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Background: The environmental stresses caused by climate change have become more severe in recent decades, affecting tree growth and physiology. Tropical forests have great potential for global carbon sequestration. However, they suffer from heavy rainfall and prolonged dry periods due to climate change. Swietenia macrophylla King and Melia azedarach L. are economically valuable trees that are widely planted in southern Taiwan. Plantations are exposed to either prolonged dry periods or heavy rainfall within the seasons of tropical monsoon areas. Photo-physiological comparisons may provide information that can improve management of S. macrophylla and M. azedarach plantations in tropical regions. Results: Both species exhibited a midday depression in leaf photosynthesis regardless of the season. The net photosynthetic rate ( P ), stomatal conductance ( g ), and transpiration rate ( E) in the dry season all significantly decreased in both tree species. In addition, M. azedarach used water more efficiently than did S. macrophylla during the dry season, but S. macrophylla had higher P compared with that in M. azedarach during the wet season. Temperature and vapor pressure deficit influenced P variation in S. macrophylla and M. azedarach, respectively. Our data suggested that the P and g of M. azedarach, but not of S. macrophylla, were linearly correlated during the dry season. The reduction of the leaf area was more sever in M. azedarach than in S. macrophylla, thus preventing water loss more efficiently. Conclusions: M. azedarach adapted to drought by reducing total leaf area and maintaining higher P , g , E, and WUE compared with those measured in S. macrophylla during the dry season. M. azedarach is more drought adaptation and more suitable for both humid and semi-humid areas than S. macrophylla, whereas the latter should be limited to more humid areas. [ABSTRACT FROM AUTHOR]

Meconopsis horridula var. racemesa Prain. is a famous alpine flower and medicinal plant native to high elevations in the Himalayas, but cultivating it at lower altitudes presents great challenges. The photosynthetic gas exchange and chlorophyll fluorescence of M. horridula were investigated at three temperatures to evaluate its photosynthetic performance and the relative importance of biochemical limitation, stomatal limitation, and mesophyll limitation to photosynthesis under different temperature regimes. Meconopsis horridula grown at 20°C could obtain the highest photosynthetic rate and photochemical efficiency among the three temperatures, and photosynthetic performance at low temperature was better than at high temperature. Non-photochemical quenching was an important mechanism protecting the photosynthetic apparatus of M. horridula under temperature stress conditions. Although mesophyll conductance was the dominant factor for limiting photosynthesis of M. horridula both at low temperature and high temperature, the photosynthesis at high temperature was also limited by stomatal conductance and biochemical efficiency. The poor photosynthetic performance at high temperature may be what limits M. horridula cultivation at low altitude. [ABSTRACT FROM AUTHOR]