Significant Root Growth Inhibition Research Articles

330 Effects of Calcium Salts on Chilling Tolerance in Cucumber Seedling Roots

The effects of calcium salts and concentrations from 25 to 200 mM on the induction of chilling tolerance in cucumber roots were studied using total root growth, electrolyte leakage, lipid peroxidation, and activities of antioxidant enzymes as indicies of chilling injury. Cucumber seeds `Poinsett 76' germinated at 25 °C for 36 h were treated with calcium sulfate, calcium nitrate, and calcium chloride for 2 h at 25 °C. After incubation, treated seedlings were rinsed with distilled H2O and chilled at 2 °C for 72 or 96 h with or without re-warming at 25 °C. Roots of CaSO4-treated cucumber seedlings exhibited less chilling injury at all concentrations, when exposed to 72- or 96-h chilling periods with a 72-h re-warming period as shown by greater root growth compared to the chilled control. Concentrations of CaCl2 and Ca(NO3)2 above 100 mM resulted in significant root growth inhibition. Electrolyte leakage (EL) was significantly reduced by CaSO4 up to 150 mM under chilling conditions and all calcium salt treatments reduced malondialdehyde (MDA) levels in seedling roots up to 150 mM. However, at 150 mM CaSO4 both EL and MDA values of 72 h chilled and re-warmed roots were at their lowest levels compared to the control and other treatments. Both superoxide dismutase and catalase activities of seedling roots decreased under chilling conditions compared to the nonchilled control, although the reduction was less in the presence of CaSO4. Peroxidase and glutathione reductase activities increased under chilling conditions and were generally reduced in the presence of calcium salts compared to the chilled control.

Variable effects of emergence-promoting rhizobacteria on conifer seedling growth under nursery conditions

Inoculation of white spruce (Picea glauca Voss.) seed with Bacillus polymyxa strain L5 under nursery conditions significantly increased the number of seedlings that emerged after sowing. No significant effects on seedling emergence were detected when white spruce seed was inoculated with Bacillus polymyxa strain L6, or when Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seed was inoculated with either strain L5 or L6. However, white spruce seedlings originating from L5-inoculated seed had significantly lower root dry weights when measured 13 weeks after sowing, and reduced shoot dry weights 26 weeks after sowing compared with uninoculated controls. Inoculation of white spruce seed with strain L6 also resulted in seedlings with decreased root dry weights compared with uninoculated controls 13 weeks after sowing, but the significant inhibition of root growth was not apparent 26 weeks after sowing. Douglasfir seedlings originating from L5-inoculated seed had significantly lower root and shoot dry weights compared with uninoculated controls 13 but not 26 weeks after sowing. Inoculation of Douglas-fir seed with strain L6 resulted in seedlings with decreased root collar diameters and shoot dry weights 13 weeks after sowing, and lower root dry weights 26 weeks after sowing compared with uninoculated controls. These results demonstrate that the effects of bacterial inoculation on seedling emergence and on plant growth are independent, and that emergence-stimulating bacteria may inhibit subsequent seedling growth.

Colonization of Winter Wheat Roots by Inhibitory Rhizobacteria

AbstractColonization of the winter wheat (Triticum aestivum L.) rhizoplane by root‐inhibiting nonfluorescent pseudomonads was investigated in laboratory and greenhouse studies with genetically marked (rifampicin resistant) isolates. Colonization was similar at root zone temperatures of 5, 10, and 15°C but was dependent on soil type, with higher populations occurring in the soil with lower organic matter and microbial biomass. Rhizoplane populations of test strains were over tenfold higher on roots in pasteurized soil than in non‐pasteurized soil or vermiculite. Deleterious effects on growth of winter wheat were related to the extent of rhizoplane colonization by the bacteria; approximately 106 colony forming units (cfu) mg−1 root were required for significant inhibition of root growth. The shoots and roots of winter wheat were significantly shorter on plants inoculated with the inhibitory bacteria in pasteurized or methyl bromide fumigated soil as well as in nonpasteurized Ritzville soil (Calciorthidic Haploxerolls). The difference in early stages of plant growth, especially shoots, between bacterized (seed treated with antibiotic‐resistant inhibitory bacteria) and nonbacterized winter wheat increased with time. The antibiotic‐resistant bacteria maintained a high population on the roots during this period. The bacteria were observed to extensively colonize winter wheat roots (rhizobacteria) and are considered a potential constraint on winter wheat yield in the Pacific Northwest.