Secondary Damage Caused by Severe Salt Stress
Salt stress, which makes excessive demands on the cell's ability to maintain homeostasis and thus leads to increased salt concentrations in the cell, results in functional disturbances. On the one hand, processes requiring balanced water relations, such as cell elongation, react particularly sensitively. Since a great deal of energy is consumed by the strain reactions and a considerable proportion of produced photosynthate is used in the production of compatible osmolytes, cell division growth is also detrimentally affected due to lack of building blocks and energy.
In salt-sensitive plants, e.g. maize and beans, even relatively low internal salt concentrations result in considerable reductions in growth (see Table 1.6.1). Indeed, the constitutive types of glycophytes and halophytes introduced at the beginning of this chapter can be relatively easily identified and even further subdivided on the basis of the sensitivity of growth to salt. Most of our highly efficient crop plants are unfortunately glycophytes which exhibit significantly reduced yields at even only relatively low salt concentrations.
In addition to growth processes, photosynthesis, particularly that of C3 plants, is detrimen tally affected by elevated intracellular salt contents. Even though enzyme activities may be influenced by salt, the salt concentrations which build up in the chloroplast are usually too low to result in significant inhibition of the Calvin cycle. On the other hand, photosynthetic electron transport is impaired at even relatively low salt concentrations, whereby the nature of the detrimental effect has not yet been clarified. It is known that enhanced ROS formation takes place in the chloroplasts of plants under salt stress upon illumination (see Chap. 1.3.5). These ROS lead to damage of the photosystems, to chlorophyll degradation and, finally, to necrotic death of cells and tissues. Typical salinity damage symptoms become apparent (see Fig. 2.3.7 G), above all necroses at the edges of leaves and in the youngest generation of needles of conifer needles. It is interesting that the inhibition of photosynthetic C02 assimilation is not primarily due to an effect of salt on the opening of the stomata. The stomata remain functional even at high salinity. It must thereby be borne in mind that the damage having been caused results from NaCl taken up by the roots, in contrast to the direct effects of salt in salt sprays and splashings (e.g. at road edges) which damage old and young tissues in the same manner.
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