Impact of substrate depth and irrigation regime on growth, flowering and physiological indices of Greek sage (Salvia fruticosa Mill.) grown on urban extensive green roof systems

The broad application of extensive green roofs in the Mediterranean region is hindered by the stressful climatic conditions of the summer period, which is characterized by drought and heat extremes that challenge plant survival and growth. The use of native plants can provide a sustainable solution in overcoming these challenges. The aim of the present study was twofold: Firstly, to investigate the growth and flowering of the native Greek sage (Salvia fruticosa Mill.) when grown as part of an extensive green roof phytocommunity under two different substrate depths (either 8 cm or 15 cm) and two levels of irrigation (either high or low). Secondly, to determine several physiological indices in order to quantify the water stress tolerance response of Greek sage when grown under the above-mentioned green roof conditions with the imposition of three drought periods (20-25 July, 22-31 August, 23 September-10 October 2014). The deeper substrate (15 cm) promoted growth regardless of the level of irrigation and the number of flowers was favored by the deeper substrate with the higher water regime treatment. Although the shallow substrate depth and the low irrigation regime reduced growth and flowering, all plants survived. The imposition of drought stress periods resulted in a gradual decrease of total soluble sugars, proline and electrolyte leakage. Chlorophyll fluorescence (Fv/Fm) fluctuated between the three drought periods and SPAD increased in the drought period that was imposed during fall. During the first drought period, plants subjected to the shallow depth (8 cm) and irrigated with the low water regime, accumulated more sugars and had a higher chlorophyll fluorescence. By contrast, plants growing in the deeper substrate depth of 15 cm provided moderate sugars, but the lowest chlorophyll fluorescence. In the second drought period, the only index that varied between treatments was proline, which was increased for the plants growing in the shallow substrate depth and irrigated with the high-water regime. The lowest proline concentration occurred in plants growing in the deeper substrate depth receiving low irrigation. SPAD increased on the third drought period for plants growing in the deeper substrate. It was concluded that Greek sage can be successfully grown in extensive green roof systems with minimal inputs of water resources even at substrate depths of 8 cm. Out of the tested physi-ological indices, none could be employed as a standalone indicator of the Greek sage water stress magnitude.