However, since the loss of the Aral Sea is due to over-intensive use of the water for farming, the gives us an idea of how much water can be contained onland in plants

Isn't it more of an indication of how much water can be contained in the Aral Sea basin? The plants don't need to contain all of the missing Aral Sea water at once, they just need to be watered faster than the Sea is being refilled by rainfall. How much water does rainfall supply every year, as a percentage of the Sea's total volume?