How to prevent and control common gas damage in greenhouse flowers?

In winter and spring, when temperatures are relatively low, greenhouse ventilation is poor, and if ventilation is not done in time, gases inside and outside cannot be exchanged promptly, causing greenhouse flowers to suffer from harmful gas damage. In mild cases, leaves turn yellow and tips dry out, growth is poor, and ornamental value is reduced; in severe cases, plants die. The common types of gas damage in greenhouse flowers and their prevention and control methods are briefly described as follows: (1) Ammonia and nitrogen oxide gas damage: These two substances are mostly produced by improper fertilization. For example, when ammonium bicarbonate is applied as a topdressing, or when urea, ammonium sulfate, and large amounts of undecomposed organic fertilizer are applied heavily, combined with high temperatures, drought, and poor ventilation, a large amount of ammonia gas is often produced in the greenhouse within a short time, causing plant poisoning. The symptoms are water-soaked spots starting from the leaf margins, resembling scalding by boiling water, which gradually turn brown, dry up, and fall off, as is often seen in cut-flower roses. Nitrogen oxide gas damage usually shows symptoms only 10 days after fertilization, with irregular green-and-white mottled spots appearing on affected leaves, as is prone to occur in Gerbera jamesonii and Gypsophila paniculata. Prevention and control methods: Ammonium bicarbonate should not be used in greenhouses; when urea and ammonium sulfate are used as topdressing, they should be applied sparingly, and it is best to dilute them with water into a 500-fold solution before watering on; organic fertilizer must be fully decomposed before application. (2) Carbon monoxide and sulfur dioxide gas damage: When coal fires or sawdust burning are used for heating in plastic greenhouses, incomplete combustion, blocked flues, or high sulfur content in the fuel cause large amounts of carbon monoxide and sulfur dioxide to be produced. Affected leaves show obvious dot-like, block-like, or sheet-like white spots, and leaf tips and margins turn yellow and dry. This occurs in Gypsophila paniculata, carnations, and others. (3) Damage from toxic gases released by film: Some plastic films have plasticizers and stabilizers (such as di-2-isobutyl phthalate) added during manufacturing, and during use they can release toxic gases such as ethylene and chlorine, as well as harmful substances such as di-2-isobutyl phthalate. Data show that when the content of harmful substances such as di-2-isobutyl phthalate in water droplets reaches 10 to 20 grams mg/L, the water droplets, after being atomized and absorbed through roots or leaf surfaces, will produce severe toxic effects. When the chlorine concentration in the air reaches 0.1 mg/L, irregular white or light brown spots will appear on leaves, and in severe cases the leaves turn white or even fall off. When ethylene in protected cultivation reaches 1 mg/L, flower leaf tips and margins turn yellow, or even turn white and dry up. Prevention and control methods: Ventilate and exchange air in time, using the midday period on sunny days to open vents so that air circulates freely; even in rainy, snowy weather, short periods of ventilation should be carried out to minimize harmful gases in the greenhouse. (4) Excessive pesticide use: Excessive use of smoke agents (such as chlorothalonil and procymidone) or fumigants (dichlorvos), as well as unreasonable application of fumigants (such as formalin and methyl bromide, which can only be used in empty greenhouses and are prone to causing phytotoxicity when used during the plant growing season), can also cause flowers and plants to show symptoms such as leaf yellowing and dry tips. Prevention and control methods: Use the above-mentioned chemicals correctly and reasonably.