Introduction
Round gourd [Praecitrullus fistulosus (Stocks) Pangalo], also known as tinda, Indian squash, apple gourd, round melon or squash melon, is an important squash-like cucurbit primarily grown for its immature fruits. It is cultivated as a vegetable in the tropical South Asia (India and Pakistan), continental central Asia (Afghanistan), tropical East Africa (Kenya, Tanzania and Uganda), tropical West Africa (Ghana) and North America (USA) (Schippers, Reference Schippers2004; Chomicki et al., Reference Chomicki, Schaefer and Renner2020). The ideal temperature requirement for its seed germination is 21–35°C. It prefers warm, bright weather with temperatures ranging from 25 to 30°C during the day, and 18°C or more at night (Schippers, Reference Schippers2004).
One of the most crucial factors influencing the plant growth and fruit yield of this crop is the sowing time or season. In the plains of North-Western India, the crop is predominantly sown in spring–summer, viz., March–April. The slow germination combined with slow initial plant growth and development, due to low temperature in March, is one of the hurdles in obtaining high early fruit yield which generally fetch high market prices. On the contrary, the crop sown in May–June faces high temperature (>35°C) stress which leads to desiccation of flower buds, ultimately reducing fruit yield. The crop can also be sown in the rainy season (June–August); however, high weed infestation and incidence of begomovirus diseases, due to the high whitefly population, are the major hurdles in its successful cultivation in this season. Fruit availability period of round gourd is short in the plains of North-Western India, and there is a need to standardize its production technologies, and extend fruit availability period.
Mulches are known to extend the harvest duration of various vegetables, to control weeds, and to reduce the incidence of viral diseases. These are mainly of two types – organic (natural) and inorganic (synthetic). Organic mulch includes crop residues, hay, straw, hardwood, sawdust, dry leaves and compost (Ranjan et al., Reference Ranjan, Patle, Prem and Solanke2017). Organic mulches have been reported to modify soil temperature, reduce evaporation losses, conserve soil moisture, promote drought tolerance, increase germination percentage, control weeds to some extent, add organic matter and other nutrients to the soil surface, reduce nitrate leaching and improve the soil physical and biological properties, thereby improving the plant growth attributes and yield of various crops, including tomato (Agele et al., Reference Agele, Iremiren and Ojeniyi1999, Reference Agele, Iremiren and Ojeniyi2000), brinjal (Singh et al., Reference Singh, Awasthi, Sharma, More and Meena2011), bell pepper (Singh et al., Reference Singh, Singh, Singh, Rawat, Kamal, Mehta and Kumar2012), okra (Adekiya et al., Reference Adekiya, Agbede, Aboyeji and Dunsin2017) and squash melon (Birbal et al., Reference Birbal, Rathore, Nathawat, Soni and Yadava2014, Reference Birbal, Soni, Rathore, Nathawat and Renjith2019). Among organic mulches, paddy straw mulch is readily available in Indian Punjab due to the extensive cultivation of paddy on about 3 million ha (Kaur et al., Reference Kaur, Sekhon, Thaman, Sidhu, Garg and Chawla2020).
Inorganic mulches include plastic (polyethylene), aluminium-coated plastic, biodegradable plastic and photo-degradable plastic mulch. Plastic mulches are readily available and commercially used by many vegetable growers in Punjab and other parts of the world, owing to their multiple benefits (Gordon et al., Reference Gordon, Foshee, Reed, Brown, Vinson and Woods2008; Mitchell et al., Reference Mitchell, Summers, Newton and Stapleton2013; El-Beltagi et al., Reference El-Beltagi, Basit, Mohamed, Ali, Ullah, Kamel, Shalaby, Ramadan, Alkhateeb and Ghazzawy2022). Plastic mulches are reported to control weeds (Johnson and Mullinix, Reference Johnson and Mullinix2008), conserve soil moisture, modify soil temperature (Moreno and Moreno, Reference Moreno and Moreno2008; Dhatt et al., Reference Dhatt, Garg, Singh and Aujla2017), enhance plant nutrient uptake from soil (Chen et al., Reference Chen, Li, Simunek, Shi, Ding and Zhang2020), induce early fruit harvest (Dhatt et al., Reference Dhatt, Garg, Singh and Aujla2016), improve plant growth (Zhang et al., Reference Zhang, You, Tian and Li2019; Lee and Park, Reference Lee and Park2020), yield attributes (Diaz-Perez, Reference Diaz-Perez2010) and reduce viral disease incidence (Stapleton and Summers, Reference Stapleton and Summers2002) in many vegetable crops. These mulches are mainly made from low-density poly-ethylene and are available in various colours, each serving different function and purpose. However, black and silver-black coloured plastic mulches are easily available, and widely used in India. The influence of plastic mulch on soil temperature, plant growth, development and yield may vary with crop, bed width, season, location, soil type, climate and mulching material (Diaz-Perez and Batal, Reference Diaz-Perez and Batal2002; Mendonca et al., Reference Mendonca, Avila, Vital, Evangelista, Pontes and Nascimento2021). Therefore, it becomes necessary to conduct studies on a particular vegetable in various seasons using different mulches at a particular location.
The literature on the effect of mulching in round gourd is scanty. The effects of straw and black plastic mulching on earliness, plant growth, fruit yield and yield components in round gourd in Bikaner, India have earlier been reported (Birbal et al., Reference Birbal, Rathore, Nathawat, Soni and Yadava2014, Reference Birbal, Soni, Rathore, Nathawat and Renjith2019). Therefore, the present study was conducted to investigate the effects of sowing time, mulching and their interaction on plant growth attributes, viral disease incidence, earliness, fruit yield and yield components of round gourd so as to identify the best mulch for different sowing times.
Materials and methods
Experimental site description
The field trials were conducted for two consecutive years i.e. 2021 and 2022 (Fig. SI-1 in supplementary information file) at Punjab Agricultural University, Regional Research Station, Jodhpur Romana Farm, Bathinda, India (30°17 ́N, 74°58 ́E, 211 m a.s.l). The site lies in the South-Western zone of Punjab, which falls in the Indo-Gangetic plains and has a semi-arid climate, with an average annual rainfall of 400–450 mm, 70–80% of which occurs during the monsoon season (July to mid-September). The mean monthly agro-climatic observations during the crop season (Table SI-1) and the initial properties of the experimental soil (at 0–15 cm depth) (Table SI-2) were recorded for both years.
Plant materials, experimental design and treatments
The round gourd cultivar ‘Punjab Tinda1’ released by the Punjab Agricultural University, Ludhiana, for cultivation in the state of Punjab, India, was used in the present study (Garg, Reference Garg2020). The field experiment was laid out in a split-plot design with three replications (Fig. SI-2 in supplementary information file). The main-plot treatment consisted of six sowing times, i.e., first week of March (DS1), April (DS2), May (DS3), June (DS4), July (DS5) and August (DS6). The sub-plot treatment comprised three mulching treatments (silver-black plastic mulch [SBPM; thickness 25 μm], black plastic mulch [BPM; thickness 25 μm] and paddy straw mulch [PSM; 6 t/ha] and a control [bare soil [BS]).
Field preparation and agronomic practices
After the application of pre-sowing irrigation, the experimental field was ploughed twice at field capacity with a disc harrow and cultivator, followed by planking to bring the soil to a fine tilth. After that, bed marks were made at a distance of 2.0 m. Mineral fertilizers, viz., urea (46% N), diammonium phosphate (DAP) (18% N and 46% P2O5), and muriate of potash (MOP) (60% K2O) (Indian Farmers Fertilizer Cooperative, New Delhi, India), were used to supply 100 kg N, 50 kg P2O5 and 50 kg K2O per hectare. Band placement of a full dose of all three fertilizers was carried out at 15 cm away on both sides of the bed mark. Fertilization was followed by the preparation of furrows (15 cm deep and 30 cm wide) in between two beds having 1.7 m wide base and 15 cm height above the ground. Plastic mulch of silver-black and black colours (2.1 m wide and 25 μm thick) was manually and tightly spread over the beds in the respective plots. Round holes of about 5 -cm diameter were made in the plastic mulch with a blade on top of the furrow on both sides of the bed at an intra-row spacing of 45 cm. Two seeds per hill were sown on pre-irrigated furrows at field capacity, and later on thinning was done to retain one plant per hill. Paddy straw mulch (6 t/ha) was applied immediately after sowing. Net plot size was 4.5 m × 2.0 m to accommodate 20 plants in two rows of ten each. Subsequent furrow irrigations were provided at weekly intervals during the summer, and on need-based in the rainy season.
Observations recorded
Soil temperature (°C)
One soil thermometer (Jain Scientific Glass Works, Ambala, India) per treatment was installed at 5-cm soil depth in-between two plants within a row (near the root zone of the plant). The soil temperature (°C) was recorded daily (0830 and 1400 h) in the mulching treatments along with bare soil. The weekly average soil temperatures for the entire crop season were calculated for both years.
Plant growth and disease attributes
Vine length (cm) of five randomly selected plants per plot was measured with a 1 m-long scale at 45 days after sowing (DAS) and at final harvest. The number of leaves per vine was recorded from five randomly selected plants per plot at peak harvest. The number of primary branches per vine was recorded from five randomly selected plants per plot at final harvest. Incidence of viral disease (%) was estimated as the proportion (%) of plants showing viral disease symptoms (interveinal chlorosis, chlorotic mottling and complete yellowing of leaves) in each plot at 60 DAS (Aguilar et al., Reference Aguilar, Abad and Aranda2006).
Earliness, fruit yield and yield components
The first flush of fruits was borne early. These fruits remained small, unmarketable and for proper vegetative growth, fruits were removed as soon as they appeared. The fruits of the later flush attained marketable size and were harvested on alternate days, when they were green, immature, tender, and hairy. At each harvest, the number and weight of fruits from each plot was noted. The number of days from sowing to the day when 50% of the plants in each plot started fruit harvest was recorded as ‘days to first picking’. The fruit yield obtained up to 55 DAS was considered as early yield (kg per plot), and was later on converted into tons per hectare. The number of fruits per vine was calculated by adding the number of fruits from each harvest, and dividing it by the number of plants per plot. To calculate fruit weight, the weight of fruits harvested from all pickings was added up and divided by the total number of fruits per plot. The duration in days from the first to last harvest was taken as harvest duration. The weight of fruits per plot from all pickings was added to estimate marketable yield (kg per plot), and was converted into tons per hectare.
Statistical analyses
The data collected during individual years were subjected to analysis of variance (ANOVA) procedure laid out for split-plot design (Gomez and Gomez, Reference Gomez and Gomez1984). The individual year data were also subjected to pooled ANOVA using mixed-model so as to study the effects of sowing date, mulching and their interaction (sowing date × mulching) on various traits. The mean values were compared using Fisher's least significant difference (LSD) test (P = 0.05). All the data were analysed employing R-studio software version 1.4.1106 using ‘doebioresearch’ package.
Results
Effect of mulching on the morning and afternoon soil temperatures (oC) at 5 cm depth
The SBPM and BPM increased the morning and afternoon soil temperatures over BS throughout the crop season during both years (Figs 1 and 2). However, the increase in soil temperature was higher with BPM compared with SBPM. Secondly, the increment in soil temperature with plastic mulching over BS was higher in the afternoon than in the morning hours. The SBPM enhanced the morning soil temperature over BS by 0.4–1.9°C (average 1.2°C) during 2021, 1.0–2.8°C (average 1.6°C) during 2022, whereas the increment in the afternoon soil temperature varied from 1.0 to 3.6°C (average 2.1°C) during 2021 and 1.8–4.3°C (average 3.1°C) during 2022. The BPM increased the morning soil temperature over BS by 0.6–2.5°C (average 1.5°C) during 2021, 1.3–3.4°C (average 2.1°C) during 2022 whereas the enhancement in afternoon soil temperature varied from 1.4 to 4.7°C (average 3.1°C) during 2021 and 2.3–5.6°C (average 4.1°C) during 2022.
Effects of mulching on weekly soil temperature (°C) (at 5 cm depth) recorded in the morning (08.30 h) (a); and in the afternoon (14.00 h) (b) during the crop season (March to early-October 2021) at Punjab Agricultural University, Regional Research Station, Jodhpur Romana Farm, Bathinda, India. SBPM, BPM, PSM and BS stand for silver-black plastic mulch (25 μm thick), black plastic mulch (25 μm thick), paddy straw mulch (6 t/ha) and bare soil, respectively.
Effects of mulching on weekly soil temperature (°C) (at 5 cm depth) recorded in the morning (08.30 h) (a); and in the afternoon (14 00 h) (b) during the crop season (March to early-October 2022) at Punjab Agricultural University, Regional Research Station, Jodhpur Romana Farm, Bathinda, India. SBPM, BPM, PSM and BS stand for silver-black plastic mulch (25 μm thick), black plastic mulch (25 μm thick), paddy straw mulch (6 t/ha) and bare soil, respectively.
On the contrary, the PSM decreased the morning and afternoon soil temperatures over BS throughout the crop season during both years (Figs 1 and 2). However, the decreased temperature was more pronounced in the afternoon than in the morning. The PSM reduced the morning soil temperature over BS by 0.8–1.4°C (average 1.1°C) during 2021, 0.5–1.8°C (average 0.9°C) during 2022, whereas the decrease in afternoon soil temperature varied from 1.5 to 3.5°C (average 2.3°C) during 2021, and 1.3–5.5°C (average 3.5°C) during 2022.
Plant growth parameters
Irrespective of sowing time, mulching significantly (P < 0.001) influenced the plant growth attributes, viz., vine length (at 45 DAS and at final harvest), number of leaves, and number of primary branches during both years (Tables 1 and 2). The highest values of these growth attributes were obtained for SBPM, which was significantly better than BPM, PSM and BS. On the basis of data pooled over the years (Table 3), the SBPM increased the vine length (at 45 DAS) over BS by 45.0%, vine length (at final harvest) by 20.8%, number of leaves by 67.4%, and number of primary branches by 18.9%. The BPM ranked second to SBPM, and it enhanced the vine length (at 45 DAS) over BS by 30.7%, vine length (at final harvest) by 11.6%, number of leaves by 46.4%, and number of primary branches by 13.6%. However, the SBPM and BPM registered similar values of the number of primary branches during 2021 (Table 1) and for pooled data (Table 3). Compared with plastic mulches, the magnitude of increase in growth attributes with PSM was lower, and it recorded values at par with BS in respect of vine length (at final harvest) during 2021 (Table 1), and the number of primary branches during both years. Based on data pooled over the years (Table 3), the PSM improved the vine length (at 45 DAS) over BS by 16.5%, and the number of leaves by 16.6%.
Effects of sowing date and mulching on plant growth, earliness, viral diseases, fruit yield and yield components of round gourd in 2021
1 DS1, DS2, DS3, DS4, DS5 and DS6 stand for first week of March, April, May, June, July and August, respectively. 2 SBPM, BPM, PSM and BS stand for silver-black plastic mulch (25 μm thick), black plastic mulch (25 μm thick), paddy straw mulch (6 t/ha) and bare soil, respectively. 3DAS stand for days after sowing. Means in a column with no letter in common are significantly different by Fisher's LSD test.
Effects of sowing date and mulching on plant growth, earliness, viral diseases, fruit yield and yield components of round gourd in 2022
1 DS1, DS2, DS3, DS4, DS5 and DS6 stand for first week of March, April, May, June, July and August, respectively. 2 SBPM, BPM, PSM and BS stand for silver-black plastic mulch (25 μm thick), black plastic mulch (25 μm thick), paddy straw mulch (6 t/ha) and bare soil, respectively. 3DAS stand for days after sowing. Means in a column with no letter in common are significantly different by Fisher's LSD test.
Effects of sowing date and mulching on plant growth, earliness, viral diseases, fruit yield and yield components of round gourd for data pooled over 2021 and 2022
1 DS1, DS2, DS3, DS4, DS5 and DS6 stand for first week of March, April, May, June, July and August, respectively. 2 SBPM, BPM, PSM and BS stand for silver-black plastic mulch (25 μm thick), black plastic mulch (25 μm thick), paddy straw mulch (6 t/ha) and bare soil, respectively. 3DAS stand for days after sowing. Means in a column with no letter in common are significantly different by Fisher's LSD test.
Irrespective of mulch, the plant growth attributes were significantly (P < 0.001) affected by sowing time during both years (Tables 1 and 2). The maximum vine length (at 45 DAS) was observed in DS3, and it was statistically at par with DS2 whereas the minimum value was recorded in DS1 (Table 3). The maximum vine length (at the final harvest) was recorded in DS1 during 2021 (Table 1), and DS2 during 2022 (Table 2), whereas for pooled data, both sowing dates registered at par values (Table 3). Vine length (at final harvest) exhibited a decreasing trend with each successive sowing date, and the minimum vine length was recorded with DS6. The maximum number of leaves and number of primary branches were recorded with DS2. The lowest number of leaves was manifested in DS6 during both years. The DS5 and DS6 registered the minimum and at par values of the number of primary branches (Table 3).
The interaction effects (sowing time × mulch) for pooled data revealed that the maximum vine length (at 45 DAS) was exhibited by DS3 + SBPM, whereas the highest vine length (at final harvest) was registered by DS1 + SBPM (Table 4). The maximum number of leaves was exhibited by DS2 + SBPM, which was at par with DS1 + SBPM. The number of primary branches was the maximum in DS2 + BPM, DS3 + BPM and DS3 + SBPM. The lowest vine length (at 45 DAS) was exhibited by DS1 + BS, which was at par with DS1 + PSM. The minimum vine length (at final harvest), number of leaves and number of primary branches were manifested by DS6 + BS.
Interaction effects (sowing date × mulching) on plant growth, earliness, viral diseases, fruit yield and yield components of round gourd for data pooled over 2021 and 2022
1 DS1, DS2, DS3, DS4, DS5 and DS6 stand for first week of March, April, May, June, July and August, respectively. 2 SBPM, BPM, PSM and BS stand for silver-black plastic mulch (25 μm thick), black plastic mulch (25 μm thick), paddy straw mulch (6 t/ha) and bare soil, respectively. 3DAS stand for days after sowing. Means in a column with no letter in common are significantly different by Fisher's LSD test.
Incidence of viral diseases (%)
Irrespective of sowing time, mulching significantly (P < 0.001) reduced the incidence of viral diseases during both years (Tables 1 and 2). The minimum incidence was recorded using SBPM followed by BPM, PSM and BS during both years. On the basis of data pooled over the years (Table 3), the SBPM, BPM and PSM decreased the incidence of viral diseases over BS by 73.8, 68.4, and 53.4%, respectively.
Irrespective of mulch, the incidence of viral diseases was significantly (P < 0.001) affected by the sowing time during both years (Tables 1 and 2). The lowest incidence was recorded in DS1 during both years and it exhibited an increasing trend with each successive sowing date. However, the highest incidence was observed in DS5 during both years.
The interaction effects (sowing time × mulch) were significant (P < 0.001) for the incidence of viral diseases during both years (Tables 1 and 2). On the basis of pooled data (Table 4), the minimum incidence was recorded in DS1 + SBPM, which was at par with DS1 + BPM, DS2 + SBPM, DS2 + BPM and DS3 + SBPM. The maximum viral disease incidence was observed in DS5 + BS.
Earliness
Irrespective of sowing time, the application of different mulches induced significant differences (P < 0.001) in the early yield of round gourd during both years (Tables 1 and 2). The highest early yield was produced with SBPM, followed by BPM, PSM and BS. On the basis of pooled data (Table 3), the SBPM, BPM and PSM enhanced the early yield over BS by 192.8, 134.6 and 26.5%, respectively.
Irrespective of mulching, the sowing date resulted in significant differences (P < 0.001) in early yield during both years (Tables 1 and 2). The highest early yield was observed in DS2 during both years. It was significantly higher compared with DS3 and DS1 during 2021 (Table 1), but was at par with DS1 during 2022 (Table 2). The lowest early yield was recorded in DS5 during both years.
The interaction effects (sowing time × mulch) were significant (P < 0.001) for early yield during both years (Tables 1 and 2). On the basis of pooled data (Table 4), the highest early yield was observed in DS2 + SBPM, which was at par with DS1 + SBPM. On the other hand, the lowest early yield was recorded in DS5 + BPM, which was at par with DS6 + BS, DS5 + BS, DS5 + PSM, DS4 + BS, DS5 + SBPM, DS6 + PSM and DS4 + PSM.
Fruit yield and yield components
Irrespective of the sowing time, mulching significantly (P < 0.001) affected the fruit yield and yield components of round gourd during both years (Tables 1 and 2). The maximum fruit yield and yield components (fruit number, fruit weight and harvest duration) were recorded using SBPM, which was superior to BPM, PSM and BS during both years. However, the SBPM performed at par with BPM in respect of fruit weight during 2021 (Table 1), and fruit number and harvest duration during 2022 (Table 2). On the basis of pooled data (Table 3), the SBPM increased the fruit number over BS by 74.4%, fruit weight by 12.8%, harvest duration by 26.7% and fruit yield by 88.6%. The BPM ranked second to SBPM, and it improved the fruit number over BS by 59.5%, harvest duration by 21.9%, and fruit yield by 63.2%. However, the BPM registered fruit weight at par with BS in 2022 (Table 2), but it increased the same by 11.9% over BS in 2021 (Table 1). The PSM ranked third in performance after SBPM and BPM. But it was at par with BS in respect of fruit weight during both years, and with respect to fruit number, harvest duration and fruit yield during 2021 (Table 1). However, the PSM increased the fruit number over BS by 18.9%, harvest duration by 12.7% and fruit yield by 26.1% during 2022 (Table 2).
Irrespective of mulch, the sowing time exhibited significant differences (P < 0.001) in fruit yield and yield components (fruit number, fruit weight and harvest duration) of round gourd during both years (Tables 1 and 2). The highest values of these four traits were observed in DS1 during 2021 and in DS2 during 2022. However, DS1 recorded fruit weight at par with DS2 during 2022 (Table 2) and for pooled data (Table 3). The minimum fruit number, harvest duration and fruit yield were recorded in DS5 during both years, and the values were significantly lower compared with other sowing dates. However, the fruit yield in DS6 was at par with DS5 during 2022 (Table 2). The lowest fruit weight was manifested in DS6, whereas the fruit weight in DS3 and DS4 was statistically at par during both years. Besides, the fruit number in DS4 and DS6 was statistically at par during both years.
The interaction effects (sowing time × mulch) for pooled data (Table 4) revealed that the highest fruit number, and yield were recorded in DS2 + SBPM, which was at par with DS1 + SBPM. The harvest duration was the maximum in DS1 + SBPM, which was at par with DS1 + BPM, and DS2 + SBPM. On the contrary, the lowest fruit number, and yield were observed in DS5 + BS.
The interaction effects also revealed that various mulches improved the fruit yield and yield components of round gourd by a variable degree in different sowing times. Based on pooled data (Table 4), the SBPM enhanced the fruit yield over BS by 324.1% in DS6, 161.5% in DS5, 155.8% in DS3, 97.8% in DS4, 78.1% in DS2 and 63.5% in DS1. Similarly, the BPM increased the fruit yield over BS by 313.0% in DS6, 114.0% in DS3, 72.9% in DS4, 57.1% in DS2 and 43.4% in DS1. Likewise, the PSM improved the fruit yield of round gourd over BS by 77.8% in DS6, 51.9% in DS5, 40.7% in DS3, 27.6% in DS4 and 15.4% in DS2.
Discussion
The plastic mulches increased the morning and afternoon soil temperatures over bare soil. The BPM caused more remarkable changes in the morning (1.5–2.1°C) and afternoon (3.1–4.1°C) soil temperatures compared with SBPM (1.2–1.6°C in the morning and 2.1–3.1°C in the afternoon). This difference in temperature modification was observed as the degree of soil warming is considered to be inversely correlated with the light reflected by the coloured plastic mulches. The BPM is reported to have the lowest light reflectance (10% photosynthetically active radiation [PAR]) (Diaz-Perez and Batal, Reference Diaz-Perez and Batal2002). In other words, it absorbs most of the incident solar radiation and transmits to the earth's surface by conductance (Kumar and Dey, Reference Kumar and Dey2011). On the other hand, SBPM is reported to reflect a considerable portion (about 55% PAR) of the solar radiation, primarily infrared radiation, thereby raising the soil temperature by a lower magnitude than BPM. Also, the broader differences in soil temperatures of these two plastic mulches during the afternoon compared with the morning hours were due to greater incident solar radiation at 1400 h than at 0830 h or, in other words, were positively correlated with air temperature. Mendonca et al. (Reference Mendonca, Avila, Vital, Evangelista, Pontes and Nascimento2021) have also reported a higher increase in soil temperature (at 5 cm depth) with BPM (2.16 and 2.37°C) compared with silver plastic mulch (1.95 and 1.37°C) over BS at 0700 and 1400 h, respectively, in tomato grown in the fall-winter season in Brazil.
On the contrary, the PSM decreased the morning (0.9–1.1°C) and afternoon (2.3–3.5°C) soil temperature (at 5 cm depth) over BS. The PSM did not allow the solar radiation to reach the earth's surface, thereby reducing the soil temperature compared with BS, which was warmed by the incident solar radiation. Here also, the differences were wider in the afternoon compared with the morning hours, as these differences were directly proportional to incident solar radiation. Mendonca et al. (Reference Mendonca, Avila, Vital, Evangelista, Pontes and Nascimento2021) have reported 3.06°C and 3.40°C reduction in soil temperature (at 5 cm depth) at 1400 h with rice straw mulch over BS in tomato grown in fall–winter and spring–summer seasons, respectively, in Brazil.
Among mulches, SBPM showed the best plant growth attributes of round gourd, followed by BPM and PSM in all sowing dates. Irrespective of sowing time, among four plant growth traits studied, the per cent improvement over BS was the highest in number of leaves (16.5–67.2%), followed by vine length (at 45 DAS) (16.4–44.9%), vine length (at final harvest) (5.6–20.6%) and number of primary branches (3.0–3.6%). The increase (%) in vine length with plastic mulching was higher at 45 DAS than that at final harvest. This differential increment suggested the role of plastic mulching in giving a high initial impetus to plant growth and development. The improvement in plant growth attributes of round gourd with plastic mulching could be primarily due to an increase in soil temperature, which influences various physiological processes in roots, such as water and nutrient uptake (Diaz-Perez and Batal, Reference Diaz-Perez and Batal2002; Diaz-Perez, Reference Diaz-Perez2009). The other reasons could be weed control, soil moisture conservation, improved soil microbial activity and reduced nitrate leaching with plastic mulching. The better performance of SBPM over BPM may be attributed to the increased photosynthetic activity of plants with the former, as the radiations reflected by the SBPM fall in PAR, thereby improving plant growth and morphogenesis (Diaz-Perez and Batal, Reference Diaz-Perez and Batal2002). While working on squash melon in Rajasthan, India, Birbal et al. (Reference Birbal, Soni, Rathore, Nathawat and Renjith2019) have also recorded 56.0 and 33.5% increase in vine length with PSM and BPM, respectively, over BS. Mulches have also been reported to increase the number of leaves in sponge gourd (Khan et al., Reference Khan, Pal and Kumar2015), and cucumber (Soleymani et al., Reference Soleymani, Hassandokht and Abdoosi2015).
The improvement in plant growth attributes of round gourd with PSM was lower compared with plastic mulching in all sowing dates. It could be due to the lowering of soil temperature with PSM over BS. Although PSM helped in controlling weeds and conserving soil moisture, but it appeared that an increase in soil temperature and the radiations reflected by plastic mulches played a more critical role in improving the plant growth attributes of round gourd. While working on okra in Nigeria, Adekiya et al. (Reference Adekiya, Agbede, Aboyeji and Dunsin2017) have reported significant increases in plant height (117.6 and 73.7%) and number of leaves (53.3 and 50.0%) over BS, with the use of grass mulches in dry and wet seasons, respectively.
All three mulches significantly reduced the incidence of viral diseases over BS in all six sowing dates. The maximum decrease in disease incidence was exhibited by SBPM, followed by BPM and PSM. The reduction in viral disease incidence could be due to the reason that plastic mulches act as a repellent for the vector (whitefly) due to modification of the light environment around the plant. However, as PSM also reduced the incidence of viral diseases over BS, modification in soil temperature, which in turn affected the plant's response to viral disease, may be the other reason for the reduction in disease incidence (Diaz-Perez et al., Reference Diaz-Perez, Gitaitis and Mandal2007). While working on late-season cantaloupe, Stapleton and Summers (Reference Stapleton and Summers2002) have reported that the number of aphids on leaves of plants on plastic film mulch was consistently lower than on plants growing over BS. Furthermore, the start of appearance of viral disease symptoms was delayed by 3–6 weeks in mulched plants, which was crucial for normal flowering and fruiting.
All three mulches significantly enhanced the early yield of round gourd over BS. Irrespective of sowing time and based on pooled data, the highest increment (192.8%) was registered by SBPM, followed by BPM (134.6%), while PSM recorded the lowest increase over BS (26.5%). The increase in early yield with mulching could be attributed to an increase in the rate of plant growth and development (faster vegetative growth, early flowering and early fruit set) due to modification in soil temperature. Ibarra et al. (Reference Ibarra, Flores and Diaz-Perez2001) have observed 266% higher early yield of muskmelon grown in Mexico with BPM compared with the control and attributed it to soil-accumulated heat. Wortman et al. (Reference Wortman, Kadoma and Crandall2016) have also observed early fruit set in cucumber with SBPM, and attributed it to higher uptake and better absorption of nutrients.
Both the plastic mulches significantly increased the fruit yield and yield contributing traits (fruit number, fruit weight and harvest duration) of round gourd over BS during both years except for BPM with respect to fruit weight during 2022. On the contrary, PSM caused a significant increase in fruit yield, fruit number and harvest duration over BS only during 2022, which was a comparatively hot year compared with 2021. It implied that the soil temperature-lowering effect of PSM proved useful in the second year as it protected the round gourd plants from high-temperature stress. During these two years, the maximum improvement in fruit yield and yield components was exhibited by SBPM, followed by BPM and PSM, in all six sowing times except in DS5, in which PSM proved superior to BPM. Irrespective of sowing time, among these four yield related traits, the maximum per cent improvement over BS was recorded in fruit yield (15.2–88.6%) followed by fruit number (13.5–73.0%) and harvest duration (8.0–26.8%), whereas fruit weight exhibited the lowest increment (3.1–12.7%). Extension in harvest duration with mulching may be the reason behind the increased fruit number. Whereas the improvement in fruit yield of round gourd with mulching was considered to be due to increased fruit number, fruit weight and early yield as these three traits have been reported to be positively correlated with the fruit yield of round gourd (Garg, Reference Garg2019). Birbal et al. (Reference Birbal, Soni, Rathore, Nathawat and Renjith2019) have also observed increases in fruit yield (26.4 and 48.8%), fruit number (17.6 and 37.0%) and fruit weight (7.9 and 13.9%) of round gourd grown at Bikaner, India, with BPM and straw mulching, respectively, over BS. Singh et al. (Reference Singh, Sharma, Kumar, Dhillon and Sekhon2017) have reported that the harvest duration of tomato was maximum with double-shaded plastic mulch (85.6 days) that was statistically at par with BPM (83.8 days), but significantly higher than with straw mulch (79.8 days) and BS (78.2 days).
In DS5, the fruit yield and its contributing traits with BPM were similar to those of the control and inferior to those with PSM. It revealed that plants faced high-temperature stress in DS5 due to an increase in soil temperature with BPM, leading to less-than-expected fruit yield. However, the cooling effect of PSM saved plants from heat stress in DS5, thereby improving yield and its contributing traits.
Among the six sowing times, DS1 (first fortnight of March) in 2021, and DS2 (first fortnight of April) in 2022 proved to be the best sowing times of round gourd because of superior crop performance with respect to most of the traits. The crop performance declined with each successive sowing date. However, crops sown in the first fortnight of August (DS6) performed better than that sown in the first fortnight of July (DS5) during both years. The probable reason was that round gourd is a short-duration (80–90 days) crop, which performs best in hot and dry weather (March–June), but its performance declines in hot and humid weather (July–August). In the first two sowing dates (DS1 and DS2), crop growth phases were completed before the onset of humid weather (monsoon rains). However in the other sowing dates (DS3–DS6), crop growth phases occurred in July and August, leading to their comparatively poor performance.
The round gourd crop sown at the optimum time (DS1 and DS2) showed improved growth attributes because it provided the plants with a longer growing period before the onset of adverse environmental conditions, such as high-temperature stress or disease pressure. This extended growth phase helped the plants establish a more robust root system, which enhanced their water and nutrient uptake, leading to vigorous plant growth and development. The increased leaf area captured more sunlight and converted it into energy, leading to better plant growth, and potentially high fruit yield. On the contrary, late-sown round gourd crop faced comparatively less favourable growing conditions, which hampered leaf development. Insufficient leaf area limited the plant's ability to photosynthesize and produce energy, potentially affecting plant growth and yield (Aniekwe and Anike, Reference Aniekwe and Anike2015). Khan et al. (Reference Khan, Iqbal, Jilani, Ghaffoor and Waseem2001) also studied the effect of six sowing dates (28 February, 10 March, 20 March, 30 March, 9 April and 19 April) on plant growth and fruit yield of round gourd at Dera Ismail Khan, Pakistan, and reported that early sowing (28 February) yielded higher than late sowing (19 April). The maximum fruit yield was recorded in 30 March sowing, which was at par with 10 March and 20 March sowings.
The DS1 recorded the lowest vine length (at 45 DAS) during both years due to below-optimum air temperatures up to mid-April. Whereas DS3 and DS2 manifested the highest vine length (at 45 DAS) which DS5 and DS4, closely followed. The probable reason for the increase in vine length during late-season sowings was high air temperature as reported by Papadopoulos and Hao (Reference Papadopoulos and Hao2000) in cucurbits.
During both years, the viral disease incidence was the lowest in DS1, but it progressed with each successive sowing date, except that it was significantly lower in DS6 compared with DS5. Whitefly, a vector of begomovirus diseases in cucurbits, has a high infestation in South-Western Punjab, India, during July to September, due to a considerable area under Bt-cotton hybrids, coupled with warm and humid weather. Therefore, viral disease incidence was high in late-sown crop of round gourd compared with early-sown crop. Secondly, late-sown crop often faces adverse climatic conditions such as high air temperature, which increase the rate of systemic movement, and decrease the time for the first appearance of viral disease symptoms (Diaz-Perez et al., Reference Diaz-Perez, Gitaitis and Mandal2007). Early-planting of tomato in spring season lead to a comparatively lower incidence of tomato spotted wilt virus compared with late-season planting (Riley and Pappu, Reference Riley and Pappu2000).
The interaction effects revealed that the best plant growth attributes, earliness, fruit yield, and yield components of round gourd, along with low viral disease incidence were achieved with DS2 + SBPM, DS1 + SBPM, DS2 + BPM and DS1 + BPM. In other words, utilization of SBPM or BPM in the cultivation of round gourd in DS1 or DS2 improved earliness, growth, fruit yield and reduced the incidence of viral diseases. However, the improvement in various traits with these mulches (over BS) was enhanced in challenging environments or in late-season sowings (DS3 through DS6) than in more favourable environments (DS1 and DS2). However, in DS4 through DS6, the absolute values of fruit yield and yield components were too low to be adopted by the growers, even with mulching. Nevertheless, sowing of round gourd in DS3 (first week of May) along with utilization of SBPM may also be recommended for the farmers of South-Western Indian Punjab, in which 70–80 days gap exists between the harvesting of wheat (mid-April), and the transplanting of paddy (mid-July). Round gourd may become a profitable option even with a moderate fruit yield as high prices in late sowings may compensate for the low fruit yield. Additionally, round gourd cultivation may provide a new crop in the lean period.
Conclusion
In North Indian plains, it is advised to sow round gourd in early-March to early-April using silver-black or black plastic mulch for improving earliness, growth and fruit yield attributes, and lowering viral disease incidence. Moreover, the silver-black plastic mulch proved beneficial in extending the crop sowing time to first week of May so as to exploit the 70–80 days gap between wheat harvesting and paddy transplanting.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0021859624000406
Acknowledgements
This work was based on the first author's MSc thesis. We are thankful to Dr Pritpal Singh, Professor of Statistics, at Punjab Agricultural University, Ludhiana, India, for his kind help in statistical analysis during manuscript revision.
Author contributions
MS conducted experiments and recorded data. NG conceived and designed the research, performed statistical analyses and wrote the manuscript.
Funding statement
This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Ethical standards
Not applicable.
Availability of data and material
All data generated and analysed in support of the findings of this research have been included in this published article and its associated supplementary information file.
