By Dr Darren Plett
Japan’s tsunami of March 11 2011 brought a wall of water laden with debris up to 5 kilometres inland from the sea. After the surge receded, the surrounding farming area was left covered in debris and a thick, black sludge. This sludge was extremely saline due to the sodium chloride from seawater.
Rice is the largest agricultural crop in Japan and the five prefectures affected by the 2011 tsunami are among the top producers of rice in Japan. Fortunately, less than 1.5% of Japan’s entire rice producing region was covered by the tsunami. Preliminary rice production statistics from the 2011 growing season show total rice production in Japan has hardly changed from 2010.
This all sounds fine on a national scale, but how did the tsunami affect the subsistence farmers in the tsunami-affected region? Reports indicate the 2011 rice production was severely decreased by salinity stress in the tsunami-affected region. This seriously affected the livelihood of these farmers.
The carpet of sludge and debris left by 2011’s tsunami wreaked havoc on paddyfields. AAP
Rice is a salt sensitive plant
Salinity stress decreases plant growth and therefore also the yield of crop plants. This happens in two ways.
Salt makes it more difficult for the plant to take up enough water. And the sodium or chloride ions can negatively affect important processes – such as photosynthesis, the process plants use to make energy from sunlight – in the leaves.
All plants have some tolerance to salinity. There are very tolerant, predominantly wild species which can grow when supplied with water as saline as sea water (saltbush is one example). Then there are the sensitive, including most common crop species.
Unfortunately for the Japanese farmers affected by the tsunami, not only is rice a salinity sensitive species, it is one of the most salinity sensitive crop species grown.
Breeding salt-tolerant crops
Plants have several defence mechanisms they use to maintain growth and tolerate salinity. The first is to keep the salt out of the sensitive leaves. Crops like rice are poor at it; crops like wheat are relatively good at it.
The second is to tolerate the salt that enters the leaves. Barley is particularly good at this. The third is the ability to survive decreased water uptake despite high levels of salt in the soil, a trait which may also be found in crops tolerant to drought stress.
Researchers know the most about keeping salt out of the leaves and the least about how plants tolerate decreased water uptake. As such, many of the genes known to be important for salinity tolerance in crops encode sodium transporters which stop sodium reaching the leaf.
Plant breeders are actively transferring these salinity tolerance genes into crops that possess other positive traits such as high yield or disease resistance. For example, the wild rice Nona Bokra, which is salt tolerant, has been crossed with the Koshihikari rice variety, which is popular with the Japanese consumer because of its superior appearance and taste. This new line has all the popular traits of Koshihikari, as well as salinity tolerance from Nona Bokra’s more efficient sodium transporter.
Lessons from previous tsunamis
The years following the December 26 2004 tsunami have given agricultural researchers and farmers valuable information about how long it takes to recover from a tsunami. Researchers have learnt the length of time required for land to return to pre-tsunami salinity levels and the best methods for accelerating this process.
Most experts estimate that within two years soil can be returned to its original salinity levels. This requires plenty of rainfall as well as irrigation with non-saline water. The rain or clean water washes the salt down through the soil profile until it is too deep for the rice roots to reach.
This prognosis is good news for the farmers in north-eastern Japan, a region which generally has plentiful rain and good sources of clean water.
Salt-tolerant crops could help farmers
Farmers in the large, relatively dry regions of the Australian agriculture zone are faced with consistently saline soil. For these farmers, crops with improved salinity tolerance are crucial.
But what about Japanese farmers? If, after a tsunami, rainfall and irrigation are sufficient to remediate the saline soil, would farmers still benefit from salt-tolerant rice?
After the March tsunami there was a very short window of time before the 2011 rice sowing season began in April. The salinity levels in the rice paddies would have been very high at sowing time. Seedling establishment is critical for developing yield potential: any increase in growth provided by salinity tolerant rice varieties would have increased yield, and that would have increased the income of farmers on very small economic margins in that difficult year.
Continued development of salinity tolerant rice varieties for release to farmers with low incomes could be vital in helping those farmers. Not only will it be important to help agriculture survive in salinity affected areas, it can also help the short-term recovery from devastating tsunamis.