Contribution of arbuscular mycorrhizal fungi (AMF) to the adaptations exhibited by the deciduous shrub Anthyllis cytisoides under water deficit Nieves Goicoechea*, Silvia Merino and Manuel Sa´nchez-Dı´az* Departamento de Fisiologı´a Vegetal, Facultades de Ciencias y Farmacia, Universidad de Navarra, c/Irunlarrea s/n, 31008 Pamplona, Spain *Corresponding authors, e-mail: niegoi@unav.es, msanchez@unav.es Received 2 March 2004; revised 20 July 2004 Anthyllis cytisoides L. is highly colonized by arbuscular mycorrhizal fungi (AMF) and behaves as a drought-avoider species in the field. Our objectives were: (1) to study the response of A. cytisoides when exposed to moderate (acclima- tion) or severe (peak) drought and subsequent rewatering under nursery conditions; and (2) to verify if AMF improved the adaptation of A. cytisoides to stress. The soil compactness in drought-acclimated treatments increased four times compared with that of well-watered controls, which could reinforce the effects of water deficit on plant physiology. Photosynthetic rates decreased by around 50% and 70% and leaf conductance decreased by 40% and 50% in drought-acclimated non-mycorrhizal and mycorrhizal plants, respectively. Peak drought limited plant growth, accelerated leaf senescence and induced the conversion of starch into soluble sugars in the leaves of stressed plants. The accumula- tion of sugars could contribute to a decrease in water potential in order to achieve the required tension to let water move from soil to shoot. Mycorrhizal plants showed a two-fold higher chlorotic leaf biomass than non-mycorrhizal plants under severe drought. Moreover, mycorrhizal A. cytisoides showed enhanced epicuticular waxes on the surfaces of the remaining green leaves. Increased leaf senescence, together with wax deposition, could reduce whole plant transpiration, thus allow- ing mycorrhizal plants to maintain a higher leaf relative water content (50%) than non-mycorrhizal plants (35%). After drought recovery, leaf abscission in stressed mycorrhizal plants was 10 times greater than that in non-mycorrhizal plants. The results suggest that AMF conferred greater responsiveness of A. cytisoides to drought. Enhanced wax deposition and leaf senescence could be an ecological adapta- tion to cope with severe water deficit. Introduction Anthyllis cytisoides L. is a widely distributed woody legume in the Mediterranean ecosystems of southern Spain. In these semiarid ecosystems, where the extreme environmental conditions and anthropogenic activity have resulted in progressive disturbance of vegetation cover and soil erosion, legumes constitute a key com- ponent of the natural succession (Herrera et al. 1993). In natural conditions, A. cytisoides is highly colonized by arbuscular mycorrhizal fungi (AMF). Mycorrhizal symbiosis is essential for the optimal growth and phos- phorus nutrition of this shrub legume (Dı´ az and Honrubia 1995). A. cytisoides appears to be less well adapted to a semiarid climate than other common perennial species growing in the same area, such as Retama sphaerocarpa. Whilst the photosynthetic rate in R. sphaerocarpa changes little with the season of the year (Haase et al. 1999), the CO 2 exchange rate in A. cytisoides progres- sively declines to practically zero during the dry season (Haase et al. 2000). Under field conditions, A. cytisoides behaves as a drought avoider, exhibits a resting period and one of its strategies to withstand prolonged water PHYSIOLOGIA PLANTARUM 122: 453–464. 2004 doi: 10.1111/j.1399-3054.2004.00421.x Printed in Denmark – all rights reserved Copyright # Physiologia Plantarum 2004 Abbreviations – ABA, abscisic acid; AMF, arbuscular mycorrhizal fungi; CER, CO 2 exchange rate; C i , intercellular CO 2 concentration; DM, dry matter; DW, dry weight; FAA, formalin–acetic acid–alcohol; FC, field capacity; FW, fresh weight; g w , leaf conductance; KPB, potassium phosphate buffer; PPF, photosynthetic photon flux; RH, relative humidity; RWC, relative water content; SEM, scanning electron microscopy; T, transpiration rate; TSS, total soluble sugars; TW, turgid weight; C, water potential; Cp, osmotic potential. Physiol. Plant. 122, 2004 453