Effects Of Soil Drought And Competition On Growth And Grain Yield Of Two Soybean Varieties

The effects of drought and competition on plant growth and grain yield formation in two soybean varieties that were recently promoted on the Loess Plateau, i.e. variety Jinda74 and Jin24, were investigated in the thesis. Studies were conducted in the conditions of pot and field experiment. De Wit replacement approach was applied to explore the relationship between the competition ability and the performance of plant biomass and grain yield in the two varieties. Drought induced root-soured signal, and other biological progresses, such as leaf gas exchange, chlorophyll fluorescence transients, plant biomass accumulation and grain yield formation were also studied under several water regimes to figure out the possible mechanisms in response to water deficit in the two soybean varieties. The main results were showed as follow:1. Performance of plant biomass and grain yields of variety Jinda74 and Jin24 were studied in monoculture and mixture under field condition. In monoculture population, plant vegetative mass (including root, stem and petiole) was higher for Jinda74 than Jin24, especially at pod-setting and seed filling growth stages. And in mixture populations of replacement series, plant aboveground biomass was deceased for variety Jin24 when that of Jinda74 increased. Grain yield, harvest index and water use efficiency of variety Jin24 were significantly higher than that of Jinda74 in monoculture, but the grain yield of Jin24 in mixture was dramatically restricted, and the harvest index was also decreased compared with that of Jin24 in monocultur. Seed number input/output model stimulated the final competition ending that variety Jin24 is to be expelled and replaced by Jinda74. Relatively great vegetative mass of Jinda74 was advantage for its competition ability but not the monoculture grain yield, while weaker competition ability in variety Jin24 coupled with higher monoculture seed yield which indicated its advantage for population yield.2. Biomass components and grain yield formations were investigated in soybean variety Jinda74 and Jin24 in monoculture and mixture in manipulated soil water deficits in pot experiment. Plant height, stem biomass weight and root biomass weight of Jinda74 was significantly higher (P<0.05) than that of Jin24 either in monoculture or mixture under wet (85% of FC) and mild drought condition (60% FC), while plant height and stem biomass of Jinda74 was significantly higher than that of Jin24 in monoculture (P<0.05) under severe drought condition (40% FC). Grain yield of Jinda74 was instinctively higher (P<0.05) in mixture but much lower (P<0.05) than Jin24 in monoculture than that of Jin24 under either wet or mild drought condition. Harvest index of Jin24 was higher than that Jinda74. Competition ability was more intensive for Jinda74, and plant growth was hence greatly restricted in Jin24. Weaker competitior (Jin24 varaiety) yield higher in monoculture.3. Non-Hydraulic Root-sourced signal (nHRS) as well as biomass and grain yield formation was compared between Jinda74 and Jin24 under pot experiment condition. In response to soil water desiccating, Jinda74 variety triggered earlier non-hydraulic root signal than Jin24 at both branching and seed-filling stages, and ended in advance of Jin24, exhibiting wider average soil water content threshold range of nHRS. Drought stress significantly decreased the aboveground biomass, root biomass and grain yield (p<0.05), more dramatically in Jinda74 variety than Jin24 variety, indicating a higher maintenance rate of grain yield in the latter than the former. Harvest index of the two varieties were rather stable among the water conditions; the root/shoot ratio of both varieties were slightly reduced under mild drought stress (P>0.05), but significantly promoted under severe drought stress (P<0.05), even more extensively for Jin24 than Jinda74. Water use efficiency was reduced in Jinda74, while increased by in Jin24 under drought stress, and WUE was significantly high under mild drought in contrast to the control for Jin24 (P<0.05).4. At pod-setting stage, physiological and biological responses to drought stress were compared in soybean plants that were formerly well watered or stressed during vegetative phase. Formerly well-watered plants triggered non-hydraulic root signal earlier and also ended later than in the formerly stressed plants, exhibiting wider average soil water content threshold range of nHRS. Drought significantly reduced the stomatal conductance for both groups of plants, but promoted the leaf fluorescence performance index and water use efficiency at leaf level. Increment in stem biomass, leaf biomass and total biomass were dramatically restricted while much slightly in pot biomass by drought for both groups. Plants in the stressed group exhibited more severe decrease in stem, leaf and total biomass and lower water use efficiency than formerly well-watered group, which might have related to its narrowed TR.5. Characteristics of gas exchange and chlorophyll fluorescence were investigated on soybean leaves during the gradually soil water declining. As drought stress developing, leaf net photosynthesis rate and stomatal conductance decreased compared to the control, so as the fluorescence induced initial fluorescence intensity Fo and relative chlorophyll fluorescence level of step J; electron transport efficiency of excited energy was reduced, while non-photochemical heat dissipation was promoted; energy absorption and trapping per cross section on the leaf was decreased, and energy transportation to electron chain was greatly reduced. Excess excitation energy was suppose to be dissipated by non-QB reduction reaction center inactivation at PSâ…¡and some high-energy quenching mechanism at harvesting antenna pigments to protect the photosynthetic apparatus from being irreversible destroyed.6. Plant growth and grain yield performances were studied in Jinda74 and Jin24 variety under changed water regimes. Vegetative phase drought caused obvious decline of photosynthesis rate and stomatal conductance in variety Jinda74, with rather tiny effect on that of Jin24. Drought stress either in vegetative phase or reproductive phase reduced plant water consumption as well as aboveground biomass, and the reduction was much greater by drought stress occurring late in the growth season. R/S ratio was increased under drought in variety Jinda74, while it was significantly increased in Jin24 under wet-dry water regime. Hundred seed weight and harvest index was reduced by drought in Jinda74; while they were much stable in Jin24 under drought compared to whole season wet treatment. Decline of grain yield was much severe in Jinda74 than in Jin24 under drought stress, and the latter exhibited higher water use efficiency. Vegetative phase drought stress reduce grain yield in Jinda74, but had little effect on that of Jin24. Improvement of water condition at reproductive phase after earlier drought stress had better compensate effect on variety Jin24 than Jinda74.