Effect Of Humic Acids On The Cucumber Seed Germination And Seedling Growth Under Exogenous Cinnamic Acid Stress

Continuous cropping obstacles of cucumber (Cucumis sativus L) could decrease yield and quality seriously. Autotoxicity is one of the most important factors of cucumber continuous cropping obstacles. Up to date, there is not report about the mechanism of humic acids increasing the ability of cucumber resistance to autotoxicity. In order to offer theory basis on conquering cucumber continuous cropping obstacles, this article studied the effects of humic acids ( HA and FA) on the physiology metabolism of cucumber seed germination from seed respiration, activity, amylase activity, Proteinase activity, sugar and starch consumption rate, and the mechamsm of humic acids enhancing the resistance to autotoxicity of cucumber seedling from active oxygen species(AOS) metabolism, the content of leaf photosynthetic pigment and the chlorophyll fluorescence under exogenous cinnamic acid(CA) stress. The main results were as follows:1 Effect of humic acids on cucumber seed germination under CA stress1.1 CA obviously restrained the seed germination. Comparing with CK, germination rates, germination index, embryo root elongation, the number of laterals root and biomass of cucumber under the treatment of 0.5mmol·L^-1(CA_0.5) were inhibited, except the germination index was not significantly decreased, others were significantly or positively significantly decreased. HA as well as FA could decrease the inhibition of CA on seed germination. Comparing with CA_0.5, adding HA or FA could increase cucumber germination rates, germination index, embryo root elongation, the number of laterals root and biomass under CA stress.1.2 Comparing with CK, seed respiration and seed activity were decreased significantly under the treatment of CA_0.5. Adding HA or FA could stimulate seed respiration and seed activity under CA stress.1.3 CA_0.5 treatment inhibited amylase,α-amylase,β-amylase activity variously and suspended the increasing of amylases activity. Adding HA or FA could enhance these amylases' activity and erase the suspending to amylases activity increasing under CA stress.1.4 Comparing with CK, proteinase activity of cucumber seed was significantly suspended under CA_0.5 treatment. Adding HA or FA could improve proteinase activity of cucumber seed under CA stress.1.5 Comparing with CK, sugar and starch consumption rates of CA_0.5 treatment decreased significantly. HA or FA could increase the consumption rates of sugar and starch in seed germination stressed by CA.2 Effects of humic acids on cucumber seedling growth under CA stress2.1 CA inhibited the growth of cucumber seedling. Comparing with CK, the fresh weight of root, root/shoot ratio, seedling biomass and the root activity decreased significantly under CA_0.5 treatment. Adding HA or FA could significantly increase the fresh weight of root, root/shoot ratio, seeding biomass and the root activity under CA treatment.2.2 Effect of CA on AOS metabolism was obvious. SOD and POD activities increased while CAT activity decreased significantly under CA_0.5 treatment. CA stress resulted in O₂^- producing rate ,H₂O₂, MDA and soluble sugar content increasing. Adding HA or FA could increase SOD and CAT activity, decreased O₂^- producing rate and the content of H₂O₂, MDA and soluble sugar under CA stress.2.3 Under CA stress, the contents of chlorophyll and carotenoid decreased while chla/chlb ratio increased. Adding HA or FA could enhance the content of photosynthetic pigment significantly under CA stress.2.4 Under CA stress, the minimal fluorescence production in cucumber leaves increased, while the maximal fluorescence production, variable fluorescence, maximal quantum field for primary photochemistry (Fv/Fm)decreased, resulting in inactivation of cucumber leaf PS II reaction centers. CA stress lead to the inhibition of transfer of linear electrons, the decrease in content of PS II acceptor, the inhibition of electron transfer from Q_A^- to Q_B, and the decrease of electron transfer rate causing the mass accumulation of Q_A^-. Adding HA or FA could increase the amount of active reaction centers of PS II, the probability of the electron transfer to other electron acceptor over Q_A^- in cucumber leaf, and reduce the proportion of inactive reaction centers. HA or FA could alleviate the damage of reaction centers acceptor side of PS II under CA stress.2.5 Under CA stress, energy pipeline of reaction centers in PS II was inhibited. Under CA_0.5 treatment, photo-energy absorption and energy dissipation in PS II reaction centers of cucumber per unit area leaf increased while the excited energy capture, electron transfer, and performance index of reaction center decreased significantly. Adding HA or FA could decrease photo-energy absorption, the ratio of excited energy capture used for energy dissipation, increase electron transfer energy in PSII reaction centers of cucumber per unit area leaf. HA or FA could alleviate the damage of the energy normal pipeline in PS II of cucumber leaf under CA stress, increased the performance index significantly.