Leaf f^rowth rate and the turgor pressure ot" cells in the e.xpanding zone of lea\-es of Lolium temulentum were measured simultaneously. Growth rate was reduced o\'er a range from 30//m min ' to zero by reducing the temperature of the expanding zone from 20 to 2 °C. Turgor pressure remained constant at 05 MPa. This implies tliat growth reduction by low temperature is due to changes in cell wall rheology. Cell membrane hydraulic eonducti\ ity (L^J was reduced with temperature as expected, but this was not sufficient to influence growth rate detectably.Key words: l^oliuin teinulcnluin, growth rate, turgor pressure, wall rheology, pressure probe, chill stress. JNTHOnUCTlONThe annual growth cycle of perennial temperate grasses involves substantial exposure to sub-optimal temperatures. These plants do not possess a true winter dormancy mechanism, but respond directly to prevailing temperatue conditions. An understanding ot the nature of tbese responses is important m impro\ing performance at low temperatures and tbence overall annual productivity (Pollock & Eagles, 1988). Grasses also provide an ideal system for studying cell expansion since, in common with other monocots, they possess a locahzed intercalary meristem and extension zone (Pollock & Eagles, 1988) allowing uneqtii\ocal identification of the position of the expanding cells. Stoddart el al., (1986) have described an instrument that allou.s the continuous and direct measurement of growth of graminaceous seedlings whilst the temperature of the leaf meristem is altered. Using seedlings of Lolium temulentum it was found that temperature perception and response occur within the extension zone over a time scale that precludes eflects on the supply of nascent cells. It has been suggested tbat tbe sensiti\ ity of extension growtb to cbilling may represent some pbysical limitation to growtb based on cbanges in tbe water * .Address tor cori'L'spoiKlcnce. relations properties of tbe plants (Kleinendorst & Brouwer, 1970; Watts, 1971; Barlow, Boersma & Young, 1977). Current models of tbe water relations of growing cells are based on tbe Lockhart equation (Lockhart, 1965; Tomos, 1985). (1)This equation relates relative growth rate (\/V) (d V/At); where V = cell volume and / = time) to the wall rheological properties

Earlier studies of murine leukemia viruses (MuLVs) have reported that a percentage of surface protein (SU) remains covalently associated with transmembrane protein (TM) through formation of disulfide bonds. Among MuLVs, there are three conserved cysteine residues within the extracellular domain of TM. These cysteine residues were substituted individually with serines to define their function and possible role in disulfide bonding with SU. Using oligonucleotide-directed mutagenesis, seven mutant constructs were generated with individual as well as multiple cysteine mutations. Transient transfection of all seven cysteine mutations resulted in nonviable virus. Analysis of intracellular proteins of producer mutant cell lines have demonstrated that precursor envelope protein (gPr80env; SU/TM) is being synthesized, but transport and processing of gPr80env is blocked in the endoplasmic reticulum. Two independent reversions of one cysteine mutation have been isolated and characterized.

SUMMARYThe ' slender' mutant of barley (Hordeum vulgare) shows high rates of leaf extension and a reduced temperature threshold for extension growth compared with normal seedlings. The turgor pressure of epidermal and mesophyll cells within the extension zone was unaffected by localized cooling. Measurements of linkage patterns in cell wall carbohydrates isolated from the extension zone showed no variation with genotype or growth temperature, suggesting that the gross chemical architecture of the cell walls was similar.Differences between genotypes were observed when the rheological properties of extending leaves were measured using an extensiometer. Leaves from mutant seedlings showed greater plastic deformation than those from normal seedlings. These differences were absent in mature leaves or in growing leaves killed in boiling methanol prior to measurement. Differences were observed in the patterns of autolytic release of carbohydrates from cell wall extracts, in the incorporation of radioactivity from ^*CO2 into specific components of the extending cell wall and in the amounts of uronic acids released following treatment with pectolytic enzymes. These results are discussed in relation to the hypothesis that the mutation acts by modifying the patterns of turnover of structural components in the extending cell wall, thereby affecting extensibility.