The effect of transplanting date and covering on the growth and development of melon (Cucumis melo L.)

The effect of different transplanting dates (May 15, May 25, and June 4) and date of polypropylene fiber removal (4 and 8 weeks after transplanting, and control without covering) on the growth, development, and yield of melon (Cucumis melo L.) were investigated. The experiment was carried out during 2008–2010 in eastern Poland (51°53'23.64" N, 22°27'47.96" E). Plants planted on May 15 were longer by 22% and 56%, weighed more by 39% and 76%, and their leaf area index (LAI) was higher by 24% and 117% compared to plants planted on May 24 and June 4, respectively. However, delayed planting reduced the period of growth and development by 7 and 17 days, respectively. When the covering application period was extended from 4 to 8 weeks, stem length increased by an average of 23% and LAI by 38%, but harvest was delayed by 6 days. The respective yields of melon fruits planted on May 15, May 25, and June 4 amounted to 5.09, 4.73, and 3.99 kg m−2. The covering of plants planted at each date contributed to an increase in yield and in the share of marketable fruit yield in the total yield. However, the length of the cover application period did not affect yield levels.


Introduction
Melon (Cucumis melo L.) is cultivated in Poland by amateurs only.However, due to its nutritional benefits, medicinal properties, and flavor-related attributes, the cultivation of this plant should be popularized and considered for commercial production.Also, the melon's biological value is much higher compared with its close relative, the cucumber [1].
Ideal growing conditions for melons, in which the plant growth stages progress best, include an air temperature of over 20°C.When temperature drops below 15°C, the growth of melon plants is inhibited.Melons are also very prone to air and soil temperature fluctuations, which cause physiological disturbances.To avoid damage caused to thermophilic vegetables by low air and soil temperatures (April, May), it is recommended to delay the transplanting date.However, there is an economic incentive to start cultivation earlier so as to accelerate the harvest, or to extend the growing season by transplanting seedlings in less favorable thermal conditions.Whereas the sowing date affects seedling quality, the transplanting date influences the growth of plants after seedlings have been transplanted [2][3][4].
In addition, the water needs of melon are high, particularly during the period of intense vegetative growth and fruit formation [5].

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The growth and development of thermophilic vegetable species in the field is improved when plants have been covered with polypropylene fiber [6][7][8].Covers reduce evaporation and decrease heat losses at night.In the study by Kosterna [9], soil temperature increased by 1.3-1.7°Cas a result of covering.A soil temperature increase due to cover application was observed in the last week of May.Moreno et al. [10] reported that the soil temperature at a depth of 5 cm was on average by 5°C higher under a nonwoven polypropylene sheet.In the study by Hamouz et al. [11], plant covering with polypropylene fiber contributed to a 1.8°C increase in soil temperature and a 2.0°C increase in air temperature, on average.Plants grown under polypropylene fiber are more uniform, taller, and their mass of aboveground parts is higher compared with cultivation without covers [12][13][14].Earlier plant development under covers contributes to greater solar radiation interception and the enlargement of the assimilation leaf area, which has the greatest impact on plant productivity [9,[15][16].
The study aimed to determine the effect of an earlier date of transplanting melon seedlings and different dates of polypropylene fiber removal on the growth and development of plants and fruit yields.

Experimental site
The experiment was carried out during 2008-2010 in eastern Poland (51°53'23.64"N, 22°27'47.96"E).According to the international system of FAO classification, the soil was classified as Podzols [17].The soil was characterized by a humus level of 37-43 cm, the average organic carbon content amounted to 2.1%, and the value of pH determined in H 2 O was 5.8.The total contents of macroelements in mg dm −3 were as follows: 14 mg NH 4 -N; 20 mg NO 3 -N; 19 mg P 2 O 5 ; 145 mg K 2 O; 797 mg Ca; 76 mg Mg.

Experimental design
The experiment was arranged as a split-block design with four replicates.The effect of transplanting date of cv.'Malaga F1' melon seedlings (June 4 -the conventionally recommended transplanting date in Central Europe, May 25 -10 days earlier, May 15 -20 days earlier compared with the recommended date) and date of polypropylene fiber removal (4 and 8 weeks after transplanting and control without covering) on the selected attributes of plant growth and development was investigated.

Field work and measurements
The field for melon cultivation was prepared following conventional cultivation recommendations.Pre-winter ploughing was performed in fall and followed by leveling in spring.Mineral fertilizers were applied at the following rates: 75 N, 140 P, and 160 K kg ha −1 , in the form of, respectively, urea, granular superphosphate and potassium sulfate.Next, they were mixed with the soil and the soil surface was smoothed using combined soil preparation equipment.Melon seedlings were grown in a non-heated greenhouse.Seeds were sown 4 weeks before the transplanting date, i.e., April 17, April 27, and May 7. Before the transplanting of seedlings in the field, they were hardened off and their vine tops were removed, so that each plant had three or four leaves, to stimulate the plants to produce side shoots on which flowers are formed.The seedlings were transplanted at a spacing of 80 × 100 cm, 16 plants per plot.Measurements were taken from four plants.The plot area for harvest was 12 m 2 .
Plant growth was determined before transplanting and 8 weeks after transplanting.The following measurements were taken: stem length (cm), weight of aboveground plant parts (g), number of leaves per plant, assimilation leaf area (cm 2 ).Assimilation leaf area was determined by the gravimetric method.The plant growth data was used to calculate leaf area index (LAI) and leaf area ratio (LAR).Also, total fruit yield (kg m −2 ), fruit weight (kg), and number of days from planting to first harvest were determined in each plot.In addition, the share of marketable yield in the total yield (%) was calculated.Fruit harvest was performed once a week as fruit ripened.

Statistical analysis
The results were statistically analyzed by ANOVA.The significance of differences was determined by Tukey's test at the significance level of α = 0.05.All the calculations were performed using STATISTICA, version 12.0, and MS Excel.

Meteorological conditions
Weather conditions varied during the study period and were not always favorable for plant growth (Fig. 1).In June 2008 and 2010, thermal conditions just after transplanting were more favorable than in 2009.Throughout all the study years, July and August were warmer compared with the long-term mean for these months.The most favorable thermal conditions in July and August were in 2010.The year 2009 was most unfavorable in terms of the amount of precipitation.Also, low temperatures in June were accompanied by very high precipitation.Moreover, high temperatures and drought in July exacerbated the situation.Unstable weather conditions in 2009 contributed to much poorer growth of seedlings.

Results
Seedlings prepared for transplanting on June 4 were significantly longer and heavier, and had a higher assimilation leaf area and LAR than those transplanted on May 15 and May 25, although in each case the time from sowing the seeds to the measurement was 28 days (Tab.1).
Air temperature and precipitation in 2009 fluctuated substantially (dry and hot July, wet and colder August).As a result, melon plants weighed less but their stems were longer than in 2008 when both rainfall and temperatures were more uniform during the growing season.The most favorable weather conditions for melon crops were in 2010 when the highest average air temperatures were recorded.Stem length, plant weight and LAI were significantly higher but LAR was significantly lower compared with the remaining study years (Tab.2).
Eight weeks after transplanting, the stem elongated more than five times and the weight of plants increased more than 30 times compared with their length and weight prior to transplanting.Stem length, plant mass, and LAI markedly declined whether the transplanting date was delayed by 10 or 20 days (Tab.3).
Irrespective of the transplanting date, plants covered with polypropylene fiber had significantly longer stems, weighed more and their LAI was higher compared with non-covered plants.At each transplanting date, plants with significantly longer stems grew in the plots where covering had been applied for 8 rather than 4 weeks.Plants transplanted on June 4 and May 25 covered for 8 weeks had a significantly higher weight (by 25%) and LAI (by 7%), respectively.Regardless of the transplanting date, non-covered plants had significantly higher LAR index values than those covered for either 4 or 8 weeks post-planting (Tab.3).
In the conditions of eastern Poland, delayed melon transplanting significantly reduced fruit yield and weight (Fig. 2).The yield of melons transplanted on May 25 and June 4 decreased by 7.6% and 29.5%, while the weight of fruits by 4.7% and 6.2%, respectively, compared to the first date (May 15).Irrespective of the transplanting date, covering with polypropylene fiber contributed to a significant increase in melon yield compared with lack of cover.At the consecutive transplanting dates, the increase due to covering amounted to 200%, 160%, and 125%, respectively.The length of covering time did not affect the yield level.The fruit weight of melon plants planted on May 15 and May 25 increased significantly as a result of plant covering.The increase amounted to 17% and 15.5%, respectively, compared with the non-covered control.
The highest (84%) and the lowest (76%) share of marketable yield in the total yield was recorded for plants planted on June 4 and May 15, respectively (Fig. 2).Covering increased the share of marketable yield in the total yield at each transplanting date.However, when the transplanting date was delayed, the effect of covering on the share of marketable yield decreased gradually.When the transplanting date was delayed by 10 and 20 days, a respective 8-and 17day decrease in the number of days from transplanting to the beginning of fruit harvest was observed.At all the transplanting dates, covering reduced the transplanting-toharvest time compared with no covering.After 4 and 8 weeks of covering, this period was shorter by 14 and 8 days, respectively (Fig. 2).
The study results revealed correlations between plant growth attributes and the beginning of harvest as well as yield and the average fruit weight.Irrespective of the transplanting date, the number of days from transplanting to the beginning of harvest was negatively correlated with stem length and plant weight (Tab.4).
The total yield was significantly positively correlated with stem length, plant weight, and LAI, but significantly negatively associated with LAR for all the transplanting dates.
For transplanting on May 15 and May 25, the average fruit weight was significantly positively correlated with stem length and plant weight, whereas for the June 4, such a relationship was not found.However, the average fruit weight was significantly negatively correlated with LAI for all the transplanting dates.Fig. 2 The effect of the investigated factors on the yields, length of period from transplanting to beginning of harvest and weight of melon fruits (C.melo) (averages for 2008-2010).
Tab. 4 The correlation coefficients between growth attributes and selected parameters of melon yield.Stem length was the only trait significantly positively correlated with the number of days from transplanting to harvest when covering with polypropylene fiber had not been applied.For the 4-week covering treatment, such a relation was found between stem length, plant weight, and LAI, whereas for 8-week covering between plant weight and the time from transplanting to beginning of harvest.

Investigated parameters (N = 12) Number of days to harvest Total yield Mean fruit weight
When no covering had been used, plant weight was significantly positively correlated with total fruit yield.For the 4-and 8-week cover application times, such a correlation was determined between stem length, plant weight as well as LAI and total fruit yield.The average fruit weight of covered plants, irrespective of the length of covering, was not significantly positively correlated with stem length, plant weight, or LAI.

Discussion
The study results showed a significant effect of weather conditions on the growth and development of melon plants.A marked influence of weather conditions prevailing in eastern Poland on tomato and potato development and yield was also reported by Wadas et al. [14] and Kosterna [18].
Seedlings transplanted at the second and third date were characterized by poorer growth.Application of covers influenced melon growth.According to Ibarra et al. [12], polypropylene fiber contributes to an increase in temperature around plants because it prevents some of the heat emitted by both soil and plants from escaping.Higher soil and air temperatures under covers provide better conditions for plants immediately after transplanting and allow them to produce a higher mass of aboveground parts [11,14,19].More favorable conditions under covers increased the leaf area index (LAI) which describes the growth of the plants.In her study, Kosterna [9] demonstrated that the LAI value of plants cultivated under polypropylene fiber was almost 1.5 as high as the index calculated for non-covered plants.higher assimilation leaf area and LAI value were reported by Gimenez et al. [13] who cultivated cabbage, beet, spinach, and lettuce under covers.According to these authors, LAI has been traditionally considered an appropriate index to characterize the interception of radiation in many crops.The larger LAI and, consequently, greater relative interception of incoming photoactive radiation as well as the higher soil temperature appear to have more than compensated for the reduction of photoactive radiation caused by the covers.Extension of the covering period from 4 to 8 weeks had a more favorable effect on the growth attributes of melon plants.An increase in melon stem length as a result of covering was also confirmed in the study by Ibarra-Jiménez et al. [20].In turn, Wadas et al. [14] reported an increase in the height, weight, and LAI of potatoes which had been covered for 3 weeks compared with a 2-week covering period.Both the transplanting date and cover application did not affect the LAR value.Plants cultivated without covers had the highest LAR values, the finding also confirmed by Wadas et al. [14].
In the present study, delaying the transplanting date contributed to a decline in fruit yield, but the yield was significantly higher due to application of covers.Previous studies by Kosterna et al. [21] and Majkowska-Gadomska [8] showed that flat covers used under moderate climate conditions helped to obtain satisfactory yields of ripe melon fruits even when the summer months were characterized by lower air temperatures.Also, Benincasa et al. [22] found in their study, conducted in central Italy, a 12% increase in melon yield as a result of covering.However, Gordon et al. [23], who carried out their study in a subtropical humid climate, reported a 30% decline in pumpkin yield due to cover application compared with cultivation without covering.Under hot climatic conditions, row covers and floating covers have been used to a limited extent to protect plants from insect attack.However, it often resulted in yield reductions of muskmelon and other cucurbits [24,25].Melon is a plant pollinated by insects only.When the period of keeping melon plants under cover is extended, fruit yields may decline because pollinating insects have more difficulty gaining access to the flowers [26].In the present study, the length of plant covering period did not affect yield levels, which is consistent with the findings by Santos et al. [27] who reported that the length of melon plant covering period (18,21,24,27, and 30 days) did not significantly influence fruit yields.In turn, Ibarra-Jiménez et al. [20] showed a significant increase in melon fruit yield following an extension of the cover application time.The marketable yield increased by 18% after 24 days and by 81% after 41 days of covering compared with non-covered control.In the current study, covering increased the average weight of melon fruit and the share of marketable yield in the total fruit yield.On the contrary, Santos et al. [27] did not find an increase in fruit weight as a result of covering.
Both delaying the transplanting date and application of covers reduced the transplanting-to-harvest period.It is consistent with the report by Ibarra-Jiménez et al. [20], in which an extension of the time when plants were kept under covers contributed to a proportional reduction in the length of melon growing season by 4 days after 24 days and by 6 days after 41 days of covering.In the work by Ibarra et al. [12], fruit harvest from plants grown under covers was conducted 24 days earlier than from the control.Thus, increases in air and soil temperatures under plastic mulch and row covers were probably responsible for the reduction in the time to anthesis and harvest, by increasing the rate of plant development.The effectiveness of covering depends largely on the growing conditions.In the colder climate, the length of the growing season as a result of covering is reduced more than in the warmer climate.
In the present study, a significant relationship was found between growth attributes, harvest earliness, and yield.An increase in yield associated with an increasing length of melon stems was also noted by Ibarra-Jiménez et al. [20].
The earlier seedlings were transplanted, the more fruit weight depended on plant size.However, irrespective of transplanting dates, the average fruit weight was significantly negatively correlated with LAI, which confirms the fact that the more nutrients the plant allocated for the development of assimilation leaf area, the less remain to be used for the development and growth of generative organs.In the cultivation of plants whose generative organs are the yield harvested by growers, it is preferable to take care of the balance between vegetative growth and generative development.Application of covers may disturb the balance as they modify the microclimate around the plants (less light, higher humidity and temperature) and thus create better conditions for vegetative growth.
The strongest positive correlations between stem length, plant weight, LAI value, and harvest earliness were found for melons covered for 4 weeks, and between stem length, plant weight, LAI value, and yield level for plants covered for 4 and 8 weeks.These associations indicate that fruit production by covered plants began later, but the total fruit yield was higher.Covered plants had better growth conditions and, therefore, intensively produced vegetative mass and bloomed later but more profusely.According to many authors, a higher assimilation leaf area and LAI value can cause an increase in yield and improve its quality [13,28,29].Gordon et al. [23] covered pumpkin plants and achieved a significant increase in plant height, although no increase in fruit yield was observed.All of the above-mentioned studies were carried out under conditions of subtropical humid climate.

Conclusions
The best growth attributes were found for seedlings transplanted at the last date.After 8 weeks of growing, plants transplanted at the earliest date were longer by 22% and 56%, heavier by 39% and 76%, and their LAI values were higher by 24% and 117% compared with 10-and 20-day delays.Delaying the transplanting date reduced the length of the growing period by 7 and 17 days, respectively.
Irrespective of the transplanting date, extension of covering period from 4 to 8 weeks increased stem length by 23% and LAI by 38% on average, but delayed harvest by an average of 6 days.
An earlier transplanting date contributed to higher yields of melon but with lower shares of marketable fruits.The respective total yields of melons transplanted on May 15, May 25, and June 4 averaged 5.09, 4.73, and 3.99 kg m −2 .
Covering of plants transplanted at each of the above dates resulted in the following respective yield increases: 200%, 160%, and 125%.Also, it increased the share of marketable fruit yield in the total yield by, respectively, 68%, 59%, and 30%.However, the length of cover application time did not affect the yield level.

Fig. 1
Fig. 1 Weather conditions during the growing season of melon (C.melo).
yield in the total yield of fruits * Values followed by the different letters are significantly different at p ≤ 0 Plant growth attributes before transplanting of melon (Cucumis melo) (averages for 2008-2010).Plant growth attributes of melon 8 weeks after transplanting of melon (C.melo).
Tab. 1Values followed by the same letters are not significantly different at p ≤ 0.05.LAI -leaf area index; LAR -leaf area ratio.
* Values within columns followed by the same lowercase letters are not significantly different at p ≤ 0.05.** Values within rows followed by the same uppercase letters are not significantly different at p ≤ 0.05.LAI -leaf area index; LAR -leaf area ratio.© The Author(s) 2017 Published by Polish Botanical Society Acta Agrobot 70(2):1699 Franczuk et al. / Transplanting date and covering impact on melon growth and development