Flow visualization of artificially triggered transition in plane Poiseuille flow in a water channel by means of 10–20 μm diameter tihnium-dioxide-coated mica particles revealed some striking features of turbulent spots. Strong oblique waves were observed both at the front of the arrowhead-shaped spot as well as trailing from the rear tips. Both natural and artificially triggered transition were observed to occur for Reynolds numbers slightly greater than 1000, above which the flow became fully turbulent. The front of the spot moves with a convection speed of about two-thirds of the centreline velocity, while the rear portion moves at about $\frac{1}{3}U_{\rm cl}$. The spot expands into the flow with a spreading half-angle of about 8°. After growing to a size of some 36 times h (the channel depth) at a downstream distance x/h of about 130, the spot began to split into two spots, accompanied by strong wave activity. The spot(s) was followed visually downstream of its origin a distance x/h of about 300. These results indicate that wave propagation and breakdown play a crucial role in transition to turbulence in Poiseuille flow.

No correlation was found between cytokinin fluxes and nodule dry weight or specific nodule activity (acetylene reduction).The timing of distinct peaks in zeatin riboside and dihydrozeatin riboside fluxes during flowering or pod formation suggests that cytokinins exported from the root may function in the regulation of reproductive growth in soybean.The roles of cytokinins in plant growth and development are not fully understood, but evidence supports the hypothesis that cytokinins are produced by the root system and transported to the shoot, where they are involved in the regulation of shoot processes (3,22,27). Transport in the xylem sap of materials with cytokininlike activity (determined by bioassay) has been demonstrated in numerous species including sunflower (22), tomato (7), and lupin (8, 9). Cytokinin-like materials have also been reported in the root nodules of Viciafaba (13) nodules, they detected Z2 and a number ofits metabolites throughout the plant.If cytokinins from the root system are involved in regulating shoot growth and function, then their supply to the shoot should fluctuate over the course of development. Seasonal variations of cytokinin-like activity in root pressure exudate have been reported in a number of genera (2, 3, 7-9, 22, 24). In Perilla, there was a 5-fold increase in cytokinin-like activity in root pressure exudate after floral induction (2, 3). Conversely, the root sap of Xanthium plants which had been induced to flower contained less than onethird as much cytokinin-like activity as did the sap of vegetative plants (24).Although cytokinins were not specifically identified as the active regulating component, several studies have shown that roots or factors from the roots are required for maintaining leaf Chl content (18) and photosynthesis (4,5,18,27). In Phaseolus, foliar applications of BA delayed Chl loss and photosynthetic decline associated with senescence (1).In A. glutinosa, cytokinin-like activity in the root nodules increased prior to the onset of nitrogenase activity (acetylene reduction) and declined by the time N2-fixing capacity had been fully established (14). Likewise, Newcomb et al. demonstrated that cytokinin-like activity in pea nodules was greatest early in nodule development and decreased thereafter (20). Henson and Wheeler hypothesized that cytokinins exported from nodules affected N2-fixation through either local action in the nodule or by a remote effect on the shoot (14).Presence of cytokinins in soybean root pressure exudate has not been demonstrated. Therefore, it was not known whether or not these hormones could be involved in the regulation of soybean development. Hence, the objectives of our research were to (a) establish the presence of cytokinins and (b) describe profiles of cytokinin flux in root pressure exudate in relation to the ontogeny of soybean plants grown in the field and in growth chambers. Root pressure exudate was collected and acetylene reduction activity was measured from late in vegetative growth through maturity. Four cytokinin...

Previous investigations have shown the feasibility of increasing pod number on legumes by the application of 6-benzylaminopurine (BA) directly to the raceme. These investiptions were designed to determine what reproductive parameter was affected by cytokinin application, and if these applications were overcoming a deficiency in root-produced cytokinins during late flowering. Five individual main stem racemes on greenhouse grown soybeans (Glycine max L. Menf.) were treated with 2 millimolar BA. A single application of BA when pods appeared at 25 to 50% of the proximal floral positions resulted in a 58% increase in pod set due primarily to a 33% reduction in floral abscission. Applications of BA at later intervals also resulted in significant reductions in total abscission. When three applications of BA were imposed on the upper five nodes of field grown soybeans, total pod number and seed weight were significantly increased in this section of the canopy by 27 and 18%, respectively. Throughout the flowering period, root pressure exudate was sampled for the subsequent separation and quantification of zeatin, dihydrozeatin, zeatin riboside, dihydrozeatin riboside, and isopentenyladenine. Total cytokinin flux peaked from 0 to 9 days after flowering began, and then dropped to one-half of this level by 15 days postanthesis. The probability that a flower would initiate a pod was directly related to the concentration of total cytokinins present in the exudate when the flower opened.Yield in soybean is determined by the number of seeds per unit area, and the average weight per seed. The number of seeds per unit area is determined by the number of flowers which initiate pods that attain maturity. Soybeans produce an abundance of flowers, but shed a rather large proportion of them before the seeds begin to fill. Estimates vary, but investigators have shown that the abscission of flowers and small pods ranges from 32 to 82% of the total flowers produced (29,30

The previously reported activity of benzyladenine and selected other cytokinin analogs to increase pod set in soybean (Glycine max [L.I Merr.) was further investigated to define the structure-activity relationship and evaluate the effects of the cytokinins on yield parameters. Enhancement of pod set was found to be greatest with N-6 saturated alkyl substituted analogs, and was only weakly associated with activity in a callus growth bioassay. The response of yield parameters to increasing pod load was evaluated by applying various cytokinin analogs having a range of pod set enhancement activity. The increased pod load at the treated nodes was not compensated by a reduction in pod number on the remainder of the plant. However, there was a compensatory decrease in seed size.Overall, a significant trend to greater total seed weight per plant was associated with the increased pod number. Initial evaluations indicated that foliar applications of select cytokinins could temporarily increase pod number. However, the increases in pod number obtained with foliar treatments were too small to be of practical utility and were not maintained to maturity.In the companion paper (1), we describe the relationship of cytokinin flux to pod set in soybean, and the use of BA as an externally supplied synthetic cytokinin to alter pod set patterns. BA has previously been reported to increase pod number of soybeans under field conditions (4, 10), as well as to increase fruit set in other species (2, 7). The close association between cytokinin flux and pod set in soybean, and the ability to enhance pod set through exogenous applications of BA, make this system especially useful for studying the natural regulation of pod set.Very few cytokinin analogs have been evaluated for pod set enhancement activity, so little is known regarding the optimum chemical structure to obtain maximal response. A diversity of cytokinin analogs have been reported in the literature (9). The activity of these compounds has usually been tested in tissue or organ bioassays, such as callus growth. Whether the activity in those assays is related to the pod set enhancement activity for the same compounds is not known.In this paper we report on the results of evaluations comparing the pod set enhancing activity of naturally occurring cytokinins to that of BA. In addition, we characterized the relative activity of a number of synthetic cytokinins, and defined chemical attributes associated with high pod set enhancing activity. Finally, by utilizing several cytokinin analogs which varied in pod set en- Plants were grown in a greenhouse with a 13.5 h photoperiod maintained with supplemental metal halide lamps, which supplied a photosynthetic photon flux density of 300 ,uE m-2 s-' PAR, and temperature was regulated to 28°C day/22°C night.Seeds were inoculated with Bradyrhizobium japonicum (Nitragin, Nitragin Co., Milwaukee, WI) at the time of planting. Plants were thinned to one plant per pot at the unifoliate leaf stage (VI according to Fehr et al. [5]). All b...