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  • Author or Editor: John Jifon x
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Use of plastic mulch to increase rhizosphere temperatures is a common practice in spring production of vegetable crops. However, supraoptimal soil temperatures during the fruit maturation period in early summer can impair root function and reduce produce quality. The effects of colored plastic mulch on rhizosphere temperature and `Primo' muskmelon root respiration were investigated in the field during Fall (Aug.-Nov. 2002) and Spring (Mar.-May 2003) seasons. Rhizosphere temperatures (measured at 0.1 m below the soil surface with thermo-couples) and respiration under four plastic mulches (black, silver, white, and clear), and a bare ground control were studied. The soil warming properties of the different mulches differed between Spring and Fall. Bare ground rhizosphere temperatures declined from ≈33 to 21°C in the Fall and increased from 14 to 26 °C in Spring. In both studies, black and clear plastic mulches had the highest rhizosphere warming effects (3-8 °C) compared to bare ground. In the Fall, average midday soil temperatures under the white and silver mulches were 2-3 °C cooler than the bare ground treatment. Canopy establishment was accelerated by plastic mulches in Spring but not in Fall. Root + soil respiration was positively correlated with measured rhizosphere temperatures (r = 0.69), with the highest respiration rates recorded under the clear and black plastic mulches. More than 80% of fruits from the clear plastic treatment were deformed and unmarketable. The number of marketable fruit was similar among the black, white and silver mulch treatments and significantly greater (32% in Spring & 12% in Fall) than in the bare ground treatments.

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Maximum CO2 assimilation rates (ACO2) in citrus are not realized in environments with high irradiance, high temperatures, and high leaf-to-air vapor pressure differences (D). We hypothesized that moderate shading would reduce leaf temperature and D, thereby increasing stomatal conductance (g s) and ACO2. A 61% reduction in irradiance under aluminum net shade screens reduced midday leaf temperatures by 8 °C and D by 62%. This effect was prominent on clear days when average midday air temperature and vapor pressure deficits exceeded 30 °C and 3 kPa. ACO2 and gs increased 42% and 104%, respectively, in response to shading. Although shaded leaves had higher gs, their transpiration rates were only 7% higher and not significantly different from sunlit leaves. Leaf water use efficiency (WUE) was significantly improved in shaded leaves (39%) compared to sunlit leaves due to the increase in ACO2. Early in the morning and late afternoon when irradiance and air temperatures were low, shading had no beneficial effect on ACO2 or other gas exchange characteristics. On cloudy days or when the maximum daytime temperature and atmospheric vapor pressure deficits were less than 30 °C and 2 kPa, respectively, shading had little effect on leaf gas exchange properties. The results are consistent with the hypothesis that the beneficial effect of radiation load reduction on ACO2 is related to improved stomatal conductance in response to lowered D.

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High temperature stress is a major limitation to commercial production of habanero pepper (Capsicum chinense Jacq.) in tropical and subtropical regions. The ability to sustain physiological activity under stress is an important trait for newer varieties. We evaluated leaf thermotolerance [based on the cell membrane stability (CMS) test] of three habanero pepper varieties to: 1) determine genetic variability in CMS among the genotypes studied; and 2) to assess correlations between CMS, photosynthesis and chlorophyll fluorescence [(CF), an indicator of membrane-dependent photosystem II quantum efficiency, ΦPSII]. The genotypes evaluated were TAM Mild Habanero (TMH, a recently developed mild habanero pepper) and its closely related parents (Yucatan and PI 543184). Net CO2 assimilation rate (An) of intact leaves was measured in the field and leaf samples collected and exposed to heat stress (55 °C for 20 min) in temperature-controlled water baths under dim light conditions. The CF was assessed before and after the heat treatment. The CMS was highest in PI 543184, lowest in TMH and intermediate in Yucatan. All genotypes maintained high An rates in the field (25 ± 6 μmol·m-2·s-1); however, correlations between An and CMS were weak. The Φ values were similar among the genotypes (∼0.8) under nonstress conditions, but differed significantly following stress exposure. PI 543184 had the highest post-stress ΦPSII values (0.506 ± 0.023), followed by Yucatan (0.442 ± 0.023) and TMH (0.190 ± 0.025). Observed differences in CMS and ΦPSII indicate plasticity in the response to heat stress among these genotypes.

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The widely observed reduction in photosynthetic (Pn) capacity following long-term exposure to elevated CO2 is believed to result from an imbalance in source–sink status. We hypothesized that nitrogen fixation in root nodules would provide a strong sink for photosynthate and lead to a sustained positive photosynthetic response to elevated CO2. Bean plants (Phaseolus vulgaris L., cv Redkloud) were grown in poly chambers at one of four combinations of temperature (35/21 or 26/15°C day/night), and CO2 (350 or 700 ppm). Half the plants in each chamber were inoculated with Rhizobium and fertilized with a complete nutrient solution lacking nitrogen; control plants received a similar solution with nitrogen. Total nitrogenase activity (acetylene reduction assay; 8 weeks after planting) of excised whole root systems was stimulated (up to 4-fold) by elevated CO2, but this response was only significant for 26/15°C-grown plants. Inoculated plants also accumulated more biomass (10%) than control plants. Nodule abundance and size were significantly higher in high CO2-grown plants than ambient CO2 plants, but the Pn capacity of inoculated plants was only slightly greater than that of control plants. Averaged across other treatments, high CO2-grown plants accumulated more biomass (42%) and had higher Pn rates (50%) than ambient CO2 plants. Treatment effects on leaf carbohydrate levels and Pn acclimation to CO2 were not consistent. The results suggest that the higher total nodule activity was due to increased nodule number and size in proportion with increased plant size under high CO2, rather than an increase in nitrogenase activity per nodule. It is also evident that plants with symbiotic nitrogen fixation capability can benefit from elevated CO2, even with reduced input of inorganic nitrogen.

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Average global surface temperatures are predicted to rise due to increasing atmospheric CO2 and other greenhouse gases. Attempts to predict plant response to CO2 must take into account possible temperature effects on phenology and reproductive sink capacity for carbohydrates. In this study, we investigated the effects of atmospheric CO2 partial pressure [35 Pa ambient CO2 (aCO2) vs. 70 Pa elevated CO2 (eCO2)] and temperature (26/15 vs. 35/21 °C day/night) on short- and long-term net CO2 assimilation (An) and growth of red kidney bean (Phaseolus vulgaris). During early vegetative development [14-31 days after planting (DAP)], An, and relative growth rate (RGR) at eCO2 were significantly greater at the supra-optimum (35/21 °C) than at the optimum (26/15 °C) temperature. At 24 DAP, the CO2 stimulation of An by eCO2 was 49% and 89% at optimum and supra-optimum temperature, respectively, and growth enhancement was 48% and 72% relative to plants grown at aCO2. This high temperature-induced growth enhancement was accompanied by an up-regulation of An of eCO2-grown plants. In contrast, during later reproductive stages (31-68 DAP) the eCO2 stimulation of An was significantly less at the supra-optimum than at optimum temperature. This was associated with reduced seed set, greater leaf carbohydrate accumulation, and down-regulation of An at the higher temperature. At final harvest (68 DAP), the eCO2 stimulation of total dry weight was 31% and 14% at optimum and supra-optimum temperature respectively, and eCO2 stimulation of seed dry weight was 39% and -18% at optimum and supra-optimum temperature, respectively. These data indicate substantial shifts in the response to eCO2 during different phenological stages, and suggest that impaired reproductive development at high temperature could reduce the potential for CO2 stimulation of photosynthesis and productivity in bean and possibly other heat-sensitive species.

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Effects of foliar sprays of a kaolin clay particle film (Surround WP) on leaf temperature (Tlf), net gas exchange, chlorophyll fluorescence and water relations of sun-exposed leaves on field-grown grapefruit trees (Citrus paradisi L.) were studied during Summer and Fall 2001. Trees were sprayed twice a week for 3 weeks with aqueous suspensions of kaolin (Surround) at 60 g·L-1. Physiological effects of kaolin application were most prominent around midday on warm sunny days than in mornings, evenings or cloudy days. Kaolin sprays increased leaf whiteness (62%), reduced midday leaf temperature (Tlf; ≈3 °C) and leaf to air vapor pressure differences (VPD; ≈20%) compared to water-sprayed control leaves. Midday reductions in Tlf and VPD were accompanied by increased stomatal conductance (gs) and net CO2 assimilation rates (ACO2) of kaolin sprayed leaves, suggesting that gs might have limited ACO2 in water-sprayed control leaves. Midday photoinhibition of photosynthesis was 30% lower in kaolin-sprayed leaves than in control leaves. Midday water use efficiency (WUE) of kaolin-sprayed leaves was 25% higher than that of control leaves. However, leaf transpiration and whole-tree water use were not affected by kaolin film sprays. Increased WUE was therefore, due to higher ACO2. Leaf intercellular CO2 partial pressures (Ci) were similar in control and kaolin-sprayed leaves indicating that stomatal conductance was not the major cause of reduced ACO2. These results demonstrate that kaolin sprays could potentially increase grapefruit leaf carbon uptake efficiency under high radiation and temperature stress.

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Roots impact plants’ capacity to absorb water and nutrients and thus play a vital role in tolerance to drought, salinity, and nutrient stress. In tomato (Solanum lycopersicum) breeding programs, wild tomato species have been commonly used to increase disease resistance and fruit quality and yield. However, tomato has seldom been bred for water/nutrient use efficiency or resilience to abiotic stress. Meanwhile, little knowledge of the genetic control of root traits in tomato is available. In this study, a mapping population consisting of 181 F2 progenies derived from a cross between an advanced breeding line RvT1 (S. lycopersicum) and a wild species Lche4 (Solanum cheesmaniae) was evaluated for root and shoot traits in the greenhouse. Root phenotypes were studied for the early seedling stage. Heritability estimates show that root traits are moderately or highly heritable. Root mass was highly correlated with root size (length, surface area, and volume). Shoot mass and chlorophyll content (SPAD) were moderately correlated with root mass and size. Genotyping-by-sequencing was applied to discover single nucleotide polymorphism (SNP) markers. Seven hundred and forty-two SNPs were successfully mapped, and a medium-dense linkage map was created that covered 1319.47 centimorgans (cM) with an average distance of 1.78 cM between adjacent markers. Using composite interval mapping, multiple quantitative trait loci (QTL) mapping and nonparametric mapping, 29 QTLs were identified for 12 root and shoot traits on eight chromosomes. Those QTLs of major and minor effect were involved in the differences among the F2 population. Two QTL hotspot regions associated with root mass, size, shoot mass and SPAD were identified on chromosomes 1 and 4, which was consistent with the correlation among traits. Five QTLs for shoot length and eight QTLs for SPAD were accounting for 40.01% and 55.53% of the phenotypic variation. Two QTLs were associated with 18.26% of the total variation for specific root length. The wild parent Lche4 has been characterized as a potential genetic donor of higher specific root length and might be a good parent to modify the root system of cultivated tomato.

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The Texas Agricultural Experiment Station/Texas A&M University announces the release of two new open-pollinated cultivars of long chile. The first, `TAM Ben Villalon,'(TBV) is a long green chile/Anaheim type, while the second, `TAM Valley Hot,' (TVH) is a large cayenne type. Both cultivars have complex pedigrees involving TAES potyvirus resistant germplasm developed by Ben Villalon. Consequently, they exhibit resistance to some strains of tobacco etch virus when mechanically inoculated. In addition, TBV exhibits resistance to several strains of pepper mottle virus. These new cultivars out-yielded their comparable commercial cultivars, `Sonora,' and `Mesilla', when grown with drip irrigation at Weslaco and Uvalde, Texas. TBV yielded 16,632 kg/ha of green pods, compared to 14,228 kg/ha for `Sonora.' Both cultivars had similar capsaicin concentrations of 30–40 ppm on a fresh-weight basis. TBV pods are significantly heavier than those of `Sonora' due to thicker flesh. It should be useful for the green chile processing and fresh market industries. TBV may also be dried at the red stage to produce chile powder, which is very similar in quality to that of `NM 6-4.' TVH pods are not significantly different from `Mesilla' for size or weight, but contain significantly more capsaicin (670 vs. 320 ppm) when grown at Weslaco. TVH should be well-suited to the cayenne mash industry for hot sauce production due to its high heat level. Both cultivars will be distributed through commercial seed companies after receiving approval for Plant Variety Protection Patents.

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Urea solutions, with or without non-ionic (X-77) and organosilicone (L-77) surfactant, were applied to Citrus leaves and isolated cuticles to examine adjuvant effects on urea uptake and leaf net gas exchange. When compared to X-77, L-77 exhibited superior features as a surfactant, resulting in smaller contact angles of droplets deposited on teflon slide. Both L-77 and X-77 had a strong effect on penetration rate of urea within first 20 min of experiment. Effect of L-77 on urea penetration rate decreased quickly within next 20 min, whereas the effect of X-77 was sustained over a 24-h period following application. When compared to solution of urea alone, addition of X-77 to urea resulted in significant increase of the total amount of urea that penetrated the cuticles. The effect of L-77 was smaller, although the total amount of urea that penetrated the cuticles within a 4-day period was similar for both surfactants. Solutions of either urea alone, urea+L-77 and urea+X-77, or L-77 alone, induced a negative effect on net CO2 assimilation (ACO2) for 4 to 24 h after they were sprayed onto leaves. X-77, when applied alone, had no effect on ACO2. Scanning electron microscopy revealed that 1 h after application, leaf surfaces treated with X-77 appeared to be heavily coated, as opposed to those treated with L-77, which appeared similar to untreated control leaves.

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Netted muskmelon [Cucumis melo L. (Reticulatus Group)] fruit quality (ascorbic acid, β-carotene, total free sugars, and soluble solids concentration (SSC)) is directly related to plant potassium (K) concentration during fruit growth and maturation. During reproductive development, soil K fertilization alone is often inadequate due to poor root uptake and competitive uptake inhibition from calcium and magnesium. Foliar applications of glycine-complexed K during muskmelon fruit development has been shown to improve fruit quality, however, the influence of organic-complexed K vs. an inorganic salt form has not been determined. This glasshouse study investigated the effects of two K sources: a glycine-complexed K (potassium metalosate, KM) and potassium chloride (KCl) (both containing 800 mg K/L) with or without a non-ionic surfactant (Silwet L-77) on melon quality. Orange-flesh muskmelon `Cruiser' was grown in a glasshouse and fertilized throughout the study with soil-applied N–P–K fertilizer. Starting at 3 to 5 d after fruit set, and up to 3 to 5 d before fruit maturity at full slip, entire plants were sprayed weekly, including the fruit, with KM or KCl with or without a surfactant. Fruit from plants receiving supplemental foliar K had significantly higher K concentrations in the edible middle mesocarp fruit tissue compared to control untreated fruit. Fruit from treated plants were also firmer, both externally and internally, than those from non-treated control plants. Increased fruit tissue firmness was accompanied by higher tissue pressure potentials of K treated plants vs. control. In general, K treated fruit had significantly higher SSC, total sugars, total ascorbic acid, and β-carotene than control fruit. Fall-grown fruit generally had higher SSC, total sugars, total ascorbic acid and β-carotene concentrations than spring-grown fruit regardless of K treatment. The effects of surfactant were not consistent but in general, addition of a surfactant tended to affect higher SSC and β-carotene concentrations.

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