[Research] The Effect of Water Temperature on Plant Growth

Introduction

The growth and development of plants can be influenced by water temperature in hydroponic cultivation. The physiological process of plants will be affected by the plant metabolic activities such as phenolic compounds, nutrient uptake, chlorophyll pigment formation, and photosynthesis. (Nxawe et al., 2011). The main function of plant roots is to absorb water and nutrients from the growing medium and later conduct them to the stem of the plant. Thus, besides electrical conductivity value, pH value and environment temperature, regulating water temperature is a crucial part in hydroponic cultivation because the temperature in the root zone may make a notable difference in plant growth.

Lettuces are the common leafy greens that grow in hydroponic cultivation, especially in commercial production. The cooling technology has been introduced for growing lettuces hydroponically in hot weather by reducing the temperature in the root zone, such as a water chiller. In this experiment, Lugano lettuce was tested to figure out the effectiveness of the use of a chiller to grow lettuce in a tropical climate.

Objective

i.   To study the difference in plant growth with different water temperatures.

ii.  To identify the ideal water temperature for growing Lugano lettuces.

iii. To determine the effects of growing Lugano lettuce with a chiller.

Material

Modified City Vertical Farm with 4 feet 2” x 4” NFT channels and 4 feet CityFarm horticulture full spectrum LED T8 growlights, chiller, submersible pump, timer, 2 week old Lugano seedlings, 47mm net pots, germination sponge, hydroponic AB fertilizer, EC meter (Hanna Instruments DiST 4 HI98304), pH meter (Hanna Instruments GroLine HI98115).

Method

This experiment was conducted in a modified City Vertical Farm (Figure 1). It has 2 layers, each layer has two 4 feet 2” x 4” NFT channels and two 4 feet CityFarm horticulture full spectrum LED T8 growlights. Each NFT channel consists of eight holes, able to plant 32 numbers of lettuces for this experiment.

Figure 1 : The Model of Modified City Vertical Farm.

The first layer was set for growing Lugano lettuce with ambient water temperature. The second layer was set for growing Lugano lettuce with 18°C cold water. A chiller was installed to produce cold water for the lettuce of the second layer. It was connected to the submersible pump in the water tank of the second layer.

The growlight of each layer was set to open for 12 hours a day (6am - 6pm) and it was controlled by a timer. Before putting the Lugano seedlings into the NFT channel, the hydroponic AB fertilizer was added to each water tank and the EC value will be controlled between 1.2-1.5 mS/cm during the experiment. While the pH value will be controlled within the range of 6.0-6.5 pH.

This experiment will be observed for 5 weeks, starting from 16 April 2022 to 20 May 2022. The experimental conditions such as the room temperature, humidity, water temperature, EC value and pH value will be recorded every 5 days during the experiment. Furthermore, pictures of lettuce were taken every week to observe the difference.

Result

Table 1 : Average Experimental Condition of Ambient Tank and Chiller Tank.

Table 2 : Final Root Weight, Shoot Weight & Total Weight of Lugano Samples

Table 3 : Root Length, Shoot Length & Width of Lugano Samples

Figure 2 : Final Shoot Weight & Shoot Length of Lugano Samples.

(i) Lugano lettuce in ambient tank   (ii) Lugano lettuce in chiller tank

Figure 3 : Final Root Weight & Root Length of Lugano Samples.

(i) Lugano lettuce in ambient tank   (ii) Lugano lettuce in chiller tank

Table 4 : The Width and Number of Leaves of Lugano Samples

Below are the pictures of Lugano lettuce growing stage during the experiment. Pictures of day 1, day 7, day 14, day 21, day 28 and day 35 will be shown to help visualise the differences of their growth.

Figure 4 : Lugano Lettuce Growing Stage (Day 1,7,14, 21, 28 & 35)

Discussion

According to Thakulla, D (2021), minor changes in nutrient solution temperature in hydroponic will have a significant impact on the lettuce growth depending on the type of lettuce. Based on Table 1, the average water temperature along the experiment duration for the ambient tank is 26.5°C and for the chiller tank is 18.4°C. The average EC value for the ambient tanks is 1.41mS/cm, while for the chiller tank is 1.34mS/cm. However, the overall Lugano samples in the chiller tank grew bigger than those in the ambient tank by seeing their physical appearance. It also is supported by the result based on Table 2 & 3 after measuring their mass and length. The Lugano samples grown in the chiller tank have a better average root weight  and root length compared to those grown in the ambient tank. Additionally, a greater proportion of roots will impact shoot growth and yield of plants, it has been shown by various observations. Finding from Cometti et al (2013) has found out that nutrient solution with a cooler temperature can increase shoot mass and water percentage in lettuce. On the other hand from observation Illahi et al (2017) reported that using cooled nutrient solution to grow butterhead lettuce in a coir-perlite mixture has increased its size index and leaf number. The result of this experiment is the same as the studies above, in which the width and the number of leaves of Lugano samples grown in chill water is slightly greater than those grown in ambient water. In contrast to this result, there is a  study that the lettuce samples have smaller fresh weight when grown in low root zone temperature. It was conducted using the red leaves lettuce samples which grow in 10°C water temperature at 20°C environment. The samples have the smallest root and shoot weight compared to the other samples that are grown in 20°C, 25°C and 30°C water temperature (Sakamoto, M. and Suzuki, T., 2015). Lettuces are able to adapt in temperature in the range of 15°C-27°C, however their growth will be stunted if it grows out of the optimal range of the temperature. The reduction in weight and size of lettuce grown in 10°C water temperature were reported from the experiment of Sakamoto, M. and Suzuki, T. (2015), while according to Thompson et al (1998), lettuces grown at 31°C water temperature face root disease.

Nutrient Absorption

The nutrient and water absorption of plants can be influenced by the temperature of nutrient solution as reported by Libia et al (2012). The development of root can affect the nutrient uptake of a plant thereby impacting on the plant shoot development. Higher numbers of lateral roots and root hairs are able to absorb more nutrients from the nutrient solution, whereas the roots of the Lugano samples in the chiller tank are longer and cause a significant difference in the shoot weight compared to the samples in the ambient tank.

Availability of Dissolved Oxygen

The Lugano samples grown in the chiller tank have a bigger appearance and a larger mass might be impacted by the availability of dissolved oxygen in the chiller water tank. Dissolved oxygen are oxygen molecules that are found inside of water, it is an important indicator for hydroponic cultivation because dissolved oxygen is able to promote the root respiration in hydroponic. One of the factors that will influence the dissolved oxygen content in hydroponic cultivation is the temperature of the nutrient solution. A study by Falah et al (2010) had proven that the temperature of nutrient solution in hydroponic can affect the amount of oxygen dissolved in the root zone. It found out that the dissolved oxygen is reduced in high temperature nutrient solution and the result of root development was poor. Deficiency in root activity can lead to the reduction of nutrient uptake for plant growth. Apart from that, there are studies that have determined that the availability of dissolved oxygen is higher in cold water temperature (Muthir et al, 2019; Cometti et al, 2013). Both studies have shown there are positive effects on plant growth and yield when the dissolved oxygen level is high in the root area.

Conclusion

The use of a chiller is effective on lettuce hydroponic cultivation in a warmer climate condition as based on this experiment, whereas it is able to optimize the root development thereby increasing the shoot weight of lettuces. The overall measure indicators of lettuces grown with a chiller in 18°C water temperature were greater than the lettuces grown ambiently. However, the temperature should be set between the optimal range when using the chiller for lettuce production, which is 15°C-27°C to avoid any negative changes that will lead to diseases and stunted growth.

Reference

1. Douglas, S. (2001). Lettuce. NC State Extension Publications. Retrieve from https://content.ces.ncsu.edu/lettuce
2. Nxawe, S. & Ndakidemi, Patrick & Laubscher, Charles. (2011). Possible effects of regulating hydroponic water temperature on plant growth, accumulation of nutrients and other metabolites. African Journal of Biotechnology. 9. 9128-9134.
3. Thakulla, D., Dunn, B., Hu, B., Goad, C. & Maness, N. (2021). Nutrient Solution Temperature Affects Growth and ◦Brix Parameters of Seventeen Lettuce Cultivars Grown in an NFT Hydroponic System. Horticulturae 2021, 7, 321. https://doi.org/ 10.3390/horticulturae7090321
4. Falah, M.A.F., Wajima, T., Yasutake, D., Sago, Y. & Kitano, M. (2010). Responses of root uptake to high temperature of tomato plants (Lycopersicon esculentum Mill.) in soilless culture. J. Agric. Technol. 2010,6, 543–558.
5. Sakamoto, M. & Suzuki, T. (2015). Effect of Root-Zone Temperature on Growth and Quality of Hydroponically Grown Red Leaf Lettuce (Lactuca sativa L. cv. Red Wave). American Journal of Plant Sciences, 6, 2350-2360.
6. Ilahi, W. F., Ahmad, D., & M. C. Husain. (2017). Effects of root zone cooling on butterhead lettuce grown in tropical conditions in a coir-perlite mixture. Horticulture, Environment, and Biotechnology, 58(1), 1-4.
7. Garden & Greenhouse. (2014). Factors that Influence the Absorption of Hydroponic Nutrients. Retrieve from https://www.gardenandgreenhouse.net/articles/hydroponics/hydroponics-101-factors-that-influence-the-absorption-of-hydroponics-nutrients/
8. Garden & Greenhouse. (2017). Maximizing the Dissolved Oxygen Content in a Hydroponic System. Retrieve from https://www.gardenandgreenhouse.net/articles/hydroponics/maximizing-dissolved-oxygen-content-hydroponic-system/
9. Cometti, N.N., Bremenkamp, D.M., Galon, K., Hell, L.R. & Zanotelli, M.F. (2013). Cooling and concentration of nutrient solution in hydroponic lettuce crop. Horticultura Brasileira 31: 287-292
10. Muthir, S., Salim, A., Yaseen, A. & Saleem K. (2019). Influence of Nutrient Solution Temperature on Its Oxygen Level and Growth, Yield and Quality of Hydroponic Cucumber. Journal of Agricultural Science; Vol. 11, No. 3; 2019
11. Thompson, Helen & Langhans, Robert & Both, A.J. & Albright, Louis. (1998). Shoot and Root Temperature Effects on Lettuce Growth in a Floating Hydroponic System. Journal of the American Society for Horticultural Science. American Society for Horticultural Science. 123. 10.21273/JASHS.123.3.361.

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