Sept 22 (Reuters) - As California struggles with another crippling drought, a Silicon Valley startup that believes robots can grow produce more sustainably said Wednesday it raised $50 million in a funding round led by Bill Gates" Breakthrough Energy Ventures. Iron Ox uses robots that are integrated with a hydroponic system consuming 90% less water than traditional farms, said CEO Brandon Alexander. The company is putting that system to work at a 10,000-square foot (930 square meter) greenhouse in Gilroy, California, where a self-driving robot named Grover moves pallets of Genovese Basil and a robotic arm system lifts the pallets for inspection. Sensors check the water for nitrogen and acidity levels for healthy growth. "Then they say, "What is missing? What does that plant need that we"re not giving it"," Alexander said. Any water not used can be pumped back into the system to be reused later.Agriculture plays an important role in California"s economy, but water usage is increasingly in the spotlight. The last major drought in 2012-2017 cut irrigation for farmers, forced strict household conservation measures and stoked deadly wildfires. Iron Ox grows Thai basil and strawberries and is working on cilantro, parsley, and tomatoes. The company is also building a new 535,000-square-foot greenhouse in Lockhart, Texas, 30 miles (48 km) south of Austin. Alexander said hydroponics - saving water by growing plants without soil - is just one piece of the puzzle for future farming. "To really eliminate waste, to really get to that next level of sustainability and impact, we have to rethink the entire grow process," he said.The funding round included investors from Crosslink Capital, R7 Partners, and Pathbreaker Ventures, among others. Iron Ox declined to comment on its valuation.