To understand the mechanisms of salt tolerance in a halophyte, sea aster ( Aster tripolium L.), we studied the changes of water relation and the factors of photosynthetic limitation under water stress and 300 mM NaCl stress. The contents of Na + and Clwere highest in NaCl-stressed leaves. Leaf osmotic potentials ( Y s ) were decreased by both stress treatments, whereas leaf turgor pressure ( Y t ) was maintained under NaCl stress. Decrease in Y s without any loss of Y t accounted for osmotic adjustment using Na + and Claccumulated under NaCl stress. Stress treatments affected photosynthesis, and stomatal limitation was higher under water stress than under NaCl stress. Additionally, maximum CO 2 fixation rate and O 2 evolution rate decreased only under water stress, indicating irreversible damage to photosynthetic systems, mainly by dehydration. Water stress severely affected the water relation and photosynthetic capacity. On the other hand, turgid leaves under NaCl stress have dehydration tolerance due to maintenance of Y t and photosynthetic activity. These results show that sea aster might not suffer from tissue dehydration in highly salinized environments. We conclude that the adaptation of sea aster to salinity may be accomplished by osmotic adjustment using accumulated Na + and Cl -, and that this plant has typical halophyte characteristics, but not drought tolerance.

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Rooftop gardening or green roof establishment, one of the afforestation methods for urban greening, has many benefits for the environment, economy, and urban landscape. Various environmental stresses including heat, strong wind, sunshine and drought prevent most plants from growing well with extensive green roof systems. For the establishment of urban rooftop gardening in summer, we evaluated ten ornamental plant species (Evolvulus pilosus, Fragaria × ananassa, Hedera helix, Lampranthus spectabilis, Ophiopogon japonicus, Pelargonium × hortorum, Petunia × hybrida, Thymus serphyllum, Verbena × hybrida, Vinca major) to use for greening with shallow soil (10 cm depth) on the flat rooftop of a four-story building. We measured the area increase in green coverage as a total growth rate, photosynthetic ability of detached leaves, stomatal conductance of an attached leaf, soil surface and canopy air temperature. Thymus, Evolvulus, Petunia, and Fragaria are excellent plant species for rooftop gardening judging from their speedy green coverage with high growth rates (90% in Thymus, 65% in Evolvulus, 60% in both Petunia and Fragaria at maximum for about three months in summer). Thymus and Fragaria showed high performance of water saving with relatively low stomatal conductance under semi-dry conditions and did not need to be replanted every year unlike Evolvulus and Petunia. Evolvulus had the highest photosynthetic activity at 40°C. Petunia and Verbena did not grow continuously but maintained active blooming without increasing their green coverage (about 60%) throughout midsummer. Lampranthus tolerated extremely hot and dry conditions on the rooftop but grew very slowly. As Vinca and Pelargonium died back due to both heat stress and strong wind, we suggested these were not suitable for rooftop gardening. All vegetations showed a 6-8°C lower temperature of soil surface than that of the concrete rooftop during summer nights. We concluded that rooftop greening has a significant effect on cooling a building and saving energy for air-conditioning in summer.

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