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Xuan Liu and Catherine Grieve

Two statice cultivars, Limonium perezii cv. Blue Seas and L. sinuatum cv. American Beauty, were grown in greenhouse sand tanks to determine the effect of salt stress on carbohydrate accumulation and partitioning. For the first experiment, irrigation waters were prepared to simulate typical saline-sodic drainage effluent in the San Joaquin Valley of California with electrical conductivities of 2.5, 7, 11, 15, 20, 25, and 30 dS·m−1. A second experiment compared responses to two types of irrigation waters with salinity levels of 2.5, 6, 8, 10, 12, 16, and 20 dS·m−1: 1) San Joaquin Valley drainage waters, and 2) solutions mimicking concentrations of Colorado River water, a major irrigation water source for southern California. In addition to the presence of myo-inositol and three common sugars (fructose, glucose, and sucrose), chiro-inositol was for the first time isolated and identified in leaf and root tissues of both Limonium species. As salinity increased from 2.5 to 30 dS·m−1, leaf chiro-inositol concentration increased from 6.4 to 52.8 and from 2.6 to 72.9 μmol·g−1 dry weight for L. perezii and L. sinuatum, respectively, suggesting that chiro-inositol contributes substantially to osmotic adjustment in the stressed plants. Meanwhile, leaf myo-inositol concentration remained low in both species and showed little response to salinity. Before salt stress, the seedlings contained little chiro-inositol, indicating that salt enhanced chiro-inositol synthesis per unit of biomass formation. Significant (P ≤ 0.05) increasing trends for fructose and glucose and a decreasing trend for sucrose with increasing salinity were observed in the leaves of L. perezii but not L. sinuatum. As a result, the leaves of L. perezii had higher glucose and fructose but lower sucrose levels than that of L. sinuatum. However, no significant (P > 0.05) salt effect was found on the sum of the three common sugar concentrations in either species. Therefore, the accumulation of chiro-inositol resulted in a change in carbon partitioning among the soluble carbohydrates (i.e., the ratio of leaf chiro-inositol over a sum of the three common sugars rose from 0.034 to 0.29 dS·m−1 and from 0.012 to 0.32 dS·m−1 for L. perezii and L. sinuatum, respectively, as salinity increased from 2.5 to 30 dS·m−1). Salt stress did not affect starch accumulation and caused no carbon reserve deficiency. Furthermore, it was observed that salinity increased chiro-inositol phloem transport. The chiro-inositol response might be a physiological process for Limonium salt adaptation. The types of saline irrigation waters (i.e., sodium sulfate-dominated waters vs. a sodium chloride system) appear to have little effect on carbohydrate accumulation and partitioning in L. perezii.

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Clyde Wilson, Xuan Liu, Scott M. Lesch and Donald L. Suarez

Over the last several years, there has been increasing interest in amending the soil using cover crops, especially in desert agriculture. One cover crop of interest in the desert Coachella Valley of California is cowpea [Vigna unguiculata (L.) Walp.]. Cowpea is particularly useful in that as an excellent cover crop, fixing abundant amounts of nitrogen which can reduce fertilizer costs. However, soil salinity problems are of increasing concern in the Coachella Valley of California where the Colorado River water is a major source of irrigation water. Unfortunately, little information is available on the response of cowpea growth to salt stress. Thus, we investigated the growth response of 12 major cowpea cultivars (`CB5', `CB27', `CB46', `IT89KD-288', `IT93K-503-1', `Iron Clay', `Speckled Purple Hall', `UCR 134', `UCR 671', `UCR 730', `8517', and `7964') to increasing salinity levels. The experiment was set up as a standard Split Plot design. Seven salinity levels ranging from 2.6 to 20.1 dS·m–1 were constructed, based on Colorado River water salt composition, to have NaCl, CaCl2 and MgSO4 as the salinization salts. The osmotic potential ranged from –0.075 to –0.82 MPa. Salt stress began 7 days after planting by adding the salts into irrigating nutrient solution and ended after 5 consecutive days. The plants were harvested during flowering period for biomass measurement (53 days after planting). Data analysis using SAS analysis of variance indicated that the salinity in the range between 2.6 and 20.1 dS·m–1 significantly reduced leaf area, leaf dry weight, stem dry weight and root dry weight (P ≤ 0.05). We applied the data to a salt-tolerance model, log(Y) = a1 + a2X + a3X2, where Y represents biomass, a1, a2 and a3 are empirical constants, and X represents salinity, and found that the model accounted for 99%, 97%, 96%, 99%, and 96% of salt effect for cowpea shoot, leaf area, leaf dry weight, stem dry weight and root dry weight, respectively. We also found significant differences (P ≤ 0.05) of each biomass parameter among the 12 cultivars and obtained different sets of the empirical constants to quantitatively describe the response of each biomass parameter to salinity for individual cowpea cultivars. Since a significant salt × cultivar interaction effect (P ≤ 0.05) was found on leaf area and leaf dry weight, we concluded that salt tolerance differences exist among the tested cultivars.

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Xuan Liu, James Sievert, Mary Lu Arpaia and Monica A. Madore

Avocado (Persea americana Mill.) tissues contain high levels of the seven-carbon (C7) ketosugar mannoheptulose and its polyol form, perseitol. Radiolabeling of intact leaves of `Hass' avocado on `Duke 7' rootstock indicated that both perseitol and mannoheptulose are not only primary products of photosynthetic CO2 fixation but are also exported in the phloem. In cell-free extracts from mature source leaves, formation of the C7 backbone occurred by condensation of a three-carbon metabolite (dihydroxyacetone-P) with a four-carbon metabolite (erythrose-4-P) to form sedoheptulose-1,7-bis-P, followed by isomerization to a phosphorylated d-mannoheptulose derivative. A transketolase reaction was also observed which converted five-carbon metabolites (ribose-5-P and xylulose-5-P) to form the C7 metabolite, sedoheptulose-7-P, but this compound was not metabolized further to mannoheptulose. This suggests that C7 sugars are formed from the Calvin Cycle, not oxidative pentose phosphate pathway, reactions in avocado leaves. In avocado fruit, C7 sugars were present in substantial quantities and the normal ripening processes (fruit softening, ethylene production, and climacteric respiration rise), which occurs several days after the fruit is picked, did not occur until levels of C7 sugars dropped below an apparent threshold concentration of ≈20 mg·g-1 fresh weight. The effect of picking could be mimicked by girdling the fruit stalks, which resulted in ripening on the tree. Again, ripening followed a decline in C7 sugars to below an apparent threshold level. Taken together, these data indicate that the C7 sugars play important roles in carbon allocation processes in the avocado tree, including a possible novel role as phloem-mobile ripening inhibitors.

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Xuan Liu, Paul W. Robinson, Monica A. Madore, Guy W. Witney and Mary Lu Arpaia

Seasonal fluctuations in nonstructural carbohydrates (starch and soluble sugars) were studied in `Hass' avocado (Persea americana Mill.) trees on `Duke 7' rootstock over a 2-year period in southern California. On a dry weight basis, total soluble sugar (TSS) concentrations ranged from 33.0 to 236.0 mg·g-1 dry weight and were high compared to starch concentration (2.0 to 109.0 mg·g-1 dry weight) in all measured organs (stems, leaves, trunks and roots). The seven carbon (C7) sugars, D-mannoheptulose and perseitol, were the dominant soluble sugars detected. The highest starch and TSS concentrations were found in stem tissues, and in stems, a distinct seasonal fluctuation in starch and TSS concentrations was observed. This coincided with vegetative growth flushes over both sampling years. Stem TSS and starch concentrations increased beginning in autumn, with cessation of shoot growth, until midwinter, possibly due to storage of photosynthate produced during the winter photosynthetic period. TSS peaked in midwinter, while starch increased throughout the winter to a maximum level in early spring. A second peak in stem TSS was observed in midsummer following flowering and spring shoot growth. At this time, stem starch concentration also decreased to the lowest level of the year. This complementary cycling between stem TSS and starch suggests that a conversion of starch to sugars occurs to support vegetative growth and flowering, while sugars produced photosynthetically may be allocated directly to support flowering and fruit production.

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Xuan Liu, Paul W. Robinson, Monica A. Madore, Guy W. Witney and Mary Lu Arpaia

Changes in soluble sugar and starch reserves in avocado (Persea americana Mill. on `Duke 7' rootstock) fruit were followed during growth and development and during low temperature storage and ripening. During the period of rapid fruit size expansion, soluble sugars accounted for most of the increase in fruit tissue biomass (peel: 17% to 22%, flesh: 40% to 44%, seed: 32% to 41% of the dry weight). More than half of the fruit total soluble sugars (TSS) was comprised of the seven carbon (C7) heptose sugar, D-mannoheptulose, and its polyol form, perseitol, with the balance being accounted for by the more common hexose sugars, glucose and fructose. Sugar content in the flesh tissues declined sharply as oil accumulation commenced. TSS declines in the seed were accompanied by a large accumulation of starch (≈30% of the dry weight). During postharvest storage at 1 or 5 °C, TSS in peel and flesh tissues declined slowly over the storage period. Substantial decreases in TSS, and especially in the C7 sugars, was observed in peel and flesh tissues during fruit ripening. These results suggest that the C7 sugars play an important role, not only in metabolic processes associated with fruit development, but also in respiratory processes associated with postharvest physiology and fruit ripening.

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Ninghang Wang, Chao Zhang, Sainan Bian, Pengjie Chang, Lingjuan Xuan, Lijie Fan, Qin Yu, Zhigao Liu, Cuihua Gu, Shouzhou Zhang, Yaling Wang and Yamei Shen

Magnolia (Magnoliaceae) is widely cultivated for its beauty; however, despite this, the components of the different flower colors in Magnolia have not been elucidated. In this study, the color parameters of 10 Magnolia petals with different colors were measured by the Royal Horticultural Society Color Chart (RHSCC) and a color reader CR-10. The composition and content of the flavonoids in the petals were analyzed by high-performance liquid chromatography coupled with diode array detection (HPLC-DAD) as well as HPLC with electrospray ionization and mass spectrometry (HPLC-ESI-MS2). All results showed that the 10 petals were divided into four color groups. Regarding the flavonoid composition, four types of anthocyanins, including Cyanidin-glucosyl-rhamnoside (Cy-GR), Cyanidin-glucosyl-rhamnosyl-glucoside (Cy-GRG), Peonidin-glucosyl-rhamnoside (Pn-GR), and Peonidin-glucosyl-rhamnosyl-glucoside (Pn-GRG), were identified, as well as 10 types of flavonols. The flavonols included isorhamnetin, quercetin, kaempferol, and their glycosides, which included rutinoside, rhamnose, and glucoside. Cyanidin and peonidin make Magnolia petals appear red-purple and purple, respectively, and the flavonols perform as evident auxiliary pigments, particularly quercetin. The Magnolia cultivar flower phenotypes sampled in this study differed by changes in their existing flavonoid content rather than by the appearance of new flavonoids. Consequently, this study provides a reference for further revealing the basis of Magnolia flower color and provides clues for color breeding.