Water relations, photosynthetic capacity, and root/shoot partitioning of photosynthate as determinants of productivity in hybrid poplar

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Abstract
Physiological determinants of productivity, including net photosynthetic, root/shoot partitioning of photosynthate, and leaf retention were investigated for three Populus deltoides Bartr. .times. nigra L. (DN 2, DN 22, DN 15) and two P. balsamifera L. .times. deltoides Bartr. (Jackii 4, Jackii 7) hybrids. Performance of the different hybrids was compared under controlled greenhouse conditions and in the field. There was a 2.4-fold difference in biomass production between the faster growing DN 2 and the slower growing DN 15 after 100 days growth in the greenhouse, and a 1.5-fold difference after the 1st year''s growth in the field. When coppice regrowth of the two Jackii hybrids was compared after 143 days under field conditions, the biomass production of Jackii 4 was 2.2 times that of Jackii 7 and net photosynthetic rate per unit leaf area was 50% higher than that of Jackii 7 over most of the summer. Many physiological and morphological factors contributed to the more vigorous growth of coppice DN 2 and Jackii 4. These included vigorous initial root growth relative to shoot growth, the ability to maintain higher xylem pressure potentials, net photosynthetic rate, and transpiration rate. These factors may have delayed leaf senescence and late-season photosynthetic decline of older leaves until later in the growing season. The possible causal sequence of physiological events contributing to the difference in productivity and the use of these factors for the early selection of productive hybrids or genotypes are discussed.