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D. Creech and Yin Yunlong

There are three botanical varieties associated with the genus Taxodium: 1) Baldcypress (BC) = Taxodium distichum (L.) Rich.var. distichum, 2) Pondcypress (PC) = T. distichum var. imbricarium (Nutt.) Croom and 3) Montezuma cypress (MC) = T. distichum var. mexicana Gordon. Taxodium hybridization promises to combine the best characteristics of superior parents. In 1988, clones T302 (a BC × MC F1 hybrid), T401 (PC × MC), and T202 (PC × BC) were selected in China primarily for growth rate and tolerance to alkaline and salt-rich coastal floodplains. T302 is recommended in China for soils with pH 8.0∼8.5 and salt concentrations <0.2%. Other attributes of T302 included 159% faster growth than BC, good columnar form, longer foliage retention in fall and early winter, and no knees. T302 has been in the USA since January 2002 and is currently under evaluation in over 30 locations in southern USA. The clone was named `Nanjing Beauty' in 2004 as a cooperative introduction of the SFA Mast Arboretum and Nanjing Botanical Garden. In March 2005, the SFA Mast Arboretum received two new clones from China. T140 and T27 are considered more evergreen than T302 and both demonstrate strong salt tolerance. The clones were selected from a field population of T302 × TM—with strong TM characteristics and improvements in growth rate, salt tolerance, form and vigor. T140 grows faster than T27, which produces a wider profile. The foundation of the most recent selections comes originally from crosses made by Professor Chen and Liu in 1992 at the Nanjing Botanical Garden. Pollen from TM was applied to a female T302 and fifteen selections were made in 1995. The main characteristics for selection were 1) fast growth rate, 2) dark green color during the growing season and a red-orange leaf color in the fall, and 3) evergreen leaves. In 2006 or 2007, the results from T140 and T27 will be reported and registered with the Chinese Forestry Department. It will be at least five years before T140 and T27 enter commerce. In June, 2005 there were <100 each of these two clones. T118, T120 and T149 have already been registered with the Chinese Forestry Department at the provincial level, while T302 has been registered at the national level.

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Qin Shi, Yunlong Yin, Zhiquan Wang, Wencai Fan and Jianfeng Hua

The physiological acclimation of Taxodium hybrid ‘zhongshanshan 118’ (T.118) plants to a progressive drought stress and drought-stressed to recovery treatment (DS-R) was investigated in this study. Plants of control (C) treatment were watered daily throughout the experiment. Results indicated that water deficit reduced stomatal conductance (g S) to improve water use efficiency (WUE) and, as a consequence, net photosynthetic rate (P n), transpiration rate (T r), and intercellular CO2 concentration (C i) were also decreased in DS-R T.118 plants compared with C plants. These reductions became more significant with decreasing soil water availability. Correlation analysis showed g S was positively correlated (P < 0.01) with the soil water content as well as leaf relative water content (RWC). There was a tendency to accumulate proline, malondialdehyde (MDA), antioxidases, and membrane electrolyte leakage as stress intensity increased. Moreover, drought stress induced significant (P < 0.05) decline in total chlorophyll contents (Chlt) and increase of nonphotochemical quenching (NPQ) on day 8 as a photo-protective mechanism. Cluster analysis distinguished the adaption of T.118 plants to water deficit in two ways. First, photosynthesis was related to thermal dissipation, and second antioxidation was related to morphology and osmosis. Furthermore, tested parameters showed a reversed tendency and restored equivalently to C levels after 9 days of rewatering. These findings suggest that T.118 plants demonstrated considerable tolerance to short-term drought stress and recovery due to a high degree of plasticity in physiological acclimation.

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Qin Shi, Yunlong Yin, Zhiquan Wang, Wencai Fan, Jinbo Guo and Jianfeng Hua

Roots are vital organs for resource uptake. However, the knowledge regarding the extent by which responses in roots influence plant resistance is still poorly known. In this study, we examined the morphological and physiological responses of lateral roots of Taxodium hybrid ‘Zhongshanshan 406’ (Taxodium mucronatum♀ and Taxodium distichum♂, T. 406) to 8 (DS-8) and 12 days (DS-12) drought. Control plants (CK-8 and CK-12) were well-watered throughout the experiment. Results indicated that drought resulted in significantly decreased root length, surface area, volume, and biomass and a relatively high death rate of roots (>2 mm). Specific root length (SRL) and specific root surface area (SRA) of drought-stressed T. 406 plants were reduced to enhance resource uptake. Meanwhile, root relative water content (RWC) of T. 406 plants in CK-12 treatment was 5.81 times of those in DS-12 treatment. Under drought stress and root superoxide dismutase and ascorbic acid (ASA) activities, proline and hydrogen peroxide (H2O2) contents consistently increased to benefit the elimination of O2 . At the ultrastructural level, the organelle structure of T. 406 plant root tip was visibly damaged because of dehydration. The nucleus swelled and then exhibited uncommon features of disorganization and disruption. In short, our results provided substantial information about lateral root traits of T. 406 plants in response to drought stress, which is crucial to improve the drought resistance of Taxodium hybrid in the future breeding.

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Lijing Zhou, David L. Creech, Ken W. Krauss, Yin Yunlong and David L. Kulhavy

Taxodium distichum (L.) Rich. var. distichum [baldcypress (BC)], Taxodium distichum var. mexicanum Gordon [Montezuma cypress (MC)], and a Taxodium hybrid (‘Nanjing Beauty’: BC × MC cross, T302) were evaluated for salt tolerance in 2006 at Nacogdoches, TX. Plants were irrigated weekly with four levels of salinity [0, 1, 3.5, and 6 ppt (0, 17, 60, and 102 mol·m−3)] for 13 weeks and then 0, 2, 7, and 12 ppt (0, 34, 120, and 204 mol·m−3) for another 12 weeks. Salinity treatments did not have a significant effect on growth rate; however, there were significant differences in growth rate among the three genotypes. Genotype T302 produced the greatest wet weight, whereas MC had stronger apical dominance and exhibited the greatest increase in height over the course of study. As expected, sodium (Na) concentration in Taxodium leaves increased as sea salt concentrations increased but did not tilt Na/potassium (K) ratios to stressful disproportions. Of the three genotypes, BC exhibited the highest leaf content of Na, calcium (Ca), sulfur (S), and iron (Fe); MC had the lowest leaf content of Na, Ca, S, and Fe; and T302 was intermediate. The benefits of using a hybrid cross (T302) that maintains greater biomass than BC or MC across a range of salinities must be weighed against the potential additional pruning and training necessary for cutting-grown clones relative to BC and MC propagated from seed and flood tolerance relative to BC. Still, combining the best characteristics of different varieties of T. distichum should facilitate the production of favorable genotypes tolerant to a number of soil physical and chemical property fluctuations for arboricultural operations.