Physiological And Biochemical Basis Of Adventitious Roots In Tetraploid Locust Cuttings

Tetraploid locust is an excellent cultivar of Robinia pseudoacacia acquired by artificially inducing the doubling of the diplod locust genome. The result of introduction field experiments showed that tetraploid locust can plant grow across large areas in China, between 23°～24°N, and 86°～124°E, including part of Tibet. It could potentially replace the ancestral diplod locust.The tetraploid locust is generally propagated by grafting. The major shortcoming of grafting is its long wait time until maturity. For example, rootstocks need to be prepared 1-2 years before grafting. Then 2～3 more years are needed before the grafted plants can be transplanted. Laboratory-based in vitro propagation is a promising method for producing clonal plants. However, this method is technically demanding and requires good laboratory setup. As a result, it’s difficult to use this method for most plants in most places. Seed propagation is another way to propagate tetraploid locust, but the superior characteristics of the parent plants can’t be guaranteed to pass onto the offspring. As a result, the offspring may not be as productive as the parents. The last method is cutting propagation, a widely used method for clonally reproducing other plants.Cuttings from tetraploid locust are often difficult to root. In some reports, high rooting rate had gotten with proper treatments. However, the rapid reproduction technique is not reality with the absent of rooting mechanism on tetraploid locust. In this study, annual branch (hardwood and softwood cuttings) in the 1-, 3-, 5-year-old cutting orchard of tetraploid locust were token as materials. The reproduction techniques for rooting hardwood and softwood cuttings were systematically studied and the adventitious roots inducing process of tetraploid locust cuttings were divided, which was depended on the studies of morphologic and anatomical characters. Then, the effect of reproduction techniques on physiological and biochemical characters and the physiological and biochemical differences of adventitious roots on rooting processes were studied. The main results are summarized below.1. The rooting percentage and propagation speed of softwood cuttings on tetraploid locust are higher than those using hardwood cuttings. The highest rooting percentage is 87.26% and propagation period is 30 to 40 days in softwood cuttings. In contras, the highest rooting percentage of 73.33% and 60 days are required for hardwood cuttings. 2. Hardwood and softwood cuttings pre-treated with the growth regulator IBA showed significant inprovement in rooting performance of tetraploid locust. We found that hardwood cuttings pre-treated with 1000 mg·L-1 IBA solution for 6 h and softwood cuttings pre-treated with 1400 mg·L-1 IBA solution for 4 h produced the best rooting regeneration.3. The present study showed that increasing ground temperature of significantly improved root generation of hardwood cuttings. The lowest effective accumulated temperature is 121.32℃for root initiation and 276.0℃for large roots formation. Rooting percentage improved by up to 13.16% with increasing effective accumulated temperature 98.26℃.4. Rooting ability of hardwood and softwood cuttings decreased with the age of the parental plant. Rooting ability of softwood cuttings also decreased with the delay of planting time. Our results showed that 1 to 3 year parent stocks produce the highest rooting survival and the optimum time for planting softwood cuttings was between mid-May and mid-July.5. The adventitious root formation of the tetraploid locust goes through three distinct phases, i.e. the callus induction phase, the root primordial formation phase, and the initiation and elongation phase of adventitious roots. There is no root primordial on hardwood and softwood cuttings of tetraploid locust before planting. Root primordial was produced from callus after planting which then initiated the adventitious roots formation.6. During root formation, the high concentration of endogenous IAA is inhibitory. Endogenous IAA concentration is negatively correlated to the rooting percentage. One of the important reasons that the rooting percentage of hardwood cuttings was lower than softwood cuttings is its higher IAA concentration. On the other hand, endogenous ZR can promote rooting of cuttings. ZR concentration is postively correlated to the rooting percentage. Another hormone ABA can assist root formation and is concentration dependent. Overall, the IAA/ABA ratio plays an important role in regulating the rooting of locust cuttings. The higher IAA/ABA ratio, the higher rooting percentage.7. IAAO, POD and PPO are three important enzymes associated with cutting rooting of tetraploid locust. Before planting, IAAO activity had the greatest influence on rooting percentage, the second was PPO activity and POD activity was the smallest. During root primordial formation, PPO activity had the greatest influence on rooting percentage, the second was IAAO activity, and POD activity was again the smallest. Thus, during the rooting process, IAAO and PPO activities in the cuttings were closely related to the development of adventitious roots. However, POD activity of cuttings had relatively little influence on the development of adventitious roots.8. During tetraploid locust rooting, the soluble protein content, soluble sugar content, and C/N ratio are all positively correlated to the rooting percentage. The starch content was negatively correlated to the rooting percentage. The total nitrogen content was not correlated to the rooting percentage.9. The effect of ground substrate temperature on hardwood cutting rooting percentage is mainly trough increasing the content of soluble proteins and soluble sugar, speeding up the degradation of starch. However, ground temperature had little influence on oxidase activity of the cuttings.