Influence of plug cell volume and substrate type on the development of cucumber seedlings for transplanting

This study investigated the influence of plug cell volume (72 plug cell volume (36 mL) and 128 plug cell volume (20 mL)) and substrate type (peat and cocopeat) on the development of cucumber seedlings. The experiment was designed in a factorial arrangement in a completely randomized design with four replications. The following growth data were measured: seedling plant height, number of leaves, stem diameter as well as the shoot and root biomasses. The result showed significant differences among the treatments with respect to the seedling plant height, stem diameter, as well as the shoot and root biomasses. Overall, it was found that growing cucumber seedlings in 72 plug cell volume with cocopeat produces a better young plant for transplanting compared to using a 128 plug cell volume containing either cocopeat or peat.


Introduction
Cucumber (Cucumis sativus) is the fourth most widely cultivated vegetable crop in the world after tomatoes, cabbage, and onions (Shetty and Wehner, 2002). China is the leading producer, contributing almost two-thirds of all world production (60%) between 2000 and 2004 (Martinez et al, 2006). Turkey is the second highest producer with approximately 5% of total cucumber production worldwide followed by Iran with an average production of 3.6%, the U.S. (2.7%) and Japan (1.9%). It has been identified as one of the cultivated exotic vegetable crops that have gained popularity in Ghana with the potential of providing foreign exchange for the country (Sinnadurai, 1992;MOFA, 2002).
The optimum yield of cucumber on the field is dependent on a lot of factors including the conditions under which seedlings are raised (NeSmith and Duval, 1998). From the studies conducted by Ray and Sinclair, (1998), it was revealed that plug cells are required to hold the substrate and are key components for the growth and survival of seedlings. According to Al-Menaie et al., (2012), plug cell size has an effect on plant growth and it may affect root and shoot growth, biomass accumulation and partitioning, photosynthesis, leaf-chlorophyll content, plant water relations, nutrient uptake, respiration and flowering. According to Schrader (2000), smaller plug cells reduce production costs.
Additionally, the type of growth substrate used also influences the production of seedlings grown in plug cells.
Hence, the use of suitable growing media or substrates is essential for production of quality horticultural crops. A good growing medium would provide sufficient anchorage to the plant, serves as reservoir for nutrients and water, allows oxygen diffusion to the roots and permits gaseous exchange between the roots and atmosphere outside the root substrate (Bhardwaj and Kendra, 2014). An ideal potting substrate should be free of weeds and diseases to facilitate handling and transportation (Landis et al., 2012). It should also be well drained and yet retain sufficient water to reduce the frequency of watering. Other parameters to consider include cost, availability, consistency between batches and the stability in the substrate overtime. Robbins and Michael (2009) noted that selection of the proper substrate components is critical to the successful production of plants.
Over the years, Ghana has been faced with low production of cucumber although the environmental conditions are favourable for large scale cucumber production. The low production of cucumber in Ghana could be attributed to several reasons including the fact that most farmers sow their seeds directly into the soil without properly nurturing the seedlings. Although it has been reported by Kumar and Raheman, (2010) that seedlings raised in plug cells with soilless media increases the proper development of the crop, this is largely not been practiced in the country. Also, the use of plug cells for raising has not been embraced by seedling producers in the country.
This study therefore aimed at investigating the influence of plug cell volume and substrate type on the development of cucumber seedlings prior to transplanting. Specifically, the influence of these factors on the cucumber seedling shoot and root growth was assessed.

The study area
The study was carried out at the A.G Carson's Technology Village of the School of Agriculture, University of Cape Coast, Ghana. The experimental area was located at the Engineering field which falls within the coastal savannah zone. The climatic condition of the area was described by Owusu-Sekyere et al (2010) to have of annual mean temperature and relative humidity of 28.2ºC and 82.8 % respectively.   Table 1 shows the mean values of key chemical properties of the substrates used in the work.

Experimental design
The experimental design used in this study was a factorial arrangement in a Completely Randomized Design (CRD) with two factors (plug cell volume and substrate type). The treatments consisted of two levels of substrate fill and two levels of cell volume in a factorial combination to obtain 4 treatment combinations. The treatments imposed were peat and cocopeat based substrate fill and different plug cell volume of 36 mL (i.e 72 plug cell volume) and 20 mL (i.e 128 plug cell volume). There were four (4) replications.

Data collection
Within five days, all the seeds had germinated and the number of leaves, plant height and shoot diameter were collected on weekly basis for three weeks on all seedlings.

Seedling plant height
The seedling plant heights were measured using a 30 cm rule from the base to the terminal leaf.

Number of leaves
The number of leaves was determined by manual counting.

Stem diameter
This was determined using GuangLu electronic caliper and the values recorded in millimeters (mm).

Shoot biomass
On the 21 st day (three weeks), all plants were removed from their individual cells and washed. The shoots were separated from the roots through the use of a pair of scissors. The shoots were placed in a beaker and weighed using an electronic balance to obtain the fresh mass.

Root biomass
The roots detached from the shoots were placed in a beaker and weighed using an electronic to obtain the fresh mass.

Root to shoot biomass ratio
This was obtained by dividing the fresh biomass of the roots by the fresh biomass of the shoots.

Statistical analyses
The results of the study were subjected to two-way analysis of variance (ANOVA) using the general linear model in Minitab (version 17) software. ANOVA was conducted to predict the effect of the interaction of the experimental factors on stem diameter, plant height, number of leaves, shoot biomass, root biomass and root biomass: shoot biomass. Mean comparisons were done using the Tukey mean comparison test at a significance level of 5 %.

RESULTS AND DISCUSSIONS
3.1 The effect of different plug cell volume and substrate type on seedling shoot growth (stem diameter, plant height and number of leaves). Favaro (2015) who stated that an increase in the container size results in plants of higher size. This is mostly due to the physical restriction of root by the use of containers with low volumes severely limit the plant growth after two weeks of sowing. Robbins and Pharr (1988) also reiterated that container size affect a number of physiological processes including nutrient efficiency and photosynthesis rates. Hence, a reduction in container cell volume limits resources such as nutrients, available growth medium and space for roots to spread and as such, root restriction will result in reduced plant growth. Generally, it was observed that cucumber transplants sown in cocopeat substrate were significantly higher than the plant height of cucumber seedlings raised in the peat substrate. The trend recorded in the seedling height could be attributed to the fact that cocopeat relatively has better chemical properties than peat as shown

Number of Leaves
The number of leaves was not significantly affected by the plug cell volume and substrate type and this is shown in

Plug cell volume (with substrate type) for days after sowing
empirical observations showed that the number of leaves increased as plant age increased (from day 7 to 21) and this is displayed in figure 4. The increase in number of leaves as the plant ages is attributed to the ability of plant roots to absorb adequate nutrients and moisture and the ability of the leaves to receive sufficient sunlight to produce photosynthate to enhance their (i.e leaves) development Bars that do not share a letter are significantly different. D stands for days after sowing.

Stem diameter
From figure 5, the plug cell volume and substrate type had significant impact on stem diameter of cucumber seedlings. Further, the 72 plug cell volume recorded the highest stem diameter when compared with the stem diameter of transplants sown in the 128 plug cell size (p<0.05). This could be as a result of the relatively adequate volume in the 72 plug cell volume in that, which allowed for more space for roots to expand and absorb nutrients and water from the substrates without much restriction. It was also observed that stem diameter increases with increasing plant age. This could be ascribed to the fact that as the cucumber seedling ages, its roots develop which Plug cell volume (with substrate type) for days after sowing seedlings raised in the 128 plug cell volume containing cocopeat. This could be as a result of high N immobilization by microorganisms and a high C:N ratio in cocopeat which has more than 50% coir (Holman et al, 2002). From figure 5, it could be observed that the stem diameter of cucumber transplants sown in 72 plug cell volume with cocopeat showed the highest stem diameter compared to those in the 72 plug cell volume of peat. This can be attributed to the high chemical properties of the cocopeat substrate than the peat substrate.
Bars that do not share a letter are significantly different. D stands for days after sowing.  Peat Coco significant response observed in the shoot biomass could be attributed to the fact that cocopeat has better nutrient content (NPK) than peat. A similar development was observed in the 128 plug cell volume. The mean shoot biomass of cucumber seedlings in the 72 plug cell volume containing cocopeat was 1.05 g whereas the shoot biomass of seedlings in 72 plug cell volume with peat was 0.85 g. The shoot biomass of cucumber seedlings grown in the 128 plug cell volume of cocopeat was 0.69 g whiles the shoot biomass of cucumber seedlings in the 128 plug cell size of peat was 0.60 g.

Shoot biomass
Bars that do not share a letter are significantly different. Figure 6: Effect of plug cell volume and substrate type on shoot biomass.

Root biomass
The root biomass of cucumber seedlings was significantly affected by the plug cell volume(p<0.05) but the substrate type did not show a significant effect on the shoot biomass(p>0.05). From figure 7, the 72 plug cell volume recorded the highest root biomass compared to the 128 plug cell size and this was evident in both peat and cocopeat growing media. The significant response revealed by the plug cell sizes could be attributed to the fact that the roots of the cucumber seedlings raised in the 72 plug cell volume had enough room to expand without being restricted by the cell size. This allowed the plant roots to absorb nutrients and water from the substrates to develop considerably.
Optimal seedling root growth depends on favourable soil or media conditions including water, fertility, and the physical rooting environment (Leskovar et al., 1990). Plug cell volume (with substrate type) higher root biomass than the former. This could be attributed to the fact that, cocopeat, generally, has high N immobilization by microorganisms and a high C:N ratio in the coir (Holman, Bugbee, and Chard, 2002). A similar pattern was observed in the 128 plug cell volume for the two substrates. The mean root biomass of cucumber seedlings in the 72 plug cell volume of cocopeat was 0.56 g whereas the root biomass of peat raised cucumber seedlings in the 72 plug cell volume was 0.44 g. The mean root biomass of cucumber plants grown in the 128 plug cell volume of cocopeat was 0.21 g whiles those in the 128 plug cell size of peat was 0.18 g.
Bars that do not share a letter are significantly different.

3.3
The optimal plug cell volume and substrate fill for producing quality cucumber seedlings.

Conclusion
From the study, it was generally evident that the plug cell volume and substrate type had a significant effect on the shoots growth of the seedlings. The 72 plug cell volume containing cocopeat recorded the highest shoot growth compared to the 128 plug cell volume containing cocopeat. Also, the shoot and root biomass of seedlings raised in substrates of 72 plug cell volume were greater than the shoot and root biomass of the seedlings raised in substrates of 128 plug cell volume. Moreso, the root to shoot biomass ratio of seedlings raised in 72 plug cell volume with cocopeat was found to be higher than that of the seedlings raised in 72 plug cell volume with peat. This shows that for the development of high quality transplants, 72 plug cell volume containing cocopeat could be a better option to use. Hence, it could be concluded that, the 72 plug cell volume and cocopeat are good container and growth substrate, respectively, for the development of cucumber seedlings for transplanting. Future work could be done to