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Fluid Journal : Spring 2015
9 in stomatal regulation. Potassium ions are actively transported (requiring bio- chemically derived energy) into guard cells around the stomata. With the change in osmotic potential, an influx of water hydrates and expands the guard cell, resulting in a "swelling" and closing of the stomata. But we also know that increasing calcium concentration in the apoplast near the guard cells leads to stomata closure and a decreasing calcium concentration leads to opening of the stomata (Figure 7). So, calcium also plays a role in plant water efficiency, like potassium, but we do not know unequivocally the mechanism involved. Some hypotheses include calcium signaling the initiating of ATP formation and energy production for active transport of potassium into the guard cells. Regardless of mechanism, it is certain that calcium controls stomatal openings. Plants with a low available calcium status cannot fully close their stomata, which results in tissue desiccation pictured in the potato photo shown at the lead of this article. Cuticle. The cuticle is a waxy resinous material covering the epidermis of leaves and other plant parts. As water vapor moves through the leaf, approximately 5 to 10 percent of the water transpired by the leaf is lost through breaks in the cuticle and is termed cuticular transpiration. Some plant species, growing in a desert environment, have thick cuticles while others do not. Generally, the thickness of the cuticle decreases the amount of water vapor lost. However, when stomata are closed, higher rates of cuticular transpiration can occur. Plant-available calcium helps reduce cuticular transpiration. Stronger thicker cell walls create a barrier to reduce water loss. Wrapper leaves of lettuce, for example, exhibit a significant water loss reduction through the cuticle with increasing calcium concentration (Figure 8). Summing up Considering all of the beneficial effects of calcium nutrition in terms of yield, fruit quality, and plant health, it is also appropriate to consider that sufficient calcium supply to plants is necessary to increase plant WUE and be incorporated into a drought stress management plan. We have observed that: • Calcium protects cell membranes