Effects of drought on water relations and nonstructural carbohydrates in cladodes of Opuntia ficus‐indica

Nocturnal acid accumulation, water content, osmotic pressure (π), and nonstructural carbohydrates were determined in the chlorenchyma and the water‐storage parenchyma of Opuntia ficus‐indica (L.) Miller for well‐watered plants and those subjected to drought for 15 weeks. During the 15‐week drought, total cladode water content decreased by 57%, the water‐storage parenchyma losing a greater fraction of water than the chlorenchyma, which most likely helped maintain nocturnal acid accumulation in the latter tissue. Despite the preferential water loss from the water‐storage parenchyma, it had a lower π than the chlorenchyma over the 15 weeks of drought, suggesting a substantial decrease in osmotically active solutes in the water‐storage parenchyma. Also, the measured π increases of both tissues were much less than those predicted based on the loss of water during drought and the initial content of osmotically active solutes under well‐watered conditions. A decrease in the amount of soluble sugars (glucose. fructose and sucrose) occurred in plants subjected to drought. accounting for 46% and 81% of the difference between the measured and the predicted increases in π of the chlorenchyma and the water‐storage parenchyma. respectively. The decrease in soluble sugars was associated with an equivalenl increase in polysaccharides, presumably starch, in the water‐storage parenchyma. but not in the chlorenchyma.