Techniques are described for quantitatively determining water movement and loss in pricklypear plants with tritiated water. Transpired vapor from plants which were injected with tritium was collected with a system that passed dry air over the pad surface and into collecting vials. Water was extracted from tissue samples by a modified freeze—drying technique. Combustion of tissue samples following extraction showed that the method was efficient for tritiated water recovery from the plant material when the extraction time was 8 hours and a warm—water bath temperature of 55°C was maintained.