植保专业英语电子书籍.docVIP

LESSON ONE WHAT PLANTS NEED To grow satisfactorily a plant needs warmth, light, water, carbon dioxide and about a dozen other chemical elements which it can obtain from the soil. WARMTH Most crop plants in this country start growing when the average daily temperature is above 6℃(42℉). Growth is best between 16℃ (60℉) and 27℃(80℉). These temperatures apply to thermometer readings taken in the shade about 4 ft above ground. Crops grown in hotter countries usually have higher temperature requirements. Cold frosty conditions may seriously damage plant growth. Crop plants differ in their ability to withstand very cold conditions. For example, winter rye and wheat can stand colder conditions than winter oats. Potato plants and stored tubers are easily damaged by frost. Sugar-beet may bolt (go to seed) if there are frosts after germination; frost in December and January may destroy crops left in the ground. LIGHT Without light, plants cannot produce carbohydrates and will soon die. The amount of photosynthesis which takes place daily in a plant is partly due to the length of daylight and partly to the intensity of the sunlight. Bright sunlight is of most importance where there is dense plant growth. The lengths of daylight and darkness periods vary according to the distance from the equator and also from season to season. This can affect the flowering and seeding of crop plants and is one of the limiting factors in introducing new crops into a country. Grasses are now being tested in this country which will remain leafy and not produce flowering shoots under the daylight conditions here. WATER Water is an essential part of all plant cells and it is also required in extravagant amounts for the process of transpiration. Water carries nutrients from the soil into and through the plant and also carries the products of photosynthesis from the leaves to wherever they are needed. Plants take up about 200 tons of water for every ton of dry matter produced. CARBON DIOXIDE (CO2)