Limitations to photosynthesis of lettuce grown under tropical conditions: alleviation by root-zone cooling.
(17/301)
Aerial parts of lettuce plants were grown under natural tropical fluctuating ambient temperatures, but with their roots exposed to two different root-zone temperatures (RZTs): a constant 20 degrees C-RZT and a fluctuating ambient (A-) RZT from 23-40 degrees C. Plants grown at A-RZT showed lower photosynthetic CO2 assimilation (A), stomatal conductance (gs), midday leaf relative water content (RWC), and chlorophyll fluorescence ratio Fv/Fm than 20 degrees C-RZT plants on both sunny and cloudy days. Substantial midday depression of A and g(s) occurred on both sunny and cloudy days in both RZT treatments, although Fv/Fm did not vary diurnally on cloudy days. Reciprocal temperature transfer experiments investigated the occurrence and possible causes of stomatal and non-stomatal limitations of photosynthesis. For both temperature transfers, light-saturated stomatal conductance (gs sat) and photosynthetic CO2 assimilation (A(sat)) were highly correlated with each other and with midday RWC, suggesting that A was limited by water stress-mediated stomatal closure. However, prolonged growth at A-RZT reduced light- and CO2-saturated photosynthetic O2 evolution (Pmax), indicating non-stomatal limitation of photosynthesis. Tight temporal coupling of leaf nitrogen content and P(max) during both temperature transfers suggested that decreased nutrient status caused this non-stomatal limitation of photosynthesis. (+info)
Relationship between the enzymatic browning and phenylalanine ammonia-lyase activity of cut lettuce, and the prevention of browning by inhibitors of polyphenol biosynthesis.
(18/301)
Cut lettuce stored at 4 degrees C gradually turned brown on the cut section after several days of storage. Three factors for enzymatic browning, the polyphenol content, polyphenol oxidase activity, and phenylalanine ammonia-lyase (PAL) activity, were examined during the cold storage of cut lettuce. A relationship between the browning and PAL activity was apparent. We tried to prevent this browning by using the two enzyme inhibitors, 2-aminoindane-2-phosphonic acid (AIP), an inhibitor of the phenylpropanoid pathway, and glyphosate, an inhibitor of the shikimate pathway. AIP and glyphosate significantly inhibited the browning of cut lettuce. The polyphenol content and PAL activity were both reduced by the treatment with AIP. These results show that regulating the biosynthesis of polyphenols is essential to prevent the browning of cut lettuce. (+info)
Production of dwarf lettuce by overexpressing a pumpkin gibberellin 20-oxidase gene.
(19/301)
We investigated the effect of overexpressing a pumpkin gibberellin (GA) 20-oxidase gene encoding an enzyme that forms predominantly biologically inactive products on GA biosynthesis and plant morphology in transgenic lettuce (Lactuca sativa cv Vanguard) plants. Lettuce was transformed with the pumpkin GA 20-oxidase gene downstream of a strong constitutive promoter cassette (El2-35S-Omega). The transgenic plants in which the pumpkin gene was detected by polymerase chain reaction were dwarfed in the T(2) generation, whereas transformants with a normal growth phenotype did not contain the transgene. The result of Southern-blot analysis showed that the transgene was integrated as a single copy; the plants segregated three dwarfs to one normal in the T(2) generation, indicating that the transgene was stable and dominant. The endogenous levels of GA(1) and GA(4) were reduced in the dwarfs, whereas large amounts of GA(17) and GA(25), which are inactive products of the pumpkin GA 20-oxidase, accumulated in these lines. These results indicate that a functional pumpkin GA 20-oxidase is expressed in the transgenic lettuce, resulting in a diversion of the normal pathway of GA biosynthesis to inactive products. Furthermore, this technique may be useful for controlling plant stature in other agricultural and horticultural species. (+info)
Overexpression of KNAT1 in lettuce shifts leaf determinate growth to a shoot-like indeterminate growth associated with an accumulation of isopentenyl-type cytokinins.
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Leaves are specialized organs characterized by defined developmental destiny and determinate growth. The overexpression of Knotted1-like homeobox genes in different species has been shown to alter leaf shape and development, but a definite role for this class of genes remains to be established. Transgenics that overexpress Knotted1-like genes present some traits that are characteristic of altered cytokinin physiology. Here we show that lettuce (Lactuca sativa) leaves that overexpress KNAT1, an Arabidopsis kn1-like gene, acquire characteristics of indeterminate growth typical of the shoot and that this cell fate change is associated with the accumulation of specific types of cytokinins. The possibility that the phenotypic effects of KNAT1 overexpression may arise primarily from the modulation of local ratios of different cytokinins is discussed. (+info)
Effect of salicylhydroxamic acid on endosperm strength and embryo growth of Lactuca sativa L. cv Waldmann's Green seeds.
(21/301)
Salicylhydroxamic acid (SHAM) stimulated germination of photosensitive lettuce (Lactuca sativa L. cv Waldmann's Green) seeds in darkness. To determine whether SHAM acts on the embryo or the endosperm, we investigated separately effects of SHAM on growth potential of isolated embryos as well as on endosperm strength. Embryo growth potential was quantified by incubating decoated embryos in various concentrations of osmoticum and measuring subsequent radicle elongation. Growth potential of embryos isolated from seeds pretreated with 4 millimolar SHAM was equal to that of untreated controls. Rupture strength of endosperm tissue excised from seeds pretreated with SHAM was 33% less than that of controls in the micropylar region. To determine if the embryo must be in contact with the endosperm of SHAM to weaken the endosperm, some endosperms were incubated with SHAM only after dissection from seeds. Rupture strength of SHAM-treated, isolated endosperms in the micropylar region was 25% less than that of untreated controls. There was no difference in rupture strength in the cotyledonary region of endosperm isolated from seeds treated with SHAM in buffer or buffer alone. SHAM therefore stimulates germination not by enhancing embryo growth potential, but by weakening the micropylar region of the endosperm enclosing the embryo. (+info)
Water content and the conversion of phytochrome regulation of lettuce dormancy.
(22/301)
In an effort to determine which biological reactions can occur in relation to the water content of seeds, the regulation of lettuce seed dormancy by red and far red light was determined at various hydration levels. Far red light had an inhibiting effect on germination for seeds at all moisture contents from 4 to 32% water. Germination was progressively stimulated by red light as seed hydration increased from 8 to 15%, and reached a maximum at moisture contents above 18%. Red light was ineffective at moisture contents below 8%. Seeds that had been stimulated by red light and subsequently dried lost the enhanced germinability if stored at moisture contents above 8%. The contrast between the presumed photoconversion of phytochrome far red-absorbing (Pfr) to (Pr) occurring at any moisture content and the reverse reaction occurring only if the seed moisture content is greater than 8% may be explained on the basis of the existence of unstable intermediates in the Pr to Pfr conversion. Our results suggest that the initial photoreaction involved in phytochrome conversion is relatively independent of water content, while the subsequent partial reactions become increasingly facilitated as water content increases from 8 to 18%. (+info)
Ion-exchange chromatography separates activities synthesizing and degrading fructose 2,6-bisphosphate from C3 and C4 leaves but not from rat liver.
(23/301)
Fructose-6-phosphate,2-kinase and fructose-2,6-bisphosphatase were separated on the basis of charge from leaves of C3 (spinach, lettuce, and pea) and C4 (sorghum and amaranthus) plants but not from rat liver--a tissue known to contain a bifunctional enzyme with both activities. [2-32P]Fructose 2,6-bisphosphate binding experiments also suggest that the major forms of these activities reside on different proteins in leaves. (+info)
Salicylhydroxamic acid potentiates germination of 'Waldmann's Green' lettuce seed.
(24/301)
A combination of salicylhydroxamic acid (SHAM) + cyanide (CN) is known to stimulate dark germination of Lactuca sativa L. seeds. Further studies were done to characterize SHAM and CN action in stimulating dark germination of lettuce seed. Germination was stimulated slightly by either SHAM or CN, whereas when SHAM and CN were combined germination was greatly enhanced. Treatment of seeds with SHAM + CN only during the first 8 hours of hydration stimulated germination as much as did treatment for 72 hours. During the first 8 hours of incubation in SHAM + CN, potentiation (i.e. domancy-breaking) of germination occurs. SHAM alone stimulated potentiation nearly to the level of SHAM + CN but inhibited subsequent radicle elongation, thereby decreasing germination when present for 72 hours. Oxygen must be present for SHAM or SHAM + CN to potentiate dark germination. The ability of SHAM and SHAM + CN to potentiate germination is influenced by O2 concentration and the timing of chemical treatment. (+info)