Plants respire, as we do, so much of that water goes into the air. Water + CO2 + light -> sugar + oxygen. Metabolized sugar -> water vapour + heat + CO2. Also, much of that water, now in the form of sugar, is used to grow the plant. Cellulose, in particular, in the cell walls, etc. There are certainly pathways to unlock the water again; burning cellulose releases water vapour, for example. But how much are you going to get for the work involved? 1 tonne of dry hay would release a few hundred litres of water if you burned it, very roughly.
Conceivably, you could grow them in a greenhouse and capture much of the exhaust, but it'd probably be cheaper and easier to desalinate, or just dehumidify air directly. Just growing them in a greenhouse at all -- with humidity and gas exchange under control -- would allow water consumption to be cut enormously.
Conceivably, you could grow them in a greenhouse and capture much of the exhaust, but it'd probably be cheaper and easier to desalinate, or just dehumidify air directly. Just growing them in a greenhouse at all -- with humidity and gas exchange under control -- would allow water consumption to be cut enormously.