Research | Effect of Oxygen Content in Root Environment of Greenhouse Crops on Crops’Growth

Agricultural engineering technology of greenhouse gardeningPublished in Beijing at 17:30 on January 13th, 2023.

The absorption of most nutrient elements is a process closely related to the metabolic activities of plant roots. These processes require energy generated by root cell respiration, and water absorption is also regulated by temperature and respiration, and respiration requires the participation of oxygen, so oxygen in the root environment has a vital impact on the normal growth of crops. The dissolved oxygen content in water is affected by temperature and salinity, and the structure of substrate determines the air content in root environment. Irrigation has great differences in the renewal and supplement of oxygen content in substrates with different water content states. There are many factors to optimize the oxygen content in root environment, but the influence degree of each factor is quite different. Maintaining reasonable substrate water holding capacity (air content) is the premise of maintaining high oxygen content in root environment.

Effects of temperature and salinity on saturated oxygen content in solution

Dissolved oxygen content in water

Dissolved oxygen is dissolved in unbound or free oxygen in water, and the content of dissolved oxygen in water will reach the maximum at a certain temperature, which is the saturated oxygen content. The saturated oxygen content in water changes with temperature, and when the temperature increases, the oxygen content decreases. The saturated oxygen content of clear water is higher than that of salt-containing seawater (Figure1), so the saturated oxygen content of nutrient solutions with different concentrations will be different.

Transport of oxygen in matrix

The oxygen that greenhouse crop roots can get from nutrient solution must be in a free state, and oxygen is transported in the substrate through air and water and water around the roots. When it is in equilibrium with the oxygen content in air at a given temperature, the oxygen dissolved in water reaches the maximum, and the change of oxygen content in air will lead to the proportional change of oxygen content in water.

Effects of hypoxia stress in root environment on crops

Causes of root hypoxia

There are several reasons why the risk of hypoxia in hydroponics and substrate cultivation systems is higher in summer. First of all, the saturated oxygen content in water will decrease as the temperature rises. Secondly, the oxygen required to maintain root growth increases with the increase of temperature. Furthermore, the amount of nutrient absorption is higher in summer, so the demand of oxygen for nutrient absorption is higher. It leads to the decrease of oxygen content in root environment and the lack of effective supplement, which leads to hypoxia in root environment.

Absorption and growth

The absorption of most essential nutrients depends on the processes closely related to root metabolism, which require the energy generated by root cell respiration, that is, the decomposition of photosynthetic products in the presence of oxygen. Studies have shown that 10%~20% of the total assimilates of tomato plants are used in roots, 50% of which are used for nutrient ion absorption, 40% for growth and only 10% for maintenance. Roots must find oxygen in the direct environment where they release CO₂. Under anaerobic conditions caused by poor ventilation in substrates and hydroponics, hypoxia will affect the absorption of water and nutrients. Hypoxia has a rapid response to the active absorption of nutrients, namely nitrate (NO₃^–), potassium (K) and phosphate (PO₄^3-), which will interfere with the passive absorption of calcium (Ca) and magnesium (Mg).

Plant root growth needs energy, normal root activity needs the lowest oxygen concentration, and the oxygen concentration below COP value becomes a factor limiting root cell metabolism (hypoxia). When the oxygen content level is low, the growth slows down or even stops. If partial root hypoxia only affects branches and leaves, the root system can compensate for the part of the root system that is no longer active for some reason by increasing the local absorption.

Plant metabolic mechanism depends on oxygen as electron acceptor. Without oxygen, ATP production will stop. Without ATP, the outflow of protons from the roots will stop, the cell sap of root cells will become acidic, and these cells will die within a few hours. Temporary and short-term hypoxia will not cause irreversible nutritional stress in plants. Because of the “nitrate respiration” mechanism, it may be a short-term adaptation to cope with hypoxia as an alternative way during root hypoxia. However, long-term hypoxia will lead to slow growth, decreased leaf area and decreased fresh and dry weight, which will lead to a significant decline in crop yield.

Ethylene

Plants will form ethylene in situ under a lot of stress. Usually, ethylene is removed from the roots by diffusing into the soil air. When waterlogging occurs, the formation of ethylene will not only increase, but also the diffusion will be greatly reduced because the roots are surrounded by water. The increase of ethylene concentration will lead to the formation of aeration tissue in roots (Figure 2). Ethylene can also cause leaf senescence, and the interaction between ethylene and auxin will increase the formation of adventitious roots.

Oxygen stress leads to decreased leaf growth

ABA is produced in roots and leaves to cope with various environmental stresses. In the root environment, the typical response to stress is stomatal closure, which involves the formation of ABA. Before the stomata are closed, the top of the plant loses swelling pressure, the top leaves wilt, and the photosynthetic efficiency may also decrease. Many studies have shown that the stomata respond to the increase of ABA concentration in apoplast by closing, that is, the total ABA content in non-leaves by releasing intracellular ABA, plants can increase the concentration of apoplast ABA very quickly. When plants are under environmental stress, they begin to release ABA in cells, and the root release signal can be transmitted in minutes instead of hours. The increase of ABA in leaf tissue may reduce the elongation of cell wall and lead to the decrease of leaf elongation. Another effect of hypoxia is that the life span of leaves is shortened, which will affect all leaves. Hypoxia usually leads to the decrease of cytokinin and nitrate transport. Lack of nitrogen or cytokinin will shorten the maintenance time of leaf area and stop the growth of branches and leaves within a few days.

Optimizing oxygen environment of crop root system

The characteristics of substrate are decisive for the distribution of water and oxygen. The oxygen concentration in the root environment of greenhouse vegetables is mainly related to the water holding capacity of substrate, irrigation (size and frequency), substrate structure and substrate strip temperature. Only when the oxygen content in the root environment is at least above10% (4~5mg/L) can the root activity be maintained in the best state.

The root system of crops is very important for plant growth and plant disease resistance. Water and nutrients will be absorbed according to the needs of plants. However, the oxygen level in the root environment largely determines the absorption efficiency of nutrients and water and the quality of the root system. Sufficient oxygen level in the root system environment can ensure the health of the root system, so that plants have better resistance to pathogenic microorganisms (Figure 3). Adequate oxygen level in the substrate also minimizes the risk of anaerobic conditions, thus minimizing the risk of pathogenic microorganisms.

Oxygen consumption in root environment

The maximum oxygen consumption of crops can be as high as 40mg/m2/h (consumption depends on crops). Depending on the temperature, the irrigation water may contain up to 7~8mg/L of oxygen (Figure 4). To reach 40 mg, 5L of water must be given every hour to meet the oxygen demand, but in fact, the irrigation amount in one day may not be reached. This means that the oxygen provided by irrigation plays only a small role. Most of the oxygen supply reaches the root zone through pores in the matrix, and the contribution of oxygen supply through pores is as high as 90%, depending on the time of day. When the evaporation of plants reaches the maximum, the irrigation amount also reaches the maximum, which is equivalent to 1~1.5L/m2/h. If the irrigation water contains 7mg/L oxygen, it will provide 7~11mg/m2/h oxygen for the root zone. This is equivalent to 17%~25% of the demand. Of course, this only applies to the situation that the oxygen-poor irrigation water in the substrate is replaced by fresh irrigation water.

In addition to the consumption of roots, microorganisms in the root environment also consume oxygen. It is difficult to quantify this because no measurement has been made in this respect. Since new substrates are replaced every year, it can be assumed that microorganisms play a relatively small role in oxygen consumption.

Optimize the environmental temperature of roots

The environmental temperature of root system is very important for the normal growth and function of root system, and it is also an important factor affecting the absorption of water and nutrients by root system.

Too low substrate temperature (root temperature) may lead to difficulty in water absorption. At 5℃, the absorption is 70%~80% lower than at 20℃. If low substrate temperature is accompanied by high temperature, it will lead to plant wilting. Ion absorption obviously depends on temperature, which inhibits ion absorption at low temperature, and the sensitivity of different nutrient elements to temperature is different.

Too high substrate temperature is also useless, and may lead to too large root system. In other words, there is an unbalanced distribution of dry matter in plants. Because the root system is too large, unnecessary losses will occur through respiration, and this part of the lost energy could have been used for the harvest part of the plant. At higher substrate temperature, the dissolved oxygen content is lower, which has a much greater impact on the oxygen content in the root environment than the oxygen consumed by microorganisms. The root system consumes a lot of oxygen, and even leads to hypoxia in the case of poor substrate or soil structure, thus reducing the absorption of water and ions.

Maintain reasonable water holding capacity of matrix.

There is a negative correlation between the water content and the percentage content of oxygen in the matrix. When the water content increases, the oxygen content decreases, and vice versa. There is a critical range between water content and oxygen in the matrix, that is, 80%~85% water content (Figure 5). Long-term maintenance of water content above 85% in the substrate will affect the oxygen supply. Most of the oxygen supply (75%~90%) is through the pores in the matrix.

Supplement of irrigation to oxygen content in substrate

More sunlight will lead to higher oxygen consumption and lower oxygen concentration in roots (Figure 6), and more sugar will make the oxygen consumption higher at night. Transpiration is strong, water absorption is large, and there is more air and more oxygen in the substrate. It can be seen from the left of Figure 7 that the oxygen content in the substrate will increase slightly after irrigation under the condition that the water holding capacity of the substrate is high and the air content is very low. As shown on the right of fig. 7, under the condition of relatively better illumination, the air content in the substrate increases due to more water absorption (same irrigation times). The relative influence of irrigation on the oxygen content in the substrate is far less than the water holding capacity (air content) in the substrate.

Discuss

In actual production, the content of oxygen (air) in crop root environment is easily overlooked, but it is an important factor to ensure the normal growth of crops and the healthy development of roots.

In order to obtain the maximum yield during crop production, it is very important to protect the root system environment in the best condition as much as possible. Studies have shown that the O₂ content in the root system environment below 4mg/L will have a negative impact on crop growth. The O₂ content in the root environment is mainly influenced by irrigation (irrigation amount and frequency), substrate structure, substrate water content, greenhouse and substrate temperature, and different planting patterns will be different. Algae and microorganisms also have a certain relationship with the oxygen content in the root environment of hydroponic crops. Hypoxia not only causes the slow development of plants, but also increases the pressure of root pathogens (pythium, phytophthora, fusarium) on root growth.

Irrigation strategy has a significant influence on the O₂ content in the substrate, and it is also a more controllable way in the planting process. Some rose planting studies have found that slowly increasing the water content in the substrate (in the morning) can get a better oxygen state. In the substrate with low water holding capacity, the substrate can maintain high oxygen content, and at the same time, it is necessary to avoid the difference of water content between substrates through higher irrigation frequency and shorter interval. The lower the water holding capacity of substrates, the greater the difference between substrates. Moist substrate, lower irrigation frequency and longer interval ensure more air replacement and favorable oxygen conditions.

The drainage of the substrate is another factor that has a great influence on the renewal rate and the oxygen concentration gradient in the substrate, depending on the type and water holding capacity of the substrate. Irrigation liquid should not stay at the bottom of the substrate for too long, but should be discharged quickly so that fresh oxygen-enriched irrigation water can reach the bottom of the substrate again. The drainage speed can be influenced by some relatively simple measures, such as the gradient of the substrate in the longitudinal and width directions. The greater the gradient, the faster the drainage speed. Different substrates have different openings and the number of outlets is also different.

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