Morphological and Anatomical Adaptations to Temperature Regime

Several key indices are used to characterize the heat supply of a territory and to assess plant response: mean annual temperature, mean temperature of the coldest and warmest months, absolute minimum and absolute maximum, as well as the duration of the frost-free period. However, the most important is the distribution of heat over time — it determines the rhythm of seasonal development (Yakovlev et al., 2006).

The growing season in temperate and high latitudes is limited to the period when temperatures consistently exceed a certain threshold (usually +5 °C for the start of growth of most crops). Outside this window, active life activity is impossible. Plants use temperature as a reliable signal to prepare for the unfavorable season: shorter day length combined with lower temperatures initiates growth cessation, formation of overwintering buds, synthesis of protective substances, and leaf fall (Serebryakova et al., 2006).

According to heat distribution on Earth, four main thermal belts are distinguished: tropical, subtropical, temperate, and cold. In accordance with evolutionary adaptation to these belts, three main ecological groups have formed in plants (Yakovlev et al., 2006):

Thus, belonging to a particular thermal group reflects not only the current distribution of a species but also its evolutionary history, fixed in hereditary features of metabolism and tissue structure.

In the annual cycle of plants of temperate and cold climates, phases of active growth and dormancy successively alternate. The transition to dormancy and emergence from it are controlled not so much by absolute temperature values as by their dynamics in combination with photoperiod (day length). This set of signals is called photoperiodic response (PPR) (Yakovlev et al., 2006).

For many species, the transition from vegetative growth to flowering is possible only at a certain day length:

Knowledge of the photoperiodic response is necessary for plant introduction and in practical crop production to control flowering times.

Special mention should be made of vernalization. In many perennial and biennial plants (winter wheat, many fruit species), flowering requires a prolonged exposure to low positive temperatures (0–10 °C) for several weeks. This prevents flowering before spring and ensures that reproductive organs form under favorable conditions. In some species (e.g., winter cereals), this mechanism is controlled by specific genes (e.g., VRN1 and VRN2), which suppress flowering until cold destroys a repressor protein (Chen et al., 2024).

As noted earlier, both excessively high and low temperatures are dangerous for plant life. High temperatures (above 40–50 °C) cause enzyme denaturation, chlorophyll destruction, and tissue burns. Leaves and young shoots are particularly vulnerable. Low temperatures (below -5…-10 °C for most temperate plants) lead to the formation of ice crystals in the intercellular space, protoplast dehydration, and mechanical rupture of membranes. However, during evolution, plants have developed complex morphological and anatomical adaptations that reduce the negative impact of extreme temperatures — these adaptations will be discussed in detail in the following sections.

Thus, the temperature factor is not only a direct physical force limiting plant distribution but also a subtle ecological signal triggering key stages of ontogeny. The ability to correctly “read” this signal and accordingly rearrange morphology and anatomy is the basis for plant survival in changing climatic conditions.

One of the most effective ways to reduce overheating is to increase reflection of solar radiation. Light (whitish, silvery, grayish-green) coloration of stems and leaves reflects a significant part of incident rays, especially in the infrared region. This effect is achieved by several means (Serebryakova et al., 2006):

In some succulents (Crassulaceae, Aloe), large, transparent, water-filled cells develop in the epidermis, acting as light filters and heat insulators, while simultaneously protecting underlying tissues from overheating (Serebryakova et al., 2006).

Spatial orientation of leaf blades plays an important role. In many plants of open habitats (steppes, deserts, savannas), leaves are not horizontal but vertical or at an acute angle to the sun’s rays at midday. This profile orientation (enantiotropy) allows the leaf to avoid peak insolation: at the hottest time, rays skim the surface, and heating is minimal. A classic example is eucalyptus, whose adult leaves hang vertically, creating a “shadeless” forest (Serebryakova et al., 2006). A similar phenomenon is observed in many grasses and acacias.

Another approach is to reduce the leaf blade area. Xerophytes and thermophytes often have small, narrow, or strongly dissected leaves (pinnately or palmately dissected). Reducing leaf size decreases heat absorption and improves convective heat exchange by increasing the relative proportion of edges. In some desert plants (e.g., saxaul), leaves are reduced to scales, and assimilation is taken over by green stems (phylloclades) (Serebryakov, 1962).

Anatomical traits of heat tolerance are closely related to xeromorphosis. In plants of hot habitats, the following are observed:

Along with permanent morphological traits, temporary adaptations also exist. In many tropical and subtropical plants, during the hottest hours, thermonasty is observed — folding of leaf blades or turning them edgewise to the sun (e.g., in mimosas, some legumes). This “daytime sleep” reduces heating. In some acacia species, during drought and heat, leaf shedding occurs (deciduousness), sharply reducing heat absorption. Finally, plants can use transpirational cooling: intensive water evaporation through open stomata lowers leaf temperature by 5–10 °C compared to ambient air (Graham et al., 2014). However, this mechanism works only when sufficient soil moisture is available.

In arid hot regions, a unique group of plants has evolved — succulents (stem and leaf succulents). They do not so much avoid overheating as store water in specialized water-storage parenchyma (Serebryakov, 1962). The thick, fleshy stems or leaves of succulents have high heat capacity and heat up slowly. In addition, many cacti, euphorbias, and crassulaceans exhibit a special type of photosynthesis (CAM metabolism), in which stomata open only at night and remain closed during the day — this sharply reduces water loss and eliminates daytime overheating. Light coloration, ribbed stem shape (increasing heat dissipation area), and dense pubescence or spines that create shade complement the adaptive complex (Serebryakova et al., 2006; Chen et al., 2024).

Thus, morphological and anatomical adaptations to overheating constitute an integrated system including changes in color, shape, and orientation of leaves, thickening of covering tissues, sunken stomata, development of sclerenchyma, and, in extreme cases, transition to a succulent appearance. All these traits allow plants to survive and maintain photosynthetic activity under conditions of intense insolation and high temperatures.

The most general approach to analyzing adaptations to cold was proposed by the Danish botanist C. Raunkiaer (Raunkiaer, 1934). He based his classification of plant life forms on one trait — the position of renewal buds relative to the soil surface during the unfavorable season (cold or dry part of the year). The higher and more exposed the buds, the more they are subject to low temperatures and drying winds; the deeper they are hidden in the soil or under snow, the better protected they are.

One of the key anatomical adaptations to cold is the formation of thick protective coverings that reduce heat loss and prevent ice from penetrating living cells. In perennial woody and shrubby forms in cold climates, periderm (cork) develops early, replacing the epidermis. Cork tissues (phellem) consist of dead, air-filled cells that are excellent heat insulators. Particularly thick cork layers develop on roots and the lower part of stems in shrubs, as well as in underground perennial organs (Serebryakova et al., 2006).

In shrubs and dwarf shrubs, xylopodia play an important role — perennial underground woody stems that connect aboveground axes into a single system. Xylopodia have thick, strongly suberized bark and developed wood, which provides them with high frost resistance and longevity (up to several decades). Aboveground shoots, living only a few years, are much thinner and less woody (Serebryakov, 1962). This differentiation into a “protected underground skeleton” and “replaceable aboveground assimilating shoots” allows the plant to remain viable throughout a long winter.

In winter, when water in the soil freezes and roots cannot absorb it, evergreen aboveground organs (needles, leaves of lingonberry, heather) continue to lose water through transpiration. Physiological drought occurs. Therefore, evergreen plants in cold regions acquire pronounced xeromorphic traits that reduce water loss. These traits include (Serebryakova et al., 2006):

A characteristic example is the leaves of lingonberry (Vaccinium vitis-idaea) and Labrador tea (Ledum palustre), which have a leathery surface, rolled margins, and a waxy coating. In many evergreen Ericaceae (Calluna, Erica), leaves are reduced to small scales tightly pressed to the stem, minimizing the transpiring surface.

In plants that overwinter with aboveground shoots (phanerophytes and chamaephytes), renewal buds are protected by dense, often suberized or resinous bud scales. Bud scales are modified leaves that lack chlorophyll and have thick, sometimes lignified cell walls. Air spaces between the scales provide additional insulation. In some species (e.g., Aesculus hippocastanum), buds are covered with sticky resinous secretions (collecters) that seal gaps and prevent moisture and pathogen entry (Serebryakov, 1962).

In shoots that die back completely in winter (hemicryptophytes and cryptophytes), anatomical protection is reduced to the presence of well-developed mechanical tissue (sclerenchyma) in the basal parts, which protects the tillering zone and underground organs from compression by ice. Underground organs (rhizomes, bulbs, tubers) have dense covering tissues (phellem), and their parenchyma often stores reserve substances (starch, inulin) and water, which also reduces the risk of freezing.

In frost-tolerant species, certain anatomical features of water-conducting elements are observed. Compared to tropical species, in woody plants of temperate and cold climates, vessels (tracheae) are generally shorter and have smaller diameters. This reduces the risk of water column rupture and air embolism formation during freezing and thawing of water in the xylem. In addition, the wood of many conifers (spruce, pine, larch) contains widespread pits with a torus, which can close when air bubbles enter, localizing damage (Mauseth, 2016). Although these mechanisms also have a physical nature, their anatomical basis (structure of cell walls) belongs entirely to morphological and anatomical adaptations.

To illustrate the diversity of anatomical adaptations to low temperatures, representatives from different climatic zones can be compared:

Thus, anatomical adaptations to cold include thickening of covering tissues, development of sclerenchyma, sunken stomata, small mesophyll cell size, and features of conducting element structure. All these traits are aimed at water conservation, mechanical protection of tissues, and prevention of the damaging effects of ice crystals. Together with biomorphological strategies (life forms, prostrate shrubs, cushions), they ensure plant survival in the harshest cold climates.

Psychrophytes (from Greek psychros — cold, phyton — plant) are plants adapted to living under constantly low temperatures (generally below +10 °C during the growing season) and capable of withstanding prolonged frosts. These include species of arctic and antarctic tundras, high mountains (alpine and nival belts), as well as some bog and heath formations. Psychrophytes are found among all life forms, but chamaephytes and hemicryptophytes predominate (Raunkiaer, 1934; Yakovlev et al., 2006).

Thermophytes (from Greek therme — heat) are plants whose life optimum is in the range of high temperatures (+25…+40 °C and above). They do not tolerate prolonged cooling and are damaged already at 0…+5 °C. Thermophytes are typical of tropical and subtropical lowlands, hot springs, and strongly sun-heated habitats (deserts, steppes). Among thermophytes, phanerophytes (trees, shrubs) and therophytes (annuals) predominate, as well as succulents (Raunkiaer, 1934; Chen et al., 2024).

It is important not to confuse psychrophytes and thermophytes with groups distinguished by their relation to water:

Thus, the concepts of psychrophytes and thermophytes reflect adaptation to the temperature factor, but specific morphological and anatomical traits (degree of xeromorphy, succulence, growth form) are determined by the interaction of temperature with water regime and other conditions. The following section considers the applied significance of knowledge about these adaptations for agriculture and breeding.

In regions with pronounced cold or dry seasons, natural selection favors species that can complete their life cycle within a short favorable period. Based on lifespan and the nature of seasonal development, several phenological groups are distinguished:

Raunkiaer’s classification, discussed in section 3.1, essentially describes a phenological strategy: the deeper the renewal buds are hidden, the longer the unfavorable period the plant can survive. However, the phenological aspect is complemented by the timing of bud initiation.

In perennial herbs and shrubs, renewal buds are initiated as early as mid-summer to early autumn, long before cold weather arrives. By autumn, they are fully formed and have either bud scales (in phanerophytes and chamaephytes) or are located in the axils of dying leaves or in underground organs. A delay in bud development (so-called endogenous diapause) prevents their premature germination during thaws. This diapause is controlled by photoperiod and temperature (Serebryakova et al., 2006).

Leaf fall is a striking example of a phenological adaptation with a clear anatomical basis. In deciduous trees and shrubs of the temperate zone, at the end of summer or beginning of autumn, an abscission layer (abscission zone) forms at the base of the petiole. This layer consists of small, thin-walled, easily separable cells. In it, dissolution of middle lamellae (pectins) occurs under the action of enzymes (cellulase, polygalacturonase). Vascular bundles (xylem and phloem) are blocked by cork tissues (lignin and suberin deposited in cells adjacent to the abscission layer), blocking water and assimilate transport. The leaf separates, and a protective leaf cushion (leaf scar) is formed in its place, covered by periderm. Leaf fall sharply reduces the transpiring surface in winter, preventing dehydration and reducing the risk of mechanical damage to branches by snow and ice (Mauseth, 2016).

In some evergreen plants (e.g., many conifers), leaves live for several years (2–5 or more), and their shedding occurs gradually, without the formation of a single abscission zone. However, they also have leaf senescence mechanisms associated with changes in cell wall anatomy and vessel blockage.

Within a single species, phenological races (ecotypes) may exist, differing in flowering and ripening times. For example, in the same species of maple or oak, populations from northern regions flower and shed leaves earlier than southern ones. This is due to genetically fixed differences in sensitivity to photoperiod and temperature. When introducing plants from southern to northern regions, they may not have time to complete vegetation before frost, and conversely, northern plants in warm autumn conditions may enter dormancy prematurely (Graham et al., 2014). Therefore, knowledge of phenological adaptations is important for introduction and breeding (see section 6).

In many perennial herbs (hemicryptophytes and cryptophytes), phenological adaptation to cold is inextricably linked to the formation of specialized underground storage organs: rhizomes, tubers, bulbs. These organs have anatomical features that ensure long-term viability: presence of periderm, large amounts of parenchyma with reserve substances (starch, inulin, fats), and dormant buds. In spring, mobilization of reserves allows the plant to quickly regrow aboveground shoots, sometimes even before the soil has completely thawed (snowdrops, scillas, corydalis). Thus, phenological plasticity complements morphological and anatomical plasticity, creating an integrated system of adaptation to temperature regime.

One of the main tasks of modern breeding is the creation of varieties resistant to extreme temperatures. Wild species — psychrophytes (for cold tolerance) and thermophytes (for heat tolerance) — are often used as donors of traits. Selection is carried out using clearly expressed morphological and anatomical markers:

The use of such morphological markers speeds up and reduces the cost of breeding, as it allows mass selection in field conditions without complex physiological tests.

When transferring plants from one climatic zone to another, their potential adaptability must be assessed in advance. For this purpose, the biological spectra of life forms of the donor and recipient regions are compared (Raunkiaer, 1934). For example, if phanerophytes predominate in a tropical region, their transfer to the temperate zone without special protection is impossible. Conversely, species with a high proportion of chamaephytes and hemicryptophytes have a better chance of acclimatization. When introducing tree species, attention is paid to the presence of dormant buds and the ability for sympodial renewal (Serebryakov, 1962).

Knowledge of phenological races (ecotypes) allows selection of forms for sowing in each region that complete vegetation before frost. For example, when moving spring wheat northward, early‑ripening ecotypes are used, in which the flowering and grain‑filling phases are shifted to earlier dates. This is directly related to inherited sensitivity to photoperiod and temperature gradient.

Agronomic techniques based on plant morphology can significantly reduce damage from extreme temperatures:

Biological spectra of life forms of natural vegetation serve as reliable climate indicators. For example, a high proportion of chamaephytes in the flora indicates a cold and windy summer (tundra or high mountain conditions), while a predominance of therophytes indicates a hot, dry summer (Mediterranean type). Agronomists and land‑use planners use these data to assess the suitability of a territory for cultivating certain crops and to select farming systems (Yakovlev et al., 2006).

In addition, anatomical traits of wild relatives of cultivated plants can indicate their potential resistance. For example, the presence of dense pubescence and a thick cuticle in a wild species of wheat or barley indicates its adaptation to drought and heat, which can be used in breeding.

Although this article does not cover biochemistry, it is worth mentioning that knowledge of morphological and anatomical target traits allows setting objectives for genetic engineering. For example, to increase drought and heat tolerance, genes controlling the development of pubescence, waxy coating, stomatal apparatus, as well as genes responsible for cutin and suberin synthesis, are introduced into plants. Using gene editing (CRISPR/Cas), rice, wheat, and tomato lines with altered stomatal density and improved cuticle have already been obtained, which increased their resistance to heat stress (Chen et al., 2024).

Thus, fundamental knowledge of morphological and anatomical adaptations of plants to temperature regime is the basis for practical crop production, breeding, and plant cover conservation. It allows not only to explain why plants survive in extreme conditions but also to actively use these mechanisms to increase the productivity and sustainability of agroecosystems.