Responses Of Maize Seedling Under NaCl Stress And The Regulatory Mechanism Of Salt Tolerance

With room water culture experiment, the physiological and biochemical characteristic of maize seedling involved the dry weight, the content of osmotic adjustment, activity of protected enzyme, absorption and transport of ion in different part of maize seedling was studied, Primarily damage and salt tolerance mechanisms of maize seedling under the salt stress was discussed and analyzed . That will provide foundation for the further research Salt tolerance of maize seedling on molecular Level. At the same time, applying proper exogenous substance such as calcium(Ca), Potassium(K), phosphorus(P), silicon(Si) and sodium nitroprusside (SNP), the effect of physiological and biochemical characteristic was analyzed. The mechanisms of enhance salt tolerance about exogenous substance was discussed, supply definite theory basis for enhance salt tolerance on maize. The main study results was as follows:1. The damage and salt tolerance mechanisms of maize seedling under the salt stressThe salt stress are mainly consist of osmotic stress and ion toxic, osmotic stress could result in difficulty of absorb moisture, the ion stress could result in the biofilms were destroyed and physiological turbulence. The damage and salt tolerance mechanisms of maize seedling can be shown in following main aspects:1.1 maize seedling have definite salt tolerance, under the salt stress (50mmol·^-1NaCl), the dry weight of maize seedling increase little, the dry weight of root increase remarkably. The dry weight of different part of maize seedling decreases along with the increase of salt concentration. The biggish apparatus decreased in dry weight was young leaf and mature leaf. The water content of different parts of maize seedlings decreases along with the increase of salt concentration, the changed extent of the water content are: root＞young blade＞mature blade＞mature sheath.1.2 Salt stress can conduce to the decrease of chlorophyll content; the decrease extent of chlorophyll a is smaller than chlorophyll b. Under the salt stress, the net photosynthesis, transpiration rate, intercellular CO₂ concentration,stomatal conductance of maize seedling decreased, but water use efficiency stomatal limited value of maize seedlings increased.This indicated that the dicrease of photosynthesis of maize seedling under salt stress is the result of stoma factor.1.3 The salt stess can induce the maize seedling Plasma membrane to be damaged, along with the increase of salt concentration, the electrolyte leakage in leaf increases, the content of MDA in leaf and root increase.1.4 Under the salt stress, the content of soluble sugar, soluble protein, free amino acids and proline in leaf and root of maize seedling increased, but the increasing extent in leaf greater than that in root.1.5 When the maize seedling was under the salt stress of 50mmol·L^-1 and 100mmol·l^-1, the activeness of SOD, CAT, POD in leaf and root all increases. But the content of MDA in leaf and root was not decreased on the contrary it was increased. It indicates that the damage of Plasma membrane in maize leaf and root have more complex mechanism. After the concentration of salt attain definite limit, the activeness of enzyme gets to be restrained, the activeness of enzyme decreases, the salt damage aggravated.1.6 The content of Na⁺, Cl^-, Na⁺/K⁺, Na⁺/Ca²⁺ ratio in different part of maize seedling was all rapidly increase along with the increase of salt concentration, the changed extent of Na⁺, K⁺ and Cl^- was: root＞mature sheath＞young blade＞mature blade, the root of maize seedling was influenced most easily by outside ion concentration, the influence on leaf by outside circumstance was on the small side.The response of salt stress on different part of maize seedling was not consistent, it cause the decrease of content of Ca²⁺ and Mg²⁺. The changed disciplinarian of Mg²⁺ content in mature blade is not obvious. But the content of Ca²⁺ and Mg²⁺ increase under the salt stress. The influence on the content of Zn²⁺ is not too. The maize seedling have the mechanism of Na⁺-exclusion and have definite salt tolerance, the salt tolerance was implemented by root and mature sheath, Na⁺ is mainly reserved in root and mature sheath, and the content of Na⁺ was less to be transported to mature blade and young blade. At the same time, mature sheath still have the capacity of Cl^--exclusion.2 Alleviative effect of exogenous substance under salt stressThe salt tolerance of plant is a integrative mechanism, its fit to salt stress was multifarious, the salt tolerance of crop is a kind of complicated physiological mechanism that is the acted together result of genetic characteristic and exoteric condition, the alleviative effect on salt stress caused by exogenous substance was realize through different approach, so it is very difficult to explain the relation of exogenous substance and salt tolerance with a given index. On the whole, the alleviative effect on salt stress caused by exogenous substance manifests as follows:2.1 The growth restrained under salt stress was the most obvious effect, several kinds of exogenous substance such as calcium(Ca), Potassium(K), phosphorus(P), silicon(Si) and NO increased the dry weight of maize seedling at the different extent.2.2 Exogenous substance increased the content of chlorophyll in leaf under salt stress, it keep higher velocity of photosynthesis, but the effect by the different matter is distinct, for example: outer Potassium (K) and NO make the ratio of chlorophyll a/b increased, but outer phosphorus (P) and silicon (Si) make the ratio of chlorophyll a/b decreased in the maize seedling.2.3 Plasma membrane is the primary position of plant damage under the salt stress, that plant electrolyte leakage increased under salt stress was one of the important characters for salt damage; appropriate amount of exogenous substance all decreased the 1 electrolyte leakage in maize seedling under salt stress.2.4 T he capacity of active oxygen scavenging was increased, exogenous calcium (Ca), phosphorus(P), Potassium(K), silicon(Si) and NO all increase the activity of SOD, CAT, POD in leaf and root at the different extent, at the same time, exogenous substance decreased the content of lipid peroxidation material-MDA.2.5 Exogenous substance accelerate transportation of osmotic adjustable substance in maize seedling under salt stress, osmotic adjustable substance can be distributed more reasonable in leaf and root, osmotic adjustment ability of maize was increased. It was lie on that whether the osmotic adjustable substance distributing in the leaf and root reasonable or not, when we estimated the magnitude of osmotic adjustment ability, and the magnitude of osmotic adjustable substance in the leaf could not completely explain the magnitude of osmotic adjustment ability. It may be an important aspect that exogenous substance improves the salt tolerance in maize seedling.2.6 Exogenous calcium(Ca), phosphorus(P), Potassium(K), silicon(Si) and NO decrease the ratio of Na⁺/K⁺ and Na⁺/Ca²⁺, maintain the ion balance in different part of maize seedling under salt stress.These effects caused by exogenous calcium(Ca), phosphorus(P), Potassium(K), silicon(Si) and NO on maize seedling under salt stress was the integrated result of improving ion absorption , distribution , growth and development in maize seedling. Whereas its mechanism of effect was diverse, but they all attach the aim which alleviative the salt damage, this illuminate that the direct mechanism of improving the salt tolerance through exogenous substance is not only.