The Impact Of Salt Uptake And Accumulation On The Drought Resistance Of Plants

The effect of different concentrations of NaCl treatment on the growth, photosynthesis,photosystem activity, antioxidation activity in leaves, MDA content, water status and Na⁺,K⁺content in both hydroponically cultured and pot grown arrowleaf saltbush (Atriplextriangularis) subjected to osmotic/drought stresses was investigated, in order to get an insightinto the regulation of environmental NaCl on the drought resistance of the plant. The resultsare as follows:(1) The impact of NaCl on the growth of arrowleaf saltbush subjected to osmotic/droughtstressesBoth10%PEG induced osmotic stress and soil drought resulted in growth inhibition inarrowleaf saltbush. In the case of10%PEG induced osmotic stress, the addition of10⁴0mmol/L NaCl into the culture medium or irrigation of the soil with10⁴0mmol/L NaClall resulted in an apparent alleviation of the growth inhibition in arrowleaf saltbush fromosmotic/drought stresses. the addition of10⁴0mmol/L NaCl into the culture medium resultedin a growth increase of58.7⁹8.4%(Fig.1) compared to the control in the case of10%PEGinduced osmotic stress, and a growth increase of29.5⁶1.0%(Fig.15) compared to thecontrol in the case of soil drought.(2) The impact of NaCl on the photosynthesis of arrowleaf saltbush subjected toosmotic/drought stresses①Net photosynthetic rateThe osmotic stress induced with10%PEG resulted in the decline in the net photosyntheticrate in arrowleaf saltbush leaves, with the major decline appearing on the4th day of theosmotic treatment and to a level of close to0on the10th day of treatment. However, when10⁴0mmol/L NaCl was added into the culture medium, the extent of decline was less thanthe control. In the case of soil drought, the major decline of photosynthesis also appeared onthe4th day of the osmotic treatment, and to a level of close to0on the14th day of treatment,but a less decline was observed in the pot grown plants watered with10⁴0mmol/L NaCl andthen subjected to drought stress.②The chlorophyll content and the ratio of chlorophyll a/bThe osmotic stress induced with10%PEG resulted in the increase in the chlorophyll content in leaves, especially with a jump up4days after the treatment, but declined greatly8days after the treatment, the ratio of chlorophyll a/b also showed a tendency of an initialincrease followed by a decline, with the major decline starting on the6th days after thetreatment. The addition of10⁴0mmol/L NaCl into the culture medium significantlyalleviated the changes in the ratio of chlorophyll a/b.③Parameters of chlorophyll fluorescenceThe osmotic stress induced with10%PEG resulted in the increase in F₀and decrease inFv/Fm and PI in leaves, but the addition of10⁴0mmol/L NaCl into the culture mediumresulted in a significant slow-down of the above changes, with the addition of40mmol/LNaCl into the culture medium showing the smallest changes of just6.12%,3.97%,43.54%,respectively. Soil drought also resulted in the increase in F₀and decrease in Fv/Fm and PI inleaves. However, soil drought after the irrigation of100⁴00mmol/L NaCl solution alsoresulted in a significant slow-down of the above changes, with the irrigation of400mmol/LNaCl showing the smallest changes of just8.18%,3.52%,45.38%, respectively. Thisindicated that both osmotic stress and soil drought could result in the injury in thephotosystems in arrowleaf saltbush leaves, the decline in the photosynthetic electrontransferring rate, while the existence of NaCl could apparently alleviate the injuries caused byosmotic/drought stresses and promote the photosynthesis in arrowleaf saltbush leaves.(3)The impact of NaCl on the anti-oxidative enzymes in arrowleaf saltbush subjected toosmotic/drought stressesThe osmotic stress induced with10%PEG resulted in the initial increase and then declinein all the three enzymes SOD, POD and CAT, with the highest activity appearing on the8thday after the treatment. However, the addition of10⁴0mmol/L NaCl into the culturemedium resulted in an overall increases in SOD, POD and CAT activity throughout theexperimental period with smaller increases compared with the control. In the pot-grownexperiments, the drought stress showed the similar effect as the osmotic stress, and irrigationof100⁴00mmol/L NaCl also reduced the stress induced increase in SOD, POD and CATactivity. In addition, the leaf MDA content was at least2times higher in the later stages inplants subjected to osmotic/drought stresses compared to the control plants. So the theexistence of NaCl could apparently alleviate the injuries caused by free radicals to themembrane system due to osmotic/drought stresses, thus elevated the ability of the plants to combat osmotic/drought stresses.(4)The impact of NaCl on the water status and the Na⁺,K⁺in plants subjected toosmotic/drought stressesThe osmotic stress induced with10%PEG resulted in the decline of the water andosmotic potentials, as well as the relative water content in the plants. However, although theaddition of10⁴0mmol/L NaCl into the culture medium or irrigation of the soil with10⁴0mmol/L NaCl resulted in a further decline in those parameters, they could render theleaves a relatively higher water content and cell turgor. In addition, the addition of10⁴0mmol/L NaCl into the culture medium or irrigation of the soil with10⁴0mmol/L NaClcould apparently promote the uptake of Na⁺in arrowleaf saltbush plants. As a osmoticregulation substance, the accumulation of Na⁺in cells could enhance the driving force of thecells for water absorption, thus enhanced the drought resistance of the plants.To sum up, the addition of NaCl into the culture medium or irrigation of the soil withNaCl under osmotic/drought stresses could promote the uptake of Na⁺in arrowleaf saltbushplants and thus enhanced the osmoregulation and water uptake in plants, so as to ensure theplants to keep a higher turgor under osmotic/drought stresses, effectively alleviated thegrowth inhibition and photosystem injuries, and avoided the damaging effect of free radicalsto the membrane system, thus effectively enhanced the resistance of the plants toosmotic/drought stresses.