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E. Crop-Weed Competition and Weed Management

E. Crop-Weed Competition and Weed Management

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(2 lakh haÀ1) combined with black plastic mulch provided eVective control

of weeds. The crop requires weed control at the early stages. Notwithstanding to this fact, work on weed management is lacking in literature.

Though there is a great deal of interest in organic cultivation, need for

chemical weed management measures cannot be kept oV. The choice of

herbicide will depend upon the weed spectrum associated with the crop.

There is a report that Stevia can tolerate trifluralin (Andolfi et al., 2002;

Katayama, 1978). At Palampur, India, crop planted during June experienced severe weed competition due to poor crop establishment (Ramesh,

personal communication). This was exacerbated due to heavy rains. There is

no published evidence regarding safe herbicides for Stevia.



F. WATER REQUIREMENT

The knowledge of water requirement of crops in diVerent growing phases

elicits higher crop yield and rational use of water resource. In natural habitat,

it occurs in areas where the sites are continuously moist but not subjected to

prolonged inundation. Stevia usually occurs on locations with high level of

underground water or with continually moistened soil. It does not require

frequent irrigation, though it is susceptible to moisture stress (Shock, 1982). It

indicated that the crop prefers moist soil. For economic crops of Stevia,

irrigation is necessary (Donalisio et al., 1982). The plant has poor tolerance

to pH, so it should not be grown with poor quality water (Shock, 1982). Plant

growth was optimal at water content in soil of 43.0–47.6%. The average water

requirement per day is 2.33 mm plantÀ1 (Goenadi, 1983). Therefore, to secure

optimum water relations for Stevia plants is one of the factors closely

connected with its cultivation (Cerna, 2000). It requires liberal watering

after transplanting, and before and after harvesting of the leaves (Andolfi

et al., 2002). The average crop evapotranspiration (Ete) was measured as 5.75

mm dayÀ1, and water consumption was high during the entire cycle. Irrigation at 117% of Ete was 13% better than 100% Ete in terms of Stevia yield

(Fronza and Folegatti, 2002a). Evapotranspiration during the cycle was

divided in to 3 parts: 6.66 mm dayÀ1 (0–25 days), 5.11 mm dayÀ1 (26–50

days), and 5.49 mm dayÀ1 (51–75 days) at Brazil (Fronza and Folegatti,

2002b).

The crop coeYcient value (Kc) is the ratio between actual Ete to potential

Ete. This could be used as a parameter to judge water requirements.

Gonzalez (2000) had reported a crop coeYcient value of 0.25 from 0 to 25

days, 0.56 from 26 to 50 days, and 0.85 from 51 to 80 days in Paraguay,

whereas Fronza and Folegatti (2003) obtained 1.45, 1.14, and 1.16 at Italy

for the said phases, respectively.



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G. SOIL REQUIREMENT

The occurrence of Stevia on acid, infertile, sandy, or muck soils with

ample supply of water is consistent with observations of plant performance

under cultivation (Shock, 1982). The plant can be grown in a wide range of

soils but has poor tolerance to salinity and so it should not be grown in saline

soils (Chalapathi et al., 1997b). This occurs on the edges of marshes or in

grassland communities on soils with shallow water tables, the soils are

typically infertile acid sands or mucks. Stevia will grow well on a wide

range of soils given a consistent supply of moisture and adequate drainage.

Stevia grows naturally on infertile, sandy acid soils with shallow water

tables. This is normally in areas like the edge of mashes and grassland

communities (Lester, 1999). But this can also grow in grasslands, scrub

forests, and alpine areas (European Commission, 1999).



H. HARVEST

The optimum time of harvest depends on the cultivar and growing season.

Leaves are harvested about 4 months after planting by cutting the plants at

about 5–10 cm above the soil level (Donalisio et al., 1982). This must

however, the maximum crop biomass stage (Fig. 9), otherwise yield reduction is possible (Shuping and Shizhen, 1995). Since the crop is highly

sensitive to low temperature, in cold areas, crop may be harvested before

or at onset of winter (Columbus, 1997).

During flowering, stevioside dissipates from leaves (Bian, 1981; Hoyle,

1992), thus leaves should be harvested at the time of the flower (Figs. 10

and 11) emergence (Dwivedi, 1999) or before flowering (Barathi, 2003).



I. GROWTH REGULATORS

1.



Foliar Application



The most eVective preparation for increasing the concentration of stevioside in leaves was application of Humiforte (synthetic amino acids, N, P, K,

and trace elements) in combination with aminol (amino acids and N).

However, Maletran (lactic and anthranilic acids) gave the highest biomass

of micropropagated plants under field conditions (Acuna et al., 1997). The

best growth (root, stem, leaf, and whole plant fresh weight) was observed in

the third harvest due to Gibberellic acid (GA3) at 50 mg literÀ1 treatment.



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Figure 9 A field view of luxuriant Stevia rebaudiana at IHBT, Palampur, India.



No trends were observed in the 10 and 20 mg GA3 literÀ1 treatments.

However, the best overall growth was exhibited by the control (Stefanini

and Rodrigues, 1999).



2.



Tissue Culture



The combination of naphthalene acetic acid (NAA) and benzyl adenine

(BA) @ 0.1–0.2 mg literÀ1 was found to induce shoot formation in Stevia

explants. Further, the addition of GA to callus and suspension cultures

resulted in a significant increase in their fresh weight (Bondarev et al.,



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Figure 10 Stevia at budding stage.



1998). Whereas growth regulators depress the content of steviol glycosides,

however, the ratio of glycosides remained the same (Bondarev et al., 2003a).



J. SEED PRODUCTION

Seed yield up to 8.1 kg haÀ1 is possible (Carneiro, 1990). However, the

climatic requirements, of day length and temperature, are diVerent for

maximum vegetative production and for maximum flowering and seed production (Hoyle, 1992) since the crop is triggered to flowering under long day

conditions. It is not the only determinant governing seed production but

nutritional requirements are also higher. Seed production in the northern

hemisphere would be best suited between 20 and 30 N latitude. The crop

could be transplanted in February–March and seed collected in late summer.

The test weight of Stevia seeds range between 0.15 and 0.30 g (Brandle et al.,

1998) and 0.30 and 0.50 g under Palampur conditions (Ramesh, personal

communication).



K. CORRELATION STUDIES

Several authors studied the yield dependence on various growth parameters

as well as stevioside content (Brandle and Rosa, 1992; Buana, 1989; Buana and



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Figure 11 A field view of Stevia flowering.



Goenardi, 1985; Chalapathi et al., 1998, 1999a; Nishiyama et al., 1991; Shu and

Wang, 1988; Shyu et al., 1994; Truong et al., 1999; Utumi et al., 1999).

Plant height and leaf number at second and fourth week after planting was

positively correlated with Stevia biomass production at 30 DAT in a greenhouse experiment (Buana and Goenardi, 1985). In another study, plant height

neither had any close relationship with production nor with leaf number or

branch number in the first 4 weeks (Buana, 1989). A positive correlation



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between total soluble carbohydrate content and stevioside content was established by Nishiyama et al. (1991). Stevioside concentrations were uncorrelated to yield or leaf:stem ratio (Brandle and Rosa, 1992). Further, dry leaf yield

was correlated with leaf size and thickness, content of rebaudioside A was

correlated with rebaudioside C, and rebaudioside A to stevioside ratio was

highly correlated with leaf thickness (Shyu et al., 1994).

The dry yield of Stevia was positively and significantly correlated with plant

height, number of branches, leaves per plant, and dry matter accumulation.

About 96.88% of the total variation in dry leaf yield was explained by a linear

function of these four characters (Chalapathi et al., 1998). The number of

branches, and yield of fresh and dry stem and leaf, was more variable than

the number of leaf pairs, number of nodes before transplanting and at harvest,

plant height at transplanting and leaf length at harvest. The characters most

closely related to yield were fresh and dry weights of leaves and stems. Step‐by‐

step regression showed that leaf dry weight/plant had the greatest eVect on yield

(Shu and Wang, 1988). Stevioside content is influenced by both leaf surface and

number of roots; however, the former has greater influence on stevioside

content than number of roots as evident from the correlation coeYcient

(Truong et al., 1999), since the chemical content of last fully expended leaf

pairs was well correlated with plant nutrient status (Utumi et al., 1999).



L. BIOTIC STRESSES

Earlier, diseases like powdery mildew (Erysiphe cichoracearum DC),

Damping oV (Rhizoctonia solani Kuehn.), and Stem rot (Sclerotium dephinii

Welch.) were reported by Thomas (2000). Two fungal diseases Septoria

steviae and Sclerotinia sclerotiorum were reported in Stevia grown in Canada

(Chang et al., 1997; Lovering and Reeleeder, 1996; Reeleder, 1999). Occurrence of stem‐rot disease was recorded by 0.1% in the crop field at Palampur,

India (Megeji et al., 2005). Incidence of insects like aphids and white flies

were observed in the experimental field at IHBT, Palampur, but these were

below the threshold level. Similarly, attack of insects like aphids, mealy

bugs, red spider mites, and whiteflies were reported by Thomas (2000).



M. CROP PRODUCTIVITY

Stevia is a semiperennial species, which can be maintained up to 5–6 years,

with 2 or 3 harvests per year. Earlier, Bridel and Lavielle (1931a,b,c) and

Metivier and Viana (1979a) reported a stevioside yield of 60–65 and 72 g kgÀ1

dry leaf, respectively. In terms of economic biomass productivity, the dry leaf

yield in the natural habitat, Paraguay, was between 1500 and 2500 kg haÀ1



CULTIVATION OF STEVIA



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under dry land conditions and around 4300 kg haÀ1 with irrigation per year

(Jordan Molero, 1984).

Leaf yields of 3000 kg haÀ1 with a stevioside concentration of 105 mg gÀ1

equivalent to 66.2 ton haÀ1 of sugar was obtained at Canada (Brandle and

Rosa, 1992).

In Japan, 1 or 2 harvests per year is possible with a dry leaf yield of 3000

and 3500 kg haÀ1 in the first year, 4000–4500 kg haÀ1 in the second, 4000–

6000 kg haÀ1 in third, diminishing to 4000 kg haÀ1 in the fourth year (Sunk,

as quoted by Taiariol, 2004).

Under agro‐climatic conditions of Palampur, first harvest is taken at 90–

110 days after transplanting during June–July. Subsequently, second harvest

is taken after 60–75 days of the first harvest in early September at the time of

flower bud initiation. In case of late transplanted crop grown for single cut,

harvesting is done after 3–4 months of transplanting and continues till flowering begins, because the maximum sweetener in the leaves is until the plant

bears flowers. Perennial crop may continue up to 4 years, once planted, in the

same field. Life span of the crop is reported to be 7–8 years and herb yield

increases up to 4 years. Maximum amount of leaves are produced in the third

or fourth year. Flowering of the plant should be avoided and pinching of the

apical bud should be done to enhance bushy growth of the plant with side

branches. In the first year, average fresh biomass yield of 15–20 ton haÀ1 was

obtained out of two harvests and increased in subsequent years up to 20–

30 ton ha–1. An average dried leaf yield of 17, 20, 23, and 25 q ha–1 could be

produced from this total biomass yield in the first, second, third, and fourth

years, respectively (Singh and Kaul, 2005).



VII. CHEMISTRY AND QUALITY

The sweetness in Stevia is attributed to the presence of ent‐kaurene

diterpene glycosides, which are water soluble (Duke and deCellier, 1993;

Lester, 1999) and 300 times as sweet as cane sugar (Metivier and Viana,

1979b). Stevia leaves accumulate a mixture of at least eight diVerent glycosides derived from the tetracyclic diterpene steviol (Brandle et al., 1998). The

leaves contain stevioside, rebaudioside A, B, C, D, and E, dulcoside A, and

steviolbioside. The sweetening potency (sucrose ¼ 1) was 250–300, 350–450,

300–350, 50–120, 200–300, 250–300, 50–120, and 100–125, respectively

(Crammer and Ikan, 1986). These products taste intensely sweet; for example, rebaudioside A has been shown to be up to 320 times sweeter than

sucrose on a weight basis (Phillips, 1987). Stevioside is a white amorphous

powder present in leaf and stem tissue, was first seriously considered as a

sugar substitute in the early 1970’s (Kinghorn and Soejarto, 1985). The



K. RAMESH ET AL.



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sweetness in the leaves is two times higher than that of inflorescence

(Dwivedi, 1999). Steviolbioside 2, rebaudioside A4, B5, C6, D7, E8, and

F9, and dulcoside A10 are other compounds present but in lower concentration (Kennely, 2002; Starrat et al., 2002). This is an alternate to artificial

sweeteners such as aspartame or sodium saccharin. There is no report of ill

eVect on human health in over 1500 years of continuous use by Paraguayans.

In Japan (the biggest consumer market), there have been no reports of side

eVects. Reports on antifertility (Planas and Kuc, 1968) and its metabolic

byproducts like steviol being highly mutagenic (but no confirmative reports

are available for harmful eVect on using this plant; Brandle and Rosa, 1992)

leads to a controversy on safety concern of this plant in humans. The sweet

compounds pass through the digestive process without chemically breaking

down; making Stevia safe for those who need to control their blood sugar

levels (Strauss, 1995). A more detailed discussion on biosynthesis, toxicity,

metabolism, and nutritional implications of stevioside was reviewed by

Geuns (2003), which contains 74 references. He concluded that most toxicity

tests performed on stevioside have been negative and the use of purified

stevioside as a food additive appears preferable from public safety point of

view. The conclusion is that Stevia and stevioside are safe when used as a

sweetener.



VIII.



RESEARCH NEEDS



Good agricultural practices (GAP) of Stevia cultivation are the need of

the hour. An integrated approach by a team of multidisciplinary scientists

is required, leading to good manufacturing practices (GMP) of desired

quality end product from this crop. Use of Stevia is intimately tied to two

major sweet glycosides, stevioside and rebaudioside A, because of the prominence of these compounds in this plant. Therefore, research should be

directed toward the improvement of stevioside and rebaudioside A through

management and crop improvement strategies. Stevia gives a new direction

for the farming community, businessmen, and also the researchers. The

possible issues are enhancing the specific enzyme responsible for the production of these glycosides so that their yield gets enhanced. Quality of sweetness is also dependent on higher proportion of rebaudioside A to stevioside

in the extracted composite powder.

In many countries, this is a crop of recent domestication. Therefore,

agronomic considerations should be of high priority to utilize its maximum

potential. Under subtemperate climate prevailing in mid hills of India and

analogous regions of the world, growth of seedlings take longer time

and vegetative propagation is restricted due to nonavailability of actively



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growing shoots. This leads to delay in large‐scale commercial plantation.

Studies on production techniques and planting through rootstock are

needed.

Water management component is considered to be critical, since the

water resources are shrinking day by day. Integrated crop management

comprising of weed, insect, disease, and nutrient management, should be

inbuilt as a part of GAP. As a system study, the suitability of this crop in the

traditional cropping systems is another determinant to avoid excess production. This complete packages of production technology will make the Stevia

cultivation socially acceptable, cheaper, and economically viable.



ACKNOWLEDGMENTS

The authors are thankful to the Director Dr P. S. Ahuja, IHBT, Palampur

for his constant encouragement for the work. They are also thankful to

Dr R. D. Singh, Scientist (Agronomy) for his constructive suggestions in

the preparation of the manuscript.



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