Calcium deficiency and gibberellic acid enhance susceptibility of pumpkin and sunflower seedlings to Sclerotinia sclerotiorum infection

Pumpkin (Cucurbita pepo L.) and sunflower (Helianthus annuus L.) seedlings were grown in culture deficient in calcium and inoculated with Sclerotinia sclerotiorum (Lib.) de Bary. Three days after inoculation the seedlings showed a susceptibility to the pathogen that was inversely proportional to the calcium content of the nutrient solution. Plants treated with gibberellic acid (GA₃) became more susceptible to infection by S. sclerotiorum. Increased addition of GA₃ resulted in increases in infection both in rate and severity. Sunflower was more susceptible to the pathogen than was pumpkin. GA₃ treatment consistently decreased the calcium content in both species. The effect of calcium deficiency on infection by S. sclerotiorum was greatly amplified by GA₃ addition.     

Nitrogen Fixation,Ureide, and Nitrate Accumulation Responses to Soybean Aphid Injury in Glycine max

There is little information available about soybean aphid (Aphis glycines Matsumura) effects on the physiology and mineral nutrition of soybean (Glycine max [L.] merr.). Controlled-environment studies were conducted to measure soybean aphid infestation effects on dry weight, nitrogen (N) fixation, ureide-N, and nitrate-N concentration and accumulation. Plants grown in perlite using –N nutrient solution culture were infested at the 3rd trifoliolate (V3) stage and measured for N fixation, nodule characteristics, and ureide-N concentration at the full pod (R4) stage. When compared to uninfested control plants, aphid infestation reduced total nodule volume per plant by 34%, nodule leghemoglobin per plant by 31%, plant N fixation rate by 80% and shoot ureide-N concentration by 20%. Soil-grown plants were infested at the first trifoliolate (V1) stage and shoots were measured for dry weight, nitrate-N, and ureide-N at the full bloom (R2) stage. Infestation reduced shoot dry weight by 63%, increased nitrate-N concentration by 75%, but did not significantly affect ureide-N concentration. Because nutrient concentration is a single-point measurement that results from the integration of two dynamic processes, nutrient accumulation and dry matter production, we conclude that aphid-induced reductions in N fixation, coupled with decreased dry weight accumulation, caused shoot ureide-N concentration to remain unchanged in aphid-injured plants when compared to uninfested plants. Because nitrate-N concentration was greater in aphid-damaged shoot tissue, we further conclude that nitrate-N accumulation was less sensitive to aphid injury than dry weight accumulation.     

Comparison of iron chelates and complexes supplied as foliar sprays and in nutrient solution to correct iron chlorosis of soybean

The application of synthetic chelates is the most efficient remedy for correcting iron (Fe) chlorosis. However, chelates are usually expensive and nondegradable products. Recently, new degradable chelates have been proposed for their use as Fe fertilizers. Also, Fe complexes cheaper than synthetic chelates and derived from natural products are also used to correct Fe deficiencies. Fifteen products, including five different synthetic chelates (Fe‐EDDS, Fe‐IDHA, and three Fe‐EDTA formulations) and ten natural complexes (humates, lignosulfonates, amino acids, glycoproteins, polyamines, citrate, and gluconate), have been compared when applied at low concentration to soybean (Glycine max L.) chlorotic plants grown in hydroponics under controlled conditions. In the first experiment, Fe compounds were applied to the nutrient solution, while in the second trial, Fe was foliar‐supplied. Dry matter, Fe concentration in shoots and roots, and SPAD values were used to evaluate the effectiveness of the Fe in the different products. In the nutrient‐solution experiment, synthetic chelates provided better plant growth, Fe concentration, and SPAD values than complexes. Among the Fe complexes, transferrin generally provided good plant responses, similar to those obtained with synthetic chelates. After foliar application, the highest regreening was observed for plants treated with synthetic chelates and amino acid complexes, but the translocation to roots only occurred for Fe lignosulfonate. Fe‐EDDS and Fe‐EDTA performed in a similar way when applied in nutrient solution or as foliar sprays.     

Effect of supplying P to a portion of the soybean root system on root growth and P uptake kinetics 1

Abstract

Knowledge of the effect of supplying P to portions of the soybean (Glycine max L. Merr) root system on P influx kinetics and root growth is important in developing P fertilizer placement practices for efficient fertilizer use. The objective of this research was to determine the effect of restricting P supply to portions of the root system on plant P status, root growth, and P influx kinetics. Two solution experiments were conducted in a controlled climate chamber. Phosphorus influx kinetics were determined on 25‐day‐old soybean plants that had been grown with 100, 75, 50, 25, and 12.5% of their roots initially exposed to P. Phosphorus influx kinetics were also measured on 25‐day‐old plants that had been P‐starved for the last 1, 2, 4, and 6 days prior to the determining P influx kinetics in order to relate plant P status to P influx kinetics.

Reducing the portion of the roots supplied with P reduced P uptake. This resulted in a reduction in plant P concentration and was related to a 3.41‐fold increase in maximum P influx measured on 25‐day‐old plants. Restricting the proportion of roots supplied with P had no significant effects on the Michaelis‐Menten constant or on the concentration in solution where net influx was zero. Root growth rate of the roots in the P containing solution was not significantly different from those in the ‐P solution.

Phosphorus uptake was correlated with final root surface area exposed to P (r² = 0.88??). Starving the plants for P reduced P concentration in the shoot and root and this resulted in as much as a 1.68‐fold increase in maximum influx.