Plant growth is dependent on the supply of water to the leaves. The ability to supply water and to withstand tension (drought stress) is a function of xylem cell structure, which affects both water flow rates (larger vessels) and drought tolerance (greater cavitation resistance).  Long-term structural acclimation is an important mechanism that enables plants to modify the water supply to the canopy.  Plants of two common bean cultivars (Othello and G4523) differing in patterns of growth and water-use, were grown in a growth chamber under three light levels (40.6, 118.6, and 449.9 µmol m^-2 s^-1) for at least four weeks.   Plant growth, water-use, hydraulic conductance, and drought tolerance (measured as the tension required for a 50 % loss in stem hydraulic conductance or P₅₀) were monitored to determine if long-term exposure to different environmental conditions can result in changes in xylem structure (hydraulic acclimation). 

Results/Conclusions

Although the two cultivars did not differ significantly in biomass production, Othello had higher root area (53.8 vs 44.5 cm²) while G4523 had higher leaf area (1588.6 vs. 1305.3 cm²).  Othello had higher hydraulic conductivity, transpiration, and plant water loss (1.0 vs. 0.66 kg*d^-1*cm^-1), while G4523 had higher P₅₀ (0.68 vs.0.46 MPa).  The light treatment resulted in differences in plant biomass, leaf area, root area, plant water loss, stomatal density , measures of stem hydraulic conductance and P₅₀.  Long term exposure to differences in light caused most measures of plant size and water use to decrease with decreasing light.  Low light plants for both cultivars had a significantly lower P₅₀ compared with medium and high light plants (0.42 vs. 0.65 MPa) indicating a shift toward lower cavitation resistance.  The two cultivars did exhibit different water use strategies, with G4523 having higher leaf area and Othello having higher stem/stomatal flow rates.  The ability of G4523 to support more leaf area with lower stem flow rates appears to be due to greater xylem resistance to cavitation.  This study confirmed that environmental factors can affect both xylem structural properties as well as having a significant impact on whole plant water flow.