Consequences of moderate drought stress on the net photosynthesis, water-use efficiency and biomass production of three poplar clones

The breeding of efficient but drought-tolerant poplar clones is very important for short-rotation planting because of frequent periods with low precipitation in Central Europe. Three clones exhibiting very different shoot-root ratios under well-watered conditions were investigated: (1) the aspen clone Grossdubrau No. 1 (Populus tremula); (2) the newly bred aspen clone L316 x L9 No. 21 Thermo (P. tremula x P. tremula); (3) and the poplar clone Max2 (P. maximowiczii x P. trichocarpa). All three clones were exposed to moderate drought stress. Photosynthesis and transpiration were measured. The influences of drought on the biometrical parameters of the plants were evaluated. The intrinsic water-use efficiency (WUEintrinsic) was calculated. Height growth was decreased in stressed plants. Photosynthesis and water conductivity were significantly decreased, which is why the WUEintrinsic exhibited a greater increase in Max2 than in the aspen clones. The poplar clone Max2 showed low WUE after sufficient watering, but this parameter exhibited a greater increase under drought stress compared with the aspen clones. Max2 was characterised by intensive root growth that was diminished under stress. In contrast, both aspen clones were less adaptive to moderate drought stress. The capability to change the WUEintrinsic under different water availabilities can be considered a possible selection criterion for breeding.