Author: Éva Zámboriné Németh

Role of the water in plant life

The water is one of the basic factors of plant life, it influences the distribution and production of species. The role of water in plant life is multiple. It is the fundamental medium of metabolic processes; it is the largest part of the cells’ cytoplasm. Green plants obtain hydrogen from dissociation of water, thus, water is also one ultimate molecule for CO₂ assimilation. It provides a solvent and transfer media for most constituents of a living organism, assures the turgidity of protoplasm which is essential also for a solid status of tissues and organs. Osmotic balance is maintained by water storage and transfer. The water column in trachea enables the uptaking of nutrients by the root. Transpiration through the stomata on the leaf surface of the plants contributes to keeping an optimal temperature of the body.

In general, it is obvious, that changes in water supply would result in changes of many processes in plant life. Modifications may occur in biomass production, often together with changes in proportion of vegetative and generative organs or proportion of root and overground shoot system as well as tissue structure (e.g. leaf thickness).

Both in natural habitats and in agricultural production, the water available for the plant depends on the water resource of the soil. It is expressed as water capacity value. Soil water capacity (SWC) shows the water storage potential of the soil. Wilting point is the second important value which shows a quantity of water of the soil practically not available for the plant because of its strong adhesion to the chemical particles. It is also called: “dyed water”. If the soil contains only the water amount of the wilting point, this results in permanent decay of the plants. Difference of these two values however, gives the usable water capacity (WC). By mathematical formula:

SWC = WC – Wilting point

In most common cases the water capacity is around 50-70%.

The water content of a given soil can be determined by different methods. One of them is the gravimetric method based on the potential of the soil to retain the water, where the tensiometric power (pF value) will be determined. Another method is the water content based on the electronic conductance of the soil, etc. Detailed knowledge on these methods can be found in books about principles of irrigation. Water content should always be measured in the depth where the majority of the roots of the given crop is growing (rooting zone).

Beside water capacity, the potential of the soil for retaining and/or supplying the water is also essential. If this latter one is weak, the plant may require a double amount of water compared to a crop growing on soils with more advantageous supply potential.

Necessary amount of water (mm) for some characteristic medicinal crops in a year with moderate precipitation in Hungary

- : cultivation not advised

 Water demand of plants

Under actual circumstances, water demand of the crop depends on several factors.

Origin: It is a basic determining factor for the water demand of any species. A lot of different forms of acclimation to water supply are known from the desert plant till the marshland species. In Central Europe, species from extreme areas do not play any role in the agricultural practice. Among wild growing plants we can find some but not many examples, like the xerophyte houseleek (Sempervivum tectorum) or the hydrophyte bogbean (Menyanthes trifoliata) which possess elongated stems and leaves are floating on the surface of water. The mentioned species are rare and under protection of law.

The original growing area of the plant basically determines the body construction, its morphological, anatomical structure, metabolism (C₃-C₄ species). Both quantitatively and qualitatively optimal drug production can be achieved practically under natural circumstances which are most similar to the original ones of the target species. Tolerance of the species is rather different from each other, nevertheless an environment largely different from the original one may always cause stress effect for the plant and this fact presents itself most frequently in a decreased biomass production. An additional water supply is necessary primarily in case of species which originate from wet or humid habitats and do not tolerate dry condition on the long run. Also drought tolerant species may have characteristic life periods (e.g. germination) when they are relatively susceptible and need more water. This must be taken into consideration in the cultivation practice.

Response of the plant to the increasing dosages of water can be described by a normal (optimum) function where each species may have a different optimum point. Moreover, the optimal amount of precipitation may differ in case of the same plant depending on the ontogenetic phases. Thus, water demand for the vegetative growth of the crop, for high green mass or for flowering and accumulation of special substances (see later on) usually is different. During introduction of wild growing plants into the agriculture, it is useful to examine and determine the optimal water requirement of each phase. However, this is not a simple task and is only possible in several steps in vitro and in vivo.

Development of tansy ( Tanacetum vulgare ) in soils of different usable water capacity (WC)

Ontogenetic phase : Water requirement of the same species or even of the same individual depends on the current status of its growth and development. Each plant cultivated in our region needs water for germination. Besides, there is usually an increased demand for water in the intensive growth periods and flower differentiation. To assure the potential for two or more harvests in the same year (regeneration of the plant), water supply is also essential. In general, it can be established that the most important and characteristic irrigation periods –although depending on species to a large extent- are the following:

Similarly, it is also advisable to mention the characteristic periods when intensive irrigation may harm and excess water may damage the crop. Irrigation during flowering may result in a sudden decay and falling down of the flowers, retard or set back pollination and fertilization, contribute to spreading and attacking of diseases. Crops providing fructus (fruit) drugs (e.g. milk thistle, borago, caraway) must not be irrigated after the formation of the fruits and during ripening. It is not a good idea to irrigate the crop providing herba or folium (shoot and leaf) drugs (e.g. lemon balm, basil, peppermint) 8-10 days before the planned harvest, because their increased water content might lead to increased costs of transport, to an easy deterioration of fresh material and also drying may be longer and more expensive.

The differences in yield and content of active ingredients due to differences in timing of irrigation periods can be observed in the following experiment with estragon. The water supply in critical periods as (development of side branches, appearance of buds and after the first cutting) resulted in multiple yields.

Plants and plant stands are able to adapt themselves to a certain extent to changes in water availability. As an effect of lack of water or minimum supply the abundance of root system, the area for water uptake starts to grow. In other cases leaf area is decreasing in order to minimize transpiration. Plants developed under continuous good water supply in their juvenile phase are more sensitive to drought stress later on than plants which could acclimatize to bad circumstances, weak water supply.

In case of mainland species optimal water uptake is in tight connection with the size and development of root system, its health and physiological status. Although formation of root system is basically a species specific trait, it is also influenced by habitat, soil type and height of soil water level. There are some agrotechnical procedures too, which are able to modify the morphology of the roots and through this its capacity for water uptake. It is known e.g. that after propagation by seed sowing a deeply growing main-root system is developing while after propagation by cuttings the developing roots are thinner and heavily branching. It has been observed in case of angelica that the formation of the roots depends also on the fact whether the plant has been propagated by direct sowing or seedling raising. In the former case the roots were longer, growing deeply in the soil and having a small number of side roots while in the latter case the roots were growing shallow, exhibiting mainly roots of similar thickness and size, which is more advantageous for an effective harvest.

Many other factors should contribute to and influence the water demand of the plant, which are usually in connection with each other. Besides the mentioned biological needs, environmental factors do play a significant role. They can be grouped according to the following principles:

Different formation of the root system of marshmallow due to propagation method

a./ Factors in connection with the characteristics of the growing area: This is the soil type which determines its water capacity and water retaining potential. This is also the relief which influences the efficacy of the precipitation and the temperature, warming of the field. The growing habitat determines also the number of sunny hours, the irradiation and through these factors indirectly the temperature and transpiration.

b./ Weather conditions: the weather conditions of the same time period at the same growing area play an important role. It includes amount, distribution, intensity and form of precipitation but also presence and strength of wind, alterations in temperature have an influence to the water supply.

c./ Technological factors: water demand and potential for the utilization of available water may be effected by the production technology, too. Propagation method (see above), propagation time, thickness of the stand (row and plant distance), intensity of cultivation (number and frequency of harvests, nutrient supply) are all such factors. Protected cultivation, cultures in greenhouses and other covered surfaces have a principally divergent demand and technological background. However, it is still very rare in medicinal plant production.

According to the above mentioned factors and their interactions, the water concentration of the tissues is changing, in consequence the turgidity stage of the plant will also be modified. Modifications include opening of the stomata, intensity of transpiration, proportion of photosynthesis and respiration, the so-called net photosynthesis. Through these modifications alterations happen in the water requirement of the plant which should be followed by appropriate irrigation.

Water demand of medicinal plant species

Information on the consequences of water supply among medicinal plants are often contradictious, which indicates a complex problem. Formerly, it has been generally accepted that a more abundant watering results in a higher vegetative mass but less effective to the accumulation of active compounds. Today it seems to be a general assumption that optimal water supply should contribute both to a higher biomass and at the same time to a higher production of biologically active ingredients.

In Hungary and the surrounding region approximately 50 medicinal plant species are regularly in cultivation and there are more than 100 species collected from wild habitats. Water requirements of cultivated and indigenous wild species have proved to be significantly different. Comparing their water demand based on the so-called W value. It could be established that cultivated species exhibit less extreme requirements compared to the wild growing ones. Cultivated species prefer basically moderate moisture content and there are no ones among them with really xerophytic or hydrophytic properties.

Water requirements of the most significant cultivated medicinal plants are summarized in next table indicating the species where an effective production without irrigation is not likely.

Water requirements of the most frequently cultivated medicinal plant species

Among others, angelica (Angelica archangelica), coneflower species (Echinacea spp.), hop (Humulus lupulus), lovage (Levisticum officinale), mint species (Mentha spp.) and valerian (Valeriana officinalis) belong to the group of species demanding higher amounts of water (humid, wet). There are numerous species known which are principally not among the ones demanding high amounts of water but under the circumstances of the region, their production is hardly profitable without regular irrigation. Species like marigold (Calendula officinalis), oil pumpkin (Cucurbita pepo var. styriaca), hairy foxglow (Digitalis lanata), marjoram (Majorana hortensis) and basil (Ocimum basilicum) belong to this group.

Basil plants supplied by different dosages of water (from left to right: irrigated every 2 ^nd day, irrigated every 4 ^th day, irrigated every 6 ^th day)

As in any other group of plants, intraspecific varieties of certain medicinal species might be different in water use and requirement. Agrotechnology and variety are usually in connection. Unfortunately, in medicinal plant production the spectrum of varieties is rather poor (see chapter 2.). There are not any cultivars especially selected for a more economic water use or drought tolerance. Therefore optimising the genotype (variety) and water supply is not possible today in the medicinal plant production. Nevertheless it is known and justified in scientific experiments that water demand and drought sensitivity of intraspecific taxa (strains, populations, accessions) of the medicinal and aromatic plants may be different from each other therefore this aspect should gain a higher significance in the future.

Effects of water supply

Effects of irrigation on drug yield

 It can be established that the goal of irrigation is assuring a production of optimal yield and drug quality taking into consideration the aspects of profitability and sustainability.

Cultivation of some species is not realizable without irrigation which means that irrigation is a fundamental condition for them. The high importance of irrigation is supported by the fact, that in Israel, on the territory of the Negev desert 1000 hectares of high intensity horticultural crops, - among other those of medicinal plants - have been established and operate prosperously.

The yield of Mediterranean species may be increased even under their indigenous conditions reducing the retarding effects of drought and high temperatures. In Bari (South-Italy) the production of glycorice (Glycyrrhiza glabra) roots could be elevated from 14.6 tonnes to 20 tonnes by irrigation. This plant should be irrigated eight times in Tadzhikistan to assure optimal yields.

It may be an even bigger challenge if the cultivation of moisture/water demanding species is established in permanently arid or semi-arid regions. An example for this is calamus (Acorus calamus) which needs thick and rich soil layer, continuous water saturation.

Diverse response reaction on the water shortage of species characterised by variable water requirement is reflected in the behaviour of basil and savory. According to the experimental results, the biomass of basil dropped to two-third if water capacity of the soil decreased from 70% to 30%. At the same time the production of savory suffered only a 20% loss.

Production of basil and savory under different water regimes of the soil

An optimal water supply is frequently a prerequisite of introduction of intensive agrotechnical methods aiming the increase of yields. Irrigation enables for example the very intensive cultivation system of poppy in Australia. It has been established that under the arid, warm climatic conditions at least 300 mm irrigation water is necessary for appropriate yields. Without this water amount the fertilizers can not be utilised either. Growth of plants is retarded, may stop, accumulation of alkaloids decreases by about 10%, capsule yield is minimised and ontogenesis shortened.

In Hungary it is well known that some spice species (e.g. marjoram, lovage) can be sown to direct place and a uniform juvenile development, necessary density of the stand can only be assured by proper water supply. In lack of regular watering small seeds are not able to germinate, the stand will be very thin. In consequence of this, the danger of weed cover increases and most likely the field must be ploughed out.

Utilization of nutrients by the plants needs a proper moisture content of the soil, too. Own studies proved, that spearmint can take up and optimally utilize 200 kg/ha K₂O in average years but it is able to utilize a higher dosage up to even 300 kg/ha if there is more precipitation during the vegetation year.

Unfortunately, abundant water supply contributes in a plant stand not only to optimal development of the cultivated crop but improves the life circumstances of many weed species, too. Therefore, irrigated cultures necessitate a well established and careful weed control system including proper crop rotation, mechanical and chemical herbicide technologies.

Water demand of medicinal plants depends also on the fact which organs provide the medicinally used drug. Most demanding are the ones giving leaf drugs (folium) like peppermint, lemon balm, plantain. In their fields it may cause a severe problem if the useful water capacity of the soil drops below 50% for a longer period (1-2 weeks). In case of species giving shoot drug (herba) the stem parts are less sensitive for drought conditions but a permanent soil water capacity value below 25% may cause severe harm. Somewhat higher soil water capacity is necessary for species providing root drug (radix). Here, a WC less than 30% may result in development of thinner, less valuable roots and deeper growth of the root system which would make the harvest more difficult. A moderate water demand is characteristic for the species giving flower drugs (flows) or even more for those which supply fruit and seed drugs (fructus and semen). For them, precipitation during flowering or ripening period might have negative effects. However, if soil WC drops below 20% even these crops require irrigation in order of proper biomass production.

Effect of irrigation on the content of active principles

There is much less information about the second most important point which is the effect of water on the accumulation of biologically active molecules in the drugs.

According to the experiences it seems, that the chemical properties of a given effective compound are not the only influencing factors of the response to water supply. It is also very important in which species we examine this response. The level of water supply will namely provoke a diverse reaction e.g. concerning the essential oil content of the hydrophyte Mentha or the xerophyte Lavandula species, respectively.

It has been demonstrated formerly, that an abundant precipitation would lead to a decreased alkaloid content of the Solanaceae species. Further research cleared however, that this decrease might be in connection also with the lack of enough light like in nature, where rainy weather comes together with clouds and decrease of sunny hours. Accumulation of tropane alkaloids hyoscyamine and scopolamine in nightshade (Atropa belladonna) which prefers humid habitats, has been however higher under stress conditions (drought). Based on detailed investigations, it seems to be most likely, that in majority of species the water supply has less influence on the formation and accumulation of alkaloid type compounds. At the same time irrigation may still be very useful when increasing the biomass production and at the same time the total production of active compounds.

Similarly, it has been shown in case of hairy foxglow (Digitalis lanata) that irrigation enhances primarily the drug yield which contributes to the elevation of yield of glycosides, however the concentration of these glycosides remains practically unaffected.

In cycle of essential oil containing species the picture is rather complex. Among the species regularly cultivated in our region, the demand of peppermint (Mentha piperita) seems to be the best known. It has already been established in the 1960s that irrigation would modify the content of essential oil depending on the phenological stage. A regular water supply in this crop is a prerequisite not only for the desired drug yield but also for the essential oil level characteristic of the variety. Our own results show that a proper watering could contribute not only to the growth of overground organs but accelerate the development of the stolons, too.

During a long term experiment in cultivation of hyssop of Mediterranean origin, measurements revealed a significant, negative correlation (r=0.93) between the amount of precipitation in flowering period and the content of essential oil in the flowers.

Connection between essential oil content of hyssop and the amount of precipitation during the flowering period

In case of another perennial spice plant, of estragon, irrigation in the optimal periods during the plant development enhanced both shoot biomass and its essential oil content. According to the studies on Taxus brevifolia it has been shown that the trees produced higher amounts of taxanes (terpene-alkaloid compounds) and abscisic acid among arid growing conditions than other ones under irrigated conditions.

In Mediterranean areas the accumulation level of both diterpenic compounds of rosemary (Rosmarinus officinalis) carnosil and carnosol decreases significantly due to the lack of precipitation and the effect of bright sunshine. Most likely, this decrease occurs in consequence of the enzymatic changes as result of the stressed environment. Similarly, in stress conditions connected to low supply of water the content of diosgenine (steroid-saponin compound) decreases in fenugreek (Trigonella foenum-graecum).

No uniform pattern could be established for the fatty acids and fatty oils, either. Water regime near the optimum level proved to accelerate the biomass of the species but had less effect on the accumulation of fatty oils. It was demonstrated in Iran by investigations on oil flax (Linum usitatissimum ): an irrigation dosage of 60 mm significantly increased the height and number of branches and that of fruits but did not influenced the quality and composition of fatty oil of the seeds.

On agricultural fields of Central-Asia even safflower should be irrigated because of the uneven distribution of natural precipitation.

Accumulation of phenolic compounds is strongly influenced by the weather. Investigating chamomile population in Hungary for several years showed, that varying weather conditions generated larger differences in the flavonoid content of the flowers than geographical and edafic differences among growing areas did.

As summary we could establish that the type and size of response of the plant on drought stress depends both on the species itself and also on the chemical features of the active compound.

Effect of drought stress on the composition of fatty acids in safflower

Other effects of irrigation

Quality of the drug is usually determined not only by the content of active ingredients but also by outside characteristics (e.g. surface, colour) and pollution by organic or inorganic materials. Water supply might have a great influence also on these features, too. Pollution by organic materials may be increased by the fact that water induce kicking up of soil particles for the ground on the surface of the leaves. This may be a severe problem especially in species of lower growth which provide shoot or leaf drug (herba, folium) like marjoram, savory, lemon balm or mint species.

Due to the irrigation, the proportion of different plant organs in the total biomass might change considerably. In medicinal plant production, a change between the proportions of vegetative and reproductive organs (leaves/flowers, fruits) has a big importance. In crops cultivated for their flowers or seeds like chamomile, marigold, borago, mustard, etc. presence and high proportion of leaves or/and stems is unambiguously a drawback.

An abundant supply of water might result in exuberant growth of plant, in a thick plant stand which further on decreases the size and mass of the leaves. An increased mass proportion of the stems in the plant material raises the transport costs and reduces the content of effective materials.

Not only morphological features (well identifiable by sight) can be modified as consequence of regular water supply but also inside composition, tissue structures, anatomical features. In researches connected with valerian are described a modification of palisade parenchym and a connected attenuating of the leaf width as consequence of irrigation technology. The acclimation potential of the plant is shown by the fact that at the same time the diameter of the transfer elements, the area of cross section of the main vessel were also modified.

It is a known fact, not only characteristic of the medicinal plants that irrigation –especially in a tight plant stand- may contribute to the spreading of diseases, fungi and bacteria. It is obviously in connection with the differences in precipitation, that fungi like Sclerotinia, Septoria and some others mean a continuous danger in caraway production in The Netherlands, while there are practically unknown in Hungary. Similarly, in rainy weather the plant protection procedures against Peronospora and Helminthosporium fungi must not be left out while in dry years such interventions may not be necessary at all.

PLANNING OF irrigation

To carry out a proper irrigation procedure assuring optimal development and drug production, in each plant stand and on each field one has to plan the followings carefully:

According to the above described, the time of the irrigation is influenced principally by the weather, by the phonological-physiological stage of the plant. During the day it is advisable to carry out the irrigation in the morning or in the evening hours, especially by rain-type equipments, as in these periods transpiration is weaker and plants are not damaged by the cold drops of water falling on the leaf surface heated by the sun. In the practice however, organisation of the work may be restricted by official labour hours.

Methods and equipments are usually applied as the possibilities allow them. Principally, it should be decided already before setting of the plantation or sowing the crop, if there is any irrigation intended. In this case, cultivation of the planned crop should be included in a crop rotation field supplied by irrigation infrastructure or at least there is some possibility for getting and transferring water to the given field if it would be necessary.

The amount of water for the irrigation may have a large amplitude, as described above. However, generally, it seems to be more advantageous to carry out the irrigation less frequently but with larger dosages of water. A smaller proportion of the larger amount of water will be wasted through transpiration and may contribute to the improvement of micro-climate. Experiences show, that at the same time the maximum amount of the irrigation water should not exceed 30 mm in order to protect the soil structure. The surplus water will flow away or cause damages. It is important to know that the dosage of water corresponding to 1 mm precipitation is 10m³ pro hectare.

Methods of irrigation

Technical carrying out of irrigation in medicinal plant cultivation depends basically on the type and size of production. On large scale fields, in cultures cultivated by so-called “cereal-technology” (anis, poppy, etc.), rain-type irrigation equipments are usually used, rotating or pulled along the field. Equipments may be self-running or transferred ones. In plant stands cultivated for the roots, furrow irrigation can also be applied, and the plants are placed on ridges (e.g. valerian, lovage).

Species cultivated in horticultural firms, may also be irrigated by different rain-type equipments supplied by dispensers –mobil, mechanically moved or automatic – but there are also examples for dropping irrigation (Video 6.6.). The necessary amount of water and its costs are variable, depending on the solution.

Video 6.6. Irrigation methods VIDEÓ ( gyogy 6.6a. AVI )

Technical details of the construction and work of irrigation equipments are not target of this training material. More information on this topic can be found in Training material entitled “Technical knowledge”.

The available infrastructure on the field and in the enterprise restricts the choice of solutions. In the frame of the realistic possibilities, however, the producer always has to optimise the agrotechnical procedures, the irrigation among others. In this training material the emphasis is on the biological and ecological background but obviously, economical considerations are also very important.

In medicinal plant production we have to plan the irrigation procedure and methods based on the type of the drug and the type and level of the active material which should be achieved. Methodology and equipments of irrigation with their special parameters may significantly influence the formation and balance of active agents of the plant organs.

There are documented cases known, that essential oil content of the species accumulating the oil in glands on their leaf surface (e.g. sage, mint) may be severely reduced by rain-type irrigation. Quality of root drug (e.g. in case of lovage, angelica) however, remained unaffected by the method of water supply.

Yield and essential oil content of basil due to different irrigation methods

Yield and essential oil content of lovage due to different irrigation methods

Water supply in GAP system

In an integrated cultivation system each procedure should be properly documented from the preparations till the ending of the work. The documentation includes many steps like the exact definition of the field, the exact state of the plant stand, the weather, the state of the soil, time of the irrigation, the amount and the quality of the water carried out to the field, persons participating in the activity, etc. The most effective equipment should be purchased and operated.

Quality of the irrigation water should fulfil the national and international (EU) requirements. The guidelines of GAP system contain also the regulations concerning water quality. According to this, irrigation water must not contain –among others- heavy metals, pesticides and other toxic materials, besides, there is a maximum level set for the content of salts and chloride ion. Also, the allowed maximum quantity of microbes is determined.

Profitability of irrigation

It is an often emphasized opinion that the cost of irrigation does not return in the large scale cultivation form of medicinal plants (i.e. crops of “cereal” technology) and in smaller surfaces (in “horticultural crop rotation) irrigation should be profitable only when it is necessary to use during germination or in extreme dry weather conditions. However, recent tendencies of the world market, intensification of concurrence and quality oriented production makes this statement rather questionable.

In the technological background, generally applied in medicinal plant cultures, the yield is drastically determined by precipitation, wind and temperature especially during emergence and shoot formation. In intensive production – as mentioned above – not only the quantity and quality of the drug is improved but also the safety and stability of the total production is growing.

Decision about introduction of irrigation technologies, its necessity, abundance and method must be well grounded. A basic requirement seems to be that excess costs of the irrigation must return by the anticipated growth of drug mass and/or improvement of its quality.

Costs of the irrigation technologies are not negligible. Purchase and building of the equipment and also its operation (labour, energy, maintenance, reparations, water charge) afterwards necessitates considerable financial investment. These costs are highly dependent on the type of the equipment. Additionally, we have to be aware that amortization, maintenance and reparation costs may be present even in years when the equipment is not used at all. Expenses will return only in that situation if an additional income can be obtained for a long run by the increased yields or by the improved drug quality.

Studies on climate change have been intensified recently. It seems that effects of warming up and the weather extremities connected to it might be manifested in the horticultural cultivation very soon.

The ratio of species of Mediterranean origin among cultivated medicinal plants is outranging. As examples we could mention Lavender species, garden thyme, coriander, marjoram. According to numerous studies, a negative effect of the warming up would be moderate in case of these crops. Besides, lack of natural precipitation can be substituted by optimized irrigation, and this is a significant advantage comparing to the situation of the wild growing, indigenous species. Although intensive production is more expensive, but at the same time more advantageous concerning yield safety and drug quality.

You can read more about climate change in the digital training material “Climate change”.

Control questions:

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Right answers

References

Bernáth, J.- Németh, É. (2004): A hazai gyógy- és aromanövény spektrum elemzése ökológiai sajátosságaik alapján. Agro 21 füzetek, 34: 79-95.

Gosztola, B.- Sárosi, Sz.- Németh, É. (2010): Variability of the essential oil content and composition of chamomile (Matricaria recutita L.) affected by weather conditions, Natural Product Communications 5 (3) 465-470.

Hoppe, B. (szerk.), (2010): Handbuch des Arznei- und Gewürzpflanzenbaus, Band 2. Grundlagen des Arznei- und Gewürzpflanzenbaus. Bernburg: Saluplanta eV., pp. 484.

Németh, É.-Bernáth, J. (2004): Az évjárat és a környezeti változások hatásai a gyógy- és aromanövények produkciójára. Agro 21 füzetek, 34: 96-107