Fifty years ago, every Indian village would probably have grown a dozen or more rice varieties that grew nowhere else. Passed down from generation to generation and family to family, there would have been a local variety for every soil and taste – rice that would grow well in droughts or deep floods, which had the aroma of mangoes or peanuts, tolerance for saltwater or medicinal value.

Back then, says the rice conservationist Debal Deb, India may have had more than 100,000 landraces, or local varieties. "Today there could be just 6,000, with fewer being grown every year. Every community had its own varieties. The rest are no longer cultivated and the knowledge of how to grow them will have been lost."

Deb, a plant scientist turned farmer, is on a mission not just to reintroduce the lost varieties but to improve agriculture for an age of climate change and scarcity. He is cultivating 920 rice varieties on just 2.5 acres in a forested area of the Niyamgiri hills, where the indigenous people last year managed to drive out the giant mining company Vedanta.

His seed bank, Vrihi, the Sanskrit word for rice, is growing fast as people bring rare seed to him. He grows it and then distributes it in 1kg packets. "Farmers take the seeds on condition they bring some back," he says. "They must return 2kg as proof they have cultivated it. Most give 1kg to other farmers so the cycle continues. In three years in Orissa, 2,000 farmers have received the seeds and 350 varieties have been distributed."

These are not just "heritage" varieties grown for the sake of growing them, he says, they are vital for food security, culture and biodiversity. Landraces perform better in marginal environmental conditions than modern cultivars produced by selective breeding. Knowledge and availability of landraces will become increasingly important, he says, as climate change shifts rainfall patterns and makes extreme temperatures a more regular occurrence, and as modern agriculture comes to rely on ever fewer varieties and so becomes susceptible to large crop losses.

Deb's search for rice varieties began nearly 20 years ago in remote areas of West Bengal largely populated by indigenous people. These marginal areas, often dismissed as backward by urban and political elites, were home to many landraces, because farmers were mostly too poor to buy the agrochemicals needed to grow modern rice varieties.

The genetic variety and the quality of what they grew amazed him. "The landraces have been developed by unknown farmers over centuries. They are all based on careful observation of their properties. One of the main problems for marginal farmers is drought.

"We have developed two varieties of drought-tolerant seeds. In addition, we have six varieties of salt-tolerant rice which we have reintroduced into the Sunderbans. They were the only varieties to survive when cyclone Aila struck the region in May 2009.

Deb, who was a Fulbright scholar and who has done post-doctoral ecology work at the University of California at Berkeley, is not impressed by GM science or hybrid rice growing. "Companies are spending billions on 'gene mining', or seeking specific genes. Yet after 60 years they still do not have one which can withstand a drought or flooding or sea water. But all of these characteristics are available in the landraces. I have varieties of rice that can grow and live for months in 12ft-deep water. There are varieties with amazing medicinal properties. The tribals know about certain dark-grained rice that give high levels of antioxidants and can prevent cancers."

He prefers a living seed bank where varieties are grown every year. "The Indian seed bank has 65,000 varieties, but 90% of them are dead and will not germinate … They are useful for big companies because the genes are still good, but they are useless on a farm. I have a living seed bank.

"High-yielding crop varieties have resulted in the loss of numerous landraces possessing important genes. With the rapid disappearance of folk varieties, farmers have become entirely dependent on commercial seed suppliers for their crop. Seeds used to be a precious gift to relatives and friends. Because crop seeds were traditionally to be belong to the community, there was no scope for commercial appropriation," he says.

Deb accuses large seed companies of trying to steal landraces. "They seem very interested in one, a three-grain rice. We have the last variety. We also have the last double grain rices. I was offered 15,000 rupees [$240] for just a handful. I just kicked the man out. Another man tried to steal some and a third tried bribes. One company man disguised himself as a farmer. I kicked him out, too."

He now keeps the rice seeds locked in a safe house, and the varieties are identified only by a number. "If they got hold of, say, three-grain rice, they would make millions. Would they share the benefits with the community who developed it? I doubt it. They would patent it. Once it became a proprietorial variety no one else would be allowed to save it. You would have to buy it each year.

"The collective knowledge about rice growing and diversity is still there but only in places which have not been industrialised. In a natural forest you can still find people who know hundreds of medicinal plants. But in a monocultural forest, people simply do not know the uses of plants. The diversity is lost. The collective memory is becoming eroded. People are being educated to think that anything traditional is bad."

He blames India's green revolution for the loss of genetic diversity and biodiversity. The development in the 1960s and 70s of new high-yield varieties of grain which needed synthetic fertilisers, pesticides and more water is popularly said to have ended famines, but for Deb it was an ecological and cultural disaster for which we are paying now.

He trains farmers and battles what he calls "developmentality", a mental "virus" of the modern world which he says has produced a collective mindlessness in India's elite and led to the crisis in rural India. He calls for a zero-growth economy and a new appreciation of how indigenous societies around the world have interacted with nature.

"I have nothing. I live by a bit of teaching. I have no institution behind me and I am limited only by the small amount of land that I have and the little money I have for research. My goal is to set up a living seed bank in every state in India and to train scientists as para-conservationists, so we can propagate and document all the landraces."