How Orchid Plants Absorb and Transport Mineral Elements

The mineral elements required by orchid plants are mainly obtained fromcultivationthe inorganic salts in the substrate. Therefore, the root system is the main absorbing organ. At the same time, Chineseorchidcultivationalso emphasizes foliar feeding, so the leaves also absorb inorganic salts more directly. Andcultivationthe inorganic salts in the substrate must dissolve in water to become ions before they can be absorbed by the roots. For example, the nitrogen absorbed by the plant is mainly ammonium ions (NH+4) or nitrate ions (NO3), the phosphorus absorbed is mainly phosphate ions (H2PO-4, HPO=4), and potassium ions (K+) are absorbed from potassium salts or calcium ions (Ca++) from calcium salts, etc. Inorganic salts must dissolve in water before they can be absorbed by the roots, but the amount of nutrients absorbed is not consistent with the amount of water absorbed. In fact, only a very small amount of the nutrients in the cultivation substrate is absorbed and utilized by the plant, part of which is nutrients dissolved in the substrate solution, and part of which is solid-phase nutrients in equilibrium with the nutrients in the substrate solution. However, when a certain ion in the substrate solution is excessive, or when only a single salt exists in the solution, the plant will be poisoned or die. Only when the proportions of various ions are appropriate can the plant grow and develop well. Therefore, there is antagonism among ions to eliminate mutual toxicity. Thus, the solution of the cultivation substrate should be made into a balanced solution, so that the cultivated plant has both appropriate mineral nutrients and no ion toxicity. Because the plant physiologically selects physiologically acidic salts or physiologically alkaline salts, the substrate solution becomes acidic or alkaline. Becoming acidic or alkaline is unfavorable to the growth of orchid plants, so balance must be provided when supplementing mineral nutrients. The absorption of mineral elements by the root system is a complex physiological process, which includes passive absorption and active absorption. When the concentration of a certain ion in the substrate solution is greater than the ion concentration inside the cell, the ion enters the plant body by diffusion, forming passive absorption. This mode of absorption does not require energy supply. But for the cell to absorb continuously, it must often maintain the ion concentration difference inside and outside the cell. Only when the absorbed ions can immediately participate in the synthesis of other substances or be rapidly transported upward and do not accumulate in the cell can such absorption proceed actively. Mineral elements enter the surface of the protoplasm by way of ion exchange, then combine with transport enzymes and enter the interior of the protoplasm. This absorption is achieved by the energy produced by respiration. This active absorption mode is the main form by which the plant root system absorbs mineral elements. Through ion exchange adsorption, the ions adsorbed by the protoplasm are transferred into the protoplasm and vacuole, causing the mineral elements to transfer from the inner layer of the protoplasm to the duct. After the mineral elements enter the duct, they rise with the transpiration stream and are transported to all parts of the plant, supplying growth and living consumption. The root system's absorption of the anions and cations of certain salts is different. For example, when (NH4)2SO4 is applied, because the plant's demand for nitrogen is greater than for sulfur, the absorption of the cation (NH+4) is always greater than that of the anion (SO=4), so that much of the unabsorbed SO= remains in the cultivation substrate, gradually acidifying its solution to become acidic. Most ammonium salts produce this situation. If NaNo3 is supplied to the plant, the amount of NO-3 absorbed by the plant is far greater than Na+, so the cultivation substrate solution gradually becomes alkaline due to the presence of excessive Na+. Most nitrates produce this result. As for salts such as NH4NO3, the plant absorbs the cation and anion in almost equal amounts, and because there is no excessive single ion, the cultivation substrate solution is mostly neutral. This selectivity for ion absorption should not be used alone for a long time during cultivation, so as to avoid making the cultivation substrate too acidic or too alkaline, which is unfavorable to plant growth. However, in the cultivation process of Chineseorchid, single chemical fertilizers are rarely used, because they decompose easily and are easily absorbed by the plant, often causing fertilizer damage. In addition to absorbing nutrient elements from the cultivation substrate through the root system, many chemical substances can be absorbed by the leaf surface in the state of aqueous solution, including mineral elements. Chinese orchids often use this method for foliar fertilization. This can solve the problem that fertilizer applied to the cultivation substrate takes a relatively long time to be absorbed and utilized by the plant, so that it cannot meet the needs of the rapid growth period. And when the root system is injured, or in the later stage of plant growth, or before new shoots have grown and root absorption is difficult, foliar fertilization can be carried out to solve this. Foliar topdressing uses a small amount and takes effect quickly. Therefore, it has become the main method of fertilization in Chinese orchid cultivation. But it cannot completely replace soil fertilization, and can only be a supplement to cultivation substrate fertilization. When carrying out foliar topdressing, the concentration of the solution, pH, spraying amount and spraying site should be considered. In addition, the influence of weather should also be considered, such as wind speed, temperature, atmospheric humidity, sunny or rainy days, etc. Generally speaking, the effect is poorer when it is too dry. In this case, the concentration of the fertilizer should be smaller, and spraying on the back of the leaves in the early morning or evening gives the best effect. The absorption of mineral elements by the plant is a physiological process related to respiratory metabolism. All factors that affect the respiration of the root system can also affect the absorption of mineral elements by the root system. (Liu Zhongjian)