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Chenping Xu and Beiquan Mou

Chitosan has become of interest as a crop biostimulant suitable for use in sustainable agriculture since it is biocompatible, biodegradable, environmentally friendly, and readily available in large quantity. Short-term (35 d after transplanting) effects of chitosan, applied as a soil amendment at 0%, 0.05%, 0.10%, 0.15%, 0.20%, or 0.30% (w/w), on lettuce (Lactuca sativa) growth, chlorophyll fluorescence, and gas exchange were evaluated in a growth chamber study. Chitosan at 0.05%, 0.10%, and 0.15% increased leaf area from 674 to 856, 847, and 856 cm2, and leaf fresh weight from 28.6 to 39.4, 39.1, and 39.8 g, respectively. Only chitosan at 0.05% and 0.10% increased leaf dry weight from 3.42 to 4.37 and 4.35 g, respectively, while chitosan at 0.30% decreased leaf number, area, fresh and dry weight. Chitosan at 0.10%, 0.15%, 0.20%, and 0.30% increased leaf chlorophyll index from 29.8 to 34.4, 35.4, 37.5, and 41.4, respectively. Chitosan at 0.20% and 0.30% increased leaf maximum photochemical efficiency and photochemical yield, and chitosan at 0.10%, 0.15% 0.20%, and 0.30% increased leaf electron transport rate. Leaf photosynthesis rate and stomatal conductance (g S) increased from 9.3 to 12.7, 14.0, and 16.6 μmol·m−2·s−1 carbon dioxide, and from 0.134 to 0.183, 0.196, and 0.231 mol·m−2·s−1, under chitosan at 0.15%, 0.20%, and 0.30%, respectively. The results indicated that chitosan, at appropriate application rates, enhanced lettuce growth, and might have potential to be used for sustainable production of lettuce.

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Chenping Xu and Beiquan Mou

Salinity and nutrient-depleted soil are major constraints to crop production, especially for vegetable crops. The effects of salinity and nutrient deficiency on spinach (Spinacia oleracea L.) were evaluated in sand cultures under greenhouse conditions. Plants were watered every day with Hoagland nutrition solution, deprived of nitrogen (N), phosphorous (P), or potassium (K) for nutrient deficiency, either with or without 20/10 mm sodium chloride (NaCl)/calcium chloride (CaCl2) for salinity treatment. Salinity significantly decreased shoot fresh weight (FW) and dry weight (DW), leaf relative water content (RWC), and specific leaf area (SLA) relative to controls after 4 weeks of treatment and increased chlorophyll content, maximum photochemical efficiency (Fv/Fm), and photochemical yield [Y(II)]. Nitrogen deficiency greatly reduced shoot FW and DW, SLA, and chlorophyll content, regardless of salt treatment. Y(II) and Fv/Fm were reduced by N deficiency and salinity treatment. Phosphorous and K deficiencies, similarly, decreased shoot FW and DW irrespective of salinity treatment and increased chlorophyll content without salt stress. Phosphorous deficiency increased Y(II) under control and Fv/Fm under both control and salt treatment. Salinity and nutrient deficiency also affected the nutritional value of spinach. Salt stress increased carotenoid and flavonoid contents, and reducing power in full-strength Hoagland solution, and decreased leaf ferrous ion chelating ability (FICA). Nutrient deficiency increased reducing power regardless of salinity treatment. Nitrogen deficiency increased anthocyanin and total phenolic contents, decreased carotenoids and flavonoids regardless of salinity treatment, and increased antioxidant capacity under no-salt conditions. Phosphorous deficiency increased carotenoid and flavonoid contents under no-salt condition and enhanced total phenolic content and reduced FICA and amino acid content under salt stress. Potassium deficiency increased total phenolic, carotenoid, and flavonoid contents and antioxidant capacity under non-salt condition, but decreased FICA regardless of salinity treatment. These results suggest that spinach nutritional value could be improved with only moderately or slightly reduced yield through cultural practices that impose either low fertilizer levels or slight salt stress.

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Chenping Xu and Beiquan Mou

Lettuce is one of the most commonly used salad vegetables and considered to be a relatively salt-sensitive crop. Salinity is a major constraint to crop production in all important lettuce growing regions of the United States, and the water quality problem is exacerbated by climate change. To identify salt-tolerant lettuce genotypes, 178 cultivars and germplasm accessions (56 butterhead, 39 crisphead, 35 romaine, 33 leaf, and 15 wild types) were selected from a preliminary screening of more than 3800 genotypes, and tested for salinity tolerance in sand cultures under greenhouse conditions. Plants were grown in Hoagland nutrient solution, either with or without 30/15 mm NaCl/CaCl2, and leaf fresh and dry mass (FM and DM), chlorophyll index, and maximal photochemical efficiency (Fv/Fm) were measured 4 weeks after plants were transplanted. Generally, salinity decreased lettuce shoot FM and DM, increased DM/FM ratio and chlorophyll index, and had no effect on Fv/Fm. Some lettuce varieties showed salt tolerance (less than 15% reduction in FM), such as PI 342515, PI 358020c, ‘Morgana’, ‘Amerika’ (butterhead), ‘Laura’ (crisphead), PI 289023, PI 273577, PI 278066, PI 177425 (romaine), PI 171676a, PI 177423, PI 342477, and PI 358018b (leaf). The results indicate that lettuce genotypes differ greatly in their salt sensitivity, which could be useful for growers to choose cultivars and for breeders to improve lettuce adaptation to salinity stress.

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Chenping Xu and Beiquan Mou

Compost is increasingly used in horticultural crop production as soil conditioner and fertilizer because of its contribution to agriculture sustainability. The short-term (35 days after transplanting) effects of composted cattle manure or cotton burr on growth, physiology, and phytochemical of spinach (Spinacia oleracea L.) were evaluated in a greenhouse. Composted cattle manure at 5% or 10% mix rate (5Ca or 10Ca) greatly enhanced spinach growth as indicated by increased leaf number, area, fresh and dry weights (FW and DW), shoot FW and DW, and root DW. They also increased water use efficiency (WUE) and shoot:root ratio, and improved the photochemistry of mature leaves. Chlorophyll content also increased under 10Ca treatment. Composted cotton burr also improved spinach growth but only at 10% amendments (10Co), and was less efficient than composted cattle manure. Specific leaf area (SLA) decreased and succulence increased under all compost amendment indicating that compost could improve spinach quality. All soil amendments reduced the content of total phenolic and anthocyanin, while only 10Co and 5Ca treatments decreased flavonoid content and total antioxidant capacity. The content of carotenoid and protein increased in 10Ca treatment and amino acid content increased under both 5Ca and 10Ca treatments. The results indicated that compost, especially composted cattle manure mixed at 10%, improved spinach production and quality, and with proper application rate enhanced nutritional value by increasing carotenoid, protein, and amino acid contents while having little effect on total antioxidant capacity.

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Chenping Xu and Beiquan Mou

Protein hydrolysates (PHs) are an important group of plant biostimulants that have received increasing attention in recent years because of their positive effects on crop performance and contribution to agroecological sustainability. The aim of the study was to determine the effects of fish-derived PHs on growth, chlorophyll content and fluorescence, and leaf gas exchange of lettuce (Lactuca sativa) grown in a growth chamber. Fish-derived PHs were drench applied (300 mL of 3 mL·L−1) three times at 0, 14, and 24 days after transplanting (DAT), and lettuce were evaluated 30 DAT. Application of PHs significantly increased the lettuce leaf number per plant from 22 to 28, stem diameter from 1.37 to 1.68 cm, shoot fresh and dry weight (FW and DW) from 59 to 89 g and 5.5 to 7.7 g, and root dry weight from 0.52 to 0.80 g. It also significantly increased the leaf relative water content (RWC) from 87% to 90% and succulence from 267 to 288 g·m−2 water, but had no effect on specific leaf area (SLA). PHs significantly enhanced chlorophyll content, photosynthetic rate, stomatal conductance, and transpiration rate although they did not alter chlorophyll fluorescence. Our study indicated that plant biostimulants and fertilizer PHs improved plant performance and might have potential to be used for sustainable production of lettuce.

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Chenping Xu and Beiquan Mou

The use of vermicompost to improve soil fertility and enhance crop yield has gained considerable momentum due to its contribution to agroecological sustainability. Short-term (35 days after transplanting) effects of vermicompost, applied either as a soil amendment (5% and 10%, v/v) or a drench (40 mL of vermicompost extract at 0, 14, 21, and 28 days after transplanting), on soil properties and spinach plants (Spinacia oleracea L.) were evaluated in a greenhouse. After harvesting, the amendments left high residual levels of nutrients, organic matter and carbon, and increased soil cation exchange capacity (CEC) and water-holding capacity (WHC). Drench treatment of unamended soil increased soil nutrients, CEC, and WHC. All vermicompost treatments, especially amendment at 10% rate, increased leaf number, area, fresh and dry weight (FW and DW), shoot FW and DW, root DW, and water use efficiency (WUE). Vermicompost increased leaf chlorophyll content, and photochemical efficiency, yield, and electron transport rate (ETR) of mature leaves, as well as increased leaf succulence, and carotenoid, protein, and amino acid content. Vermicompost soil amendment reduced phenolics and flavonoids, leading to lower antioxidant capacity, whereas drench treatment only decreased betacyanin content. Vermicompost improved soil fertility, prompted leaf production, delayed leaf senescence, and enhanced growth of spinach. It also favorably influenced spinach quality by increasing leaf succulence and carotenoid, protein, and amino acids content, although it, as soil amendment, reduced flavonoid content leading to low antioxidant capacity.

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Zhongchun Jiang, Chenping Xu, and Bingru Huang

Low nitrogen (N) rates are recommended for creeping bentgrass (Agrostis stolonifera) putting greens to prevent excessive shoot growth and potential nitrate leaching, but low N rates could lead to N deficiency, which induces leaf senescence. This study was conducted to examine the effects of N deficiency on two enzymes involved in organic N metabolism as well as amino acid (AA) and soluble protein (SP) contents in both young and old leaves and roots of creeping bentgrass. Creeping bentgrass plants (cv. Penncross) were grown in a nutrient solution containing either 6 mm nitrate (+N plants) or zero N (−N plants), and each of the two treatments had four replicate pots. Young leaves on upper portions of the stolons and old leaves on lower portions of the stolons were separated and sampled at 14, 21, and 28 days of treatment, and roots were sampled at 28 days. Nitrogen deficiency increased glutamine synthetase (GS) transferase activity in all three tissues and at all three dates and GS biosynthetic activity in young leaves at all three dates. Prolonged N deficiency at 21 and 28 days increased glutamate dehydrogenase (GDH) deamination and amination activities in old leaves. In the roots, N deficiency at 28 days increased GS transferase activity but decreased GDH deamination activity. The N deficiency decreased AA content in all three tissues and at all three dates and SP content in young leaves at all three dates and in old leaves at 21 and 28 days. Decreasing organic N reserves in AA and SP and increasing GS and GDH activities in senescing leaves may be adaptive responses to N deficiency.

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Chenping Xu, Zhongchun Jiang, and Bingru Huang

Nitrogen (N) deficiency inhibits plant growth and induces leaf senescence through regulating various metabolic processes. The objectives of this study were to examine protein changes in response to N deficiency in immature and mature leaves of a perennial grass species and determine major metabolic processes affected by N deficiency through proteomic profiling. Creeping bentgrass (Agrostis stolonifera cv. Penncross) plants were originally fertilized with a diluted 36N–2.6P–5K fertilizer. After 14 days acclimation in a growth chamber, plants were grown in a nutrient solution containing 6 mm nitrate (control) or without N (N deficiency). Immature leaves (upper first and second not yet fully expanded leaves) and mature leaves (lower fully expanded leaves) were separated at 28 days of treatment for protein analysis. Two-dimensional electrophoresis and mass spectrometry analysis were used to identify protein changes in immature and mature leaves in response to N deficiency. The abundance of many proteins in both immature and mature leaves decreased with N deficiency, including those involved in photosynthesis, photorespiration, and amino acid metabolism (hydroxypyruvate reductase, serine hydroxymethyltransferase, alanine aminotransferase, glycine decarboxylase complex, glycolate oxidase), protein protection [heat shock protein (HSP)/HSP 70, chaperonin 60 and FtsH-like protein], and RNA stability (RNA binding protein). The reduction in protein abundance under N deficiency was greater in mature leaves than in immature leaves. The abundance of small HSP and metalloendopeptidase increased under N deficiency only in immature leaves. These results suggest that N deficiency accelerated protein degradation in immature and mature leaves of creeping bentgrass, particularly those proteins associated with energy and metabolism, but to a lesser extent in immature leaves. Immature leaves were also able to accumulate proteins with chaperone functions and for N reutilization, which could protect leaves from senescence under N deficiency.

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Daniel I. Leskovar, Chenping Xu, and Shinsuke Agehara

Globe artichoke is typically grown in Mediterranean and coastal areas. Because of the high profitability as a specialty crop, demand to develop production systems optimized for other semiarid and water-limited regions is rising. Field experiments were conducted over three seasons (2008–09, 2010–11, and 2011–12) in southwest Texas to investigate plant growth, physiology, and yield of artichoke grown as an annual system. Three strategies were evaluated: planting configuration (single and double lines per bed), plasticulture (bare soil and black plastic mulch), and cultivars differing in maturity (‘Imperial Star’, early; ‘Green Globe Improved’, late). Each fall, transplants were established in the field at 2.03 m between rows and 0.90 m between plants (single line) or 4.06 m between rows and 0.90 m between plants (double line). In both cultivars, black plastic mulch enhanced plant growth (leaf number, plant height and width) and increased early yield; however, its effect on total yield and yield components was not consistent. Single line per bed significantly increased head number of jumbo and large size per plant as compared with double line in the 2009 season. Chlorophyll index was unaffected by either planting configuration or plastic mulch. Comparing cultivars, ‘Green Globe Improved’ had lower marketable yield but bigger head size than ‘Imperial Star’ in one and two seasons, respectively. Our results indicate that single line with black plastic mulch can be recommended to improve earliness and water savings as compared with the bare soil system for annual artichoke production.

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Tatsiana Espevig, Chenping Xu, Trygve S. Aamlid, Michelle DaCosta, and Bingru Huang

Cold acclimation improves freezing tolerance in various plants, including perennial grass species. The objectives of this study were to determine protein changes in crowns of velvet bentgrass (Agrostis canina) during cold acclimation in association with freezing tolerance. Treatments consisted of: 1) nonacclimated (NA) plants maintained at 18/12 °C (day/night); 2) plants acclimated at a constant 2 °C for 4 weeks with a 10-hour photoperiod [A4 (cold acclimation)]; and 3) plants acclimated at a constant 2 °C for 4 weeks with additional subzero acclimation (SZA) at a constant –2 °C for 2 weeks (A4 + SZA2). Exposing plants to A4 significantly increased freezing tolerance, but additional SZA had no further beneficial effects on freezing tolerance, as demonstrated by the lethal temperature for 50% of the test population (LT50). Thirteen protein spots with increased abundance (up-regulated) or with decreased abundance (down-regulated) during cold acclimation were identified for biological functions. Proteins up-regulated after cold acclimation (A4 or A4 + SZA2) included methionine synthase, serine hydroxymethyltransferase, aconitase, UDP-D-glucuronate decarboxylase, and putative glycine-rich protein. Cold acclimation-responsive proteins involved in amino acid metabolism, energy production, stress defense, and secondary metabolism could contribute to the improved freezing tolerance induced by cold acclimation in velvet bentgrass.