Assessment of physiological parameters to determine drought tolerance of plants for extensive green roof architecture in tropical areas

Abstract

Extensive green roof architecture is an effective and low-maintenance solution to reduce energy cost of heating or cooling the buildings, and can be used to mitigate several urbanization problems such as heat island effect, lack of green space, and excessive stormwater runoff. However, designs and vegetation selections of the extensive green roofs have primarily been based on the studies conducted in the temperate areas. This study aimed to assess the suitability of ten groundcover plants in a tropical area using quantitative physiological parameters of plants, including relative water content (RWC), stomatal opening rate, maximum quantum yield of photosystem II (Fv/Fm), soil moisture, and leaf surface temperature. Upon severe drought stress, RWC could distinguish non-tolerant from tolerant species as early as 3 days after drought treatment (DAD). The drought-tolerant species had >80 % RWC at 5 DAD. For non-tolerant species, Fv/Fm decreased to an undetectable level, whereas only 5.45 % reduction was found for the tolerant species at 5 DAD. Three plant species Sesuvium portulacastrum, Evolvulus nummularius and Callisia repens maintained >80 % stomatal opening at 5 DAD, but E. nummularius was not considered a tolerant species based on the RWC and Fv/Fm characteristics. Interestingly, under drought stress C. repens maintained >50 % soil moisture at 7 DAD and maintained canopy surface temperature comparable to the well-watered condition. Together the results suggest that these physiological parameters are useful for the assessment of drought tolerance ability of groundcover plants, and S. portulacastrum and C. repens should be considered for extensive green roof architecture in the tropical areas.