The Physiology of Flowering Plants - KHAM PHA MOI

354 RESISTANCE TO STRESS13.3.5 Salinity: a special case of water-deficit stressMost of the water on the earth’s surface is salt water containing NaClat about 3% or 0.5 mol L 1 and with a C of c. 2.5 MPa. Very fewflowering plants are marine, but around sea coasts terrestrial salinehabitats supporting an angiosperm flora occupy large stretches assalt marshes of temperate regions, and as mangrove swamps of thetropics. There exist also salt-rich inland areas – salt deserts andregions around salt lakes, e.g. the Dead Sea and the Great Salt Lake.Further areas have become saline as a result of irrigation. A commondefinition of a saline habitat is one where the concentration of NaClequals or exceeds 0.5%. Plants that are able to grow normally at 0.5%(or more) NaCl are classed as halophytes as opposed to salt-sensitiveglycophytes. Halophytes may be obligate or facultative. Nearly allagricultural crops are glycophytes and salinity limits the utilizationof considerable expanses of land.The ions of the salt do exert direct toxic effects, but the majorcomponent of salinity stress is water deficit. Halophytes accumulatehigh levels of Na + and Cl . This enables them to maintain a lowinternal C and avoid water deficits; for instance Salicornia europaea,grown in a saline soil at C 1.6 MPa, can maintain a shoot C of 6.5MPa. In the halophyte tissues, the ions are segregated away from thecytoplasm in vacuoles and cell walls; in the cytoplasm, high concentrationsof ions would interfere with protein structure and enzymeactivity owing to their electric charges. The ions in the cell wall andvacuolar compartments are osmotically balanced by correspondingcytoplasmic concentrations of compatible solutes, especially prolineand QAC. In plants capable of hardening towards salt stress, suchcompounds are synthesized in response to exposure to salt. Othercommon halophytic characters are succulence, the very large vacuolespermitting a dilution of the salt, and the possession of salt glands. Theseglands are found on the aerial parts of many halophytes and eliminateexcess Na + and Cl ions by secretion onto the epidermal surface.Some species achieve a certain degree of avoidance of salinity bysalt exclusion, i.e. by avoiding the uptake of excessive amounts.Varieties of barley (Hordeum vulgare) and the grass Agrostis stoloniferadiffer in their resistance to salinity; in saline media, the more resistantvarieties take up less Na + and Cl than the more sensitive ones.In some plants salt is absorbed into the root system but very little istransported into the shoots. The extent of resistance that can beachieved by salt exclusion is, however, limited, permitting the plantsto endure only moderate levels of salinity.13.4 Low-temperature stress13.4.1 Reactions to low temperature in the fieldMost plants do not possess any specific system of temperature regulation.In the centres of tree trunks, temperatures as much as 10 8C