Using non-invasive techniques (CO2 gas exchange, light scattering, light absorption, chlorophyll fluorescence, chlorophyll luminescence), we have analysed the response of respiration and photosynthesis to dehydration and rehydration of leaves of the resurrection plants Craterostigma plantagineum Hochst., Ramonda mykoni Reichb. and Ceterach officinarum Lam. et DC. and of the drought-sensitive mesophyte spinach (Spinacia oleracea L.). The following observations were made: (i) The rate of water loss during wilting of detached leaves of drought-tolerant resurrection plants was similar to that for leaves of the sensitive mesophyte, spinach. Leaves of Mediterranean xerophytes lost water much more slowly. (ii) Below a residual water content of about 20%, leaves of spinach did not recover turgor on rewatering, whereas leaves of the resurrection plants did. (iii) Respiration was less sensitive to the loss of water during wilting in the resurrection plants than in spinach. (iv) The sensitivity of photosynthesis to dehydration was similar in spinach and the resurrection plants. Up to a water loss of 50% from the leaves, photosynthesis was limited by stomatal closure, not by inhibition of reactions of the photosynthetic apparatus. Photosynthesis was inhibited and stomates reopened when loss of water became excessive. (v) After the leaves had lost 80% of their water or more, the light-dependent reactions of photosynthetic membranes were further inhibited by rewatering in spinach; they recovered in the resurrection plants. (vi) In desiccated leaves of the resurrection plants, slow rehydration reactivated mitochondrial gas exchange faster than photosynthetic membrane reactions. Photosynthetic carbon assimilation recovered only slowly.

The effects of freezing and desiccation of spinach leaves (Spinacia olracea L. cv Yates) on the thylakoid membranes were assessed using antibodies specific for thylakoid membrane proteins. The peripheral part of the chloroplast coupling factor ATPase (CFI) was used as a molecular marker for chemical membrane damage by chaotropic solutes. Plastocyanin, a soluble protein localized inside the closed thylakoid membrane system, was a marker for damage by mechanical membrane rupture. After freezing and wilting of leaves which resulted in damage, very little CF1 was detached from the membranes, whereas almost all plastocyanin was released from the thylakoids. It is suggested that in vivo dehydration both by freezing and desiccation results in membrane rupture rather than in the dissociation of peripheral thylakoid membrane proteins.

The stress stability of membranes from two drought-tolerant plants (Craterostigma plantagineum andCeterach officinarum) was compared with that of a drought-sensitive plant (Spinacia oleracea) in model experiments. Thylakoids from these plants were exposed to excessive sugar or salt concentrations or to freezing. All stresses caused loss of membrane function as indicated by the loss of cyclic photophosphorylation or the inability of the membranes to maintain a large proton gradient in the light. However, loss of membrane functions caused by osmotic dehydration in the presence of sugars was reversible. Irreversible membrane damage during freezing or exposure to salt was attributed mainly to chaotropic solute effects. The sensitivity to different stresses was comparable in thylakoid membranes from tolerant and sensitive plants indicating that the stress tolerance of a plant can hardly be attributed to specific membrane structures which would increase membrane stability. Levels of membrane-compatible solutes such as sugars or amino acids, among them proline, were much higher in the drought-tolerant plants than in spinach. Isolated thylakoids suspended in solutions containing an excess of sugars remained functional after dehydration by freeze-drying. This indicates that membrane-compatible solutes are important in preventing membrane damage during dehydration of poikilohydric plants.

No abstract

Visual transduction in the compound eye of flies is a well established model system for the study of G proteincoupled transduction pathways. To characterize key components of the phototransduction cascade we performed substractive hybridization screening. We cloned the cDNA coding for the visual G␥ (G␥ e ) subunit from Drosophila which had so far eluded identification at the molecular level. Northern blot analysis revealed the presence of a major, 1.4-kilobase(kb) G␥ e transcript and two minor transcripts of 1.8 and 6 kb in size. The major 1.4-kb mRNA is expressed preferentially in the eye. The spatial expression pattern determined for G␥ e as well as co-immunoprecipitation experiments demonstrated that G␥ e dimerizes with G␤ e to form the heterodimeric G␤␥ subunit which functions in visual transduction in the Drosophila compound eye. G␥ e shares common characteristics with the visual G␥ subunits of human rod and cone photoreceptors although different classes of G␣ subunits are employed in vertebrate and invertebrate phototransduction. By the molecular cloning and characterization of the visual ␥ subunit of Drosophila one of the few missing links in the well studied Drosophila phototransduction cascade has been characterized to complete our knowledge about the Drosophila visual transduction pathway.