is higher than in dense (classical) crowns.

Tomato in greenhouse: The use of supplementary lighting systems (LED lamps with optimised spectrum) allows controlling morphogenesis: blue light promotes the formation of compact sun leaves (short internodes, thick blade), while red light promotes shoot elongation. The agronomist can consciously model the habitus of the crop (Ghorbel et al., 2023).

Vegetable crops in open field: When growing broccoli cabbage under excessive light (southern regions), the use of 25% shading nets reduces the number of burns and improves head quality, because leaves form a thicker cuticle and better developed palisade tissue.

6.7. Economic aspect

Proper consideration of light adaptations allows:

• Reduce yield losses from lodging by 15‑30%.
• Increase fruit quality (sugar content, colour) by 20‑40% due to uniform crown illumination.
• Increase planting density per unit area (without loss of productivity) by 20‑50% (vertical farms, compacted sowings).
• Reduce manual labour costs (thinning, pruning) due to correct initial plant placement.

Thus, theoretical knowledge of morphological and anatomical adaptations of plants to light serves as the basis for developing effective agronomic practices aimed at maximising the use of solar energy (or artificial light) and increasing yield and product quality.