exposure to various Cd concentrations and the interaction of Cd uptake with P nutrition on trace metal accumulation were studied. High P availability (1 mM) stimulated high cell growth and consequently greater total uptake of Cd from the growth medium, however, the Cd cell load (atoms/cell) was significantly higher when cultures were P limited (10 μM). Significant changes in cell metal content were observed when P availability was limited, including a large increase in Zn content. Moreover, Cd treatments appeared to alter the accumulation of some metals. Further investigations using a Chlamy- domonas mutant (psr1) which has an impaired P starvation response, will reveal whether these changes in metal uptake are linked to known mechanisms of P scavenging. Additionally, experiments are assessing gene expression changes in response to these stresses to determine the pathways responsible for the metal changes. Email Address for correspondence: rachel.e.webster@manchester. ac.uk doi:10.1016/j.cbpa.2009.04.413 P1.15 13:45 Monday 29th June 2009 Delayed leaf senescence induces extreme drought tolerance in crop plants Eduardo Blumwald (Department of Plant Sciences University of California Davis), Rosa M. Rivero (Department of Plant Sciences University of California Davis), Harkamal Wadia (Department of Plant Sciences University of California Davis), Zvika Peleg (Department of Plant Sciences University of California Davis), Mark Szczerba (Depart- ment of Plant Sciences University of California Davis), Rosa N. Jauregui (Department of Plant Sciences University of California Davis), Liu Li (Department of Plant Sciences University of California Davis) We investigated the effects of the expression of IPT (isopentenyl- transferase) and cytokinin production on several aspects of photo- synthesis in transgenic tobacco plants grown under optimal or restricted (30% of the optimal) watering regimes. There were no significant differences in stomatal conductance between leaves from wild-type and transgenic plants expressing the IPT gene under the control of the SARK promoter grown under optimal or restricted watering. On the other hand, there was a significant reduction in the maximum rate of electron transport as well as the use of triose phosphates only in the wild-type plants during growth under restricted watering, indicating a biochemical control of photosynth- esis during the growth under water deficit. The transgenic plants displayed an increase in catalase inside peroxisomes, a physical association between chloroplasts, peroxisomes and mitochondria and an increase in the CO 2 -compensation point, indicating the cytokinin- mediated occurrence of photorespiration in the transgenic plants. The contribution of photorespiration to the tolerance of the transgenic plants to water deficit was also supported by the increase in transcripts coding for enzymes involved in the conversion of glycolate to RuBP. Moreover, the increase in transcripts was further enhanced in the transgenic plants grown under restricted watering conditions, indicating a cytokinin-induced increase in photorespiration and the contribution of photorespiration to protecting photosynthetic pro- cesses and playing a beneficial role during water stress. Our results indicate the possibility of generating transgenic plants with increased water use efficiency and increased tolerance to water deficit. Email Address for correspondence: eblumwald@ucdavis.edu doi:10.1016/j.cbpa.2009.04.414 P1.16 14:30 Monday 29th June 2009 Differential regulation of the genes encoding the high-affinity K + transporters HAK5 of Thellungiella halophila and Arabidopsis thaliana in response to salinity Fransico Rubio (CEBAS-CSIC), Fernando Alemán (CEBAS-CSIC), Man- uel Nieves-Cordones (CEBAS-CSIC), Vicente Martínez (CEBAS-CSIC) One of the biggest challenges for a plant growing under high salinity is to ensure the uptake of essential mineral nutrients such as K + while restricting the accumulation of potentially toxic ions such as Na + . When in addition K + is a limiting factor, high-affinity K + uptake systems with a high discrimination between K + and Na + are crucial to maintain K + supply and K + /Na + homeostasis. Group I HAK K + transporters are major contributors to high- affinity K + uptake. The regulation of these genes is complex and may be mediated by responses to the tissue K + concentrations, hormones, reactive oxygen species or the plasma membrane potential. These genes are highly induced in K + -starved roots. However in tomato plants, salinity represses LeHAK5 expression. To characterize the regulation of this type of genes under salinity in plants differing in their salt tolerance Arabidopsis thaliana and its halophyte relative Thellungiella halophila have been employed. The results show that, as in glycophytes, ThHAK5 is a major contributor to high-affinity K + uptake in T. halophila. However, whereas ThHAK5 transcripts are detected in K + -starved plants grown with NaCl, transcripts of the A. thaliana homologue AtHAK5 are absent. In parallel, high-affinity K + uptake is reduced to a lesser extent in T. halophila than in A. thaliana when plants are grown with NaCl. The results presented here indicate that plants which differ in salt tolerance show differential regulation of the expression of genes encoding K + transporters. Email Address for correspondence: frubio@cebas.csic.es doi:10.1016/j.cbpa.2009.04.415 P1.17 15:30 Monday 29th June 2009 Systems analysis of membrane transport and homeostasis in stomatal guard cells Zhonghua Chen (Glasgow University), Adrian Hills (Glasgow Uni- versity), Virgilio L. Lew (Cambridge University), Michael R. Blatt (Glasgow University) Guard cells play a vital role in regulating photosynthetic CO 2 uptake and transpirational water loss from plants. The mechanisms that drive stomatal movement have been intensively studied at the level of the guard cells in epidermal peels and in intact leaves, protoplasts and membrane patches. However, our knowledge of the internal mechanisms that control the dynamic continuum of stomatal apertures is very poor. Using systems analysis, we are bridging this gap in understanding of the dynamic range of stomatal apertures and its regulation by plasma membrane and tonoplast transport. An important focus for us is to understand how transport integrates with homeostasis in response to environmental stress and how such integration determines the ‘set point' for stomatal aperture. To this end, a software platform is under development for quantitative mathematical modelling of guard cell membrane transport and homeostasis involving ion and solute channels, pumps, carriers and co-transporters. Individual models of the key transporters are being programmed in the C++ language, based on the detailed kinetic information of guard cell transporters. Concurrently, we are using Abstracts / Comparative Biochemistry and Physiology, Part A 153 (2009) S184–S194 S188