Difference In Adaptability To Drought Stress Between Different Species Of Populus Section Tacamahaca Spach

Currently, drought is one of the most serious environmental stresses. And in arid and semi-arid regions, drought is a major constraint imposed on tree survival and growth. Poplars (Populus spp.) are important components of ecosystem and, due to their fast growth, represent optimal species for the production of biomass suitable as a source of fuel, fiber, lumber and plywood. In this experiment, different species of Populus section Tacamahaca Spach were used as model plants to study the adaptability to drought stress and interspecific differences in adaptability, and the effects of exogenous abscisic acid (ABA) application and fertilization on the drought tolerance. Our results could provide a strong theoretical evidence and scientific direction for the afforestation, prevention of desertification and rehabilitation of ecosystem in the arid and semi-arid area, and provide a strong evidence for adaptive differentiation of different species, and so may be used as criteria for species selection and tree improvement. The results are as follows: 1. The growth and morphological response to drought stress (1) Adjustment in growth rate. The relative growth rate of plant height and basal diameter significantly reduced under drought stress, and the extent of adjustment became larger with the drought stress became severer; (2) Modification of plant structure. Drought stress significantly reduced the mass of all parts of plant, and increased at least relatively dry matter allocation into the root fraction. 2. Photosynthetic response to drought stress Drought stress not only significantly decreased gas exchange under controlled optimal conditions, but also altered the diurnal changes of gas exchange, chlorophyll fluorescence and intrinsic water use efficiency (WUEintr). On the other hand, drought stress also affected the photosynthesis PAR response curve. At midday, there was a depression in net photosynthesis rate (Pn) for water-stressed plants, but not for well-watered plants. There were stomatal and non-stomatal limitations to photosynthesis. Stomatal limitation was dominating in morning, and low Pn at midday was caused by both stomatal and non-stomatal limitation, whereas non-stomatal limitation was dominating in afternoon. In addition, drought stress also increased light compensation point (LCP) and dark respiration (Rd), and decreased light saturation point (LSP) and maximum net photosynthesis rate (Pmax). 3. A large set of parallel responses to drought stress (1) Drought stress caused pronounced inhibition of the growth and increased at least relatively dry matter allocation into the root; (2) Drought stress caused pronounced inhibition of photosynthesis rate and the efficiency of photosystem II, and that the stomatal limitation to photosynthesis was dominant. The decrease in stomatal conductance (gs) effectively controlled water loss and increased water use efficiency (WUE); (3) Drought also affected many physiological and biochemical processes, including increases in free proline, malondialdehyde (MDA) and ABA content, and superoxide dismutase (SOD) activity. On the other hand, the ABA content of leaves was significantly higher than that of stem and roots under all watering regimes; the high level of ABA in the leaf may result from the large import of ABA to leaves from other organs. These results demonstrated that there was a large set of parallel changes in the morphological, physiological and biochemical responses when plants are exposed to drought stress; these changes may enhance the capability of plants to survive and grow during drought periods. 4. Difference in adaptation to drought stress between P. cathayana and P. przewalskii There were significant interspecific differences in early growth, dry matter allocation and WUE between two sympatric Populus species. Compared with P. cathayana, P. przewalskii showed higher shoot height, dry matter accumulation, number of leaves, total leaf area, fine root mass, fine root/total root ratio (Ft) and WUE under both well-watered and water-stressed treatments. On the other hand, P. przewalskii also showed higher root mass/foliage area ratio (Rf), root/shoot ratio (Rs) and carbon isotope composition (δ13C) than P. cathayana under water-stressed treatment. The results suggested that there weredifferent water-use strategies between two sympatric Populus species, P. przewalskii with higher drought tolerance may employ a conservative water-use strategy, whereas P. cathayana with lower drought tolerance may employ a prodigal water-use strategy. The findings confirm the existence of interspecific genetic differences in early growth, dry matter allocation and WUE as affected by water stress, these variations in drought responses may be used as criteria for species selection and tree improvement. 5. The effects of exogenous ABA application on the drought tolerance Exogenous ABA application significantly decreased dry mass accumulation and gas exchange, and significantly increased root development and δ13C under well-watered and water-stressed conditions. However, distinct interspecific differences were found in ABA-induced growth reduction, gas exchange decrease and δ13C increase. P. kangdingensis was more responsive to applied ABA than P. cathayana, resulting in larger decreases in growth and gas exchange, and larger increases in Rs and ABA. In addition, P. kangdingensis also exhibited higher Ft and δ13C than P. cathayana under all experimental treatments. Our results provide strong evidence for adaptive differentiation between two contrasting poplar species. 6. The effects of fertilization on the drought tolerance Fertilization promoted plant growth under well-watered condition, while there was negatively effect on plant growth under drought stress. Fertilization increased water use efficiency (instantaneous water use efficiency (WUEinst) and δ13C) under well-watered and water-stressed treatment. There were differences in response to different treatment between two species: compare with P. przewalskii, P. cathayana had higher dry mass accumulation under well-watered and fertilization treatment and lower dry mass accumulation under drought-stresses and non-fertilization treatment, the results showed that compare with P. cathayana collected from lower altitude area, P. przewalskii collected from higher altitude area had slower growth rate and the stronger adaptability to drought and sterility.