The physiological and morphological responses to water stress induced by polyethylene glycol (PEG) or by withholding water were investigated in Aegilops biuncialis Vis. genotypes differing in the annual rainfall of their habitat (1050, 550 and 225 mm year–1) and in Triticum aestivum L. wheat genotypes differing in drought tolerance. A decrease in the osmotic pressure of the nutrient solution from –0.027 to –1.8 MPa resulted in significant water loss, a low degree of stomatal closure and a decrease in the intercellular CO2 concentration (Ci) in Aegilops genotypes originating from dry habitats, while in wheat genotypes high osmotic stress increased stomatal closure, resulting in a low level of water loss and high Ci. Nevertheless, under saturating light at normal atmospheric CO2 levels, the rate of CO2 assimilation was higher for the Aegilops accessions, under high osmotic stress, than for the wheat genotypes. Moreover, in the wheat genotypes CO2 assimilation exhibited less or no O2 sensitivity. These physiological responses were manifested in changes in the growth rate and biomass production, since Aegilops (Ae550, Ae225) genotypes retained a higher growth rate (especially in the roots), biomass production and yield formation after drought stress than wheat. These results indicate that Aegilops genotypes, originating from a dry habitat have better drought tolerance than wheat, making them good candidates for improving the drought tolerance of wheat through intergeneric crossing.

The functional state of the photosynthetic apparatus of flowering homoiochlorophyllous desiccation tolerant plant Haberlea rhodopensis during dehydration and subsequent rehydration was investigated in order to characterize some of the mechanisms by which resurrection plants survive drought stress. The changes in the CO2 assimilation rate, chlorophyll fluorescence parameters, thermoluminescence, fluorescence imaging and electrophoretic characteristics of the chloroplast proteins were measured in control, moderately dehydrated (50% water content), desiccated (5% water content) and rehydrated plants. During the first phase of desiccation the net CO2 assimilation decline was influenced by stomatal closure. Further lowering of net CO2 assimilation was caused by both the decrease in stomatal conductance and in the photochemical activity of photosystem II. Severe dehydration caused inhibition of quantum yield of PSII electron transport, disappearance of thermoluminescence B band and mainly charge recombination related to S2QA- takes place. The blue and green fluorescence emission in desiccated leaves strongly increased. It could be suggested that unchanged chlorophyll content and amounts of chlorophyll-proteins, reversible modifications in PSII electron transport and enhanced probability for non-radiative energy dissipation as well as increased polyphenolic synthesis during desiccation of Haberlea contribute to drought resistance and fast recovery after rehydration.

Accumulation of DLS (possibly phenolics) in the thylakoid lumen is demonstrated and is proposed as a mechanism protecting the thylakoid membranes of H. rhodopensis during desiccation and recovery under LL. Disappearance of DLS during desiccation in ML could leave the thylakoid membranes without protection, allowing oxidative damage during dehydration and the initial rehydration, thus preventing recovery of photosynthesis.

In intact plants, Cd-induced Fe deficiency is thought to play a role in the toxic effects of Cd on photosynthesis. To investigate the contribution of the Cd-induced Fe deficiency to Cd stress symptoms we studied the composition and organization changes of thylakoid pigment-protein complexes by twodimensional Blue Native-SDS gel electrophoresis and mass spectrometry, in parallel to functional changes, using Beta vulgaris plants grown in hydroponics. Plants were treated by withdrawing of Fe or with 10 µM CdCl2 for 10 days. Both metal stresses caused a marked decline in leaf chlorophyll concentration and chloroplast Fe content, as well as a loss in photosystem I (PSI) and light harvesting complex II (LHCII) particles. Furthermore, organizational changes of the photosynthetic apparatus were found, including a decrease in the ratio of the PSII mega-/supercomplexes and an increase in the monomeric form of the LHCII antennae, with the extent of these changes being similar under both stresses. This supports that Fe deficiency responses have a major role in the responses of plants under Cd stress. In the Fe-deficient thylakoids, an increase in the ratio of PSI supercomplexes and degrading PSII particles was more pronounced, together with a higher zeaxanthin content. Under Cd stress, a stronger inhibition of PSII activity and enhancement of thermal dissipation of the inactive PSII complexes were observed. The differences detected under the two metal stresses lead to the conclusion that both local Fe deficiency in chloroplasts and other direct or indirect inhibitory effects of Cd are behind the response mechanisms of plants grown under Cd stress. Our results appreciably contribute to the sparse structural information on thylakoid complexes affected by Cd toxicity and Fe deficiency. EÖTVÖS LORÁND UNIVERSITY INSTITUTE OF BIOLOGY DEPARTMENTAll previously published work cited in the manuscript has been fully acknowledged.I am looking forward to hearing from you at your earliest convenience.With best regards, Yours faithfully, Brigitta BasaHighlights:-Cd-induced chloroplast Fe deficiency is a prime trigger for thylakoid acclimation to Cd excess.-Both Cd stress and Fe deficiency induce PSII supercomplex and LHCII disassembly.-Thermal dissipation by inactive PSII complexes is markedly induced by Cd stress.-The amounts of PSI supercomplexes and zeaxanthin rise with Fe deficiency.-Cyclic electron flow and zeaxanthin are likely to be energy quenchers under low Fe.

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