| Regulation of maize plant growth and water use by varying soil bulk density under different soil water conditionsPh.D. Student: Liu Wan-gou Directed by: Prof. Shan LunAbstractSoil bulk density, has consistently been considered an important soil physical property, and it has a direct relationship to other properties such as soil mechanical resistance, and also affects the root growth and function and consequently the shoot growth. Some reports have confirmed that mechanical resistance, as water stress, can induce root-sourced signal that would regulate the growth of plant. Water stress signal has been extensively studied and also successively been used to engineer stomatal opening so that control plant water loss in agricultural practices. The objective of the study presented here was to evaluate the effects of soil bulk density and hence mechanical resistance on maize (Zea Mays L.) plant growth under controlled conditions, and thereby to probe the possibility of the exploitation of soil mechanical resistance for manipulating crop performance.Split-root experiments were conducted on maize under both low (-0.86MPa) and high (-0.17MPa) soil matric potential conditions, in which seminal roots were divided equally between two compartments in partitioned galvanized steel containers containing Lou soil packed to a bulk density of 1.2, 1.33 or 1.45g cm"3. Four types of soil treatments were imposed: low density (both compartments packed to 1.2 g cm"3), middle density (both compartments packed to 1.33 g cm"3), high density (both compartments packed to 1.45 g cm"3), and mix density (one compartment packed to 1.2, the other to 1.45 g cm"3). Plants were grown in a walk-in growth chamber under constant climatic conditions for 60 d with a relative humidity 75%, a day/night temperature 28/23 癈, a photoperiod of 12 h of light and a photosynthetic photon flux density of 500 umol m"2s"'. The main results are as follows:The soil mechanical resistances to root penetration measured as penetrometer resistance increased with both increasing soil bulk density and decreasing soil water content. The mechanical resistance of soil with bulk density of 1.2, 1.33 and 1.45 g cm"3 under high soil matric potential (-O.lTMPa) was 0.51, 0.93 and 1.38 MPa, respectively, whereas under low soil matric potential (-0.86MPa) was 0.68, 1.74 and 3.37MPa, respectively.High soil bulk density and low matric potential effects on both root and shoot growth were independently significant, but low matric potential effects was more profound, and there was also an interactive negative effect of them upon the growth. There was a significant decrease in root length and also in root dry matter and shoot dry weight, but shoot growth was more reduced than root growth when plants are grown on compacted soils or the soil matric potential reach -0.86MPa from -0.17MPa. In high density soils,not only was there a reduction of root length but there was increase in the diameter of roots. Both leaf expansion rate and plant size reduction occurred at high soil mechanical resistance regardless of whether high soil resistance was caused by increasing bulk density or lowering soil water content. Smaller plant size in compacted treatments was due not only to slower leaf expansion rates, but also smaller maximum size of individual mature leaf. These results suggest some similarity in plant response mechanism to water deficit and mechanical resistance. In contrast, when plants were grown in soil of mixed bulk density, there was enhanced growth of root in low-density soil to compensate or even overcompensate for reduced growth in high-density soil and so that plant growth were comparable with those in low bulk density.Plants grown in high bulk density soil exhibited reductions in both stomatal conductance and transpiration rate (Tr) without any detectable decrease in leaf water potential and photosynthetic rate (Pn) compared with plants grown in low-density soil. Stomatal conductivity also reduced when only part of root system was. in high compacted soil. Th...
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