Physiological foundation for the differences of long-term water use efficiency among Populus deltoides clones

With randomized complete block design, long-term water use efficiency (WUEL), photosynthetic parameters (net photosynthetic rate Pn, transpiration rate Tr, stomatal conductance Gs, instantaneous water use efficiency WUEi, maximum photosynthetic rate P max, light saturation pointLSP, carboxylic efficiency Ce), stomatal properties (frequency, size, superior/inferior stoma amount ratio) and root/shoot ratio of twelve Populus deltoids clones were studied under water stress in a greenhouse. Four water treatments were designed, namely, well-watered condition, slight water stress, moderate water stress, and severe water stress. Although the volumes of irrigated water for different treatments were the same, the irrigation intervals were strictly controlled. The results showed that long-term water use efficiency (WUEL) differences among tested clones were obvious and became greater with the increase in water stress. J2, J6, J7, J8, and J9 were excellent clones with high WUEL. Stomatal frequency, stomatal size, Gs, Tr, Pn, and root/shoot ratio of J2et al were moderate, whereas the superior/inferior ratio, Pmax, LSP, Ce of J2et al were higher than other clones. The interrelations between WUEL and the physiological parameters were analyzed. The results revealed that the WUEL differences were induced by a series of physiological parameters, and clones with higher WUEL always had strong photosynthetic capacity, higher WUEi, and optimum root/shoot ratio.