Before flowering, food molecules originally formed in the leaf are stored in the stem and the leaf itself. But after flowering, the food storage organ is the rice grain. Food molecules from the leaf move to the grain where they accumulate (Figure 12.16).
Since the amount of food stored in grains is largely dependent on photosynthesis, several farming activities done to increase harvest actually help enhance the photo-synthetic process.
In lowland rice farming, land preparation is done in flooded soil. Plowing breaks the soil into large clumps (Figure 12.17). Harrowing changes the soil further into small clumps which are thoroughly mixed with the water. A thick mixture results.
Plowing and harrowing are done for the following reasons.
• To kill weeds--Weeds compete with rice plants for sunlight and nutrients. Leaves of weeds may cover the rice leaf from sunlight. Weeds absorb soil nutrients that should otherwise go to the rice plants. If weeds are destroyed, more sunlight and nutrients become available to rice.
• To mix weeds and parts of the previous rice crop with the soil—These eventually decompose and provide nutrients for the use of new rice plants.
• To convert the soil into a soft puddle that makes transplanting easier—A soft puddle also allows uniform distribution of water after irrigation.
Most varieties of rice grow best in flooded soil. For many lowland varieties, a water depth of 5 cm is kept in the rice field from transplanting up to the fruiting stage.
When the grains are filling up, very little water is needed. After the grains have turned yellow, the field is drained of water.
Water is important to rice and other plants because water
• makes up about 85 percent of the plant cells and tissues;
• is a raw material for photosynthesis and participates in other chemical reactions;
• carries nutrients and manufactured food molecules to different parts of the plant, and
• makes the plant erect and rigid.
Have you seen a rice field after being newly transplanted with rice seedlings? The rice seedlings are arranged in neat rows. Viewed from different angles, the rice plants, usually in groups of two, are in straight lines (Figure 12.18). But a neat appearance is not the purpose of transplanting rice. Rather, transplanting is done with these objectives in mind.
• To make weed control easier—it is simpler to pull out weeds when there are enough spaces between plants.
• To allow for maximum growth of new shoots (tillers), and at the same time avoid overcrowding. Measured spaces between plants permit good development of tillers. Yet there is no overcrowding of plants because of enough spaces between them.
Seventeen elements are required by rice plants for proper growth and development. Of this number, three elements, namely, carbon (C), hydrogen (H) and oxygen (0) come from air and water. The rest are taken by plants from the soil.
Rich soil contains all the 14 essential elements. However, due to continuous planting and harvesting of crops, the soil will lack some of the elements after sometime.
For example, nitrogen (N), phosphorus (P) and potassium (K) are used in large amounts by rice and other plants. Usually, these elements become deficient in the soil after continuous harvests. Hence, they are supplied to the soil in the form of fertilizer (Figure 12.19).
in rice farming, fertilizer is applied soon after transplanting and just before flowering.
The three essential elements play a role in photosynthesis, directly or indirectly.
• N is part of the protein molecule. Proteins are a major component of the cell. As enzymes, they control the rate of chemical reactions.
• P is needed to form ATP. ATP, a high-energy molecule, is one of the products of the light reactions.
• K is involved in the opening and closing of the stomata. Therefore, it is related to the entry and exit of carbon dioxide, water vapor and-other gases into and out of the leaf.
Controlling Insect Pests
Rice has many insect enemies. Some examples are the leafhoppers, plant hoppers and stem borers (Figure 12.20). Leafhoppers feed on leaves and upper parts of the rice plant. Plant hoppers eat the lower parts of the plant.
Moreover, both groups of insect pests carry a certain kind of virus that causes the dreaded rice disease, known as tungro. Plants affected with tungro are shorter, with yellowing leaves and poor growth.
Another kind of insect pests are the stem borers. Larvae of stern borers feed on the leaf sheaths, causing the central leaves to dry up. During the fruiting stage, grains fail to form.
By destroying insect pests, a better yield or harvest results. This is because:
• Leaves and the whole plant, in general, exhibit good growth.
• Rice grains are allowed to develop.
In our country, most farmers use chemicals (pesticides) to kill insect pests. However, this method has dangerous effects on the human body, on other organisms and on the environment. The farmer has to be very careful in using pesticides.
Researchers of the International Rice Research Institute (IRRI) and other research units in our country are lookingg for other methods of fighting insect pests. One method they are investigating is biological control. In this method, other organisms are used such as other insects that feed on, or destroy the pests.