1 Introduction

Mustard plant is a cruciferous vegetable [1], its color varies i.e., yellow, green, red, and brown due to phytochemicals compositions such as minerals, vitamins, chlorophylls, dietary fiber, polyphenols, glucosinolates and volatile components (3-butyl isothiocyanate, allyl isothiocyanate, etc.) [2]. In Asian countries, B. juncea is used as food and folk medicine. B. juncea productivity is improved by the usage of organic fertilizers (green manure) or chemical fertilizers [3]. It contains seeds and leaves and is widely used in Asian food products or as a traditional medicinal plant [4]. B. juncea leaves are more popular and contained high phenolic content [5] which is responsible for biological activities i.e., antioxidant [5], antimicrobial [6], anticancer [7], and anti-hyperglycemic [8].

Continuous and intensive cropping often led to excessive build-up of pests and diseases in vegetable production areas [9]. Nowadays pesticides are common for the protection of plants, fruits, vegetables and turfs from numerous fungal diseases, and pathogens infections and to improve the productivity and quality of crops [10]. B. juncea is a common plant for pest attacks, which is the main cause of the reduction in productivity [11]. Apart from fertilizers, pesticides were also used to improve crop productivity [12, 13]. Unfortunately, the injudicious use of pesticides by farmers led to the presence of unwanted residues in crops produced which may pose risks to human health [14, 15]. Besides, extensive violation of pesticide usage may also cause deleterious effects on the environment. Pesticides that entered the soil via spray drift, wash-off from plant and soil drenching had been reported to cause environmental problems such as water contamination and alteration of soil natural properties [16]. The pesticides of choice mostly belonged to the organophosphorus (OP) and synthetic pyrethroid (Py) groups. According to the World Health Organization, profenofos and λ-cyhalothrin specifically belong to Class II respectively [17].

Numerous studies had been reported on the dissipation and residue levels of profenofos and λ-cyhalothrin in different types of vegetables (i.e., kumquat onions, and tomatoes) in USA, Italy and China [12, 18,19,20].

Nevertheless, based on our literature studies, no research is available in the literature on the comparative dissipation of profenofos and λ-cyhalothrin in B. juncea in the open field (O.F.) and green house (G.H.) under humid tropical climates. There are many different aspects and processes that influence pesticide dissipation rate. For example, the frequency and rate of application, pesticide formulation, crop morphology and weather conditions. Therefore, the pesticide dissipation needs to be evaluated individually on specific crops and under specific growing and environmental conditions through field trials [14].

The present study aims to investigate the influence of different growing systems and seasons on the dissipation rate of two different types of heavily used pesticides namely profenofos (1) and λ-cyhalothrin (2) (Fig. 1) on B. juncea. Half-life values obtained from this study will be used to estimate a suitable pre-harvest interval for profenofos and λ-cyhalothrin in B. juncea.

Fig. 1
figure 1

Pesticides profenofos (1) and λ-cyhalothrin (2)

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2 Materials and methods

2.1 Reagents and materials

Commercial profenofos, ELAK 45 EC (profenofos 45% w/w), commercial λ-cyhalothrin and ALERT 2.8 EC (λ-cyhalothrin 2.8% w/w) were purchased from the local market. Profenofos standard (98.2%) was obtained from Sigma Aldrich, Steinheim, Germany. λ-cyhalothrin (purity 98.2%) standards were obtained from Dr. Ehrenstorfer GmbH, Ausburg, Germany. Acetonitrile, dichloromethane, n-hexane, glacial acetic acid, sodium chloride, and anhydrous magnesium sulfate were purchased from J.T. Baker, Philipsburg, USA. The silica gel and primary-secondary amine (PSA) were purchased from Merck, Darmstadt Germany and Varian, California USA, respectively.

2.2 Experimental details

Field trials setup and sampling for B. juncea followed as it is from earlier paper [21]. Homogenized sample (10 g) was weighed into a 50 mL Teflon centrifuge tube. Acetonitrile (15–20 mL) containing acetic acid (1%) was added to the sample and for 1 min it the sample extract was vigorously shaken by hand. Sodium chloride (1.5 g) and anhydrous magnesium sulfate (5–6 g) were added to the sample. The sample extract was vortexed and centrifuged at 2500–3000 rpm for 1 min each. The supernatant was transferred into another Teflon centrifuge tube, shaken with anhydrous magnesium sulfate (3 g), vortexed and centrifuged at 3000 rpm for 1 min. The extract (2 mL) was taken and eluted through activated silica gel (0.2 g packed in a 2 mL glass Pasteur pipette). The eluate was left in the fume hood to dry, makeup with 2 mL n-hexane and injected into a gas chromatograph-electron captured detector (GC-ECD) for qualification and quantification of λ-cyhalothrin.

For profenofos determination, extract was eluted through primary-secondary amine (PSA), 0.2 g packed in a 2 mL glass Pasteur pipette. The eluate was left in the fume hood to dry, makeup with 2 mL acetonitrile and injected into a gas chromatograph-flame photometric detector (GC-FPD) qualification and quantification.

Moreover, the pH and salinity are not suitable for this type of study. The field trials (green house, open field) and lab experimental as a control study were conducted to address the pesticides residue problems from the two commonly used pesticide in vegetables planted by farmers in the reported location which is the main supplier to nearby house whole vegetable product. The data obtained are useful in future determination of exact PHI for the two pesticides when it is applied by local farmers whom usually practice growing vegetables under the studied conditions. GC was recommended to be set to suit the suitability with the extracted pesticides from the soil and plants.

2.2.1 Gas chromatography conditions

Gas Chromatography equipped with a Flame Photometric Detector (GC-FPD) was used for the determination of profenofos. GC-FPD is preferred for analysis of organic and inorganic phosphorus compound. It was equipped with a non-polar fused-silica capillary column, HP5, 1.5 m × 0.53 mm × 1.5 μm obtained from J&W Scientific, Folsom, California, USA and was used with a nitrogen gas carrier at a flow of 4.0 mL min−1. A more polar capillary column, DB1701, 15 m × 0.25 mm × 1.0 μm obtained from J&W Scientific, Folsom, California, and USA were used for the confirmation of pesticides in vegetable samples. The column temperatures were maintained at 120 °C for 1 min and then programmed at 30 °C min−1 to 150 °C followed by another temperature ramp of 5 °C min−1 to 270 °C and held at 270 °C for 10 min. The injector and detector temperatures were maintained at 260 °C and 250 °C respectively. The air and hydrogen flows were set at 80 mL min−1 and 67 mL min−1 respectively. Gas Chromatography equipped with an Electron Capture Detector (GC-ECD) was used for the determination of λ-cyhalothrin. Optimized instrument setting followed from earlier paper [21]. GC-ECD is preferred for λ-cyhalothrin analysis due to better sensitivity achieved compared to GC-FPD used for Profenofos analysis.

2.2.2 Method validation

Validation of the in-house modified QuEChERS method was carried out by assessing few parameters such as linearity, accuracy, and precision. The limit of detection and limit of quantification of the method were also determined [21].

3 Results

3.1 Method validation

The methods used in this trial were validated by assessing their linearity, accuracy, and precision following SANCO/12,571/2013 and SANTE 11,312/2021 [22, 23]. Individual standard of profenofos and λ-cyhalothrin were prepared in B. juncea extract and in acetonitrile (for profenofos) and n-hexane (for λ-cyhalothrin) in the range of 0.01 to 5 mg kg−1. Each concentration was injected three times (n = 3) and the averaged peak areas obtained plotted against concentration analysed. Good linearity was achieved with a correlation coefficient of 0.997 and 0.999 for profenofos and λ-cyhalothrin, respectively. A linear response is essential to achieve good analytical results of pesticide residue analysis [22, 23]. The retention time for profenofos and λ-cyhalothrin were 18.73 and 8.52 min respectively [24, 25] (Fig. 2). The results obtained for calibration standard prepared in B. juncea extract and blank solvent showed that the matrix effect is not significant when using the modified QuEChERS method. There were also no interfering peak observed at the targeted retention time in both blank and spiked matrices.

Fig. 2
figure 2

Pesticides prefenofos (1) and λ-cyhalothrin (2) retention time

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The limit of detection (LOD) of profenofos and λ-cyhalothrin in this study was estimated by analysis of the few lowest concentration that is expected to produce a response equivalent to 3 times the baseline noise. The limit of detection for profenofos and λ-cyhalothrin was 0.006 mg kg−1 and 0.005 mg kg−1, respectively (Table 1). The limit of quantification (LOQ) is defined as the concentration giving a signal-to-noise ratio of 10 which produces an acceptable recovery and precision [26]. The LOQ of the method was set at 0.01 mg kg−1 for the two pesticides in B. juncea having a precision (% RSD) of less than 20% and % recovery within the ± 30% range.

The accuracy was expressed by percentage recovery of spiked samples [27]. The accuracy and precision of this method were estimated by analysis of 5 replicates of spiked samples. B. juncea was spiked with profenofos and λ-cyhalothrin at 5 concentration levels (1.0, 0.5, 0.1, 0.05 and 0.01 mg kg−1). Recoveries obtained for profenofos and λ-cyhalothrin in B. juncea fortified at 1.0, 0.5, 0.1, 0.05, and 0.01 mg kg−1 are shown in Table 2. Average recoveries obtained for profenofos and λ-cyhalothrin in B. juncea were in the range of 80.4-97.7%. The recoveries obtained for profenofos and λ-cyhalothrin fall within the acceptable range of 70–120% in accordance to the analytical method quality control and validation procedures for pesticide residue analysis in food and feed [22]. Method precision was determined from multiple injections of spiked samples and was expressed as relative standard deviation, RSD. The acceptable value of RSD for repeatability is < 20%. The RSD (n = 5) obtained in this study was in the range of 2.87–6.43%, which is well below the acceptable value limit of RSD, < 20% [22].

Table 1 Linearity, LOD, LOQ and maximum residue limit of profenofos and λ-cyhalothrin in B. juncea
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Table 2 Recovery data of profenofos and λ-cyhalothrin in B. juncea
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3.2 Dissipation of profenofos and λ-cyhalothrin in B. juncea

A field study to evaluate the dissipation of profenofos and λ-cyhalothrin in B. juncea was conducted at the Agriculture Research Centre Semongok, Sarawak (Table S1–4). As climate conditions such as sunshine, rainfall and surrounding temperature were important in this study; meteorological data was therefore collected throughout the experimental period of 22 days. Maximum air temperature recorded during dry season was 35 °C, while the lowest temperature recorded was 24 °C with average daily sunshine of 4.5 h (Fig. 3). Rainfall volume collected (in mm) was measured daily at 8 a.m. Average daily rainfall volume collected during dry season was 12.03 mm, with raining day frequency 15 out of 22 days (Table S5–9).

Fig. 3
figure 3

Sunshine, rainfall and surrounding temperature data throughout the experimental period

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Meanwhile, during the wet season, the highest air temperature recorded was only 34.5 °C and the lowest was 24 °C. Average daily sunshine was 2.9 h, shorter compare to dry season (4.5 h). Average rainfall volume measured was 11.80 mm with raining day frequency of 17 out of 22 days. This showed it rains more frequently during wet season compare to dry season. Nevertheless, no significant difference in the volume of rainfall (in mm) collected during both seasons. Humidity data was also measured for G.H and O.F for both dry and wet season (Table 3). Statistically, there were no significant difference of % humidity measured during both seasons under G.H. and O.F.

Table 3 Average humidity recorded under green house and open field during the dry and wet season
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3.3 Dissipation pattern of profenofos and λ-cyhalothrin in B. juncea during dry season

The dissipation of profenofos and λ-cyhalothrin in B. juncea planted under the green house (G.H.) and open field (O.F.) during the dry season are shown in Fig. 4. Initial concentration of profenofos and λ-cyhalothrin in B. juncea planted under G.H. were 8.35 and 1.65 mg kg−1. In O.F, initial deposit of profenofos and λ-cyhalothrin were 7.14 and 1.51 mg kg−1 (Table 4). The lower initial concentration of λ-cyhalothrin in B. juncea compared to profenofos was due to the lower active ingredient (a.i) percentage in the commercial formulation used for this trial. Profenofos commercial formulation used in this study contain 45% w/w profenofos while λ-cyhalothrin was 2.8% w/w. Profenofos and λ-cyhalothrin however showcased a biphasic exponential dissipation trend with rapid dissipation taking place during the first 7 days after application followed by a slower and steady rate of dissipation from day 9 to day 22. Certain residues could not be quantified by the adopted method as the pesticide’s residue dissipated below the detectable limit.

Fig. 4
figure 4

Dissipation curve of profenofos and λ-cyhalothrin in B. juncea under G.H. and O.F. during dry season 

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Table 4 Dissipation of profenofos and λ-cyhalothrin in B. juncea under green house and open field cultivation system during dry season
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Within the first 24 h following the final application, 43.7% of profenofos and 61.8% of λ-cyhalothrin had dissipated in B. juncea under G.H. while in the O.F, 31.82% of profenofos and 48.34% of λ-cyhalothrin had dissipated. The initial dissipation of profenofos and λ-cyhalothrin could be due to the high surrounding temperature (29.5 °C) and longer sunlight radiation (7.6 h). As volatilisation is correlated with the specific vapour pressure of a substance, profenofos was suspected to dissipate via this route due to its considerably high vapour pressure [20]. Physicochemical properties of profenofos and λ-cyhalothrin were listed in Table 5. λ-cyhalothrin has lower vapour pressure as compared to profenofos [28]. Dissipation via volatilisation or evaporation may have occurred but could be at a lower rate compared to profenofos. Although λ-cyhalothrin has a lower vapour pressure compared to profenofos, rapid dissipation was observed where a higher percentage of dissipation took place compared to profenofos. This could be associated to photodegradation. λ-cyhalothrin was reported to be a photosensitive compound [28]. It is therefore more susceptible to degrade via photodegradation [29]. The photodegradation of λ-cyhalothrin was documented to be initiated mainly by the presence of reactive species generated from UV radiation of sunlight [30, 31]. The metabolites formed undergo further degradation into non-toxic compounds such as CO2 and water [28].

Table 5 Physicochemical properties of profenofos and λ-cyhalothrin
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After 7 days, 96.8% of profenofos and 98.8% of λ-cyhalothrin had dissipated under G.H. While in the O.F., 93.0% of profenofos and 93.2% of λ-cyhalothrin had dissipated. λ-cyhalothrin exhibits faster dissipation rate than profenofos under both G.H. and O.F. Both pesticides however show slightly higher dissipation rate under G.H. compared to O.F. Elevated temperature as a result of heat accumulation due to the hermetic condition of G.H has been reported [32]. A slightly higher dissipation rate in G.H. can be associated to this condition. Besides, rapid growth of the B. juncea plant within the first week may have also contributed to the dilution of both pesticides by plant growth. As the B. juncea grows larger, the plant weight and the leaf surface area will increase, causing a decrease in observed pesticide concentration even if there is no degradation occurred [33].

Profenofos dissipated completely in O.F. and G.H. on day 22, while λ-cyhalothrin was still detected at 0.01 mg kg−1. λ-Cyhalothrin is a lipophilic substance with low solubility in water. It was reported to tend to diffuse into cuticular waxes or the inner part of the plant which prevents further dissipation of its residue via physical processes such as wash-off by rainfall [34]. The fact that the ability of λ-cyhalothrin residue to be held within the plant cuticular wax might have led to the detectable amount on day 22. Even though the residues were still detected, it complies with the national tolerance level of 0.02 mg kg−1 for λ-cyhalothrin in B. juncea.

3.4 Dissipation pattern of profenofos and λ-cyhalothrin in B. juncea during wet season

A similar exponential dissipation trend was observed for profenofos and λ-cyhalothrin in B. juncea planted during the wet season (Fig. 5). Initial profenofos concentration was 6.60 mg kg−1 and 4.72 mg kg−1 while λ-cyhalothrin initial concentration was 0.39 and 0.34 mg kg−1, under G.H. and O.F. respectively (Table 6).

Fig. 5
figure 5

Dissipation curve of profenofos and λ-cyhalothrin in B. juncea under G.H. and O.F. during wet season

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Samples collected 24 h after the final spray showed only a 25.8% reduction of profenofos and a 38.5% reduction of λ-cyhalothrin under G.H. On the other hand, profenofos and λ-cyhalothrin residues in B. juncea planted under O.F. had only dissipated about 28.5% and 5.9%. Compare to dry season, only a small percentage of residue had dissipated 24 h after application during wet season. There was no rainfall recorded within the first 24 h. Sunshine availability was recorded to be 4 to 7 h and the surrounding temperature was recorded to be 29.3 °C. Profenofos dissipation here can be associated with volatilisation while λ-cyhalothrin dissipation can be associated with photodegradation [28]. λ-Cyhalothrin was reported to be a photolabile pesticide and its main degradation pathway was ascribed to photodegradation, which could explain its slightly higher dissipation percentage compare to profenofos [28]. A reduction in sunshine availability to 4.75 h during the wet season seems to minimised photodegradation and volatilisation processes [35]. Therefore, observed dissipation percentage of the two pesticides during the wet season was quite low compared to dry season (Table 6).

Table 6 Dissipation of profenofos and λ-cyhalothrin in B. juncea under net house and open field cultivation system during wet season
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There were 20.9 to 38.7 mm of rainfall recorded from day 1 until day 5. On day 7, the total profenofos and λ-cyhalothrin residue that had dissipated was 97.8% and 92.3% in G.H. While in O.F. profenofos and λ-cyhalothrin had dissipated at about 86.2% and 85.3%. Profenofos is a highly water soluble pesticides and has high vapour pressure [32]. Because of its high solubility in water, profenofos was envisaged to dissipate more rapidly under O.F. conditions that was exposed to direct rainfall compared to G.H, however rapid dissipation was observed under G.H. where rainfall intensity was reduced. A similar pattern was also observed for λ-cyhalothrin. Although λ-cyhalothrin is a hydrophobic pesticide and intense rainfall would be needed to effectively dislodge it from the leaf surface, its dissipation was more rapid under G.H. Similar observation was made during dry season where during rainfall, hermetic condition and limited airflow under G.H. led to heat accumulation [30]. Heat accumulation under G.H. might accelerate several processes involved in pesticide dissipation [36]. It has been noted that processes like hydrolysation, for instance, are more active in regions with higher temperatures and high humidity levels, like G.H. [36]. During the wet season, profenofos and λ-cyhalothrin were observed to dissipate completely on day 22. On a contrary, during dry season, only profenofos had fully dissipated while λ-cyhalothrin was still detectable on day 22. As the amount of residue was at a trace level, the differences were statistically insignificant.

3.5 Half-life variation of profenofos and λ-cyhalothrin in B. juncea

The dissipation of profenofos and λ-cyhalothrin in B. juncea under G.H. and O.F. cultivation system was fitted into the first-order kinetic equation, Ct = Ct0 e−kt, employing a non-linear least-squares regression analysis of residue concentration against time, where Ct is the concentration at time t, Ct0 is the initial concentration and k is the degradation rate [37]. The degradation rate k and the correlation coefficient (R2) were obtained from regression analysis of the concentration plotted against time (day) after application for each curve [27] (Table 7).

Table 7 Linear regression equation and coefficient of determination (R2) of profenofos and λ-cyhalothrin in B. juncea cultivated under G.H. and O.F. conditions in the dry and wet season
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The slope parameter was used as the least-squares estimate of k. The half-life (t1/2, day) was estimated from the equation, t1/2 = ln 0.5 k−1. He and his coworkers (2008) even noted that the photodegradation of λ-cyhalothrin followed first-order kinetic behaviour [28].

The calculated half-life (day) of profenofos and λ-cyhalothrin estimated from the equation, t1/2 = ln 2/k in B. juncea during the dry season was 0.71–1.18 days under G.H. and 0.66–1.25 days under O.F, respectively (Table 8). For the wet season, the t1/2 was slightly longer for both pesticides i.e. 1.28–1.76 days for profenofos under G.H. and O.F. and 2.51–3.7 days for λ-cyhalothrin under G.H. and O.F. Field dissipation of profenofos and λ-cyhalothrin in B. juncea varied depending on the physicochemical properties of pesticides which determine the fate and behaviour of these compounds under different climate conditions [38]. Highly soluble pesticides such as profenofos for instance was predicted to dissipate faster with the presence of rainfall. However, this field trial results were suggesting otherwise where profenofos half-life was longer in wet season where rainfall frequency was higher.

Table 8 Initial deposit and half-life variation of profenofos and λ-cyhalothrin in B. juncea cultivated under G.H. and O.F. conditions in the dry and wet seasons
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Lower half-life for both pesticides under G.H. during both dry and wet seasons, except profenofos during dry season may suggests the important role of high surrounding temperature in accelerating processes involved in the dissipation of pesticides [38]. Profenofos however showed a slightly longer t1/2 under G.H. (0.72 days) during the dry season compared to O.F. (0.66 days). Longer sunlight radiation and the higher surrounding temperature might have led to a faster dissipation of profenofos in B. juncea under O.F. conditions. This could imply that volatilisation and photodegradation are the dominant processes involved in profenofos residue dissipation in B. juncea [30]. Shading under G.H. was suspected to impede profenofos dissipation and led to its longer t1/2 compared to O.F.

The λ-cyhalothrin, on the other hand, has shorter t1/2 (1.18 and 2.51 days) under G.H. compared to O.F. (1.25 and 3.69 days) in both wet and dry seasons. This suggested that the dissipation rate of λ-cyhalothrin was faster in G.H compared to O.F. Faster rate of dissipation for λ-cyhalothrin under G.H appeared to be related to the lipophilicity property of λ-cyhalothrin which was explained by its high octanol-water coefficient (log Kow) value. Lipophilic pesticides tend to be retained by the fatty plant surface tissue which favours the partitioning of the active ingredients into inner plant cells where enzymatic transformation took place [39]. Higher relative humidity under the G.H allowed this partitioning to occur as higher air humidity improves the adsorption affinity of λ-cyhalothrin to plant surfaces [30]. Therefore, the dissipation pathways of λ-cyhalothrin in B. juncea could be associate to plant metabolism and growth dilution in addition to rainfall and sunlight radiation [36].

As profenofos has higher water solubility and vapour pressure compared to λ-cyhalothrin, profenofos was predicted to show a higher rate of dissipation than λ-cyhalothrin. The water solubility of profenofos and λ-cyhalothrin is 28 mg L−1, and 0.004 mg L−1, respectively. From this study, the half-life of profenofos was found to be lower than λ-cyhalothrin which confirmed the significant effect of rainfall washed-off onto the dissipation rate of these pesticides from B. juncea. Apart from that, a high vapour pressure led to rapid volatilisation profenofos on plant surfaces [36]. This can be affirmed by the vapour pressure value where profenofos (1.24\(\times\)10−1 mPa at 25 °C) has higher vapour pressure than λ-cyhalothrin (2.00\(\times\)10−7 mPa at 20 °C). Results obtained from this trial confirmed our prediction that profenofos has shorter t1/2 compared to λ-cyhalothrin in B. juncea during both dry and wet seasons. Other OP insecticides were also reported to have shorter t1/2 in crops compared to Py insecticides. Acephate and chlorpyrifos (both OP) for example were reported to have shorter t1/2 than cypermethrin (Py) in B. juncea [40].

The half-life of profenofos in this study (0.72–1.76 days) was comparable to those reported for non-leafy vegetables such as okra, which is 1.35 days, and 2.7 days for bitter gourd [41]. The t1/2 of profenofos in/on plant or vegetation may vary not only according to the climatic conditions and types of formulation used or dosage applied, but also depending on the growth rate, plant matrices, sizes and surface areas [41]. The initial deposit of profenofos in bitter gourd planted under subtropical conditions was 1.29 mg kg−1 which was lower than the initial deposit detected in this study (4.72–8.35 mg kg−1). It is because bitter gourd has a smaller surface area and waxy peel, while B. juncea has a larger surface area. Therefore, B. juncea can trap more pesticide compared to a bitter gourd, leading to a higher initial concentration detected. However, the residues persisted for up to 15 days in both B. juncea and bitter gourd. Therefore, it could be concluded that the profenofos dissipation rate in vegetables under humid tropical conditions was faster than in the subtropical condition. Due to high solar intensity and high rainfall frequency in a tropical climate, the t1/2 of a pesticide is shorter compared to a temperate region [40].

The dry season in subtropical conditions has lower humidity, surrounding temperature and sunlight intensity, compared to tropical conditions. Therefore, t1/2 of λ-cyhalothrin reported by Seenivasan and Muraleedharan (2.8–3.5 days) was longer than those reported in this study for dry season, which was 1.16 days. Albadri et al., (2012) have also reported a longer t1/2 of λ-cyhalothrin in tomatoes (5.7 days) compared to our findings. It can be further evidence that for a tropical climate with high solar intensity and high rainfall frequency, the t1/2 of λ-cyhalothrin reported in this study was shorter [40]. Apart from high temperature and longer sunlight availability which promotes volatilisation and photodegradation, physical dissipation processes such as plant penetration and growth dilution need to be highlighted as well [42]. It is worthwhile to note that plant growth dilution may have also play an important role in overall dissipation processes here. With longer sunshine radiation, plant growth rate increased leading to rapid dissipation via growth dilution. This can be another factor to be weigh as important as volatilisation and photodegradation in promoting faster pesticides dissipation from plants.

Maximum residue limits (MRLs) of profenofos and λ-cyhalothrin in B. juncea were 0.01 mg kg−1, and 0.02 mg kg−1 [43]. Based on the MRLs given, calculated pre-harvest interval (PHI) required for the safe consumption of profenofos and λ-cyhalothrin treated B. juncea are listed in Table 9. The pre-harvest interval calculation was based on PHI = [ln A – ln (MRL)/K] [44] (Table S10). B. juncea has a short crop cycle of 21 days. Repeated application of profenofos and λ-cyhalothrin, therefore, is not advisable since the residues persist until up to 15 days, especially λ-cyhalothrin (0.01–0.02 mg kg−1) which is higher than the tolerable residue allowed. Therefore, harvest at the optimum pre-harvest intervals obtained from this study is not favourable as the plant has been overly matured at the time of harvest.

Table 9 Pre-harvest interval (Days) of profenofos and λ-cyhalothrin in B. juncea cultivated under G.H. and O.F. conditions in the dry and wet seasons
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4 Conclusion

The dissipation rate of profenofos and λ-cyhalothrin in B. juncea depends greatly on their physicochemical properties. Dissipation rate was faster as compared to λ-cyhalothrin in B. juncea due to higher water solubility and higher vapour pressure. Profenofos and λ-cyhalothrin dissipated faster during the dry season due to high temperatures and longer sunshine availability throughout the day. Rainfall event during the dry season seems to enhance the dissipation rate of both pesticides in B. juncea. It is worth noting that both pesticides dissipated more rapidly in O.F. compared to G.H. during the dry season. On the contrary, the dissipation rate of these pesticides is higher in G.H. than in O.F. during the wet season. This finding suggests the significant impact of surrounding temperature on pesticide dissipation rate in vegetables. The PHI obtained from this study suggests that both pesticides have to be applied to B. juncea at least 2 weeks before harvest to ensure the residue has enough time to dissipate below the tolerance level during harvest.