Improving Maize (Zea May s L.)Yield Through Substrate-Dependent Microbially-Derived Plant Hormones

Abstract

Azotobacter and Azospirillum, inoculation has successfully been used for improving growth and yields of various cereals in different parts of the world. Supplementation of inoculation with L-tryptophan (an auxin precursor) and adenine (a cytokinin precursor) may further improve these beneficial effects on crop growth and yield due to substrate-dependent inoculum-derived plant growth regulating substances in the rhizosphere. This study consisted of a series of laboratory, pot and field experiments to evaluate the potential of Azotobacter and Azospirillum in the presence of L-tryptophan (L-TRP) or adenine (ADE) for improving the growth of maize (Zea mays,,) According to the plan of work (as given in PC-1). During the first year ten strains of each bacterium Azotobacter and Azopirillum were isolated from maize rhizosphere soil and their auxin production was measured by colorimetry. Efficient auxin producers’ strains Z_1, Z_3 and Z_4 (Azotobacter) and S_2,S_8 and S_9 (Azospirillum) were selected for further experimentation. Auxin production by these selected cultures were determined both in the presence and absence of filter sterilized L-TRP @ 10^(-3) and 10^(-4) M. Azotobacter culture Z_4 and Azospirillum culture S_9 were the most prolific auxin producers and these strains were used in subsequent experimentation (PI. See.Section-1). Two plate experiments were conducted in the laboratory to see the effects of three selected cultures of both Azotobacter (Z_1, Z_3 and Z_4) and Azospirillum (and S_2,S_8 and S_9) on germination percentage by sowing maize seeds on the moist filter paper. Germination rate were found to be maximum with the inoculation of seeds with Azotobacter cultures Z_1 and Z_4 and Asopirillum culture S_9 (see section 1). Selected Azotobacter strain Z_4were tested in the presence and absence of L-TRP and ADE on rot growth of three maize varieties (Golden, EV.6089 and C-17) conducting plate experiments. Maize variety Golden gave better response than other two varieties and was selected for subsequent studies. Same experiments were repeated twice either with Azotobacter strain Z_4 or Azopirillum S_9 using more responsive variety Golden. Most promising results in terms of root length and weight of maize were obtained with combined application of Azotobacter/azospirillum with 10^(-4) to 10^(-3)M L-TRP/ADE. Further, plate experiments were conducted to study the response of maize (Var. Golden) to Azotobacter or azospirillum inoculation in the presence or absence of L-TRO and ADE application at various concentration (10^(-5) to 10^(-3)M) under normal, salinity stressed (at EC-3.0 and 6.0 DS m^(-1) ) and fertility stressed (at a1/2 and 1/6 strength Hoagland solution) conditions. In most of the cases, L-TRP and ADE concentration of 10^(-4)M in combination with Azotobacter/azopirillum gave significantly positive effects on root length and weight under normal, salinity and nutrients stressed conditions and this concentration was selected for Leonard jar experiment (P1 see section-II). Leonard jar experiment were conduct to study the effect of Azotobacter and L-TRP/ADE on the growth of maize shoots under normal, salinity stressed (at EC=3.0 and 6.0 m^(-1)) and fertility stressed (at a1/2 and 1/6 strength Hoagland solution) conditions. Maximum shoot length (45.3 and 62.6% dS m^(-1) greater than control, respectively) was recorded with treatments: Azotobacter inoculation + 10^(-4)M L-TRP or ADE Response in shoot Weight was also encouraging with the same treatments (see section-II). Like the Aboobacker studies, Leonard jar trials were conducted during second year of the project to evaluate the effectiveness of substrate (L-TRP/ADE)-dependent Azospirillum inoculation -derived plant growth regulators on shoot growth of maize under normal, salinity stressed (at EC=3.0 and 6.0dS m^(-1)) fertility stressed (at a1/2 and 1/6 strength Hoagland solution) conditions. In these trials’ response was more obvious in combined application of Azospirillum and L-TRP/ADFE compared to uninoculated/untreated control and inoculation alone (see section-II). Pot experiment were also conducted during the second half of the 2nd year of the project to further evaluate the effectiveness of approach in soil conditions. In these trials, selected Azotobacter/azospirillum strains and various levels of L-TRP/ADE were applied to maize growth under normal, salinity (at original EC=3.0 and 6.0 dS m^(-1)) and fertility stressed (at a1/2 and 1/6 strength Hoagland solution or full dose of recommended fertilizer vs 1/3 of recommended fertilizer dose) conditions. Results revealed that Azotobacter/azopirillum, L-TRP and ADE in wirehouse experiments when applied alone or in combination had significant effects on maize growth under normal (original EC of soil i.e.,1.5 dS m^(-1)) and salinity stressed (and 6.0 dS m^(-1)) soil conditions. However, combined applications of Aboobacker and L-TRP @ 10^(-4)M was found most effective in increasing fresh and dry shoot weight, fresh and dry root weight, root shoot ratio and stem diameter at original EC of soil and similarly inoculation-interaction with 10^(-4)M ADE also significantly increased he above parameters as compared to control. At original EC of sol 1.5 dS m^(-1), combined applications of azopirillum inoculation plus L-TRP or ADE had more pronounced effects on plant height, stem diameter, fresh and dry shoot weight, fresh and dry weight of maize as compared to control and other treatments. In case of EC 6.0 dS m^(-1), combination of Azotobacter/azospirillum + L-TRP or ADE also significantly promoted the growth parameters as compared to control (P1 see section-III). Under normal nutrients conditions, combined applications of Azotobacter with L-TRP (10^(-4) and 10^(-3)M) significantly affected maize growth by improving fresh (14.6 and 13.7%) and dry shoot weight (22.1 and 20.10%) fresh (17.4 and 22.1%) and dry root weight were increased significantly in response to Azotobacter+ L-TRP (10^(-4) M) by 8.9, 51.6, 42.0 and 107.0% respectively compared to control. However, under nutrients rich environment, this treatment although had positive effect on growth parameters but significant response was only observed in case of dry root weight (90.3% greater than control). Similarly effect of Aboobacker +ADE as also positive in most of the growth parameters at both nutrient poor and rich environments. Overall, treatments response was more pronounced under low fertility conditions (see section-III). During the last (Third) year of the project, field experiments were conducted to test the effectiveness of approach under field conditions. The effects of Azotobacter/azospirillum and various levels of L-TRP/ADE on maize growth under normal and fertility stressed (recommendation full dose fertilizer and 1/3rd of the full dose) conditions were evaluated under field conditions, Azotobacter inoculation had maximum improvement in grain yield (18.4%), fresh biomass (16.7%), cob weight (20.4%),1000-grain weight (14.5%) and dry shoot weight (15.4%) when supplied along with 10^(-4)M L-TRP over control, while maximum increases in straw yield, cob length, grain rows 〖cob〗^(-1) and fresh shoot weight were observed by Azotobacter+10^(-3)M L-TRP, under normal fertility conditions. However, at low fertility, effect of L-TRP and ADE @ 10^(-4)M was also observed and most effective response in plant height,1000-grain weight and grain yield was found by the application of ADE @ ADE 10^(-4)M (5.4, 10.2, and 34.0% respectively, greater than control). L-TRP Tryptophan @ 10^(-4)M caused maximum increases of 14.7, 22.0, 4.8 and 10.6% in fresh shoot weight cob length, con weight and number of rows 〖cob〗^(-1), respectively, over control: however, under nutrient rich environment, this treatment improved plant height, fresh shoot weight, cob length, con weight and grain yield by 5.6, 8.5, 15.6, 9.7 and 11.5% respectively. Moreover, number of rows 〖cob〗^(-1) and 1000-grains weight were significantly increased by ADE @ 10^(-4) which were 12.5 and 10.2% respectively, higher than control at rich fertility condition PI. See. Section-IV). It is obvious from the series of trials conducted in the lab, wirehouse and field that the goals outlined in the project synopsis have been almost achieved. Results demonstrated the success of a precursor-inoculum, interaction for the growth response obtained with maize crop under axenic, pot and field conditions which could be attributed to the hormones produced by Azotobacter or Azospitrillum derived from L-TRP and AD, under normal and stressed (salinity and fertility stressed) conditions. The work reported in this manuscript clearly demonstrated that substrate-dependent microbially-derived plant hormones could be utilized for promoting growth and yield of treated plants. However, this is a recently emerging technology, so intensive work is needed on its various aspects to make this technology farmer’s friendly for commercial use to boost up agriculture production in Pakistan.