Assessment of growth of Mentha pulegium in a green roof system through the study of PSII photochemical parameters

The extensive use of green roofs in recent years has created several concerns regarding parameters such as plant material, substrate composition and depth, because of the special conditions found on a green roof (e.g., temperature extremes, intense solar radiation). In vivo chlorophyll fluorescence was used in the present study to assess growth of Mentha pulegium plants grown in three different substrates based on pumice (P) added at 60% and additionally on the following addends: soil (S) or compost (C). Rooted cuttings of M. pulegium were planted on April 2010 in plastic containers (40×60 cm) following a typical green roof infrastructure. The substrate depth was 7 cm and three different substrate compositions were tested: A = 6:4 (P:C), B = 6:4 (P:S) και C = 6:2:2 (P:C:S) (v/v). Assessment of plant growth was based on photosystem II (PSII) photochemical parameters and biometric characteristics during and at the end of the experimental period (6 months), aiming to test the suitability of the particular plant species for green roofs. The soil, as a component of substrate B (mainly) and of substrate C (secondarily), had the largest effect on plant growth inducing taller plants and greater final above ground biomass compared to the soilless substrate (Α). In plants of treatment B and C, values of ETR (electron transport rate), in June, were higher compared to those of plants in treatment A, while values of ΦPSII (effective quantum yield of PSII photochemistry) did not differ among treatments. During summer, plants showed values of the ΦPSIIo parameter above 0.7 in the majority of cases indicating that no critical damage had been established. In conclusion, the addition of soil in the plant growth substrate resulted in higher growth and better response to stress due to xerothermic conditions. The photochemical parameters of PSII can be used as a sensitive probe of the physiological condition of plants under green roof growth conditions.