Salt of the Earth

In Chapter 6 we discussed the role of long-term changes in the distribution of Earth’s continents and seas, which has shaped the diversity of coastal wetland floras over the past >66 Ma. In fact, environmental salt is a fundamental legacy of Earth’s evolution for more than 550 million years. As far back as Pangean time when there was only one supercontinent, there have been major sea level changes like those described in Chapter 2, and these have required ongoing adaption to alternating terrestrial and tidal regimes for coastal plants. Both aquatic algae and land plants (excluding mosses) responded to these shoreline changes during their evolution by developing salt-tolerance mechanisms across a wide range of taxonomic groups (Plants in Action, 1999). This convergent evolution in various mangrove trees, intertidal salt marsh and salt desert halophytes is manifest as similar mechanisms for salt exclusion by roots, salt excretion by leaves and as compartmentation of salt within fleshy leaf or stem tissue across widely separated taxonomic groups (Chapter 3). Other less well-developed mechanisms for salt exclusion and cellular compartmentation are also found in some non-halophytic wild plants and their cultivated varieties. This genetic variation in salt tolerance can be exploited for genetic improvement of commercially significant species or cultivars, especially when the mechanisms and inheritance of the salt tolerance is well understood.