How do leaves absorb nutrients?

The mechanism of nutrient uptake in leaves

The leaves of aquatic plants are important parts for absorbing mineral nutrients. On the other hand, there are waxy layer and cuticle layer on the outer wall of leaf epidermal cells of land plants. The waxy layer is highly hydrophobic, which is not conducive to the absorption of water-soluble nutrients. Water and its dissolved inorganic ions reach the stratum corneum through the Spaces on the waxy layer. Then it reaches the cell wall through a microporous channel (ectoplasmic desmosis). The nutrients then travel through the cell wall to the plasma membrane just as they do in the roots. The transmembrane process of nutrients is similar to that of roots.

Characteristics of leaf surface nutrition

Compared with root nutrition, leaf fertilization is a fast and efficient way of fertilization. Foliar spraying can also prevent the fixation of nutrients in the soil, which is very beneficial for the application of trace element fertilizers such as zinc, copper, iron and manganese. In addition, leaf nutrition is an effective fertilizer supplement method in some special situations, such as decreased root absorption capacity, soil drought, soil fertilization difficulties, etc. However, the application amount of foliar spraying fertilizer is limited, the fertilizer effect is short, and it needs to be sprayed several times. Therefore, extranuclear nutrition cannot replace root nutrition, and can only be used to solve some special plant nutrition problems.

Influencing factors of leaf surface nutrition

① Blade type

Aquatic plants and plants growing in humid environments, thin waxy layer, easy to absorb nutrients; However, xerophyte leaves have a thick waxy layer, which makes it difficult for them to absorb nutrients. Dicotyledonous plants have large leaf area, thinner leaf cuticle, and easier nutrient penetration. Monocotyledonous plants have small leaves, thick cuticle, and nutrients are not easily absorbed. Compared with field crops, crops in greenhouses have delicate leaves and less cuticle. Therefore, the nutrient concentration and fertilizer spraying times should be appropriately increased for xerophytes and monocotyledonous plants.

② Types and concentrations of mineral elements

The absorption rate of different kinds of mineral nutrients in plant leaves is different. The nitrogen absorption rates of leaves were urea > nitrate > ammonium salt. The absorption rate of potassium fertilizer was potassium chloride > potassium nitrate > potassium dihydrogen phosphate. Within a certain range, the proper increase of nutrient concentration is conducive to nutrient absorption. Nitrogen, nutrient concentration must be determined by plant species, growth environmental conditions, nutrient types, fertilization purposes. If the concentration is too high, it will burn the leaves and damage the roots, which is more likely to occur in the seedling stage.

(3) The adsorption capacity of leaves for nutrients

The longer the attachment time of nutrient solution on the leaves, the better the nutrient absorption. If surfactant is added to the solution, the adhesion time of the solution on the leaf can be extended.

④ Spraying period

Generally choose the morning after the dew dries in the morning, or before the sun sets in the afternoon, or a windless cloudy day. Do not spray before heavy rain, or under the hot sun, or when the wind is strong.

Nutrients enter mesophyll cells from leaf surface through three main ways: directly through the cuticle, through stomata, and through the crevices at the junction of leaf hairs and leaves. In the leaves of most crops, nutrients can enter mesophyll cells directly through the cuticle. There are colloidal substances passing through the cuticle on the surface of the leaves, which can be used as a channel for water-soluble nutrients to enter the inside of the leaves. Some small holes and channels were found in the cuticle of apple leaves.

The distribution of stomata on the leaf surface is very dense, about 100 to 500 per square millimeter. The surface of stomata is covered with cuticle, but the cuticle covering the pore cavity has high polarity and no wax layer, and nutrients can directly enter the leaf interior. The speed of nutrients passing through the stomatal cuticle was higher than that of the non-stomatal cuticle. The wax layer and cuticle layer on the lower surface of the leaves are less than that on the upper surface of the leaves, the stomata are relatively more, and the nutrient absorption rate is also higher than that on the upper surface. It is recommended to spray foliar fertilizer and internal fungicide on both sides.

The waxy layer and cuticle layer are not evenly distributed at the intersection of leaf hairs and leaves, and water-soluble nutrients can enter the interior of leaves through this part. Nutrients enter the leaf epidermal cell wall after passing through the waxy layer and the cuticle layer, and this absorption process is less hindered. Nutrients are transported through the cell wall across the membrane to the inside of the cell in the same way as root cells. There was little difference between plant utilization and root absorption of nutrients in the physiological activities of mesophyll cells.

The process of water soluble nutrient ions or molecules passing through the stratum corneum is generally a diffusion process, and the penetration rate and quantity are closely related to the nutrient concentration, temperature and ionic properties. The utilization of nutrients by crops depends on the amount of nutrients entering mesophyll cells. The permeability of elements is an important factor affecting blade surface absorption, and it is necessary to refer to the permeability of each element and the safe density of solution.