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  • Author or Editor: David W. Wolfe x
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Field studies conducted in 1993 on an Eel loam soil compared the growth and yield response of direct-seeded cabbage, cucumber, snap bean, and sweet corn, and transplanted cabbage, to a compacted soil layer (>2.5 MPa penetrometer resistance) at the 15 - 30 cm depth. Direct-seeded cabbage and snap bean were most severely affected by compaction, with 50% yield losses, and much smaller cabbage head size in compacted plots. Transplanted cabbage had a 30% lower yield in compacted compared to uncompactcd plots. Early vegetative growth of cucumber was less stunted by compaction compared to snap bean and cabbage, but compaction nevertheless resulted in a 50% reduction in total cucumber yield. Compaction delayed maturity and reduced early yield of cabbage, snap bean, and cucumber. Sweet corn yield was reduced by only 10% when grown on compacted soil, and there was no delay in maturity. Sweet corn responded more negatively to compaction in a 1992 field experiment,

Greenhouse studies found a reduction in total plant biomass at 21 days after planting of 30%, 14%, 1%. and 3% for snap bean, cabbage, cucumber, and sweet corn, respectively, in pots compacted at the 10 cm depth. Sweet corn had a significantly higher proportion of root biomass in the compacted zone compared to the other crops. For all species, the growth reductions could not be attributed to reductions in leaf turgor, photosynthetic rate per unit leaf area or leaf nutrient status.

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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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Long- and short-term physiological responses of pak choi (Chinese cabbage, Brassica campestris cv. `Hypro') to elevated CO2 and light environments were evaluated in the series of growth chamber experiments. Plants were grown hydroponically (Nutrient Film Technique) at 25/18°C (day/night) temperature, a 16-h photoperiod, and at three CO2 levels (350, 700, 1400 ppm) and two light levels (200 and 400 μmol·m–2·s–1 PPFD). Relative to 350-ppm CO2 treatment, the final total plant dry mass in low light increased by 37% and 38% at 700 and 1400 ppm CO2, respectively. In high light the increase was 7% and 13% at 700 and 1400 ppm CO2, respectively. Light response curves showed a positive CO2 effect on light compensation point, a slight increase in quantum yield and increase in maximum Pn rates at elevated CO2. Carbon dioxide response curves (measured at saturating PPFD of 1600 μmol·m–2·s–1) showed no effect of growth light treatment on the CO2 compensation point, but a 20% to 30% higher maximum Pn rate at saturating CO2 in plants grown at the higher light level. Overall, the highest Pn rates and the highest plant dry mass at final harvest were found in plants grown at the 400 μmol·m–2·s–1 PPFD and 1400 ppm CO2. Relative beneficial CO2 effects, however, were the most pronounced in low light conditions.

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Cucumber (Cucumis sativus L. cv. Marketmore 80) plants were exposed to a soil water deficit and subsequently rewatered. Maximum stress intensity was -1.5 MPa midday leaf water potential compared to -0.6 to -0.8 MPa in the well watered control, eight days after withholding water. Midday stomatal conductance {ks), leaf turgor potential and water potential decreased in the stress treatment compared to the control beginning at the first sampling, two days after withholding water. The decrease in all three was approximately linear with time over the stress. Decreased leaf elongation was observed at the second sampling, three days after the initial decline in ks and five days after withholding water. At similar relative water content {RWC), osmotic potentials of the stress and control treatments were the same throughout most of the stress. Further, there was no difference in osmotic potential, at the same RWC, between the stress and control treatments 12 - 16 hours after rewatering. Split-root experiments were also conducted to examine a possible role of a non-hydraulic signal from roots in drying soil in the regulation of ks and leaf elongation in cucumber. No conclusive evidence of a signal was found despite significant decreases in soil water potential of one-half of the root system of the stress plants. However, fluctuating vapor pressure gradients (vpg) may have obscured evidence of a signal.

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Two presowing seed treatments—pregermination and osmoconditioning—were examined for their effects on emergence, seedling growth rate, and yield of ‘UC 82’ tomato (Lycopersicon esculentum mill.). Seeds presoaked in a −5 bar solution of polyethylene glycol-6000 for 7 days prior to planting and fluid-drilled pregerminated seeds had significantly faster emergence rates than the control. There was no treatment effect on total yield, but fluid-drilled seeds which had been presoaked in the osmoticum maintained a developmental advantage over controls throughout the growing season and had a significantly higher percentage of breaker-red fruit at harvest.

Open Access

Fruit of tomato (Lycopersicon esculentum Mill.) hybrids heterozygous for the alc ripening mutation stored on average 60% (3.6 days) longer at 20C than that of their normal-ripening parents. There were no detrimental effects of the alc heterozygous condition on fruit color, firmness, or size. The background into which alc was introduced also affected fruit quality and shelf life. These results indicate hybrids heterozygous for the alc ripening mutant can produce commercially acceptable fruit with significantly longer shelf life than their normal-ripening parents.

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