Electron microprobe analyses of salt distribution in the halophyte Salicornia pacifica var. utahensis

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The halophyte Salicornia pacifica var. utahensis grows in the desert saline playa. The fused leaves form succulent stems and have apparently isolated tracheids in the palisade region as observed by scanning electron microscopy. Frozen shoots were fractured under liquid nitrogen and scanned for Na+, K+, and Cl− with an electron microprobe X-ray analyzer. In young shoots, the palisade cells were low in salts, and the spongy cells had higher concentrations. The salt in the spongy cells provides a high osmotic pressure permitting the plant to absorb more water from the soil. As the shoots matured, the concentration of salts increased in the spongy cells, and the amount of salt in the palisades also increased. The salt ions in the palisades were excluded from the organelles and were mainly present in the vacuoles. Eventually, the leaf section collapsed because of the high salt in the palisade and spongy cells, but the vascular region in the shriveled section continued to function. The sections adjacent to the dead shriveled section remained green and succulent. The salt tolerance appeared to be based on the exclusion of the salt from the photosynthetic cells and on the ability of the succulent stem to function even though sections were dead owing to high salt concentration.