Environmental Engineering Reference
In-Depth Information
Osmotic Regulation under Salt Stress
Osmotic stress on plants dose not only occur in drought land, as physiological drought,
hard to absorb water, can also be induced by salinity. Large quantity of ions in the saline land
reduces its water status, so it is difficult for roots to collect water. In consequence, most
important metabolism including photosynthesis and cell division in plants would be affected
or inhibited. Active responses are necessary for plants to survive under this adverse condition.
Lots of metabolites are accumulated in plants, which are considered as ―compatible
(organic) solutes‖ in the cytoplasm to increase their hyperosmotic tolerance for avoiding
water loss from the cells induced by salinity. Various compounds have been suggested to
accomplish this function in plants, and they include sugars, sugar alcohols, complex sugars,
quaternary amino acid derivatives, tertiary amines and sulfonium compounds (Flowers and
Colmer 2008). The compatible solutes are mainly distributed in cytosol to balance the high
concentration of salt outside the cell and compensate the high concentrations of salt ions in
the vacuole. In contrast to many halophyte plants, the capacity to accumulate compatible
solutes in glycophyte plants is not enough, and as a result, they are more susceptible to salt
stress.
Proline and glycinebetaine belong to tertiary amines. Ashraf and Foolad (2007) have
illustrated their synthesis process in detail and summarized their important protective function
under abiotic stress. The major role of proline and glycinebetaine is accepted as the
metabolites in plants grown in saline soil (Venkatesan and Chellappan 1998; Mansour 2000;
Mohanty et al. 2002; Yang et al. 2003; Yang and Lu 2005; Koskeroglu and Tuna 2010;
Chakraborty et al. 2012). Glycinebetaine is synthesized in chloroplast and it possesses
protective function for thylakoid membrane and photosynthetic apparatus. Increase in
glycinebetaine concentration was commonly observed under salt stress in many crop plants,
such as sugar beet (
Beta vulgaris
), spinach (
Spinacia oleracea
), barley (
Hordeum vulgare
),
wheat (
Triticum aes-tivum
), and sorghum (
Sorghum bicolor
) (Weimberg et al. 1984; Fallon
and Phillips 1989; Yang et al. 2003). Exogenous application of glycinebetaine can help
reduce adverse effects in maize subjected to salt stress (Yang and Lu 2005). Similarly,
increase in proline content was observed in plants under salinity condition, and it is involved
in alleviating cytoplasmic acidosis and sustaining NADP
+
/NADPH ratios at required levels
for metabolism and thus supporting redox cycling (Babiychuk et al. 1995; Hare and Cress
1997). Salt stress remarkably increased proline accumulation in leaves of two rice cultivars
with different salinity tolerance, and the increase rate was higher in the tolerant one (Demiral
and Turkan 2004). Our recent study also illustrated that proline concentration was
accumulated to a higher level in sorghum leaves subjected to salt stress, and the increased
proline help to counteract the negative effects of high temperature stress on sorghum (Yan et
al. 2012). Similarly, this cross tolerance was also demonstrated in the study on salinity
tolerance in dogwood (Renault 2012). In conclusion, proline and glycinebetaine are
appropriate indicators for osmotic regulation in plants grown in saline soil.
In order to maintain normal water content, plants accumulate high concentrations of
compatible solutes to regulate water status in cells in response to the salt-induced osmotic
stress. Water potential, glycinebetain and proline contents in leaf and root are important
candidates for detecting osmotic regulation under salinity stress. Water potential can be
detected by using dew point water potential instrument, while glycinebetain and proline
