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Robert P. Sabba and Bill B. Dean

Potato tubers (Solanum tuberosum) of genotypes that vary in resistance to dark pigment formation when damaged, characteristic of the physiological disorder blackspot, were assayed for free tyrosine. The tubers were also assayed for relative levels of chorismate mutase and proteinase activities, which can regulate free tyrosine levels. The susceptibility of potato tubers to blackspot was shown to be correlated to the amount of free tyrosine by third order regression (R = 0.88). Tyrosine was found to be a limiting factor in pigment development. Chorismate mutase activity (CMI and CMII) was not correlated to blackspot susceptibility of the genotypes studied. Proteinase activities of Atlantic, TXA 763-5, Ranger Russet, Russet Burbank, and Lemhi Russet tuber protein extracts measured with synthetic substrates correlated with blackspot susceptibility. This suggests that the high free tyrosine levels associated with blackspot susceptibility may be due to high levels of proteinase activity in the tuber, rather than tyrosine synthesis.

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Oded Sagee and Carol J. Lovatt

Maximum leaf NH3-NH4 + content and activity of the de novo arginine biosynthetic pathway occurred during the 1st week after transfer of 5-year-old rooted cuttings of the `Washington' navel orange (Citrus sinensis L. Osbeck) from 8 weeks of low-temperature stress [8-hour days (500 μmol·s-1·m-2) at 15 to 18C/16-hour nights at 10 to 13C]. Both aspects declined in parallel during the subsequent 4 weeks of 12-hour days (500 μmol·s-1·m-2) at 24 C/12-hour nights at 19C, which culminated in maximum bloom. Apical flowers of inflorescences initiated in response to 8 weeks of low-temperature stress exhibited maximum tissue concentrations of NH3-NH4 + and putrescine, and maximum activity of the de novo arginine biosynthetic pathway 1 week after transfer of the trees from the low-temperature induction to the higher temperature (flower buds were 7 × 5 mm, length/width). All three criteria decreased in parallel as flowers developed through Stage V (petal fall). In contrast, spermine concentration increased 7-fold during Stage IV of flower development (flower opening). By Stage V, ovaries contained about equal concentrations of putrescine, spermidine, and spermine. The activity of the de novo tyrosine biosynthetic pathway exhibited a pattern of change independent of flower NH3-NH4 + concentration. Observed changes were not due to increased organ weight or size and persisted when the data were expressed per milligram protein. The results of this study demonstrate that leaves and floral buds undergo parallel changes in N metabolism in response to low-temperature, stress-induced flowering and provide evidence that flower NH3-NH4 + content and putrescine synthesis via argine are metabolically correlated during flower development in C. sinensis.

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Matthew A. Escobar, Andrew Shilling, Pine Higgins, Sandra L. Uratsu, and Abhaya M. Dandekar

protein (10 to 100 μg) was separated on a 10% polyacrylamide gel lacking SDS (a native gel). After electrophoresis, the gel was incubated in a staining solution containing 500 mM NaPO 4 (pH 7.0), 3.3 mM L-tyrosine (PPO substrate), and 0.16 mM CuSO 4 for

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Weiting Huang and Zhongming Fang

, tryptophan, or methionine), polar neutral amino acids (serine, threonine, glutamine, tyrosine, asparagine, or cysteine), polar acidic amino acids (glutamate or aspartate), or polar basic amino acids (histidine, arginine, or lysine). Media were also

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Stephen L. Love, Asunta Thompson-Johns, and Timothy P. Baker

Eight hundred and fifty-three clones of Russet Burbank and 1012 clones of Lemhi Russet were obtained from Native Plants, Inc. in 1988. The clones were produced via a tissue culture system designed to produce somoclonal variants. Four cycles of selection were completed from 1988-1991. Selection was based on resistance to blackspot bruise, a tuber flesh discoloration caused by condensation of free tyrosine; or the ability to produce light french fry color following cold storage. At the end of the four selection cycles all but six Russet Burbank clones and seven Lemhi Russet clones were eliminated. ANOVA across years was completed for the eleven somaclonal variants and Russet Burbank and Lemhi Russet checks.

Of the Russet Burbank clones, three were significantly (p = .05) more resistant to blackspot bruise and one had significantly better fry color after cold storage. All four clones had significantly reduced yield in comparison to the check clones. Of the Lemhi Russet clones, three were significantly more resistant to blackspot bruise, and four had significantly better fry color than the check clone. Only one of the seven clones (one with superior fry color designated L1908) did not show a significantly lower yield potential.

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Galen Peiser, Gloria López-Gálvez, Marita Cantwell, and Mikal E. Saltveit

Russet spotting is a physiological disorder of lettuce (Lactuca sativa L.) caused by exposure to hormonal levels (<1 μL·L-1) of ethylene in air at ≈5 °C. Enhanced phenolic metabolism and the accumulation of phenolic compounds accompany the appearance of brown, oval lesions on the leaf midrib. Phenylalanine ammonia-lyase (PAL) is the first committed enzyme in the phenylpropanoid pathway. Three inhibitors of PAL activity [2-aminoindan-2-phosphonic acid (AIP), α-aminooxyacetic acid (AOA), and α-aminooxi-β-phenylpropionic acid (AOPP)] greatly reduced the accumulation of phenolic compounds and browning of lesions. At a concentration of 50 μm, AIP inhibited the formation of chlorogenic and dicaffeoyl tartaric acids in cut midribs of iceberg lettuce by 92% and 98%, respectively. AIP competitively inhibited PAL activity from a lettuce midrib homogenate with an apparent Ki of 22 nm. While the formation of phenolic compounds was strongly inhibited by AIP, the number of lesions associated with russet spotting was not affected. Only the color of the lesions was affected by AIP. In control midribs the russet spotting lesions were brown while those in the AIP-treated midribs were initially olive green and after 3 to 7 days these lesions turned the characteristic brown color. No tyrosine ammonia-lyase activity was detected in a homogenate of lettuce midrib tissue. These results indicate that the early development of russet spotting lesions is independent of the increase in PAL activity and phenolic compounds rather than an effect of these increases as previously suggested. However, accumulation of phenolic compounds does contribute to the subsequent browning symptoms indicative of russet spotting.

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Xiaoning Li, Xiaoyan Sun, Guangyang Wang, Erick Amombo, Xiuwen Zhou, Zhaohong Du, Yinkun Zhang, Yan Xie, and Jinmin Fu

Phosphorus (P) is an essential nutrient element that is necessary for plant growth and development. However, most of the P exists in insoluble form. Aspergillus aculeatus has been reported to be able to solubilize insoluble forms of P. Here, to investigate the P-solubilizing effect of A. aculeatus on the performance of perennial ryegrass (Lolium perenne) under P-deficiency stress, we created four treatment groups: control [i.e., no Ca3(PO4)2 or A. aculeatus], A. aculeatus only (F), Ca3(PO4)2 and Ca3(PO4)2 + A. aculeatus [Ca3(PO4)2 + F] treatment, and Ca3(PO4)2 at concentrations of 0 and 3 g per pot (0.5 kg substrate per pot). In our results, the liquid medium inoculated with A. aculeatus exhibited enhanced soluble P and organic acid content (tartaric acid, citric acid, and aminoacetic acid) accompanied with lower pH, compared with the noninoculated regimen. Furthermore, A. aculeatus also played a primary role in increasing the soluble P content of substrate (1 sawdust: 3 sand), the growth rate, turf quality, and photosynthetic capacity of the plant exposed to Ca3(PO4)2 + F treatment, compared with other groups. Finally, in perennial ryegrass leaves, there was a dramatic increase in the valine, serine, tyrosine, and proline contents, and a remarkable decline in the glutamic acid, succinic acid, citric acid, and fumaric acid contents in the Ca3(PO4)2 + F regimen, compared with other groups. Overall, our results suggested that A. aculeatus may play a crucial role in the process of solubilizing Ca3(PO4)2 and modulating perennial ryegrass growth under P-deficiency stress.

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Cécile Bertin, Andy F. Senesac, Frank S. Rossi, Antonio DiTommaso, and Leslie A. Weston

of a chewing's fescue, cultivar Intrigue, revealed an amino acid analog, m -tyrosine, that is strongly phytotoxic to numerous weedy dicots and monocots encountered in turf settings ( Bertin et al., 2007 ). We speculate that differences observed in

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Jinyu Wang, Bo Yuan, Yi Xu, and Bingru Huang

amino acids (tyrosine, phenylalanine, and tryptophan) serve as precursor for numerous metabolites involved in stress defense, including auxin, melatonin, phenolic compounds, and alkaloids ( Dixon, 2001 ). Glycine is known to be the substrate for

Open access

Nihad Alsmairat, Philip Engelgau, and Randolph Beaudry

, and tyrosine) in the pulp exhibited a modest to pronounced peak on day 2, 1 d after the highest internal CO 2 concentration was recorded, reflecting the climacteric peak ( Fig. 4 ). This may simply reflect a period of elevated metabolic activity and a