able and subtle differences due to treatments are difficult to identify as a result of the high variances (Murphy Accurate cover estimates in turfgrass research plots are often diffiet al., 1995). The line-intersect method is commonly cult to obtain because of the time involved with traditional sampling and evaluation techniques. Subjective ratings are commonly used to used for ecological studies in which the occurrence of estimate turfgrass cover, but the data can be quite variable and difficult plants or the distribution of plant types within a plot to reproduce. New technologies and software related to digital image are required (Laycock and Canaway, 1980; Kershaw, analysis (DIA) may provide an alternative method to measure turf-1973). The line-intersect method involves setting up a grass parameters more accurately and efficiently than current techgrid system over an entire plot or a quadrat within the niques. A series of studies was conducted to determine the applicabilplot and counting the number or types of plants found at ity of DIA for turfgrass cover estimates. In the first study, plots each intersection on the grid. The number of intersects containing a range (1-16) of bermudagrass [Cynodon dactylon (L.) where the desired plant material is found is then Pers.] plugs of specific diameter (15.0 cm) were established to repremultiplied by the area of each grid section and divided sent values of turfgrass cover from 0.75 to 12%, by 0.75% increments.by the total sample area for a percentage of each species. Digital images (1280 by 960 pixels) were taken with a digital camera and processed for percent green color to a software package. Estimates

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Decreased N 2 fixation in soybean (Glycine max) L. Merr. during water deficits has been associated with increases in ureides and free amino acids in plant tissues, indicating a potential feedback inhibition by these compounds in response to drought. We evaluated concentrations of ureides and amino acids in leaf and nodule tissue and the concurrent change in N 2 fixation in response to exogenous ureides and soil-water treatments for the cultivars Jackson and KS4895. Exogenous ureides applied to the soil and water-deficit treatments inhibited N 2 fixation by 85% to 90%. Mn fertilization increased the apparent catabolism of ureides in leaves and hastened the recovery of N 2 fixation following exogenous ureide application for both cultivars. Ureides and total free amino acids in leaves and nodules increased during water deficits and coincided with a decline in N 2 fixation for both cultivars. N 2 fixation recovered to 74% to 90% of control levels 2 d after rewatering drought-stressed plants, but leaf ureides and total nodule amino acids remained elevated in KS4895. Asparagine accounted for 82% of the increase in nodule amino acids relative to well-watered plants at 2 d after rewatering. These results indicate that leaf ureides and nodule asparagine do not feedback inhibit N 2 fixation. Compounds whose increase and decrease in concentration mirrored the decline and recovery of N 2 fixation included nodule ureides, nodule aspartate, and several amino acids in leaves, indicating that these are potential candidate molecules for feedback inhibition of N 2 fixation.Water-deficit stress is often a primary constraint of soybean (Glycine max) yields (Purcell and Specht, 2004). N 2 fixation in soybean is reportedly more sensitive to moderate water deficits than are many other plant processes including transpiration (Sall and Sinclair, 1991;Serraj and Sinclair, 1996) and leaf gas exchange (Durand et al., 1987;Djekoun and Planchon, 1991). It has been proposed that increased tolerance of N 2 fixation to water deficits would increase overall water-deficit tolerance in soybean.Despite much research effort in the area, the mechanisms influencing N 2 fixation response to limited soil water are not well understood. Accumulating evidence indicates that the decline in N 2 fixation during water deficits and genotypic differences in sensitivity to drought may be associated with levels of nitrogenous compounds, such as amino acids or ureides, in leaves or nodules of N 2 -fixing plants (Silsbury et al., 1986;Parsons et al., 1993;Oti-Boateng and Silsbury, 1993;Serraj et al., 1999a;Purcell et al., 2000;. How specific compounds regulate nodule activity is unclear, although several hypotheses have been proposed (Serraj et al., 1999b).The ureides, allantoin and allantoate, are the final products of N 2 fixation that are exported from soybean nodules to the shoot (McClure and Israel, 1979), where they are catabolized. While N 2 fixation declines in response to water deficits, shoot ureides often increase (de Silva et al., 1996; due to a decrease...

Glyphosate [N‐(phosphonomethyl)glycine] inhibits 5‐enolpyruvylshikimate‐3‐phosphate synthase, EC 2.5.1.19 (EPSPS), thereby blocking aromatic amino acid synthesis. While glyphosate‐tolerant (GT) soybean [Glycine max (L.) Merr.] contains resistant EPSPS, the N2‐fixing symbiont in soybean root nodules, Bradyrhizobium japonicum, does not contain a resistant enzyme, and glyphosate spray to GT soybean may interfere with the symbiotic relationship. Glyphosate‐tolerant soybean was treated with glyphosate at several different stages of development to evaluate N2 fixation, growth, and yield in a series of greenhouse, growth chamber, and field experiments. Early applications of glyphosate generally delayed N2 fixation and decreased biomass and N accumulation in the cultivar Terral TV5866RR (TV5866RR) harvested at 19 d after emergence (DAE), but plants had recovered by 40 DAE. The biomass and N content of GT soybean were also decreased by glyphosate in plants that were grown with available soil N. There were differences in sensitivity to glyphosate among GT cultivars, with biomass decreases in response to glyphosate ranging from 0 to 30% at 40 DAE for the most tolerant and sensitive cultivars that were evaluated. In growth chamber studies, N2 fixation was more sensitive to water deficits in glyphosate‐treated plants. In field studies, there was no measured effect of glyphosate on GT soybean at Fayetteville, AR where there was adequate soil water throughout the growing season. However, glyphosate tended to decrease biomass and seed yields under conditions of limited soil water at Keiser, AR.

Maize (Zea mays L.) grain yield responds greatly to plant density (D). However, the hybrid-plant density interaction usually found is not well understood. The objective of this work was to analyze responses of different maize hybrids to D considering their biomass plasticity and reproductive partitioning. Responses to D were ana-