Effect of organic mulch and LEDs light colors on green bean plants ( Phaseolus vulgaris L.)

Studying the positive role of applying organic mulch and different colors of LEDs light, on the growth and yield of green bean plants, were carried out during 2019/2020 and 2020/2021 seasons, at a privet farm within Suez Governorate. Seedlings of Bronco cultivar were transplanted at the 15 th of October of both seasons. Two types of organic mulch i.e., chopped maize straw and chopped rice straw, addition to bare soil (control) were tested as a soil mulch treatment (factor 1). In addition, LEDs light colors i.e., red, blue and "red + blue" and without LED light were also tested (factor 2). Finally, interaction between both studied factors was also studied. Studied factors were arranged in split plot design with three replicates. Organic soil mulches were arranged in the main plots. However, LEDs light colors were arranged within the sub main plots. Vegetative growth parameters i.e., plant height, number of leaves/plant, stem diameter, number of branches/plant, fresh and dry weight of plants were measured. Moreover, percentages of nitrogen, phosphorus and potassium contents in leaves, as well as, chlorophyll reading (SPAD) were also measured. In addition, pod characteristics i.e., number of pods/plant, average pod weight and fiber content in pods, were studied as well. Finally, both early and total yield of green bean were studied. Changes in soil temperature under organic mulch treatments, as well as, bare soil were daily measured during the growing seasons. Results indicated that, in general, applying tested types of organic mulch modified soil temperature. Moreover, both tested factors were enhanced measured characters of vegetative growth, leaves content of nitrogen, phosphorus and potassium, as well as yield and its components. Furthermore, both treatments of chopped maize straw and "red + blue" LEDs light, as well as the interaction between both treatments were the most suitable for increasing productivity and quality of green bean plants.


Introduction
Green beans (Phaseolus vulgaris L.) is important vegetable crop belongs to legumes, with high protein contents, fiber, phosphorus, iron, vitamin B1, high concentrations of essential nutrients, such as ascorbic acid, Vitamin A, Vitamin B and calcium [1]. It regulates serum cholesterol and lipid profiles, and inhibits precancerous growths [2]. Furthermore, increasing yield of green beans with quality characteristics is vital in increasing the consumption and export value of the crop. There is a need to evaluate more modern technologies that can help to meet markets and consumers requirements [3].
Covering agricultural bare soil is a practice to enhance soil properties, plant growth and crop yield [4]. Soil mulching strongly connected with different elements and factors that are used in vegetable crop intensive production systems and increase its efficiency. Many materials are valid to be used as soil mulch in respect to its impact on the characteristics of soil and plant growth [5]. Furthermore, organic mulch application is a good energy source for the microorganisms in the root zone. In addition, it plays an important role in water penetration [5]. Organic mulch decomposition promotes soil nutrients through slow release from organic mulch, as well as, nourishing the agricultural soil gradually during the season [6]. From the climatological point of view, organic mulch, such as rice straw mulch, performs like an air conditioner for the soil, whereas, it makes soil temperature higher during the colder seasons and lower during the warmer seasons when compared with bare soil [7]. Temperature extremes affect plants in early stage of growth negatively through stopping nutrient uptake by the new developed roots [8]. So, adjustment and maintaining soil temperature to suit the cultivated crop is vital for avoiding such stressful conditions. Straws from any crop are intensively available locally after harvesting. Recently, rice straw is used as soil mulch. It provides all advancement of polyethylene mulch. In addition, it presents better conservation of water through the early stage of crop growth [9]. Abd El-Wahed et al., [10] reported a significant enhancement in green bean yields (by 11.5, 30.8 and 40.2 %) as a direct result of using rice straw mulch of the following thickness 3, 6 and 9 cm, respectively, compared to bare soil.
On the other hand, light is very important factor that control plants performance. Plants showed different response to light according to the type of photo-sensory receptors. Reaction of plants to light are depending on light quality (color or wavelength), light quantity (fluency rate) and finally light direction and duration [11]. Wavelengths, affecting plant growth and development, are in the range between 300-700 nm, and called Photosynthetically active radiation (PAR) [12]. Moreover, inside the light spectrum both blue and red regions are basically producing the quantum yield of photosynthesis [13]. Red wavelength (600-700 nm) and blue wavelength (420-450 nm) are considered as the most effective regions of light for controlling plant growth and yield [14]. The mix between red and blue LEDs give a higher result related to accumulation of chlorophyll, antioxidants and carotenoids [15, 16, 17 and 18].
Light-Emitting Diodes (LEDs) work as an innovative tool for achieving desired performance of horticulture crops throughout fine tuning of the light conditions for cultivated plants [19]. In addition, LED light represents advancement in relation to higher efficiency in converting energy into photosynthetically active radiation, lower cooling requirements, robustness and cost effectiveness as compared to other source of light that usually used in horticulture production [20]. To adjust a specific characteristic of horticulture production, LED customized the distribution of a specific wavelength that is able to govern growth, development or metabolic processes [12].
The aim of this study is to determine the positive role of two types of organic mulches (maize straw & rice straw) and three different wavelengths (Red, Blue and "Red + Blue) on enhancing the growth and productivity of green beans.

Experimental layout
Experiments were conducted at a privet farm in Suez Governorate, during 2019 and 2020 seasons. Seedlings of Bronco green bean cultivar were transplanted at 15 th of October in hills (30 cm apart) on ridges (each is 50 cm width). Plot area was 30 m 2 (30m  1m). A distance of 150 cm was left between each treatment. Treatments of two types of organic mulch, three LED light colors, and their interaction, were carried out, compared with natural open field conditions, to investigate the growth and productivity of green bean plants.

Soil preparation
Recommended amount of fertilizers (17 Kg of calcium super phosphate, 4.5 Kg of ammonium sulphate, 8.5 Kg of Sulphur, 4.5 Kg of potassium sulphate and 2 m 3 of compost) were mixed perfectly to create a starter. Such mix was divided into eight equal quantities; each was added to each treatment two weeks before transplanting [21]. The drip irrigation system was used.

Organic mulch preparation
Using a cutting machine, the two tested organic mulches were shredded to 2-4 inches. Soil beds were covered by the two types of organic mulches (chopped maize straw and chopped rice straw), by spreading them manually on the soil surface (forming 3 cm thickness above bar soil) after seed germination.

LEDs light colors preparation
Three LEDs light colors were used to light up tested plants i.e., Red (660 nm), Blue (440 nm) and "Red + Blue". The LED light was provided by LED inter-lighting lamps, and installed 0.60 m above tested plants. LED light was applied to plants daily from 6 pm to 5 am.

Treatments
Two tested factors were applied, and their interaction as treatments, as follows:  Organic mulch factor Two types of organic mulch were tested i.e., (chopped maize straw and chopped rice straw), compared with natural open field conditions as control treatment.
 LED light color factor Three LEDs light colors were used i.e., (Red, Blue and "Red + Blue", compared with natural open field conditions as control treatment.

Climatic conditions
Soil temperature: Maximum and minimum soil temperatures were daily recorded under organic mulch treatments using digital thermometer. The results were calculated as an average of every 10 days.

Recorded Data
A sample of 3 randomized green bean plants, from each replicate in different treatments, were collected after 70 days from transplanting, to determine the following parameters i.e., plant length, number of leaves per plant, stem diameter, number of branches/plant and fresh and dry weights of total plant. In addition, number of pods per plant and average pod weight for each treatment were estimated from the first harvest to the end of each season, after that, the early and total yield per plant were estimated.
The percentage of N, P and K contents were determined in leave samples after drying at 70 o C in an air forced oven for 24 hours. Dried leaves were digested in H2SO4, the following mineral contents were estimated: phosphorous and potassium in the acid digested solution by colorimetric method, ammonium molybdate by spectrophotometer and flame photometer [22]. Total nitrogen was determined by Kjeldahl method according to the procedure described by [23]. The total fibre content was also estimated, in green pods on dry weight basis, according to [24].

Experimental design
The experiment was split plot design with three replicates. The organic mulch type treatments were arranged as the main plots, whereas, the LED light colors were arranged as the sub plots.

Data analysis
Obtained data were statistically analyzed using the analysis of variance method. Duncan's multiple range tests at 5% level of probability were used to compare treatment means [25].

Soil temperature
Data in Figure (  The highest recorded average maximum soil temperature was observed in plots covered with maize straw followed by those cover with rice straw. Whereas, maize straw mulch increased the average maximum soil temperature by 2.5 to 3.3 o C above average bare soil temperature. However, rice straw mulch increased average maximum soil temperature about 1 o C above average bare soil temperature. The same range of increase was observed in both studied seasons. Similarly, application of maize straw soil mulch increased average minimum soil temperature by 3.5 to 5 o C above average minimum temperature of bare soil. Moreover, rice straw mulching increased average minimum soil temperature by 1.5 to 2.5 o C above average minimum temperature of bare soil. The same range of increase was observed in both studied seasons. Obtained result was in harmony with Horton et al., [26]. They indicated that organic mulch has a special mechanism to modify soil temperature during winter season by reducing heat loss from soil providing thermal insulation layer. Moreover, organic mulch reflects the short waves (Albedo effect) more than soil. Both thermal insulation and albedo effect reduce soil temperature capacity and this resulted in less soil temperature extremes under organic mulch. In addition, Zhao [27] reported that, mulches change soil temperature by about 0.5 to 3.0 o C depending on many factors.

Number of leaves
As shown in Table (

Steam diameter
Data illustrated in Table (

Number of branches/plant
Data in Table (

Fresh weight of total plant
Illustrated data in Table (

Chlorophyll reading
Treatments means in Table (7) showed that organic mulch type treatments had highest chlorophyll reading in leaves compared to control (without mulch) treatment. Moreover, there is no significant difference between applying maize straw and rice straw on chlorophyll contents in green bean leaves.
On other hand, blue LED light color and "red + blue" LEDs colors treatments lead to increase chlorophyll reading in leaves of green bean without any significant difference between them (

Nitrogen, phosphorus and potassium contents
Results in Tables (8, 9 and 10) indicated that organic mulch treatments increased N, P and K % contents compared to control (without mulch     As for the effect of their interaction noticed that application of maize straw plus "red + blue" LEDs light colors treatment increased green bean leaves contents from N, P and K more than other treatments (

Number of pods/plant
Data in Table ( 11) showed that the two types of organic mulch treatment increased number of pods per plant, without any significant difference between them, compared to control treatment (without mulch).
On the other hand, the blue LED light color and the "red + blue" LEDs light treatment recoded the greatest number of pods per plant, without any significant difference between them (

Average pod weight
Result in Table ( Concerning LEDs light colors effect, data in Table ( The interaction between organic mulch types and LEDs light colors showed that, "red + blue" LEDs light interacted with maize straw treatment had higher average pod weight than other treatments (4.12 & 4.26 g/pod; respectively during 2019 and 2020). While, lowest values of this parameter were obtained with the control (bare soil without LED light) treatment (2.93 & 3.07 g/pod; respectively, during 2019 and 2020).

Fiber content in pods
Table (13) showed the role of organic mulch types, LEDs light colors and their interaction on fiber contents in green bean pods. In general, fiber contents in pods decreased significantly by applying both types of tested organic mulch compared to control treatment (without mulching).
Therefore, plants grown with maize straw treatment produced the lowest value of fiber contents, followed by rice straw treatment ( Positive role of interaction was obtained through the results illustrated in Table ( 13). Applying maize straw together with "red + blue" LEDs light lead to decrease green bean pod contents of fiber (7.11 & 7.14 %; respectively during 2019 and 2020). On the other hand, control treatment (without mulch and without LED light control) increased this parameter (10.80 & 10.83 %, respectively, during 2019 and 2020.

Early yield per plant
Data in Table ( 14) showed that, applying organic mulch lead to enhancing positively early yield per plant of green bean, compared to the control treatment (without mulch

Total yield per plant
Data in Table (

Discussion
Obtained results indicated the superiority of both organic mulch treatments (rice straw & maize straw), compared with bare soil, concerning the enhancement of all measured characteristics related to plant growth (plant length, number of leaves per plant, stem diameter, number of branches/plant and fresh and dry weights), nutrient uptake (percentage of N, P and K content) and crop yield (number of pods, average pod weight, early and total yield per plant).
The above-mentioned trend of results was confirmed by several authors [5, 7, 4 and 28]. These obtained enhancements were explained by Barche et al., [29] on bases of positive effect of the organic mulch that appear in three different dimensions (physical, chemical and biological).
Physical effect of organic mulch due to its ability to change soil structure in the root zone area, which is reflected in increasing the growth of roots and nutrient uptake. In addition, organic mulch improves the soil aeration as well as water holding capacity [29,28). Moreover, process of organic mulch decomposition presents a reach source of soil granulation throughout activation sticky substances secretion by micro-organisms.
Chemical effect of organic mulch presented through controlling pH of the soil to acidity or alkalinity. Mentioned effect concerning soil pH is mainly depending on kind of applied organic mulch. Most of organic mulch kinds slightly increase the pH [29]. Moreover, the organic mulch play a good role in adds a little amount of nutrient elements during process of decomposition.
Biological role of organic mulch became from its role as food for beneficial microorganism. As well as, stimulation microorganism's community through adding extra quantities of organic matter to the soil [30]. On the other hand, organic mulch modifies soil temperature to be constant which guarantee the continuous activity of beneficial microorganisms and new roots activity [31, 26, 8 and 7].
Focusing on the results related to the effect of tested LED light colors, it's noticeable that mixing both red and blue LED lights presents a superiority in large number of studied characteristics i.e. number of leaves, plant fresh weight, plant dry weight, N (%), P(%), K (%), number of pods, early yield and total crop yield. Such excellence explained by both of [32] and [33] by the ability of both LED lights together (red and blue) to activate large number of photoreceptors compared to the effect of individual colors. Whereas, blue LED light affects cryptochromes, phytotropins and Flavinbinding Kelch (FKF1) that are initiated and involved in biosynthesis of hormones and secondary metabolites, stomatal activity and cell wall elasticity [34]. In addition, it leads to earliness in bud outgrowth, internode elongation, flowering and protection against biotic and abiotic factors. However, red LED light has the highest RPE, about 90%, meaning the plants under red light gained more energy than the others [35].
Concerning effect of the solo blue LED light, it was found that the blue LED light increased both of plant height and chlorophyll reading (SPAD) compared to other tested LED light colors. Briggs and Christie, [36] reported that the blue color wavelength is between 400 -520 nm where, Chlorophyll pigments obtain peak energy absorption at these wavelengths and strongly influence vegetative growth and development. More reports related to effect of blue light were presents by [32, 37, 38, 39, 40 and 41]. They indicated the ability of blue light to affect both of primary and secondary metabolism, volatile production, carotenoid and chlorophyll pigment bioaccumulation and intermodal length.
Similarly, red and far-red (700-800 nm) wavelengths are able to activate other photoreceptors (phytochromes) involved in the regulation of different processes through the plant life cycle (including seed germination, shade avoidance, flowering time, vegetative development, root growth and nutrient uptake) as reviewed by Demotes-Mainard S et.al [42].

Conclusion
Covering bare soil by organic mulch led to modified soil temperature and increased tested characteristics of common bean plants, when compared with bare soil, as open field condition. The greatest values of most studied parameters observed with chopped maize straw followed by chopped rice straw. Applying LEDs light colors had positive role on enhancing vegetative growth parameters, uptake nitration elements, pods characteristics and produced heavy early and total green pods more than open field conditions. Furthermore, the most favorable effect of LEDs light colors ranked as "red + blue", "blue color" and "red color", respectively, for most of the tested parameters. Combination between organic mulch types and LEDs light colors achieved highest results for most parameters, especially, chopped maize straw plus "red + blue" LEDs light treatment.

Disclosure of conflict of interest
No conflict of interest.