Plant growth and development are closely associated with the environmental conditions and nutrient supply. Similarly, hydroponic systems are also mainly dependent on three key factors which are the electrical conductivity (EC), pH and dissolved oxygen level of the nutrient solution. In NFT hydroponic systems, plants are grown in pots sitting in channels whereby the lower part of the root system is immersed in a thin film and constant passing flow of nutrient solution. Hence, the healthy root growth has become an important determinant of both plant growth and yield.

Dissolved oxygen (DO) acts as the life force for plants by promoting healthy root development and maximizing nutrient uptake in plants. According to Kubota (2020) , DO is an essential abiotic factor influencing the conditional of root-zone environments by which plants able to develop more finely branched roots with root zones that having higher air porosity. Moreover, sufficient DO levels not only can improve plant quality but also establish an aerobic environment favoring beneficial microorganisms. Conversely, the depletion of DO levels favor harmful anaerobic organisms that cause common root rot diseases, leading to nutrient deficiency and stunted plant growth. Here, this project aims to study the effect of DO on the growth of Lollo Rossa. We also hypothesized that Lollo Rossa samples grown in oxygenated nutrient tank have better plant growth than in control nutrient tank.

Modified City Vertical Farm, Lollo Rossa seedlings (3-week old), 4ft CityFarm Horticulture Full Spectrum LED T8 Growlight, hydroponic fertilizers A and B, pH down (food-grade phosphoric acid and water), 4ft 2" x 4" NFT channels, 47 mm net pots, germination sponge cubes, cooling fans, digital DO meter (JPB-70A pen-type intelligent dissolved oxygen analyzer), EC meter (Hanna Instruments HI98304 DiST 4), pH meter (Hanna Instruments HI98107 pHep®), air compressor pump connected with air stones, weighing balance, ruler and measuring cup.

The experiment was conducted in a modified City Vertical Farm (Figure 1.0) consisting of two 4ft 2" x 4" NFT channels that connected to the control nutrient tank (blue tank) and oxygenated nutrient tank (grey tank), respectively. Each channel was occupied with eight Lollo Rossa seedlings and was continually supplied with nutrient solution containing hydroponic fertilizer A and B. The farm was also installed with three 4ft CityFarm Horticulture Full Spectrum LED T8 grow lights in which the lighting duration was set to 10 hours in a day, as well as two cooling fans were installed to facilitate ventilation of the farm.

The NFT channels were connected to the control nutrient tank (blue tank) and oxygenated nutrient tank (grey tank), respectively.

In the oxygenated nutrient tank, the nutrient solution was constantly oxygenated throughout the experiment by installing an air compressor pump connected with multiple air stones, as well as the outlet pipe was situated above the water surface to enhance gas exchange rate. On the other hand, the outlet pipe in control nutrient tank was designed to be immersed into the nutrient solution as to limit the flow rate, minimizing the surface agitation in the tank. The concentration of nutrient solution was increased from 1.4 to 1.7 mS/cm, according to the plant growth whereas the pH value was varied within the range of 6.0 to 6.7. Besides, the DO level and temperature in oxygenated nutrient tank was maintained within the range of 7.4 to 7.2 mg/L and 27.5 to 27.8 ℃; in control nutrient tank was kept between the range of 4.7 to 4.9 mg/L and 28.0 to 28.3 ℃, respectively. Furthermore, the plant growth was measured by recording the fresh weight, plant height and number of leaves every interval of five days. Additionally, the root and shoot mass were also recorded as the final measurement upon harvesting on Day 52 since seed germination. A statistical hypothesis test, pooled t-test was also carried out to further verify the effect of DO on the final root and shoot mass of the samples.

Lollo Rossa samples of (a) 5 days, (b) 15 days and (c) 25 days after transplantation into NFT system.

Table 1.0: Average experimental conditions of oxygenated and control nutrient tank

Tank	Average experimental conditions
	EC (mS/cm)	pH	DO (mg/L)	Temperature (℃)
Oxygenated	1.52	6.4	7.3	27.6
Control	1.45	6.4	4.8	28.2

 

Table 2.0: Final root and shoot mass of Lollo Rossa samples

Sample	Oxygenated tank		Control tank
	Root mass (g)	Shoot mass (g)	Root mass (g)	Shoot mass (g)
1	27	69	23	67
2	29	90	24	56
3	35	114	30	80
4	32	101	30	90
5	30	109	29	83
6	32	118	32	103
7	34	113	28	80
8	27	71	28	74
Average mass	30.8	98.1	28.0	79.1

Note: Both root and shoot mass of samples included the net pot and water content in the sponge.

 

Table 3.0: Independent sample t-test results of the samples’ final root mass

Group	n	Mean	SD	df	tcal	t0.05	Result
Oxygenated	8	30.75	3.012	14	1.808	1.761	Accept
Control	8	28.00	3.071	14

Note: The alternative hypothesis specifies that the final root mass of oxygenated group is greater than control group. (Significance level, α = 0.05)

*n = Number of samples; SD = Standard deviation; df = Degree of freedom

 

Table 4.0: Independent sample t-test results of the samples’ final shoot mass

Group	n	Mean	SD	df	tcal	t0.05	Result
Oxygenated	8	98.13	19.44	14	2.232	1.761	Accept
Control	8	79.13	14.21	14

Note: The alternative hypothesis specifies that the final shoot mass of oxygenated group is greater than control group. (Significance level, α = 0.05)

*n = Number of samples; SD = Standard deviation; df = Degree of freedom

Figure 3.0: Final shoot mass and root mass of Lollo Rossa samples. 

The (a) shoot and (b) root mass of Lollo Rossa samples collected from oxygenated nutrient tank were greater than (c) shoot and (d) root mass samples from the control.

Figure 4.0: Effect of DO on the growth of Lollo Rossa samples grown in oxygenated and control nutrient tank. 

(a) The DO levels in oxygenated nutrient tank (blue line) was practically constant at 7.4 mg/L, whereas the DO levels in control nutrient tank (orange line) remained constant at 4.8 mg/L. The (b) average of plant fresh weight, (c) plant height and (d) leaf number of Lollo Rossa samples grown in oxygenated and control nutrient tank throughout the experimental period.

Notes:

1 The fresh weight of samples has included the net pots and water content in the sponge.

2 The height of samples was measured from the base of net pot up to the tip of leaf.

Discussion:

Dissolved oxygen is simply the presence of free oxygen (O2) molecules dissolved in water and is a critical parameter to be optimized in assessing water quality as it indirectly influences any organisms living by the water. Likewise, in horticulture, DO plays important roles in enhancing both plant quality and crop yields. There are two factors which are temperature and salinity greatly influence the DO content in hydroponic system. Between these two factors, DO content is very temperature-dependent as the temperature inversely regulating the solubility of oxygen in water. Generally speaking, cold water can hold more dissolved oxygen than warm water and vice versa (Becker, 2016).

Based on Table 1.0, although the average temperature of the oxygenated nutrient tank is 0.6 ℃ lower than the control nutrient tank, there is a significant difference in the DO level whereby the oxygenated nutrient tank is 2.5 mg/L higher than the control. Due to the presence of air stones and surface agitation in oxygenated nutrient tank, air bubbles were produced and dispersed evenly in the tank, causing a higher rate of gas exchange at water interface and a higher concentration of DO. In term of the average EC value of nutrient solution, oxygenated nutrient tank demonstrated a higher EC value than the control in the end of the experiment as the plants grown in oxygenated nutrient tank had higher transpiration rate than nutrient uptake rate.

Additionally, based upon Table 2.0, Lollo Rossa samples grown in oxygenated nutrient tank had greater average root and shoot mass in compared with the control. By conducting pooled t-test, the results had revealed that both final root and shoot mass of samples grown in oxygenated nutrient tank were significantly greater than the control samples (Table 3 and Table 4). As proof, Suyantohadi, Kyoren, Hariadi, Purnomo, and Morimoto (2010) also elucidated that DO had positively impacted on both plant and root development in which plants grown in nutrient tank with saturated DO displayed better root and shoot characteristics. Besides, as illustrated in Figure 3.0, samples grown in oxygenated nutrient tank were compact and good in shape. Also, the overall root mass of samples grown in oxygenated nutrient tank was visibly denser and longer than the control samples. Thus, high levels of DO largely not only promote healthy root formation but foster plant development process, producing substantial and healthier plants.

Apart from that, Figure 4.0 shown the different effect of DO on plant fresh weight, height and number of leaves. In respect of plant fresh weight, samples harvested from oxygenated nutrient tank had greater weight, as well as the number of leaves was greater than the control samples. As to the average plant height, the control samples were slightly elongated and higher than the samples treated with high DO level. The stem elongation of control samples was likely correlated with high temperature. On account of the exposure to higher temperature, the occurrence of stem elongation in lettuce was facilitated, resulting in loose leaf structure which can be observed in the control samples (Iqbal, 2018). Therefore, temperature can inversely regulate the solubility of oxygen in water and indirectly affect the morphology of plant.

Conclusion:

To conclude, Lollo Rossa samples grown in oxygenated nutrient tank had significantly final root and shoot mass than the control samples. Over and above, the presence of DO has also proven to improve both plant growth development and characteristics with respect to fresh weight, height and leaf number. More importantly, DO is temperature-dependent and serves as a basic measurement to be taken in consideration for horticulture, optimizing both root and plant development. In the future, an extensive amount of research is required to intensify the understanding of the mechanism involving DO levels against both root and plant development.

References

Becker, K. (2016). Understanding dissolved oxygen. Retrieved from

https://www.growertalks.com/Article/?articleid=22058

Iqbal, Q. (2018). Effect of high temperature and exposure duration on stem elongation of iceberg lettuce. Pakistan Journal of Agricultural Sciences, 55, 95-101. doi:10.21162/PAKJAS/18.6554

Kubota, C. (2020). Chapter 13 - Growth, development, transpiration, and translocation as affected by abiotic environmental factors. In T. Kozai, G. Niu, & M. Takagaki (Eds.), Plant Factory (Second Edition) (pp. 207-220): Academic Press.

Suyantohadi, A., Kyoren, T., Hariadi, M., Purnomo, M. H., & Morimoto, T. (2010). Effect of high consentrated dissolved oxygen on the plant growth in a deep hydroponic culture under a low temperature. IFAC Proceedings Volumes, 43(26), 251-255. doi:https://doi.org/10.3182/20101206-3-JP-3009.00044