Comparative Study On Electrical Impedance And Physiologic Indexes In Pinus Bungeana Zucc. Seedlings Under Drought Stress

Pinus bungeana Zucc. is a Chinese special precious species. It is a kind of main landscaping species in the northern part of China and the excellent tree of afforestation in arid region and mountainous areas. Three or four year-old container seedlings are suitable for the forestation. In this experiment, we treated 4-year-old P. bungeana container seedlings with preplanting (soil relative water content was B1: 75-80% (normal irrigation), B2: 55-60% (light drought), and B3: 35-40% (severe drought)) and postplanting (A1: 75-80%, A2: 55-60%, A3: 35-40%, and A4: 15-20%) drought, measured a series of physiologic indexes (Electrolyte leakage, soluable sugar, PV curve parameters and so on), aiming to find what scope of drought the P. bungeana could tolerate before and after planting, and offer the theoretical guide for scientific forestation. Based on the analysis of the physiologic indexes, it is aimed to explore the mechanism of drought resistance of P. bungeana. In addition, after preplanting and postplanting drought treatments, we measured the electrical impedance parameters of needles and stems of P. bungeana, and followed the change rules of the parameters. The correlation between electrical impedance parameters and various physiological parameters was studied to find the best impedance parameters which can estimate the plant drought-resistance degree, and provide a technical method for fast monitoring and assessing plant drought-resistance. Main results are as follows:1. Under preplanting B3 treatment (severe drought), water content of needles did not decrease significantly, electrolyte leakage of needles increased, chlorophyll content and maximal PS II efficiency (F_v/F_m ) of needles declined significantly. These changes suggested that needles were damaged to some extent. Such effects would be ignored if normal irrigation was given after planting. However, if postplanting drought happened, injury degree of B3 treatment was not deeper than that of B1 and B2. So for preplanting severe drought of container seedlings would not affect the seedlings performance after planting. After 4 weeks A3 and A4 severe drought treatments, water content of needles decreased, electrolyte leakage of needles increased, and F_v/F_m of needles declined significantly, indicating that needles were injured. We therefore suggest that in the early phase after planting, more than 4 weeks severe drought should be avoided.2. Preplanting and postplanting drought caused decrease in soluble sugar and starch content of needles, suggesting that the carbohydrates in needles outputed under the drought stress. Under postplanting severe drought A3 and A4 treatments, Superoxide dismutase (SOD) changed with a trend of"rise– decrease– rise", then decreased in long-term extremely severe drought stress (A4). So SOD is thought as main protective enzyme in needles of P. bungeana seedlings. Peroxidase (POD) activity decreased with drought stress in the early four weeks, and r_ised only after five weeks severe drought treatments (A3, A4). Thus, POD activity is not regarded as key enzyme of protective enzyme system in needles of P. bungeana seedlings. The total protein content increased in severe drought stress (A3, A4) through producing new protein in the early drought phase, and decreased after four weeks ser_ious drought injury, showing that protein was an important regulating substance. Super oxide anion always remained high in severe drought stress (A4), showed that P. bungeana seedlings was damaged by active oxygen under A4 treatment.3. Effects of preplanting and postplanting drought on water content and electrolyte leakage of needles in P. bungeana seedlings were earlier and larger than those of stems. Soluble sugar and starch produced in needles transfered to stems under drought stress, to make the soluble sugar content of stems increased, and to strengthen drought resistance of the stems. Starch content of stems reduced under drought stress, probably because starch content in stems transformed to soluble sugar to increase the osmotic adjustment ability. Preplanting and postplanting drought caused the enhance of root activity.4. The relative water content at incipient plasmolysis (RWCtlp) of P. bungeana seedlings was mostly higher than 90%. So we think endurance dehydration capability of P. bungeana seedlings was not strong. Under A3 and A4 drought stress, P. bungeana seedlings mainly decreased osmotic potential at incipient plasmolysis (ψwtlp) and maximum osmotic potential at saturation point (ψssat) to maintain minimum turgor pressure and maximum turgor pressure, thereby increasing drought resistance. Under A2 and A3 drought strss, P. bungeana seedlings increased drought resistance through increasing bound water volume and slightly raising endurance dehydration capability. Under A1 and A4 treatments, P. bungeana seedlings also adjusted cell wall elasticity to increase the drought resistance. 5. After preplanting drought treatments, electr_ical impedance spectra and parameters of needles did not change a lot. Postplanting drought treatments caused the changes in electr_ical impedance spectra and parameters. With prolonged drought, intracellular resistance (r_i) and high frequency resistance (r∞) of needles r_ised under A1 and A2 treatments, and changed little under A3 and A4 treatments. Extracellular resistance (re) and membrane time constant (Ï„_m) decreased greatly under A4 treatment. Some impedance parameters closely related to water content, relative electr_ical conductivity, chlorophyll content, F_v/F_m , and soluble sugar content in needles. Among these impedance parameters, re andÏ„_m correlated better, and had linear positive correlations with F_v/F_m (r=0.54～0.89), water content (r=0.74～0.98), and soluble sugar content (r=0.52～0.96) in three weeks, while had negative linear correlations with relative electr_ical conductivity (r=-0.54～-0.97), and chlorophyll content (r=-0.62～-0.8). Therefore, re andÏ„_m could be regarded as the best parameters to estimate the drought resistance of needles in P. bungeana seedlings.6. Impedance parameters r₁, r2, re and relaxation timeÏ„2 of stems decreased, whereas r and r_i did not change a lot under severe drought stress (A4). Relaxation timeÏ„1 of stems under A4 treatment was higher than that under A1 in the first four weeks, and decreased significantly after five weeks drought. After five weeks drought stress, distr_ibution coefficient (ψ2 ) of relaxation timeÏ„2 of stems was higher under A2, A3 and A4 treatments than that under A1 treatment. Among the impedance parameters, r₁ and re changed regularly under A4 drought treatment, and had significant correlations with water content, relative electrical conductivity, soluble sugar content, starch content and root activity in three weeks (R²>0.5), and had positive linear correlations with starch content (r=0.51～0.97) and water content (r=0.71～0.97), negative linear correlations with relative electrical conductivity (r=-0.75～-0.99) in three weeks. So r₁ and re could be regarded as the best parameters to estimate the drought resistance of stems in P. bungeana seedlings.