| The plant photosynthesis and transpiration can alleviate the urban "heat island effect" effectively. However, the land area which can be used for urban landscape plants is limited,due to the high urban land price. Thus, the green roof is attracted more and more attentions in the high populated and density-building cities, because it does not need to occupy the extra land area and also can improve urban ecology and environment. Because the lawn-style green roof are light-weighted and do not require heavy building load capacity, it almost can be constructed on all kinds of building roofs and was the dominant green roof style in China. The lawn-style green roof requires the extreme drought and heat resistances and year-round stability in performance in plants, since it only has thin growth media layer (less than 15cm in general), and can not apply to heavy maintenance operations. Few of landscape plants can adapt and grow well under such extreme roof environmental conditions. The plant which has been only popular used in lawn-style green roofs nowadays in China is Sedum lineare Thunb. Long-term single species planted on the roofs has gradually raised some insect and disease problems, and the landscape scenery of green roof become worse and worse. Therefore, it is necessary to select more suitable plants which can be grown on thin-layer media to enrich the green roof landscape styles and maximize the ecological, economic, social and ornamental value of green roofs. In this paper, four candidate green roof plants which are Sedum hispanicum L., Sedum spurium M. Bieb., Atenia cordifolia (L. f.) Schwant, Sedum lineare Thunb, which were screened from the long-term performance in the year-round green (or color) in local area, were tested for their drought and heat resistances in growth cambers. Four warm-season turfgrasses overseeded with perennial ryegrass (Lolium perenne) for the year-round green turfs were evaluated for the species transformation during spring, and the possibility to achieve year-round green turfs by the mixtures of Kentucky bluegrass (Poa pratensis L.) with centpedegrass (Eremochloa ophiuroides (Munro) Hack.) Or seashore paspalum (Paspalum vaginatum Sw.) were also evaluated. The main results are as follows:(1) The growth and physiological permeters changed with the processes of heat stress in all four tested candidate green roof plants, and there were significant differences among the plants. Sedum lineare performed the worst in heat stress. The visual quality, canopy coverage, leaf relative water content decreased and leaf electrolyte leakage increased more than other three plants. After 15 d heat stress, Sedum lineare plant was dead. The decreases of the visual quality, canopy coverage and the increase of leaf electrolyte leakage in Atenia cordifolia were less than Sedum lineare, showed its higher heat tolerance than Sedum lineare. The decreases of the visual quality, canopy coverage, leaf relative water content and the increase of leaf electrolyte leakage in Sedum spurium were less than Sedum lineare and Atenia cordifolia, showed its higher heat tolerance than Sedum lineare and Atenia cordifolia. Sedum hispanicum showed the best stabilities in the visual quality, canopy coverage, leaf relative water content and leaf electrolyte leakage, showed that Sedum hispanicum has the best heat tolerance among these four candidate plants.(2) The growth and physiological permeters changed with the processes of drought stress in all four tested candidate green roof plants, and there were significant differences among the plants. Sedum spurium showed the most decrease in its relative water content and increases in electrolyte leakage and MDA content, but its plant height, canopy coverage, visual quality showed the significant decline until 35 d and 21 d of drought stress respectively, which was later than the other three plants, and it can be quick recovered after re-water treatment, indicating that Sedum spurium has strong physiological reactions under drought stress, and drought damage repairing capacity and better drought tolerance. Atenia cordifolia showed the least decrease of plant height under drought stress, The decreases of canopy coverage and visual quality were mainly due to the wilting and exfoliation of the old leaves. Its new leaves still maintain relatively high leaf water content after 42 d drought stress, and the least increases of leaf electrolyte leakage and MDA content, indicating its better drought tolerance. In addition, Atenia cordifolia can be recovered within 3 d of re-watering, which showed it is the best in drought resistantance among these candidate plants. Sedum lineare showed the late significant changes in physiological parameters under drought stress, its leaf electrolyte leakage and MDA content increased significantly at 21d and 35d of drought stress respectively, However, it can not be recovered after rewater treatment and plant died at the end of the experiment, indicating that Sedum lineare got the most severe drought injury and such injury was hard to be repaired. Sedum hispanicum showed the earliest morphological response to drought stress in four plants, and the all leave wilted at 7 d drought stress. Its leaf electrolyte leakage increased with the process of drought stress, and the decreases of plant height, canopy coverage and visual quality were significantly lager than Sedum spurium and Atenia cordifolia. All plant died after 42 d drought stress and can not be recoverd after rewater treatment.For the pathway of drought resistence, Sedum spurium showed strong repairing capacity of drought damage, and Atenia cordifolia showed strong drought tolerance. These two plants have higher drought resistance capacity than Sedum lineare and Sedum hispanicum.(3) Warm-season turfgrasses overseeded with perennial ryegrass is one of the major methods to maintain year-round green turfs in climate transition region. The key for the overseeding success is laid on smooth transformation of perennial ryegrass to the warm-season grasses during spring. All four warm-season turfgrasses overseeded with perennial ryegrass can achieve the smooth species transformation during spring, and maintained their visual quality above 7.1 during the season, which were higher than the acceptable visual quality (6.0). From the species transformation speed, there were significant differences among four warm-season turfgrasses. Seashore paspalum was the fastest in species transformation, followed by Sports bermudagrass and Tifdwarf bermudagrass, and centepedegrass was the slowest. By July 5th, the covery of four warm-season grasses has reached 100%, 97%, 91%, 82%, respectively.(4) Two warm-season grasses, seashore paspalum and centepedegrass were planted into Kentucky bluegrass turf in the spring of 2007. The warm-season grasses performed comparative advantages in the competition with cool-season grass, and by the summer of 2007, the canopy coverages of seashore paspalum and centepedegrass reached 98% and 99% respectively. For turf quality performance in seasons, the mixed grasses did not showed significant differences in visual quality during summer and autumn, to compared with their warm-season turf control (seashore paspalum and centepedegrass), but significantly higher than Kentucky bluegrass control. In winter and spring, the visual quality of the mixed grasses was significantly higher than their warm-season turf control, but lower than Kentucky bluegrass control significantly. Because of the creeping growth habit of seashore paspalum and centepedegrass, the mixed turf can easily developed to warm-season grass patches. When the warm-season grass went into dormant during the winter, the Kentucky bluegrass grass remained uneven, resulting in the poor visual quality, with only 2.5 and 2.0, respectively, which were significantly lower than Kentucky bluegrass control. It still need further studies on the inhibition of creeping growth and patch development in warm-season grasses by cultivation operations during spring and summer to achieve the possibility of stable year-round turf quality in mixed turf. In conclusion, Sedum spurium and Sedum hispanicum showed the significantly higher heat tolerance than Sedum lineare, and Atenia cordifolia also showed higher heat tolerance than Sedum lineare. Sedum spurium and Atenia cordifolia showed significant higher drought tolerance and drought damage repairing capacity than Sedum lineare, and Sedum hispanicum showed lower drought resistance than Sedum lineare. The results indicated that Sedum spurium, Sedum hispanicum and Atenia cordifolia could be served as candidate plants for lawn-style green roof. Seashore paspalum, Sports bermudagrass, Tifdwarf bermudagrass, centepedegrass autumn overseeded with perennial ryegrass can smoothly achieve species transformation during the spring, providing the possibility to used on the green roof as year-round green turf technology. Kentucky bluegrass mixed with seashore paspalum or centepedegrass showed low competitive capacity of Kentucky bluegrass in the mixture, the visual quality of mixture turfs was still unacceptable during the winter, indicating the further studies are needed to inhibit the creeping growth and patch development of warm-season grasses in spring and summer.
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