Detailed Introduction to the Functions and Roles of Rice Leaves

The rice leaf is composed of two main parts: the leaf blade and the leaf sheath, in addition to accessory organs such as the auricle and ligule. The junction where the leaf sheath connects to the stem is called the leaf node. The junction where the leaf sheath connects to the leaf blade is called the leaf collar. When the seed germinates, the first thing to grow out is the coleoptile that protects the young shoot as it emerges, also called the sheath leaf. The sheath leaf differs from other leaves in that it is white and shaped like a cylinder. The second leaf has only a leaf sheath and no leaf blade, and is called an incomplete leaf. Leaves beginning from the third node have both a leaf sheath and a leaf blade, and are called complete leaves. Counting the number of leaves of a variety should start from the first complete leaf. The number of leaves from the first complete leaf up to the stage of young panicle differentiation is related to the maturity period of the variety. Early-maturing varieties have fewer leaves, while late-maturing varieties have more leaves. After young panicle differentiation begins, the number of leaves that grow out is three for any variety. Therefore, when the third leaf counted from the top begins to appear, that is also the time when young panicle differentiation begins.

The leaves that rice grows before young panicle differentiation serve root development, tillering and the formation of later leaves; after young panicle differentiation, the nutrients required by the rice roots are produced and supplied by the fourth and fifth leaves counted from the top. According to research, two-thirds to three-quarters of the nutrients needed for grain filling come from the photosynthesis of the last few leaves. Therefore, strengthening field management in the later stage and protecting the last few green leaves, especially the topmost flag leaf, from damage by diseases and pests, so as to extend their lifespan, is an important measure for improving grain filling.

The importance of rice leaves lies in their photosynthesis. Using carbon dioxide absorbed through the stomata on the leaf surface and water absorbed by the roots and transported to the leaves as raw materials, with chlorophyll as the medium, rice leaves use the sun's thermal energy to synthesize carbohydrates (glucose). This physiological activity is called photosynthesis, or assimilation. Of the dry matter of the grains and straw produced by rice, 90-95% is organic matter generated by photosynthesis. The photosynthetic amount on cloudy days is about one-half that on clear days. The photosynthetic amount on rainy or dark days is about one-third that on clear days. If grain filling and maturation occur under clear weather, the grains are plump. When the light received on both sides of a rice leaf is equal, photosynthesis on both sides is also equal. Therefore, upright leaves have high photosynthetic efficiency, while drooping leaves have low efficiency. If water inside the leaf is insufficient, the stomata on the leaf surface close, which hinders the leaf's absorption of carbon dioxide and thus affects photosynthesis. Therefore, in cultivation techniques, field drying at the mid-stage should not be too severe, and water should not be cut off too early in the later stage, so as not to affect yield.

The growth of rice leaves is closely related to temperature, light, water and nutrients. It is most suitable at an air temperature of 32°C and a soil temperature of 30-32°C; below 7°C or above 40°C, leaf growth stops. If early rice seedlings encounter short-term low temperatures (below 5-7°C), the young leaf forming and elongating inside the leaf sheath will lose its ability to form chlorophyll. After this young leaf emerges, it will be entirely white, or partly turned white, and it is not easy for it to turn green again after the weather warms up.

Among the nutrient requirements of rice leaves, nitrogen fertilizer is the most important. When nitrogen fertilizer is sufficient, the leaf color is dark green, growth is vigorous, and photosynthesis is strong. When nitrogen is deficient, the leaf color is pale yellow, the leaves are narrow and small, and photosynthetic products are few; conversely, if nitrogen fertilizer is excessive, the leaves expand excessively, become soft and drooping, causing shading, which also affects photosynthesis and makes the plants prone to disease. When phosphorus is deficient, the leaf color is dark green, and the leaves are narrow and small, and upright. When potassium is deficient, the leaf color is dark green, the leaf tips turn yellow, and the leaves are broad, thin, short and drooping. Therefore, in cultivation techniques, appropriate fertilizer and water management measures should be adopted according to leaf color and leaf posture, so as to obtain more photosynthetic products and achieve the goal of stable and high yield.