Germination Inhibition of Undesirable Seed in the Soil using Microwave Radiation

ARTICLE IN PRESS Biosystems Engineering (2006) 93 (4), 365–373 doi:10.1016/j.biosystemseng.2006.01.005 PA—Precision Agriculture

Germination Inhibition of Undesirable Seed in the Soil using Microwave Radiation B. Vela´zquez-Martı´ 1,; C. Gracia-Lo´pez1; A. Marzal-Domenech2 1

Department of Mechanisation and Agrarian Technology, C/ Camino de Vera no 14 CP 46022 Valencia, Spain; e-mail of corresponding author: [email protected] (B. Vela´zquez-Martı´ ) 2 Department of Vegetal Production, Polytechnic University of Valencia, C/ Camino de Vera no 14 CP 46022 Valencia, Spain (Received 8 April 2005; accepted in revised form 18 January 2006; published online 3 March 2006)

Research was carried out to determine the effects of thermal treatment with microwaves on the germination of weed seeds in different growing conditions. Firstly, the elimination of weed seeds buried in the ground at several depths was evaluated using a waveguide designed to obtain a wide superficial distribution of the irradiated energy. Secondly, flowerpots and trays being treated with soils or substrata that will subsequently be employed for ornamental plant cultivation were considered. A modular prototype oven that can be placed in an automatic sowing line was tested to disinfect seedbed trays for horticultural plants. To inhibit germination of buried seeds in fields requires greater exposure times or power, a minimum of 21 MJ m
2. However, germination inhibition in flowerpots was achieved with short exposure energies, of between 60 and 80 MJ m
3, since the use of microwave radiation was viable for this application. The modular oven offers a practical application for seed elimination in the soils or substrata placed in sowing trays for either horticultural nurseries or ornamental plant nurseries, where the soil can be disinfected in the module introduced in the automatic sowing line. r 2006 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd

1. Introduction The search for alternatives to chemical treatment methods to eliminate undesirable soil microorganisms or weeds is a major challenge for agricultural research. Thermal treatment with microwaves for soil disinfection leaves no chemical residues (Olsen & Hammer, 1982; Nelson, 1985; Mavrogianopoulos et al., 2000). Radiating high-frequency electromagnetic waves (1–1000 GHz), this physical disinfection method increases the temperature of pathogens and weed seeds (Davis et al. 1971; Nelson, 1996). The principle of microwave heating is based on tuning the frequency with the oscillation resonance range of molecules, such as water. This leads to strong molecular shaking (resonant critical frequency), and consequently to the production of heat within the material (Metaxas & Meredith, 1993; Krasewski & Nelson, 1995). Thermal methods for disinfection are based upon the systematic increase of plant temperature, reaching the diverse thermal death points 1537-5110/$32.00

to eliminate the vegetation and some pathogenic agents that can exist in the ground (Fujiwara et al., 1983; Diprose et al., 1984; Barker & Craker, 1991). At such a temperature, proteins are denaturalised and they become inactive, leading to the destruction of living matter (Diprose et al., 1984; Nelson, 1996; Catala´-Civera & de los Reyes, 1999a, 1999b). Although a temperature increase within certain limits can produce an increase in germination (Brown et al., 1957), research has verified that germination is inhibited when seed is exposed to a determined threshold of microwave radiation, both in vitro experiments (Davis et al., 1971; Menges & Wayland, 1974), and in treatments with seeds buried at 2 cm in soils of low depth in static conditions (Barker & Craker, 1991). One important aspect is considered in Olsen’s research (1975), who discovered ‘selective heating’ when he was researching the inhibition of seed germination in soil irradiated by microwaves. The temperature reached by the seeds in the radiation process can be greater than

365

r 2006 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd

ARTICLE IN PRESS 366

B. VELA´ZQUEZ-MARTI´ ET AL.

that in the soil under certain conditions. Although this phenomenon occurs, it depends on two factors: firstly, the power absorption in the seed may be higher than in the soil; and secondly, the thermal insulation in the seed may also be high. In other words, the soil thermal conductivity may be lower than seed thermal conductivity. According to this phenomenon, it may be possible to use this type of selective heating in soil radiation either by taking better advantage of the energy or by reaching higher temperatures in organisms and germinated weed seeds when their water content is greater than soil humidity. Using less energy may result in similar organism elimination by favouring the disinfection time. The main objective of this study is to determine if microwave radiation can feasibly eliminate undesirable seeds in two growing conditions: weed seeds buried directly in the fields and weed seeds placed in flowerpots and trays for greenhouse seedbeds. Firstly, the elimination of weed seeds buried in the ground at several depths was evaluated using a waveguide designed to obtain a wide superficial distribution of the irradiated energy. Secondly, flowerpots and trays being treated with soils or substrata that will subsequently be employed for ornamental plant cultivation were considered using a prototype of continuous oven. The trays were radiated once circulating array of microwave radiators for a certain time. This modular prototype oven is tested to be placed in an automatic sowing line to disinfect seedbed trays for horticultural plants. On the other hand, the selective heating is also evaluated in the present study. 2. Materials and methods

2005), a waveguide was designed to achieve a uniform radiation distribution over a large area (Vela´zquezMartı´ & Gracia-Lo´pez, 2004a). The waveguide consisted of a hollow pipe made out of a conductive material capable of guiding the electromagnetic energy in a specific direction. The propagation is achieved through multiple reflections between the waveguide walls (Fig. 1) (Krauss & Fleisch, 1999). Since it is slotted, the waveguide irradiates uniformly over a relatively large surface (Vela´zquez-Martı´ & Gracia-Lo´pez, 2004a). After several tests, the waveguide dimensions were designed to minimise the reflection of the emitter system in Industrial, Scientific and Medical applications bands (ISM), specifically between 915 MHz and 2450 MHz. This waveguide was a rectangular hollow pipe, with a 100 mm by 70 mm section, measuring 850 mm in length. It had six slots on the lower side, parallel to the soil. The slots were angled at 451 with respect to the longitudinal axis, and there was a 100 cm separation between them. The slotted waveguide was fed with a 4 kW magnetron located at its end. It was placed longitudinally over an octagonal, sectioned, metallic container that measured 1000 mm long, 580 mm wide and 380 mm high, with a storage capacity of 200 kg of soil. The waveguide was placed at a distance of 8 cm from the soil surface (Fig. 2). The metallic prism that forms the oven was perforated with 36 circular holes of 3 cm2, located on its lateral sides, forming a matrix of six rows separated by 5 cm and six columns separated by 10 cm. The holes provided direct access, to the inside of the oven and allowed soil samples to be collected and the soil moisture content measured in each experiment without having to open the oven lid. Thermocouples were inserted at 72 positions

In order to determine the effects of microwave radiation on the inhibition of seed germination, two types of applicators were tested. (a) Firstly, a microwave distribution system with a waveguide fed by one 4 kW magnetron was used, designed to treat a large soil surface and volume. This system was placed over a 30 cm deep container filled with soil, which allowed for the study of the radiation effects on seeds buried at several depths as well as the evaluation of its possible application in the field. (b) Secondly, the radiation of seeds buried in trays was tested using a prototype continuous oven with four lined magnetrons of 1 kW each. A medium-textured soil of 638% sand, 208% silt and 154% clay was used for both test. 2.1. Treating seeds directly in the soil By referring to dielectric property values obtained for the different agricultural soils, (Vela´zquez-Martı´ et al.,

Fig. 1. (a) Scheme of waveguide reflections and (b) view of the slotted waveguide applicator; y is the inclination of the propagated wave

ARTICLE IN PRESS GERMINATION INHIBITION IN UNDESIRABLE SEEDS

367

Fig. 2. Schematic views of the waveguide applicator; all dimensions in mm

within the irradiated soil volume in order to measure the temperature distribution. To determine the temperature distribution for each radiation time, the thermocouples were placed in direct contact with the soil on two vertical planes inside the applicator. Half of them (36) were placed in a vertically-longitudinal plane under the slotted waveguide, and the other 36 were placed into a parallel plane with a 200 cm separation at the side. The thermocouples were distributed into columns with a 10 cm separation, beginning below the first feeding slot. Twelve of them were placed the surface, 12 at 5 cm in depth, 12 at 10 cm in depth, 12 at 15 cm in depth, 12 at 20 cm in depth and another 12 at a 25 cm depth (Fig. 3). The temperature was recorded every 20 s. The soil was irradiated through its surface for 6, 8, 10 and 15 min exposure times for each experiment. These measurements allowed for a temperature map for different depths to be obtained. With the data for these temperatures, it was possible to determine the soilspecific heat, and thus the power absorbed by each sector of the ground surface was calculated, to be 35 kW m
2. The temperature was measured at 72 points in the irradiated soil volume located at depths of 0, 5, 10, 15, 20 and 25 cm, and also at distances of 0, 10, 20, 30, 40 and 50 cm from the first waveguide slot. In each experiment, the soil was homogeneously moistened until values of 10%, 14% and 18% moisture content were achieved. The soil moisture content m was expressed as a percentage of the water mass over the total soil mass, calculated as: (1)

Fig. 3. (a) Oven view full of soil with its lid open before radiation and (b) thermocouples placed in a holes column of the lateral side matrix of the container

where: mw is the weight of water in the soil and md is the dry soil weight. The soil was then radiated through the surface during 6, 8, 10 and 15 min periods and the temperatures were recorded. The thermocouples were removed 15 min after having finished the heating treatment. Therefore, the temperature variation was

measured in two phases, heating and cooling, with a different thermal kinetic. The seeds were placed inside Teflon (tetrafluoroethylene) tubes which crossed the oven side by side at different depths. For each seed group, one thermocouple

mw  100 m¼ mw þ md

ARTICLE IN PRESS 368

B. VELA´ZQUEZ-MARTI´ ET AL.

was placed with the seed inside the Teflon tubes, another thermocouple was placed into the soil next to the Teflon tubes. The tubes enabled the seeds to be inserted consistently at different depths. The Teflon material is practically transparent to microwaves; therefore, the tubes have a negligible influence on the absorption and reflection of microwaves, but allow for thermal insulation which prevents measurement distortions in the heat conduction from both the soil to the seeds and also from the seeds to the soil (Catala´-Civera & de los Reyes, 1999a). Seeds of Lolium perenne (ryegrass), a monocotyledon, and Brassica napus var. oleifera (oilseed rape), a dicotyledon, were used to determine the influence of radiation on the germination in two situations: dried seeds, and seeds that were moistened for 1 h. The weed seeds were located on the surface and at depths of 5 cm and 10 cm. Thirty experiments were performed for each seed type. After radiation, the seeds were placed in germination chambers over filter paper, with climate control set at 25 1C with 8 h light and an 80% relative humidity. The germination was checked every 2 days over a period of 14 days. A seed was considered germinated when its rootlet and plumule were clearly visible. Three sets of experiments were performed. (1) Seeds were radiated without being moistened, moisture content ms lower than 10%, at several energy levels, and then immediately withdrawn from the soil after treatment. (2) Seeds were radiated without being moistened at several energy levels, then remained in the soil for 15 min after treatment. (3) Seeds were radiated after being previously moistened for 1 h, and then withdrawn from the soil immediately after treatment. Thermal variation (temperature versus time) in the seeds can influence during germination, either positively eliminates latencies or negatively inhibits the germination (Barker & Craker, 1991; Cavalante & Muchovej, 1993; Morozov et al., 1999). By comparing seeds immediately withdrawn after treatment with those left in the soil 15 min following treatment, it is possible to determine whether germination is affected either by the maximum temperature reached inside the seeds, or by the influence of the radiation and the subsequent temperature variation after treatment. If germination is affected only by the temperature reached inside the seeds, the germination would be modified equally as in the first set, by withdrawing the seeds immediately after treatment, whereas in the second set, the seeds remained 15 min in the soil inside the oven after the treatment had ended. The seed moisture content effects on the treatment due to the microwave affinity to the water molecules

(Gracia-Lo´pez & Vela´zquez-Martı´ , 2002), and the phenomenon of selective heating occurred on seed radiation were determined by the third set of experiments. After treatments, germinated seeds were counted every 2 days in the germinative chamber to determine whether the germinative vigour had been modified during the different treatments. The vigour is defined as the time taken for 50% of the viable seeds to germinate (Morozov et al., 1999). The energy doses that need to be applied to the soil to achieve the maximum germination inhibition were also analysed.

2.2. Treating seeds in trays This applicator prototype consists of a rectangular multimode cavity measuring 150 cm by 50 cm by 30 cm, with four microwave emitter heads of 1 kW each. The heads were fixed on the upper wall cavity, arranged vertically and separated by 35 cm.Three mode shakers were placed between each emitter head (Fig. 4). The mode shakers vary the particular conditions of Maxwel differential equations, by rapidly changing the electromagnetic field distribution. This fast distribution variation aims to produce a better heat uniformity (Vela´zquez-Martı´ & Gracia-Lo´pez, 2004b). The trays were filled with the substratum to be treated which measured 50 cm by 40 cm by 6 cm. In each treatment, the tray was inserted and withdrawn from the mode cavity through two gates 42 cm wide by 65 cm high. Each gate was protected by microwave filters, which allowed a continuous functioning of the system (Fig. 4). The exposure time was varied for each experiment (0, 120, 180, 240, 300, 360 and 480 s), and the temperature measurements were carried out before beginning and after ending the treatment. Measurements were taken at three points on the tray: at the centre and at two at a distance of 125 cm from the centre during heating and cooling. The heat distribution obtained from the temperature measurements was 172 kW m
2. The experiments were carried out with ryegrass and oilseed rape to determine the influence of the radiation in two different situations: dried seeds and seeds moistened for 1 h. In both conditions the seeds were radiated with different levels of energy, placed in trays in 12 sets that were uniformly distributed at a depth of 5 cm. The seeds remained in the soil in the tray during a 15 min cooling after treatment. The soil moisture content was determined in each experiment by collecting several samples. After treatment, the seeds were placed in germination chambers with the same atmospheric conditions as those

ARTICLE IN PRESS GERMINATION INHIBITION IN UNDESIRABLE SEEDS

369

Fig. 4. Modular microwave oven

3. Results and discussion

120 Relative germination, %

for the previous treatments. The number of germinated seeds was counted every 2 days, as in previous experiments. Thus, ratio of germination and germinative vigour were determined according to the energy required and the specific soil moisture content.

3.1. Treating seeds directly in the soil

80 60 40 20 0

0

5

10 15 20 Absorbed energy, MJ m−2

25

0

5

10 15 20 Absorbed energy, MJ m−2

25

(a)

120 Relative germination, %

Figure 5 depicts variation of the relative germination for ryegrass and oilseed rape without moistening (ms o10%), with energy level radiated over the soil and the seeds being immediately withdrawn from the soil after treatment. The unmoistened ryegrass seeds decrease their germination from 10 MJ m
2. Oilseed rape seeds decrease their germination from 15 MJ m
2. On the other hand, it is necessary to apply around 15 MJ m
2 to achieve a germination inhibition of approximately 50% for ryegrass located in the first 5 cm of depth and 20 MJ m
2 for the rape variety. In neither species, was the elimination of germination 100%. Figure 6 provides variation of the relative germination for ryegrass and oilseed rape, without moistening (ms o10%), with energy level radiated over the soil, but the seeds remaining in the soil for 15 min after microwave treatment. When the seeds are withdrawn from the soil 15 min after treatment, it can be observed that the germination begins to decrease from 10 MJ m
2, as ryegrass and oilseed rape. Moreover, the treatment was completely effective in the germination inhibition at 21 MJ m
2 for all seeds located in the surface. With this applied energy none of oilseed rape seed germinated at

100

100 80 60 40 20 0

(b)

Fig. 5. Variation of relative germination with the energy exposure for dried seeds of (a) ryegrass and (b) oilseed rape buried in the soil at three depths and withdrawn from the soil immediately after a radiation: , surface; , 5 cm depth; , 10 cm depth

5 cm depth and ryegrass seeds germinated only 20% in relation to not irradiated group. The germination inhibition experiment was less efficient in the rape seeds located at a depth of 10 cm which germinated 35%, and

ARTICLE IN PRESS B. VELA´ZQUEZ-MARTI´ ET AL.

370

120

120

Relative germination, %

Relative germination, %

140

100 80 60 40 20 0

0

5

(a)

10 15 20 25 Absorbed energy, MJ m−2

30

35

100 80 60 40 20 0

0

5

0

5

(a)

10 15 20 Absorbed energy, MJ m−2

25

80 60 40 20 0

(b)

120

100 Relative germination, %

Relative germination, %

120

0

5

10 15 20 25 Absorbed energy, MJ m−2

30

35

it was completely inefficient for ryegrass, where germination increased 20% in relation to not irradiated group. Figure 7 depicts the relative germination percentage for moistened ryegrass and for moistened oilseed rape (ms410%), which are removed from the soil immediately after treatment. When the seeds are moistened, the energy required to decreases germination is only 10 MJm
2. From 20 MJm
2, germination is reduced between 70% and 100%. It may be observed that when seeds are moistened, the treatment is equally efficient in the three studied depths. This fact verifies the existence of selective heating in moistened seeds, as reported by Olsen (1975). In short, germination decreases as the level of microwave energy is applied to the buried seeds is increased. This decrease is bigger for moistened seeds located higher than at a depth of 5 cm, because the microwave energy acts directly on water molecules present in the seeds. The fact that germination decreases when seeds remain buried after treatment indicates the temperature influence on the inhibition process. This

80 60 40 20 0

(b)

Fig. 6. Variation of relative germination with the energy exposure for dried seeds of (a) ryegrass and (b) oilseed rape buried in the soil at three depths and withdrawn from the soil immediately after a radiation: , surface; , 5 cm depth; , 10 cm depth

100

10

15

Absorbed energy, MJ

20

25

m−2

Fig. 7. Variation of relative germination with the energy exposure for wet seeds of (a) ryegrass and (b) oilseed rape buried in the soil at three depths and withdrawn from the soil immediately after a radiation: , surface; , 5 cm depth; , 10 cm depth

will favour the germination inhibition in usual treatments due to seed remain buried in the soil. In spite of a minor efficiency in seeds without moistening located at 10 cm depth, it is important to point out that those seeds have a low probability of emerging due to the long distance that their stems must cover. Table 1 provides the average of maximum temperature reached by seeds buried at different depths as moistened as not moistened in the soil being radiated during 15 min of radiation. The temperature reached by both tested species did not show a significant difference. If the seeds are wet, the temperature reached is about 5 1C higher in regards, to the temperature reached by dried seeds with the same radiation energy. Figure 8 shows the heating–cooling process undergone by the ryegrass seeds and the oilseed rape seeds being radiated during 15 min but remaining in the soil for 15 min after microwave treatment. The temperatures recorded for both species followed a practically linear increase with time. Nevertheless, this increase of temperature loses its linearity when the seeds are located at a greater depth, where the temperature reached is also

ARTICLE IN PRESS 371

GERMINATION INHIBITION IN UNDESIRABLE SEEDS

lower. Therefore, the seeds placed on the surface reached the highest temperature. This fact is due to the attenuation by the microwaves when they are propagated by the soil, causing the seeds that were buried deeper to receive lower levels of radiation (Vela´zquez-Martı´ & Gracia-Lo´pez, 2004b). This attenuation may vary between 006 and 046 dB mm
1, depending on the soil type and its moisture content (Barker & Craker, 1991). On the other hand, the

Table 1 Average of maximum temperatures reached by seeds buried in the soil during 15 min of radiation Specie

Location

Ryegrass Oilseed rape

Average of maximum temperature, 1C Unmoistened seeds

Moistened seeds

Surface 5 cm depth 10 cm depth Surface

144 104 79 142

150 110 84 149

5 cm depth 10 cm depth

106 75

113 79

superficial seeds undergo a faster cooling. During the heating process by radiation, the soil moisture content at the surface decreases from 11% to 3% in several areas, while the moisture content remains constant at a depth of 5 cm. The correlation coefficients for the all studied factors with ryegrass and rape as both dried and wet seeds are shown in Table 2. A negative correlation can be observed between the temperature reached and germination. It is also apparent that when the soil moisture content is higher, the seed has less heat, and therefore germination is higher (positive correlation with the soil moisture content). This is due to the fact that when the moisture content in the soil is higher, the energy of the radiation is employed in order to heat the soil more than the seeds, thus preventing the selective heating phenomenon. The correlation coefficients between soil moisture content and temperature, and between soil moisture content and germination, are low due to the fact that the sampled range was not very large (8–14%). In any treatment, the vigour of the germinated seeds varied significantly for radiations lower than 20 MJ m
2.

3.2. Treating seeds in trays 160

Temperature, °C

140 120 100 80 60 40 20 0

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 Time, min

Fig. 8. Variation of temperature for seeds buried in the soil at three depths, radiated for 15 min at 35 kWm
2 and withdrawn from the soil 15 min after radiation: , surface; , 5 cm depth; , 10 cm depth

Figure 9 provides data on the relative germination compared to the absorbed energy to the radiation. It is observed that with both, the moistened and the dried ryegrass seeds, germination decreases significantly at 40 MJ m
3 (2 min radiation). For a germination inhibition of 100%, with both the wet and dried ryegrass seeds, it is necessary to apply about 60 MJ m
3 (3 min radiation). For the rape seed, it is necessary to apply about 80 MJ m
3 (4 min radiation) if the seeds are dried for a germination inhibition of 100%. However, if the seeds are wet, it is necessary to apply only about 60 MJ m
3 (3 min radiation). The continuous modular prototype is more efficient than the prototype with the slotted waveguide. It seems that the energy employed in heating layers deeper than 7 cm, is now used on increasing the temperature of a

Table 2 Correlation coefficients for the factors studied in the seeds radiated by microwave Ryegrass seed Dried

G Tmax m

Oilseed rape seed Wet

Dried

Wet

G

Tmax

m

G

Tmax

m

G

Tmax

m

G

Tmax

m

1
074 055

— 1
069

— — 1

1
089 040

— 1
060

— — 1

1
073 050

— 1
066

— — 1

1
088 038

— 1
0.58

— — 1

G, Germination in %; Tmax, maximum temperature in 1C; m, soil moisture content in %.

ARTICLE IN PRESS B. VELA´ZQUEZ-MARTI´ ET AL.

372

Relative germination, %

120 100 80 60 40 20 0

0

20

40 60 80 100 120 Absorbed energy, MJ m−3

140

160

0

20

40 60 80 100 120 Absorbed energy, MJ m−3

140

160

(a)

Relative germination, %

120 100 80 60 40 20 0 (b)

Fig. 9. Variation of relative germination with the energy exposure for seeds of (a) ryegrass and (b) oilseed rape buried in trays at 5 cm: , dried seeds; , wet seeds

narrow layer of material which circulates through the cavity of continuous applicator. For the continuous modular prototype, the temperature increase was also linear until a maximum value of 100 1C, where the temperature was subsequently constant. This fact is due to evaporation processes as observed by Barker and Craker (1991). The minimum time to reach this temperature depends on the soil volume and the power employed. The time used in this prototype was 3 min which also lead to 100% germination inhibition. The germinative vigour was not modified in any of the prototype results.

minimum 21 MJ m
2. This is due to the high wave attenuation produced by the soil properties and the water present at the most superficial layer in the soil. The results of this study demonstrate that a negative relationship exists between the microwave energy absorbed by the seeds buried in the soil and the germination capacity. The extent of germination reduction depends on the temperature reached within the seeds. In moist soils, the energy consumption during the evaporation of the water present in the superficial soil layer has as result that the energy absorbed by the seeds located at deeper layers is lower. This is proved in the exposed experiments where the temperature reached by the seeds at 5 cm was 70% of the temperature reached in the surface, soil moisture content being 8–14%. Therefore, although the germination ratio has a negative correlation with the final temperature reached, it has a positive correlation with regard to soil moisture content. Nevertheless, the germinative vigour is not modified. When the water content in the seeds is higher than the soil moisture content, for example in the germination process, a selective heating is produced. This means that the temperature reached by the seeds is higher than the soil temperature for the same employed energy, and it is possible to reduce the power or the exposure time by 25% to obtain the same decrease in germination. Accordingly, to improve the efficiency of the microwave application to eliminate undesirable seeds located at the surface, it is better to irrigate 4 or 5 days prior to treatment to activate seed germination. The continuous modular oven offers a workable practical application for seed elimination in either the soils or substrate destined to seedbeds placed in sowing trays where the soil layer is shallow enough to permit a short exposure time. This system is a valid alternative to chemical treatments for soil disinfection in greenhouses at horticultural nurseries or ornamental plant nurseries, where the soil can be disinfected in a module introduced in the automatic sowing lines.

References 4. Conclusions The use of the microwave radiation to eliminate weed seeds present in the substrate for ornamental plants cultivated in flowerpots was examined. In this application if the microwave energy is distributed uniformly, the exposure energy needed to eliminate seeds present in 6 cm deep soil trays is about between 60 and 80 MJ m
3. However, the energy required to inhibit the germination of those seeds buried in the ground is too great,

Barker A V; Craker L E (1991). Inhibition of weed-germination by microwaves. Agronomy Journal, 83, 302–305 Brown O A; Stone R B; Andrews H (1957). Low energy irradiation of seed lots. Agricultural Engineering(September), 666–669 Catala´-Civera J M; de los Reyes E (1999a). Enzyme inactivation analysis for industrial blanching applications employing 24580 MHz monomode microwave cavities. Journal of Microwave Power and Electromagnetic Energy, 34, 239–252 Catala´Civera J M; de los Reyes E (1999b). Microwave inactivation of polyphenoloxidase for mushroom blanching. Automatika, 39, 57–62

ARTICLE IN PRESS GERMINATION INHIBITION IN UNDESIRABLE SEEDS

Cavalante M J B; Muchovej J J (1993). Microwave irradiation of seeds and selected fungal spores. Seed Science and Technology, 21, 247–253 Davis F; Wayland J; Merkle M (1971). Ultrahigh-frequency electromagnetic fields for weed control: phytotoxicity and selectivity. Science, 173, 535–537 Diprose M F; Benson F A; Willis A J (1984). The effect of externally applied electrostatic fields, microwave radiation and electric currents on plants and other organisms, with especial reference to weed control. Journal The Botanical Review, 50(2), 171–223 Fujiwara O; Goto Y; Amemiya Y (1983). Characteristics of microwave power absorption in an insect exposed to standing-wave fields. Electronics and Communications in Japan, 66-B, 46–54 Gracia-Lo´pez C; Vela´zquez-Martı´ B (2002). Effects of microwave energy for agricultural soil processing. EurAgEng Paper No. 02-AE-026, International Conference on Agricultural Engineering, AgEng2002, Budapest, Hungary, July 1–4 Krasewski A W; Nelson S O (1995). Application of microwave techniques in agricultural research. SBMO/IEEE MTT-S International Microwave And Optoelectronics Conference, pp 117–126 Krauss J D; Fleisch D A (1999). Electromagnetics with Applications, 5th Edn. McGraw-Hill, New York, 152 pp Mavrogianopoulos G N; Frangoudakis A; Pandelakis J (2000). Energy efficient soil disinfestations by microwaves. Agricultural Engineering Resources, 75, 146–153 Menges R; Wayland J (1974). UHF electromagnetic energy for weed control in vegetables. Weed Science, 22(6), 584–590

373

Metaxas A C; Meredith R J (1993). Industrial Microwave Heating. IEE Power Engineering Series 4, Peter Peregrinus Ltd., London Morozov G A; Sedelnikov Yu E; Stakhova N E (1999). Microwave seeds treatment. International Conference on Microwave and High Frequency Heating, Valencia, Spain, September 13–17, pp 193–196 Nelson S O (1985). RF and microwave energy for potential agricultural applications. Journal of Microwave Power, 20(2), 65–70 Nelson S O (1996). A review and assessment of microwave energy for soil treatment to control pests. Transactions of the ASAE, 39(1), 281–289 Olsen R (1975). A theoretical investigation of microwave irradiation of seeds in soil. Journal of Microwave Power, 10(3), 281–296 Olsen R G; Hammer W C (1982). Thermographic analysis of waveguide-irradiated insect pupae. Radio Science, 17, 95–104 Vela´zquez-Martı´ B; Gracia-Lo´pez C (2004a). Evaluation of two microwave superficial distribution systems designed for substratum and agricultural soil disinfection. Spanish Journal of Agricultural Research, 2(3), 323–333 Vela´zquez-Martı´ B; Gracia-Lo´pez C (2004b). Thermal effects of microwave energy in agricultural soil radiation. International Journal of Infrared and Millimeter Waves, 25(7), 1109–1122 Vela´zquez-Martı´ B; Gracia-Lo´pez C; Plaza-Gozalez P J (2005). Determination of dielectric properties in the agricultural soils. Biosystems Engineering, 91(1), 119–125