Martin P.N. Gent

Influence of Citric Acid Amendments on the Availability of Weathered PCBs to Plant and Earthworm Species

A series of small and large pot trials were conducted to assess the phytoextraction potential of several plant species for weathered polychlorinated biphenyls (PCBs) in soil (105 μ g/g Arochlor 1268). In addition, the effect of citric acid on PCB bioavailability to both plants and earthworms was assessed. Under small pot conditions (one plant, 400 g soil), three cucurbits (Cucurbita pepo ssp pepo [zucchini] and ssp ovifera [nonzucchini summer squash], Cucumis sativus, cucumber) accumulated up to 270 μg PCB/g in the roots and 14 μg/g in the stems, resulting in 0.10% contaminant removal from soil. Periodic 1 mM subsurface amendments of citric acid increased the stem and leaf PCB concentration by 330 and 600%, respectively, and resulted in up to a 65% increase in the total amount of contaminant removed from soil. Although citric acid at 10 mM more than doubled the amount of PCB desorbed in abiotic batch slurries, contaminant accumulation by two earthworm species (Eisenia foetida and Lumbricus terrestris) was unaffected by citric acid at 1 and 10 mM and ranged from 11–15 μg/g. Two large pot trials were conducted in which cucurbits (C. pepo ssp pepo and ssp ovifera, C. sativus) and white lupin (Lupinus albus) were grown in 70 kg of PCB-contaminated soil. White lupin was the poorest accumulator of PCBs, with approximately 20 μ g/g in the roots and 1 μ g/g in the stems. Both C. pepo ssp ovifera (summer squash) and C. sativus (cucumber) accumulated approximately 65–100 μ g/g in the roots and 6–10 μ g/g in the stems. C. pepo ssp pepo (zucchini) accumulated significantly greater levels of PCB than all other species, with 430 μg/g in the roots and 22 μ g/g in the stems. The mechanism by which C. pepo spp pepo extracts and translocates weathered PCBs is unknown, but confirms earlier findings on the phytoextraction of other weathered persistent organic pollutants such as chlordane, p,p′-DDE, and polycyclic aromatic hydrocarbons.

Phytoextraction of weathered p,p'-DDE by zucchini (Cucurbita pepo) and cucumber (Cucumis sativus) under different cultivation conditions.

ABSTRACT Previous studies have shown that zucchini (Cucurbita pepo) and cucumber (Cucumis sativus) under field conditions are good and poor accumulators, respectively, of persistent organic pollutants from soil. Here, each species was grown under three cultivation regimes: dense (five plants in 5 kg soil); nondense (one plant in 80 kg soil); and field conditions (two to three plants in approximately 789 kg soil). p,p′-DDE and inorganic element content in roots, stems, leaves, and fruit were determined. In addition, rhizosphere, near-root, and unvegetated soil fractions were analyzed for concentrations of 11 low-molecular-weight organic acids (LMWOA) and 14 water-extractable inorganic elements. Under field conditions, zucchini phytoextracted 1.3% of the weathered p,p′-DDE with 98% of the contaminant in the aerial tissues. Conversely, cucumber removed 0.09% of the p,p′-DDE under field conditions with 83% in the aerial tissues. Under dense cultivation, cucumber produced a fine and fibrous root system not observed in our previous experiments and phytoextracted 0.78% of the contaminant, whereas zucchini removed only 0.59% under similar conditions. However, cucumber roots translocated only 5.7% of the pollutant to the shoot system, while in zucchini 48% of the p,p′-DDE in the plant was present in the aerial tissue. For each species, the concentrations of LMWOA in soil increased with increasing impact by the root system both within a given cultivation regime (i.e., rhizosphere > near-root > unvegetated) and across cultivation regimes (i.e., dense > nondense > field conditions). Under dense cultivation, the rhizosphere concentrations of LMWOAs were significantly greater for cucumber than for zucchini; no species differences were evident in the other two cultivation regimes. To enable direct comparison across cultivation regimes, total in planta p,p′-DDE and inorganic elements were mass normalized or multiplied by the ratio of plant mass to soil mass. For cucumber, differences in total p,p′-DDE and inorganic element content among the cultivation regimes largely disappear upon mass normalization, indicating that greater uptake of both types of constituents in the dense condition is due to greater plant biomass per unit soil. Conversely, for zucchini the mass normalized content of p,p′-DDE and inorganic elements is up to two orders of magnitude greater under field conditions than under dense cultivation, indicating a unique physiological response of C. pepo in the field. The role of cultivation conditions and nutrient availability in controlling root morphology, organic acid exudation, and contaminant uptake is discussed.

GROWTH AND COMPOSITION OF SALAD GREENS AS AFFECTED BY ORGANIC COMPARED TO NITRATE FERTILIZER AND BY ENVIRONMENT IN HIGH TUNNELS

Form of fertilizer affects composition of salad greens. This response may vary with growth rate or environment during growth. The interaction of environment and form of fertilizer was examined for salad greens grown as successive plantings in unheated high tunnels. Eight species, including eleven plantings of lettuce and spinach, were grown with either a nitrate-based complete soluble fertilizer in perlite or with an organic fertilizer, namely leaf compost: perlite 1 : 1 v/v amended with cottonseed meal. For each species, relative growth rate and specific leaf area varied with time of year. Fertilizer had little effect on growth rate, but specific leaf area was generally greater for plants with organic fertilizer. For many species, leaf concentrations of nitrogen (N), phosphorus (P), potassium (K), and calcium (Ca) were raised by 10 to 20% with organic, compared to nitrate-based, fertilizer. How-ever, some species responded differently. For instance, organic fertilizer lowered Ca in spinach and nitrate and K in kale. The nutrient concentrations varied with time of year, but each nutrient had a different pattern. The seasonal variation was similar in magnitude to that due to fertilizer. Interaction of effects of season and fertilizer were significant only for N and P in lettuce. Salad greens grown in high tunnels had low concentrations of nitrate, and nitrate varied less with fertilizer or season than did total or reduced N. *Approved by the director of the Connecticut Agricultural Experiment Station.

A carbohydrate supply and demand model of vegetative growth: response to temperature and light

Photosynthesis is the limiting factor in crop growth models, but metabolism may also limit growth. We hypothesize that, over a wide range of temperature, growth is the minimum of the supply of carbohydrate from photosynthesis, and the demand of carbohydrate to synthesize new tissue. Biosynthetic demand limits growth at cool temperatures and increases exponentially with temperature. Photosynthesis limits growth at warm temperatures and decreases with temperature. Observations of tomato seedlings were used to calibrate a model based on this hypothesis. Model predictions were tested with published data for growth and carbohydrate content of sunflower and wheat. The model qualitatively fitted the response of growth of tomato and sunflower to both cool and warm temperatures. The transition between demand and supply limitation occurred at warmer temperatures under higher light and faster photosynthesis. Modifications were required to predict the observed non-structural carbohydrate (NSC). Some NSC was observed at warm temperatures, where demand should exceed supply. It was defined as a required reserve. Less NSC was found at cool temperatures than predicted from the difference between supply and demand. This was explained for tomato and sunflower, by feedback inhibition of NSC on photosynthesis. This inhibition was much less in winter wheat.

Persistence of triazole growth retardants on stem elongation of Rhododendron and Kalmia

Abstract. Triazole growth retardant chemicals may inhibit stem elongation of woody ornamental species for several years after application. Potted plants of large-leaf Rhododendron catawbiense and Kalmia latifolia were treated with a single spray application of paclobutrazol or uniconazole in the 2nd year from propagation. They were transplanted into the field the next spring. The elongation of stems was measured in the year of application and in the next 2–4 years. Treatments with a wide range of doses were applied in 1991, 1992, or 1995. For all except the most dilute applications, stem elongation was retarded in the year after application. At the highest doses, stem growth was inhibited for 2 years after application. The results were fit to a model of growth regulator action which assumed that stem elongation was inversely related to the amount of growth regulator applied. For paclobutrazol, the dose per plant that inhibited stem elongation half as much as a saturating dose was tenfold that for uniconazole, about 0.5 and 0.05 mg, respectively. For both chemicals, the dose-response coefficient decreased exponentially with time after application, with an exponential time constant of about 2 year−1. A dose of growth regulator which reduced stem elongation by half immediately after application would only inhibit 12% of stem elongation the next year. However, a tenfold greater dose would result in less than half the stem elongation of untreated plants in the next year.

Composition of hydroponic lettuce: effect of time of day, plant size, and season.

BACKGROUND The diurnal variation of nitrate and sugars in leafy green vegetables may vary with plant size or the ability of plants to buffer the uptake, synthesis, and use of metabolites. Bibb lettuce was grown in hydroponics in a greenhouse and sampled at 3 h intervals throughout one day in August 2007 and another day in November 2008 to determine fresh weight, dry matter, and concentration of nitrate and sugars. Plantings differing in size and age were sampled on each date. RESULTS The dry/fresh weight ratio increased during the daylight period. This increase was greater for small compared to large plants. On a fresh weight basis, tissue nitrate of small plants was only half that of larger plants. The variation in concentration with time was much less for nitrate than for soluble sugars. Soluble sugars were similar for all plant sizes early in the day, but they increased far more for small compared to large plants in the long days of summer. CONCLUSION The greatest yield on a fresh weight basis was obtained by harvesting lettuce at dawn. Although dry matter or sugar content increased later in the day, there is no commercial benefit to delaying harvest as consumers do not buy lettuce for these attributes.

Growth, Yield and Nutrition of Potato in Fumigated or Nonfumigated Soil Amended with Spent Mushroom Compost and Straw Mulch

Four cultural amendments; spent mushroom compost, straw mulch, both compost and straw mulch, or neither, were applied to soils that were either fumigated or not fumigated in a field of potatoes subject to early dying and Colorado potato beetle defoliation. Two plant samples were harvested at two week intervals to measure shoot and tuber growth and mineral nutrition, and two rows were harvested for yield at maturity. Amending the soil with compost increased vegetative growth and shoot weight more than final yield of tubers. Compost amendment delayed tuber filling by several days. Fumigation partly controlled the loss of leaf area due to early dying, but it did not increase tuber yields, and in 1994 fumigation reduced tuber yield in compost amended soils. The effects of compost and straw mulch on tuber yield were related to the concentrations of N and P in leaves. The potato crop did not benefit from compost amendment combined with fumigation, because in fumigated soil there was no improvement in plant nutr...

Effect of planting date, ventilation and soil temperature on growth and nutrition of tomato in high tunnels

Growth rates and tissue nutrient concentrations were measured in tomato (Lycopersicon esculentum Mill) grown in unheated high tunnels in the spring in the northeast USA. Two weeks after transplant on 3 April, seedlings had low concentrations of Nitrogen, Magnesium and other nutrients, while later plantings on 17 April and 1 May had adequate nutrition. The low yield and small fruit of the 3 April planting, compared to the later plantings, was likely related to this nutrient stress soon after transplant. Air and soil temperatures were less than 10°C at planting on 3 April. Air and soil were warmed during the day to different extents in tunnels vented at different temperatures. Over all plantings and ventilation regimes, relative growth rates over the two weeks after transplant were correlated to average air temperature. However, there was little uptake of P, N and Mg, when soil was cooler than 12°C. Nutrient concentrations in the shoot became very low because shoot growth continued when soil temperature limited nutrient uptake.

Paclobutrazol or uniconazol applied early in the previous season promotes flowering of field-grown Rhododendron and Kalmia

Field-grown large leaf Rhododendron and Kalmia latifolia were treated with one of three concentrations of paclobutrazol or uniconazol in April, June, or August in the second year from propagation. The elongation of stems was measured as was the number of flower buds initiated. Spray applications in April or June increased flowering at the lowest concentrations tested; 4 mg · L−1 paclobutrazol and 1.5 mg · L−1 uniconazol. Flowering was induced in cultivars that normally did not flower, and the number of flower buds per plant was increased in cultivars that normally flowered. All treatments that induced flowering also reduced stem elongation. Spray application in August failed to enhance flowering. At the concentrations tested, uniconazol was more effective than paclobutrazol in increasing the number of flower buds and reducing stem elongation of Rhododendron. For Kalmia, there was less response to the concentration of growth retardant, and the two chemicals enhanced flowering equally.

Uptake and transpiration of 1,2-dibromoethane by leaves

Tobacco and radish leaves sorbed 1,2-dibromoethane (EDB) in the vapor phase or took up EDB dissolved in water. Sorption of EDB vapor by the leaf blades was controlled primarily by diffusion resistance of the leaf stomata, but was also influenced by the nature of the leaf surface. EDB taken up from solution through the petiole was transported to the leaf blade, from where it exhibited a high escaping tendency relative to the transpiration of water. These processes may affect transport and persistence of soil fumigants.