Influence of water stress, nonstructural carbohydrates and free amino acids on control of root and shoot growth of Ligustrum japonicum Thunb

A more profound knowledge of the factors affecting root and shoot post-transplant growth would enable more conscientious decisions on plant management and on practices to be adopted. The experimental system required to impose precise drought treatments, and to make possible observation of natural growth of Ligustrum japonicum Thunb. under favorable conditions was developed. Influence of different intensities and duration of water stress on plant growth was tested. Plant architecture was described for undisturbed conditions with and without moderate water stress. Interactions between labile forms of carbon and nitrogen within different meristem tissues, and control of growth initiation or cessation was studied in a whole plant approach.;The final rhizotron designed allowed exceptional root observation, provided a near-uniform profile of soil moisture, and was easily manageable for precise long-term data acquisition. This rhizotron had eight independent viewing/sampling windows and held 0.16 m3 of soil. An electric powered root separator was developed that sped sample preparation for root dry mass determination with a capacity of 40 L of container substrate or 32 kg of sandy soil. No water was required and a four-fold reduction in total processing time was achieved with a >98% root mass recovery.;Excessive water availability resulted in marked reductions of carbon allocation towards roots, consequently luxury shoot growth was observed. Constant or intermittent moderate water stress resulted in total biomass reductions of approximately 20%, which disproportionally reduced shoot mass, particularly diminishing leaf number and size. Prolonged, severe water stress resulted in a 40% reduction of total biomass. This reduction was also disproportionally greater for shoot biomass production, with smaller reductions in root biomass production. Plant responses to water stress differed with time of exposure and degree of stress. Low correlation of root-to-shoot ratio and irrigation frequency suggested that root-to-shoot ratio may not be the best indicator of water status during the plant growing period.;Following transplant into rhizotrons, root growth began before shoot growth and the first flush of shoot growth was mostly basipetal. Water stress altered shoot architecture by enhancing apical dominance. As plants adapted to the stress imposed, indeterminate growth was triggered more often in meristematic regions of terminal buds. At the second flush, which occurred later in the stress treatment, old buds expanded more frequently than the newly formed apex lateral bud. Temporal variations of moisture caused by wetting and drying cycles resulted in continuous growth for portions of the root system, with quiescent periods observed for shoot growth. Conversely, continuous high moisture levels resulted in roots exhibiting quiescent periods in some plants. Patterns of shoot and root growth varied considerably between these clonal plants, which may be an important consideration for analyses of populations of woody plants.;Free amino acid levels at the shoot tip were more decisive for initiation of meristem growth or quiescence than the quantity of total nonstructural carbohydrates or nonstructural carbohydrate-to-free amino acid ratio. In roots, this ratio was a good predictor of root growth. Valine, leucine, tyrosine, cysteine, metionine, and arginine increased significantly with bud set, compared with growing shoot tips. Root tips contained abundant fructose, stachyose, and myoinositol. Mannitol was the major transport sugar and glutamine, valine and histidine were the main free amino acids transported in xylem fluid. Water stress resulted in increases in the concentration of some amino acids in growing shoo tips, such as arginine, valine, and histidine, and especially valine in developing buds at the beginning of root flush.;The results observed from these experiments suggest important considerations for woody plant management. Since it appears that water stress can increase apical dominance; the necessity for pruning may increase if the plant is to be used as a shrub rather than as a small tree. Conversely, well-irrigated plants show basipetal branching in the beginning of the growing season, which can also affect pruning decisions. Wetting and drying cycles result in continuous growth of portions of the root system; thus, establishment can benefit by temporal variations of soil moisture. Fertilization during the growing period prior to transplant into a landscape is an important factor for building up internal nutrient reserves to support initial root growth and bud expansion at the beginning of the growing season. These observations suggest that woody plant management can benefit from appropriate decision making of pruning, frequency of irrigation, and timing of fertilization and transplant.