PHOTOSYNTHESIS, WATER RELATIONS, AND GROWTH OF 2 HYBRID POPULUS GENOTYPES DURING A SEVERE DROUGHT

During the 1988 growing season in East Lansing, Michigan, United States, only 1.53 cm of rain fell from mid-May to mid-July, causing a severe drought. Then, beginning in mid-July, a period of near-record precipitation commenced; 30.4 cm of rain fell from July 19 to October 4. Growth, photosynthesis, and water relations of hybrid poplar cultivars 'Eugenei' and 'Tristis', which had been established in the spring of 1987 in 114-L plastic pots buried in the ground, were measured on several sunny days during the 1988 growing season. Pots were irrigated at two different rates, and half the pots received supplemental nitrogen fertilizer. On a seasonal basis, photosynthesis and water-use efficiency in both genotypes peaked in early July and declined thereafter. Stomatal conductances were low during the drought but increased substantially when the rains commenced. Whereas nitrogen level had little effect on leaf physiology, the low water treatment produced significant reductions in photosynthesis and conductance. Diurnal measurements were made on June 17 and July 12. On both days photosynthesis and conductances were higher in 'Tristis' than in 'Eugenei', especially for plants in the high water treatments and on July 12, the most extreme period of the drought. Drought produced both stomatal and mesophyll limitations to photosynthesis in both clones, though these responses were more pronounced in 'Eugenei'. This clone also showed very low water-use efficiencies in the low water treatment on July 12. Even though the physiology of 'Eugenei' was more impacted by drought than 'Tristis', it still produced two to three times more biomass over the 2-year period of the study than did 'Tristis'.