Abstract—Water availability in soil is a key aspect in the context of global change. More frequent and more severe extreme events including droughts, flooding and heat waves must be expected for the next decades in some regions. Since a high percentage of the area used for agriculture is covered by grasslands, the response of grass and legume species to severe drought is important for the evaluation of drought impacts. Differences between the species considered (Lolium perenne L., Dactylis glomerata L., Phleum pratense L., Trifolium repens L., Trifolium pratense L.) in their response to a low water potential in the root medium were evident. In general the number of photosynthetically active leaves per plant decreased under drought. This decrease was partially due to a smaller number of new leaves produced and partially to senescence of the oldest leaves. The strongest reduction in the number of active leaves was observed in Trifolium repens, while Trifolium pratense was less susceptible. The grasses were even less affected than the clover species. Temperature of sun-exposed leaves was increased in drought-stressed plants as a consequence of decreased transpiration. The elevated temperature may cause additional effects on leaf metabolism (e.g. inactivation of Rubisco activase). Modifications in leaf senescence and leaf emergence under drought stress are species-specific and influence plant performance during the stress and the subsequent recovery phase. Keywords—Climate change, drought stress, grassland plants, recovery I. INTRODUCTION shorter version of this paper was presented orally at the 9 th International Conference on Energy & Environment (EE ’14) in Geneva, Switzerland (December 29-31, 2014) and was included in the Conference Proceedings [1]. Water availability in agriculturally used soil is besides an increasing CO 2 level in the atmosphere and an increase in the average This work was supported in part by Swiss National Science Foundation (NCCR “Climate”, Project “Plant Soil”). R. M. Blösch is with the Institute of Plant Sciences at the University of Bern, Switzerland (e-mail: regula.bloesch@ips.unibe.ch). O. Riesen was with the Institute of Plant Sciences, University of Bern, Switzerland and is now with the Pädagogische Hochschule Bern, Switzerland (e-mail: olivier.riesen@phbern.ch). U. Feller was with the Institute of Plant Sciences, University of Bern, Switzerland and is now retired (corresponding author, phone: +41-31-302-21- 09; e-mail: urs.feller@ips.unibe.ch). ambient temperature an important aspect in the context of global change caused by the combustion of fossil energy sources [2, 3]. More frequent and/or more severe drought periods must be expected during the next decades in some regions as a consequence of climatic changes, while other regions may be characterized by increased precipitation with favorable (increased water availability and plant productivity) or unfavorable (waterlogging) effects on crop plants [2-5]. Even in cases when the yearly precipitation will remain quite constant, a shift from the main growth period to the cold season may cause summer droughts and negatively influence plant productivity. Such changes are of ecological and economical relevance. Economic consequences of global change on agriculture were recently discussed by Lanfranchi et al. [6] and possible impacts on sustainable development and tourism were listed by Mazilu [7]. Various species present in grasslands may be influenced differently by such extreme events [8-10]. A series of reports refer to the overall productivity of grasslands [9, 11], to the species composition [11-14], to the expansion of weed populations [15] and to gas exchange with the atmosphere [16]. The weed Rumex obtusifolius may be less affected by drought than the desired grassland species and cause serious problems for farmers [17]. On the level of individual plants, stomatal regulation [18 and references therein], photosynthetic activities [19-21], summer dormancy [22, 23] and symbiotic nitrogen fixation in legumes [24, 25] were key aspects addressed during the past decades. Stomatal and non-stomatal limitations may negatively influence CO 2 assimilation [26]. Water availability in the root medium influences stomatal opening, transpiration and as a consequence leaf temperature [18]. Physiological processes on the whole plant level are less well investigated, but leaf production/expansion and senescence were identified as crucial points in this context [27, 28]. The experiments reported here are focused on the number of active leaves per plant and its dependence on the formation of new leaves and senescence of older leaves. A direct comparison of selected grass and clover species was envisaged by growing them on the same pots either in soil culture with natural illumination in a green house or in hydroponic culture Extended drought periods in grasslands: impacts on the number of photosynthetically active leaves and on leaf senescence in grass and clover species Regula M. Blösch, Olivier Riesen and Urs Feller A INTERNATIONAL JOURNAL OF ENERGY and ENVIRONMENT Volume 9, 2015 ISSN: 2308-1007 147