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Do you still believe that Rice grows in water is because it likes water?

2020-05-12
Oryza is a kind of grass and energy source for half of the world's people. 90% of the world's rice is eaten in Asia.


 
Since it's rice, and it seems that there is always water in the rice field from small to large, many people take it for granted that rice is an aquatic plant that likes water

Actually not. The main reason why rice is irrigated is to  kill weeds.
 
According to the international food and Agriculture Organization (FAO), rice is the only cereal crop that can survive in the water. Irrigation is an important weed control method to avoid weed breeding and herbicide use. The International Rice Research Institute (IRRI) also said that flood irrigation is the best way to control weeds in rice fields.
 
Barnyard grass, small flower umbrella plant and other weeds are all weeds that like to play with rice. They don't care about social distance at all.
 
If the weeds are not weeded in time, the rice will not grow handsome due to the competition of weeds. How terrible are weeds? If there are only 10 weeds per square meter, the yield of rice will drop by 25%. Moreover, if weed seeds are mixed with rice, the quality and price of rice will be affected.
 
Although these weeds, which have been mixed with rice for a long time, are more or less adaptable to shallow water, no rice can adapt to flooding. So people have already found out the way to grow rice in paddy field.
 
In this way, under the control of Fick's first law, the diffusion coefficient in water is 10000 times smaller than that in air, so it is difficult for plants to breathe underwater. The seeds of weeds also need oxygen to germinate. Watering them will kill them in the bud.

Of course, rice can't stand to soak for a long time.
 
During the period of being water flooded, the rice looks pale, weak and hard. Generally speaking, rice can endure 48 hours of flood irrigation, and then the water should be drained in time. When they harden, repeat the flooding several times.
 
Different rice varieties can bear different water depth, usually the water depth of rice field is about 5cm ~ 20cm. Of course, deep water can drown stronger weeds, but it will also spiral rice into the sky, so farmers need to master the delicate balance between water depth and the number of weeds.

So, how does rice adapt to the water environment?

 

Rice was first domesticated in Asia 9000 years ago. In fact, although people began to grow rice by flood irrigation for a long time, the understanding of the principle of this method is less than 80 years.

 

In the 1940s, during World War II, Dutch botanist MH van Raalte first studied the nature of rice flood tolerance in the Bogor botanical garden of Java (now Indonesia).

 

After decades of research, scientists have finally discovered the following water tolerance skills of rice.

 

First of all, after soaking in water, the rice will grow and leave its leaves out of the water, which is called "bubble seedling".


Second, after soaking in water, rice will spontaneously cause some cells in the root to die and form aerenchyma.

The aeration tissue of the root is connected with the leaf, which is equivalent to a straw, so that the root can breathe freely and freely under the water, so as to retain the root in this way. But the seedlings can also produce coleoptile similar to phreatic breathing tube under the condition of hypoxia.

 

The coleoptile (red circle) used for respiration in Rice Seedling

Another skill of rice is that when their leaves are soaked in water, they can absorb a layer of air on the surface of the leaves, so that the leaves can exchange air indirectly with water.

Semi aquatic plants, such as rice, form gas film on the surface of their leaves when they are immersed in water. Photo source: (DOI) 10.1104/pp.112.208173

In addition, rice's "alcohol" is not bad.

Although the plant can also carry out anaerobic respiration in a short period of time, the toxic metabolic waste ethanol produced by anaerobic respiration will quickly accumulate, which will lead to the death of the plant.

However, the root of rice is very tolerant to ethanol, so that it can endure hypoxia for a period of time before it grows out of aeration tissue.

Of course, there are also a few weeds, such as rough seed bulb, which is the same as rice and can endure deep water, so they are the main application objects of rice herbicides.

 

It took a lot of human effort to tame such strong grass. You probably don't know that human beings are still fighting with rice that wants to be wild.
Have you eaten the so-called red rice? Is this a special rice variety? Not really. Red rice is the wild rice, that is, the prodigal son who wants to return to his ancestors.




Red rice image source: Wikipedia
 
According to a study published in Journal of evolutionary biology in 2013 by Kenneth Olsen, a rice scientist at the University of Washington, Louise, and colleagues, rice has experienced two times of emergence and two times of accession to the WTO: first it was independently domesticated in Africa and Asia, then it experienced de domestication in the next 1000 years, that is, it returned to the wild.
 
So what is rice domesticated and what is wild?
 
In this way, all domesticated animals and plants will have domestication syndrome, which is mainly for rice that does not shed grains.

 

Pictures of wild rice (left) and domesticated rice (right) with and without falling grains from (DOI) 10.5334/ai.1314

When the ancestor of rice was still a wild product, the immature seeds would automatically bounce and fly. How can the farmers clean up after breaking them off, right? After domestication, the seeds will hang until they are mature.

In addition, the domesticated rice seeds are larger, and there will be no dormancy period like wild goods when they germinate, so that the seeds can germinate and grow together, and the whole family can be harvested neatly.

 

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Some rice glumes in Africa and other places are colored, but most rice glumes are Khaki colored and look golden when they are put in the fields.

However, after the wild, the glumes will turn black and return to their ancestors, turning into strange colors such as black or red.

A study published on Weed Science in 2005 found that as long as there are 25 red rice plants per square meter, the yield of rice field will drop by 60%. Therefore, red rice is properly labeled as "weed".
Olsen said that after the harvest of rice fields polluted by wild goods, the White Rice turned into red rice, so many farmers had no way to sell it. Fortunately, the anthocyanin content of red rice is relatively high, so it has a good health care selling point.


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