The roles of inorganic nitrogen salts in maintaining phytochrome- and gibberellin A3-mediated germination control in skotodormant lettuce seeds

Skotodormant seeds of Lactuca sativa Grand Rapids imbibed in darkness for 10 days (10-day DS) germinated poorly upon terminal treatment with red light (R) or gibberellin A₃ (GA₃). Inorganic nitrogen salts in the imbibition solutions reduced seed skotodormancy. Ten-day DS seeds, imbibed in 25 mm salt solutions followed by terminal R, germinated 99% if imbibed in NH₄NO₃, 70% if imbibed in KNO₃ or NH₄Cl, and 55% if imbibed in NaNO₃. Seeds imbibed in higher salt concentrations germinated fully upon terminal R treatment. Seeds imbibed in 25 mm NH₄Cl or in 50 mm NH₄NO₃ germinated completely upon GA₃ treatment. Osmotic effects of imbibition media accounted for only part of the effect, since seeds imbibed in 50 mm CaCl₂ or NaCl germinated poorly following R or GA₃ treatment. Seeds imbibed in 500 mm polyethylene glycol (PEG) 1000 or mannitol solutions for 10 days still exhibited skotodormancy. Treatments of R or GA₃ did not stimulate germination in seeds imbibed in mannitol, but germination was complete if seeds were given 1-h acid immersion plus a water rinse before the terminal R or GA₃ treatment. Seeds imbibed in 50–500 mm PEG during 10-day DS germinated significantly better in response to terminal R. Terminal GA₃ significantly improved germination only in seeds imbibed at 500 mm PEG. P_fr appeared to function in mannitol-imbibed seed only after an acid treatment. Seed exposure to inorganic nitrogen salts during the 10-day DS maintained seed sensitivity to terminal R or GA₃ treatment. The depth of seed skotodormancy was related to the availability of inorganic nitrogen and also involved the levels of P_fr or endogenous GA₃.Abbreviations FR far red - DS dark storage - R red - GA₃ gibberellin A₃ - PEG polyethylene glycol - SHAM salicylhydroxamic acid - ANOVA analysis of variance - GLM general linear model - LSD least squares difference - P_fr far-red absorbing form of phytochrome     

Silicotungstate lowers dramatically the quantum yield of chlorophyll fluorescence in situ without affecting the rate of electron transport

Electron-transport and fluorescence properties of chloroplasts in the presence of silicotungstate are studied by O₂-evolution and fluorescence-quenching techniques. Results show that silicotungstate lowers the fluorescence quantum yield of closed units, without affecting electron-transport activities of open units. Open and closed units are units with oxidized and reduced primary electron acceptor (Q), respectively.     

Cultivar-level genotype differences influence diversity and composition of lettuce (Lactuca sp.) phyllosphere fungal communities

Different lettuce genotypes supported significantly different phyllosphere fungal communities. Phyllosphere fungal diversity was low and fungi fell into five similarity groups. These groups were represented in significantly different proportions throughout 26 lettuce accessions indicating cultivar-level variation in the fungal colonization of the lettuce phyllosphere. Significant differences in the proportions of the two dominant groups (with similarity to Cladosporium spp. and Sporobolomyces roseus) were identified between parental lines of two lettuce mapping populations providing opportunities to further investigate the genetic control of cultivar-level variation in fungal phyllosphere colonisation.     

Site-specific cleavage of double-strand DNA by hydroperoxide of linoleic acid

The breakage of double-strand (ds) DNA by 13-L-hydroperoxy-cis-9,trans-11-octadecadienoic acid (LAHPO) was investigated by agarose gel electrophoresis of supercoiled pBR322 DNA and the site of cleavage on the DNA molecule was determined by the method of DNA sequence analysis using 3'-end and 5'-end-labeled DNA fragments as substrates. LAHPO caused cleavage at the position of guanine nucleotide in dsDNA. LAHPO caused dsDNA breaks at specific sites, but linoleic acid (LA) and 13-L-hydroxy-cis-9,trans-11-octadecadienoic acid (LAHO) have no such effects on dsDNA. The active oxygen atom of the hydroperoxy group of LAHPO was perhaps responsible for the site-specific cleavage of dsDNA.     

Effects of Low pH on the Induction of Root Hair Formation in Young Lettuce (Lactuca sativa L. cv. Grand Rapids) Seedlings

Received 13 August 1999/ Accepted in revised form 22 December 1999