Increased water shortage and land degradation pose challenges to dryland conventional agriculture [1
]. In recent years, the development of light-emitting diode (LED) technology has enabled low-cost indoor cultivation using artificial light [2
] and soilless culture [3
] to enable plant production in places that are unsuitable for crop growth.
Hydroponics is a water-saving cultivation method [4
] that is often used in dryland farming. Compared with traditional soil culture and substrate culture, hydroponics uses different culture media (nutrient solution) and incorporates strict environmental control technologies to regulate pH, electrical conductivity (EC), and temperature [5
]. Due to its flowable culture media, hydroponics also displays different nutrient migration patterns than does traditional soil culture.
Plant nutrients in the soil typically reach the root surface by root extension, mass flow, and diffusion [6
]. In hydroponic cultivation, flowable nutrient solution cultivation substrates also allow nutrients to be transported to the root surfaces via turbulent diffusion [7
], which transfers nutrient ions to the root surface by the irregular motion of fluid particles. Turbulent diffusion is affected by a container’s flow rate, which determines the circulation and diffusion of nutrient ions in the container and the resulting nutrient uptake and plant growth.
Recently, various researchers have investigated the effect of nutrient solution flow rate on crop growth. Dalastra et al. [8
] evaluated hydroponic lettuce nutrition and production based on nutrient solution flow rates of 0.5, 1, 2, and 4 L/min applied to separate cultivation channels. The greatest nutrient accumulation was found in the plant shoots, with the highest lettuce yield obtained at a nutrient solution flow rate of 1 L/min. Soares et al. [9
] evaluated two nutrient solution flow rates (1.5 and 2.5 L/min) using brackish water in hydroponic vegetable production, with the flow rate of 1.5 L/min showing the best results in shoot fresh weight, shoot dry weight, leaf area, number of leaves, plant height, and shoot diameter. Genuncio et al. [10
] studied the fresh weights of three different lettuce varieties (Lucy, Izabela, and Veneza) grown at three different flow rates (0.75, 1, and 1.5 L/min) and nutrient ion concentrations (the 50%, 75%, and 100% concentration of the standard nutrient solution in that study). Nutrient solution with a flow rate of 1.5 L/min and 100% ionic concentration increased the fresh weight of Izabela and Veneza lettuce varieties. Finally, Al-Tawaha et al. [11
] investigated the effect of three different nutrient solution flow rates (10, 20, and 30 L/min) on lettuce growth, finding that the lettuce weight increased most under a 20 L/min flow rate. The results of these studies indicate that flow rate affects hydroponic plant growth, and that regulating flow rate is recommended to increase hydroponic vegetable yields.
The influence of nutrient solution flow rate on plant growth is related to the plants’ physical environment. The flow of nutrient solution not only promotes nutrient ion diffusion, but also increases the kinetic energy (dynamic pressure) available to plant roots. Previous researchers have demonstrated that nutrient solution flow rate affects hydroponic plant growth; however, the mechanism of this phenomenon has not been explained in detail. In particular, the response of hydroponic plant roots to physical stimulation by water flow has not yet been described in detail. The regulation of flow rate in hydroponic production affects plant growth, which in turn affects crop yield and quality. Therefore, it is necessary to explain the effect of flow rate on plant growth to better regulate hydroponic crop quality.
We performed a hydroponic cultivation experiment under artificial lighting to investigate how plants respond to different nutrient solution flow rates. To do this, we evaluated the effect of different nutrient solution flow rates on plant growth, nutrient uptake, root morphology, and cell wall composition.
The leaf is the most important organ for plants to transfer light energy to chemical energy by means of photosynthesis. Photosynthetic capacity is affected by leaf area and is closely related to plant growth [12
]. Fresh and dry plant weights are considered the main indexes for measuring yield.
In this study, we found that plant growth was promoted by flow rates ranging from 2 L/min to 4 L/min, and a flow rate of 4 L/min optimized the maximum plant yield. Beyond this optimal flow rate, the plant yield decreased. We directly observed the effects of flow rate on plant growth as seen by the plant growth indexes of dry weight, fresh weight, leaf area, nutrient uptake, root length, root surface area, and root volume.
Nitrogen (N) is considered to be the most important nutrient element. N is essential in making sure plants are healthy as they develop [13
]. In this study, we used N as a representative to investigate the effect of flow rate on nutrient uptake by plants. It is worth mentioning that although each plant growth index varied in its specific values, they all shared similar trends in plant growth index and N uptake with increased flow rate. Generally, these indexes increased as the flow rate increased from 2 L/min to 4 L/min, and decreased as the flow rate increased from 4 L/min to 8 L/min.
These results indicate that for a hydroponic crop, there is an optimal flow rate that will maximize root growth by increasing root length, surface area, and volume over other flow rates. Root length and root surface area are related to plants’ nutrient absorption capacity [14
]. Therefore, the appropriate mechanical stimulation from the flow rate will increase root growth and facilitate higher nutrient absorption to promote overall plant growth.
We found that root VFW was significantly higher at low flow rates (2–4 L/min) than at high flow rates (6–8 L/min). Additionally, the root SAFW decreased significantly at 8 L/min compared with the other flow rates. This indicates that the roots initiated a morphological response to adapt to the flow rate environment. Decreasing VFW corresponds to increasing density and the plant becoming more compact [15
]. Consequently, roots become more compact at high flow rates, which reduces the root SAFW. Decreased root surface area therefore causes the roots to uptake less N at higher flow rates.
Cellulose and hemicellulose are essential components for maintaining the root cell wall framework. Thus, the structural and compositional changes in cellulose and hemicellulose are also the main factors affecting the cell wall’s extension, elongation, and mechanical strength [16
]. In this study, we found that the CFW and the CV were significantly smaller at low flow rates than at high flow rates. Moreover, the root strength was highly correlated with cellulose content [17
], indicating that plants grown under high flow rates will develop stronger and more compact roots.
Plant responses are triggered following a range of biological, chemical, and physical stimuli [18
]. The stress theory indicates a dose-response relationship between the strength and duration of the stimulus and the corresponding plant response. Moderate stimuli positively affect plant growth (eustress), whereas excessive stimuli negatively affect plant growth (distress) [20
]. Several studies [21
] have reported beneficial effects from applying biological, chemical, and physical eustressors in horticulture. Managed eustress dose and duration may stimulate the plant response to improve overall plant yield and quality [20
]. Such managed eustress may also apply toward enhancing hydroponic production.
In hydroponics, the flow of nutrient solution provides mechanical stimulation to the plant roots. Previous studies have shown that mechanical stimulation is related to thigmomorphogenesis [15
]. In this study, low flow rates (2–4 L/min) can be regarded as eustress, and increasing the flow rate within this range can provide the roots with the appropriate mechanical stimulation for promoting root growth. Increased root growth allows the root system to absorb more nutrients, which in turn increases plant growth. Conversely, high flow rates (6–8 L/min) may be regarded as distress. To adapt to the high flow rate environment, the roots become compact, and the root surface area and overall root growth become inhibited. Root length and root surface are the key factors that determine nutrient ion absorption, and in turn, overall plant growth. Smaller root surface areas result in lower nutrient absorption, which in turn reduces overall plant growth. Our results (Figure 3
) indicate that regulating the nutrient solution flow rate can regulate plant thigmomorphogenesis and nutrient absorption, and thereby increase hydroponic crop quality.
To our knowledge, there have been no previous studies on plant morphological formation as affected by nutrient solution flow rate in hydroponics. It is regrettable that we cannot compare the results of our study with others. However, reviewing previous studies on the effects of flow rate on plant growth has led us to reach similar conclusions with other researchers (Table 1
). We have determined that there is an optimal flow rate for different plants to achieve the highest yield. Combined with these studies, we confirm that it is possible to regulate crop quality by regulating the flow rate’s mechanical stimulation of hydroponic crops.
In this study, according to the result of dry weight under different flow rates, the optimal flow rate is considered to be 2–6 L/min. That being said, it should be mentioned that plant growth is not only affected by the flow rate. The growth of plants is affected by a combination of the biological environment, the physical environment, and the chemical environment, which means that the optimal flow rate in this environment may not be the same in other environments (such as different nutrient solutions, light environment, cultivation container, and plant species, etc.). It is very important to find the ideal flow rate for hydroponic production because it is helpful to increase the yield [11
]. At the same time, based on the above reasons, it is not easy to determine the ideal flow rate, which still needs specific analysis for different specific situations. To investigate the coupling effect of flow and other environmental factors on plants is a future topic for us. Furthermore, concerning the plant characteristics, cultivation container shape and environmental characteristics, to put forward a comprehensive index not only including flow rate to guide ideal flow regulation is also a topic for us in the future [7
In drylands, plants are exposed to different biotic and abiotic stress factors such as drought, salt stress, and water shortage. These stress factors limit plant yield, growth, quality, and appearance. Agricultural and horticultural practices for food production in dryland aim to minimize stress by using suitable production systems, either under controllable or open-field environments. Hydroponic cultivation is widely used in drylands as a form of controlled environmental agriculture to alleviate stresses during crop production. The flow of nutrient solution in the hydroponic substrate mechanically stimulates plants and affects plant thigmomorphogenesis.
A suitable flow rate can be regarded as a eustress. In the range of suitable flow rates, increased flow provides plant roots with an appropriate level of mechanical stimulation to promote root growth. Increased root growth allows the root system to absorb more nutrients, which makes the plant grow better. Conversely, excess flow rates may be regarded as stress. To adapt to the high flow environment, plant roots become compact, which inhibits the root surface area and root growth. Root length and surface area are key factors that determine a plant’s capability for nutrient ion absorption, which in turn affects the overall plant growth compared with optimal flow rate conditions. Our results indicate that regulating flow rate can regulate plant thigmomorphogenesis and nutrient absorption, and therefore affect crop quality in hydroponics.
Determining the ideal flow rate for hydroponic production may help increase yield. However, such a determination requires specific analysis of each crop and growing environment. We intend to investigate the coupling effects of flow rate and other environmental factors on plants in future studies, and to develop a comprehensive index of other plant characteristics, including cultivation container shape and environmental characteristics, to guide the ideal flow regulation under different growth scenarios.