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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5217_Библиотеки_им_академика_М_И_Перельмана.pdf
Figure 5.1: Changes caused by drought in plants above and below ground.
Chapter 5 Impact of drought stress on the medicinal and aromatic plants’ biochemistry 203
Studies have shown that drought affects the physiological, biochemical, and morphological properties of medicinal plants both qualitatively and quantitatively, but the effect varies, depending on the genotype, and irrigation regime and characteristics. Considering global climate change, it becomes clear how important it is to include
drought-resistant medicinal plant genotypes in future selection and breeding programs.
Drought stress causes oxidative stress in plants. During periods of insufficient
water, light-chlorophyll interactions in the chloroplast cause oxidative stress in the
vegetative tissues of the plant. Plants have a complex defense mechanism consisting
of lipid-soluble and membrane-bound antioxidants, water-soluble antioxidants, and
enzymatic antioxidants against the harmful effects of oxidative stress. Plants exposed
to drought stress can combat oxidative stress as a result of the activation of some or
all of their antioxidant defense systems [38-41]. Increasing the synthesis and accumulation of osmolytes such as proline, glycine betaine, and polyamines are other defense
mechanisms that reduce osmotic stress in plant cells [42]. Protein content decreases
in plants under drought stress, which is associated with increased activity of protein
degrading enzymes and accumulation of free amino acids such as proline [43]. If the
balance between the production of free radicals and the plant antioxidant defense
system is disrupted, oxidative stress destroys cell membranes and other organelles
[44]. However, long-term and sometimes even short-term stress can cause visible
damage to plants and even death if the capacities of defense mechanisms are [45, 46].
The effects of drought on plants and the defense mechanisms formed by plants are
summarized in
Figure 5.2.
Figure 5.2: Negative effects and adaptations of plants to drought stress, modified from Seleiman et al.
[47]; (–) means decrease and (+) means increase.
204 Gülen Özyazıcı and Negar Valizadeh
There are more than 200 species of the Thymus genus, which is an important medicinal and spice plant, and these species respond differently to water deficiency. Moradi
et al. [48] conducted a study to determine and physiologically evaluate the response
of eleven populations of various species of thyme (Thymus daenensis, T. kotchyanous,
T. vulgaris, T. serpyllum, T. capitata, and T. zygis) to water deficit stress. The findings
showed that populations had significantly different root/shoot ratios under drought
conditions, with leaf water potential decreasing from −3.4 bar in irrigated plants to
−10.5 bar in droughted plants. Moradi et al. [48] found a significant negative relationship between water content and water potential and determined that T. serpyllum
was more resistant than other thyme species and that the Spanish population of
T. vulgaris was susceptible.
5.4 Effect of drought or water deficiency on secondary metabolites of medicinal plants
Droughts occur annually in many parts of the world and often cause significant damage to crop production. Drought, one of the important abiotic stresses, is known to
increase the amount of secondary metabolites in plants. On the other hand, drought
stress can cause oxidative stress due to its formation. In order to protect against the
harmful effects of active oxygen species, plants have developed a complex antioxidant
system that includes enzymatic antioxidants and nonenzymatic antioxidants. Accumulation of secondary metabolites is known as a defense mechanism of plants, and
Figure 5.3: Chemical structures of some common secondary metabolites produced in medicinal and
aromatic plants.
Chapter 5 Impact of drought stress on the medicinal and aromatic plants’ biochemistry 205
plants can respond and adapt to water stress by changing their cellular metabolism
under stress conditions [49]. The chemical structures of some common secondary metabolites produced in medicinal and aromatic plants are presented (Figure 5.3).
In studies conducted with medicinal and aromatic plants, it is thought that the
genetic characteristics of the plant, and its anatomical and morphological development stages, as well as stress factors play a role in the formation of its phytochemical
composition, unlike traditional plant products [50]. Since stress-related metabolism
largely affects all other metabolic events, it is known to also affect the synthesis and
accumulation of secondary metabolites [51].
Many studies have shown that drought increases the amounts of secondary metabolites in a wide range of plant species, including hesperidin in Rehmannia glutinosa
[52], indole alkaloid in leaves and roots of Catharanthus roseus [15], rutin, quercetin,
betulinic acid in H. brasiliense Choisy [11], and saikosaponin a and c in Bupleurum chi-
nense DC [13]. In S. miltiorrhiza, whose roots are widely used in traditional Chinese
medicine, it was determined that drought stress reduced both shoot and root dry
weight and water stress reduced the yield of tanshinone IIA; on the other hand, the
contents of other active components, except rosmarinic acid, and the yield of salvianolic acid B increased under water stress [53].
Drought stress in the early vegetative stages of Spigelia anthelmia, which is used
locally as an anthelmintic, reduced its growth but did not affect the alkaloid content
[54]. Superoxide dismutase and peroxidase antioxidant enzyme activities of Hyoscya-
mus niger increased in root and leaf under water deficit, while hyoscyamine and scopolamine decreased under moderate and severe water deficit. The use of plant-
Figure 5.4: Diagram showing the effect of drought stress on DNA, proteins and lipid modified from
Bistgani et al. [59].
206 Gülen Özyazıcı and Negar Valizadeh
growth-promoting rhizobacteria (PGPR) reduced the negative effect of drought severity on alkaloid abundance [55]. Similarly, studies revealed that water stress increased
the tannin, saponin, and flavonoid content of Bryophyllum pinnatum but decreased
the alkaloid content [56].
In order to increase the quantity and quality of secondary metabolites that determine the economic value of medicinal plants, the water content available to plants
should be kept under control. Studies have reported that appropriate levels of water
stress increase secondary metabolite content in medicinal plants [43, 57, 58] (Figure 5.4).
The amount and composition of essential oils are affected by various abiotic stress
factors. In Dracocephalum moldavica L. plants exposed to different drought treatments, the highest essential oil content of 0.58% was detected in the moderate drought
treatment [60]. When three different irrigation regimes were applied to two different
Salvia species, it was found that the highest essential oil content (2.20%) was in the
moderate drought application [61].
Some phytochemicals can only be seen as a product of the response mechanism
under stress conditions, depending on their synthesis. For example, Kılıç and Kaya
[62], although α-pinene, sabinene, limonene were not detected in basil essential oil
under normal conditions, they were determined at different rates under drought
stress. However, it has been reported that some plants respond differently to drought
stress; the oil yield of Lavandula latifolia and Salvia sclarea decreases and it has no
Chapter 5 Impact of drought stress on the medicinal and aromatic plants’ biochemistry 207
effect on Mentha piperita, Salvia lavandulifolia, Thymus capitatus, and Thymus mastichina [63]. In general, drought stress or water deficiency can affect plant growth, vola-
tile oil content, and components, depending on the species (Table 5.1). The appearance
of Melissa officinalis and Rosmarinus officinalis plants is given in Figures 5.5 and 5.6.
Table 5.1: The effect of non-drought stress and severe stress on essential oil percentage (%).
Scientific
name
Cymbopogon nardus . . . – [21]
Melissa officinalis . . . . [64]
Lavandula latifolia . . – – [63]
Mentha piperita . . – –
Salvia lavandulifolia . . – –
Salvia sclarea . . – –
Thymus capitatus . . – –
Thymus daenensis . – . . [65]
Origanum vulgare . – – . [66]
Tagetes minuta . . . . [26]
Ocimum basilicum – . . . [67]
Ocimum × africanum – . . .
Ocimum americanum . . .
Rosmarinus officinalis . – . . [68]
Dracocephalum moldavica . . . [69]
Control Mild
drought
stress
Moderate
drought
stress
Severe
drought
stress
Reference
Sage (Salvia officinalis L.) is a species sensitive to drought, and severe drought can cause
a decrease in the activity of enzymes involved in the biosynthesis of phenolic compounds
[70]. The main components of the essential oil of peppermint, one of the most important
and widely used medicinal and aromatic plants worldwide, show different responses to
drought stress at different growth stages (Figure 5.7). The essential oil content of plants
exposed to mild water stress (60 ± 5% field capacity) increases, while moderate water
stress (40 ± 5% field capacity) significantly reduces the essential oil content [71]. Drought
stress decreased the growth, seed yield and yield components, total fatty acid content,
and especially petroselinic acid content of cumin (Carum carvi L.), while it increased the
essential oil components [72].
The effects of drought stress on alkaloid, glaucoside, and glucosinolate components
of some medically and economically important plants vary, depending on the plant species and the type of component. When plants are exposed to various stress conditions,
alkaloid concentrations often increase. This is well known, probably due to the passively
Figure 5.5: Melissa officinalis.
Figure 5.6: Rosmarinus officinalis.
208 Gülen Özyazıcı and Negar Valizadeh
increased biosynthesis rate caused by greatly elevated NADPH concentrations in
stressed plants [73]. In Papaver somniferum plant, drought stress caused an increase in
the concentration of alkaloids (morphine, codeine, and papaverine) [74]. In response to
Figure 5.7: Peppermint (Mentha piperita) and sage (Salvia officinalis).
Chapter 5 Impact of drought stress on the medicinal and aromatic plants’ biochemistry 209
water scarcity, plants usually change their secondary metabolism, which is leading to an
increase in nitrogen-containing compounds such as alkaloids. Morphine biosynthesis depends on nitrogen availability and is therefore affected by drought conditions through
changes in enzymatic activity and metabolic pathways. Moderate drought stress can increase morphine accumulation by upregulating essential biosynthetic genes, while severe
stress can inhibit growth and reduce overall alkaloid yield. In addition to this, nitrogen
metabolism plays an important role in alkaloid production because nitrogen-containing
precursors such as tyrosine and ornithine are essential for morphine biosynthesis. The
interaction between drought stress and nitrogen availability suggests that optimizing
water and nutrient management may be a strategy to increase alkaloid production in
Papaver somniferum) under drought-prone conditions.
poppy (
In the plant Withania somnifera, different effects were observed on different secondary metabolites. Withanolide compounds increased, while withanolide and 12deoxywithastramonolide levels decreased. For all that, the concentration of Withaferin A compound increased in the roots and leaves. An increase in asiaticoside and
madecassoside components was observed in the leaves of Centella asiatica under low
temperature and drought conditions. In cassava (Manihot esculenta) plant, drought
stress appears to cause a strong increase in the concentration of cyanogenic glycosides, especially in tuberous roots and leaves. In Lupinus angustifolius (narrow-leaved
lupin), a significant increase in quinolizidine alkaloids in seeds occurred. In Eucalyp-
tus cladocalyx, drought conditions increased the levels of cyanogenic glycosides in
dried leaves and oil content. A strong increase in the concentration of indole alkaloids
was recorded in Catharanthus roseus. In general, it appears that drought stress significantly increases the production of certain secondary metabolites in these plants, and
this may be related to the defense mechanisms of the plants [75]. In Brassica napus,
drought stress caused a large increase in the concentration of glucosinolates [76]. In
rapeseed, the biochemical properties of the seed were greatly altered in plants exposed to drought during flowering. Drought during the early vegetative and flowering
stages caused a slight increase in seed protein concentration. Depending on its timing,
significant effects of drought stress were observed on the accumulation of secondary
Figure 5.8: Safflower (Carthamus tinctorius) flower.
210 Gülen Özyazıcı and Negar Valizadeh
metabolites (i.e., phenolics and glucosinolates) that are of great importance for rapeseed meal quality. Achnatherum inebrians is a grass species that produces alkaloids
such as ergonovine and ergine (lysergic acid amide). These alkaloids, which are classi
fied as nitrogen-containing compounds, play an important role in plant defense by
deterring herbivores and insects. Ergonovine and ergine biosynthesis is affected by
environmental factors such as drought stress and nitrogen availability. In response to
drought stress, the plant can increase alkaloid production, resulting in increased accu
mulation of these nitrogenous metabolites [77]. However, the extent of alkaloid production depends on the balance between nitrogen uptake and stress adaptation. Understanding the relationship between drought, nitrogen metabolism, and alkaloid
production in
Achnatherum inebrians is important to manage its ecological impact
and potential applications in biotechnology.
The biosynthesis and accumulation of active substances such as silymarin, found
in milkthistle (
Silybum marianum
(L.) Gaertn) seeds in plant tissues, are highly affected by environmental conditions [78]. Moderate and severe drought stress increases silymarin content, which is attributed to more silymarin, silybin, isosilybin,
and silychristin content in stressed plants, while silydianin content decreases.
Safflower (Carthamus tinctorius L.) is an important plant both as a medicinal and
oil plant (Figure 5.8). The pharmacological properties of the safflower plant are
mainly due to its ability to accumulate some active secondary metabolites, mainly
phenolic and flavonoid compounds. Drought stress decreases the seed yield and oil
content of the safflower plant; on the contrary, the amounts of vanillic and caffeic
acids, and rutin and quercetin in flower and seed extracts increase. The presence of
these compounds causes an increase in antioxidant capacity [
Carthamus tinctor-
79].
ius is a multipurpose plant that can grow in arid and semiarid environments due to
its tolerance to drought stress, salinity, and low and high temperatures.
Although
saf-
-
-
Chapter 5 Impact of drought stress on the medicinal and aromatic plants’ biochemistry 211
flower can grow in arid and semiarid climates, drought stress reduces plant height
and yield, leaf chlorophyll content and leaf area, photosynthetic rate, yield components, oil content and yield, and fatty acid composition. Increased root/shoot ratio and
root growth are some of the drought adaptation mechanisms of safflower.
5.5 Different approaches to mitigate the negative effects of drought stress on plants
Drought stress is an inevitable factor that exists without obvious warning in various
environments that inhibits plant biomass production, quality, and energy. Its cumulative and subtle effect seriously affects plant morphological, physiological, biochemical, and molecular characteristics with negative impact on photosynthetic capacity.
Plants that are coping with water limitation develop a variety of complex resistance
and adaptation mechanisms, including physiological and biochemical responses that
differ, depending on the species level. The strategies adopted by water deficient plants
are reduction in transpiration loss by changing stomatal conductance and distribu
tion, leaf curling, change in root-shoot ratio, increase in root length, accumulation of
solutes, and osmotic and hormonal regulation. Planting time, plant genotype, and soil
and nutrient management practices can help reduce yield losses in plants exposed to
drought stress. Nevertheless, the use of drought-tolerant transgenic plants is the most
popular approach to reducing drought stress.
Various strategies can be used for drought stress resistance. Among these, preplanting or post-planting chemical applications, plant growth regulators (PGR), and
bacterial inoculations are of great importance to increase drought resistance in different growth stages of the plant. Another application is the use of nanoparticles (NPs).
NPs with sizes ranging from 1 to 100 nanometers have high surface energy and surface-to-volume ratio, which makes them highly efficient for many purposes by enhancing their other biological activities [80, 81]. Nanoparticles have emerged as a promising
tool to reduce the negative effects of drought stress in plants. Due to their physicochemical properties, nanoparticles can enhance plant growth, improve water use efficiency, and regulate stress-related biochemical pathways. Nanoparticles can enhance
water exchange, increase root growth, and improve water absorption efficiency. Silicon (SiO₂) and carbon-based nanoparticles increase the water retention capacity of soil
and reduce water loss due to drought. It regulates stress-related hormones; silver (Ag)
and zinc oxide (ZnO) nanoparticles balance the drought response by affecting abscisic
acid (ABA) and cytokinin levels. Silicon (Si) nanoparticles prevent premature aging by
reducing stress-induced ethylene production. Copper (Cu) and selenium (Se) nanoparticles increase antioxidant enzyme activity (SOD, CAT, and POD). Iron oxide (Fe₃O₄)
nanoparticles improve proline, glycine, and betaine synthesis, which helps maintain
cell hydration. Nanoparticles are applied as seed priming, controlled nutrient release
-
212 Gülen Özyazıcı and Negar Valizadeh
nanofertilizers in drought stress management. Ghavam [82] reported that silver NP applications in Thymus daenensis and Thymus vulgaris L. increased the ability to withstand drought stress and increased germination and root length in saline conditions
(200 mM). Titanium dioxide (TiO
the negative effects of drought [83]. Application of titanium dioxide (10 ppm) to the
leaves of Dracocephalum moldavica L., growing under drought conditions, resulted in
increased shoot dry biomass and essential oil content [84].
) NPs applied to Verbascum sinuatum plant alleviated
2
5.6 Case studies
Due to a worldwide water shortage and the rising use of herbal medicines, studies on
drought stress affecting the composition of secondary metabolites in medicinal plants
is crucial. Even though drought stress is generally considered as the main factor responsible for serious yield losses in agricultural production, this is different for medicinal and aromatic plants [85].
Total flavonoid and rosmarinic acid contents were not affected in Melissa officinalis
L. and Thymus vulgaris L., grown under different water stress conditions. On the other
hand, Melissa officinalis L. plants gave lower biomass weight under low water stress [86].
Tatarai et al. [16] investigated the effects of drought stress on two-year-old Thy-
mus citriodorus plants by treating them with different concentrations of polyethylene
glycol (PEG-6000) (0%, 2%, and 4%) for 15 days under greenhouse conditions. Thymus
citriodorus exhibited a morphological drought avoidance mechanism by reducing
shoot fresh weight to protect root system development, which enhanced root absorptive capacity and sustained plant growth. Additionally, thyme plants minimized tissue
dehydration through stomatal closure and improved root water uptake. Regarding essential oil composition, the levels of geraniol and diisobutyl phthalate increased
under drought stress, while pseudophytol content decreased. Although thymol was
not the main component under control or mild stress conditions, its content increased
under severe drought stress. Furthermore, carvacrol levels rose by 31.7% under severe drought stress compared to control plants.
Lotfi et al. [87] investigated the effects of drought stress on morphological traits,
proline accumulation, soluble carbohydrates, and yield to determine the drought tolerance threshold of tarragon (Artemisia dracunculus L.). Stress treatments were applied
at four levels: T1 (100% field capacity), T2 (80% field capacity), T3 (60% field capacity),
and T4 (40% field capacity). The study revealed that drought stress significantly impacted morphological traits, flowering shoot yield, proline accumulation, and soluble
carbohydrate content. As drought stress increased, plant height, crown diameter, leaf
length, leaf width, leaf surface area, stem diameter, longest lateral shoot length, root
length and development, shoot yield, and dry leaf yield decreased. The highest values
for plant height, crown diameter, leaf length, leaf width, leaf surface area, stem diame-