Photosynthate and nitrogen requirements for seed production by various crops.

Seed biochemical composition was the basis for segregating 24 crops into four distinct groups. Nitrogen requirements of pulses and soybeans were so great that sustained seed growth demanded continued nitrogen translocation from vegetative tissues. This translocation must eventually induce senescence in these tissues, restrict the duration of the seed-fill period, and limit seed yield.

Soybean (Glycine max) Nodule Physical Traits Associated with Permeability Responses to Oxygen.

Nodule permeability (P) controls the amount of O2 entering the nodule and is an important determinant of N2 fixation. Modulation of water volume in the intercellular spaces of the nodule cortex was hypothesized to change the effective thickness of a diffusion barrier and account for changes in P. This hypothesis was examined by evaluating physical traits of nodules that may affect P. The first test of the hypothesis was to determine whether alterations in P may result in changing both the density and the air space content of nodules as the water content of intercellular spaces was varied. Density of nodules exposed to 21 kPa O2 increased as the time following detachment from the plant increased from 5 to 60 min. Nodules from soybean (Glycine max [L.] Merr.) plants shaded for 48 h had a lower fractional air space content than nodules from control plants. Nodule detachment and prolonged shading decreased P, and the increase in density and decrease in fractional air space content associated with decreased P in these treatments supports the proposed hypothesis. The second test of the hypothesis was to determine whether nodules released water easily in response to water potential gradients. The intrinsic capacitance of nodules determined by pressure-volume analysis was 0.29 MPa-1 and indicated that the tissue can release relatively large amounts of water from the symplast with only small changes in total nodule water potential. Estimates of the bulk modulus of elasticity ranged from 0.91 to 2.60 MPa and indicated a high degree of elasticity. It was concluded that the physical properties of nodules were consistent with P modulation by the release and uptake of intercellular water in the nodule cortex.

Osmolyte accumulation: can it really help increase crop yield under drought conditions?

Osmolyte accumulation (OA) is frequently cited as a key putative mechanism for increasing yields of crops subjected to drought conditions. The hypothesis is that OA results in a number of benefits that sustain cell and tissue activity under water-deficit conditions. It has been proposed as an effective tolerance mechanism for water deficits, which could be enhanced in crops by traditional plant breeding, marker-assisted selection or genetic engineering, to generate drought-tolerant crops. However, field studies examining the association between OA and crop yield have tended to show no consistent benefit. The few, often-cited, investigations with positive associations were obtained under severe water deficits with extremely low yields or conditions with special water-supply scenarios when much of the benefit is plant survival. Under conditions where water deficits threaten crop survival, yields are so low that even large fractional yield gains offer little practical benefit to growers. Indeed, the often-cited benefit of turgor maintenance in cells is likely to result in crop behaviour that is exactly opposite to what is beneficial to crops. The one clear mechanism identified in this review for beneficial yield responses to OA is in the maintenance of root development in order to reach water that may be available deeper in the soil profile.

Soybean genotypic difference in growth, nutrient accumulation and ultrastructure in response to manganese and iron supply in solution culture.

BACKGROUND AND AIMS: The objective of this research was to characterize the physiology and cell ultrastructure of two soybean genotypes subjected to nutrient solutions with increasing concentrations of manganese (Mn) at two contrasting iron (Fe) concentrations. Genotypes 'PI227557' and 'Biloxi' were selected based on their distinctly different capacities to accumulate Mn and Fe. * METHODS: Bradyrhizobium-inoculated plants were grown in hydroponic cultures in a greenhouse. Nutrient solutions were supplied with Mn concentrations ranging from 0.3 to 90 microm, at either 5 or 150 microm Fe as FeEDTA. * KEY RESULTS: For both genotypes and at both Fe concentrations, Mn concentrations from 6.6 to 50 microm did not affect shoot, root and nodule mass, or leaf and nodule ureide concentration. Mn concentrations of 70 and 90 microm did not result in visible toxicity symptoms, but hindered growth and nodulation of 'Biloxi'. An Mn concentration of 0.3 microm was, however, deleterious to growth and nodulation for both genotypes, and caused an accumulation of ureides in leaves and major alterations in the ultrastructure of chloroplasts, nuclei and mitochondria, regardless of the Fe concentration. In 'PI227557', there was also a proliferation of Golgi apparatus and endoplasmic reticulum in the cytoplasm of leaf cells, and nodules showed disrupted symbiosomes lacking poly-beta-hydroxybutirate grains concomitantly with a proliferation of endoplasmic reticulum as well as arrested bacterial division. At 15 microm Fe, ferritin-like crystals were formed in the lumen of chloroplasts of 'PI227557' plants. For both genotypes, there was an antagonism between the Fe and Mn concentrations in leaves, the higher values of both microelements being detected in 'PI227557'. The absence of any detectable relationship between Fe or Mn and zinc, phosphorus and copper concentrations in leaves ruled out those micronutrients as relevant for Mn and Fe nutrition in soybeans. * CONCLUSIONS: The results confirmed the greater capacity of 'PI227557' for Mn and Fe accumulation than 'Biloxi' for most nutrient treatments. Hence, 'PI227557' may be a very useful genetic resource both in developing soybean cultivars for growth on low nutrient soils and in physiological studies to understand differing approaches to nutrient accumulation in plants.

Oyxgen and temperature effects on soybean seed coat respiration rates.

Soybean (Glycine max (L.) Merr) seed coat respiration rates in response to changing O(2) concentration and temperature were examined experimentally and with a mathematical analysis. The experimental observations showed seed coat respiration rates were sensitive to O(2) concentration below 0.25 micromole O(2) cm(-3). There was a steady decline in respiration rates from the saturating O(2) concentration down to about 0 to 0.03 micromole O(2) per cubic centimeter. Seed coat respiration rates were found to change linearly with temperature between 8 and 28 degrees C. The explanation for these results was sought by examining the diffusion of O(2) into the vascular bundles of the soybean seed coat. Differential equations describing O(2) uptake in two distinct zones of the vascular bundle were solved. The outer zone was assumed to be O(2) saturated and respiration proceeded at a constant rate per unit volume. The inner zone was assumed to have respiration rates which were linearly dependent on O(2) concentration. The solution of this mathematical model showed considerable similarity with the experimental results. Respiration rates were predicted to saturate at about 0.31 micromole O(2) per cubic centimeter and to decrease curvilinearly below that concentration. While the mathematical model predicted an exponential response in respiration rate to temperature, it was found that the exponential response is difficult to distinguish from a linear response in the temperature range studied experimentally. Consequently, both the experimental and theoretical studies showed the importance of O(2) diffusion into soybean seed coat vascular bundles as a potential restriction on respiration rates. In particular, it was suggested that increases in the total length of the vascular bundles in the soybean seed coat was the major option for increasing the total respiratory capability.

Nitrogenase Activity and Nodule Gas Permeability Response to Rhizospheric NH(3) in Soybean.

This study was conducted on soybean (Glycine max L. Merr.) nodules to determine if exogenous NH(3) exerts a controlling influence over nitrogenase activity through changes in nodule gas permeability (P), and if decreasing carbohydrate availability, as a result of low-light treatment, increases the sensitivity of root nodules to NH(3). Nodulated root systems of intact plants were exposed to one of several NH(3) concentrations ranging from 0 to 821 microliters per liter for an 8-hour period. Treatments were conducted under high-light (2300 micromoles per square meter per second) or low-light (800 micromoles per square meter per second) conditions. Increasing the NH(3) concentration and length of exposure of NH(3) caused a progressive decline in acetylene reduction activity (ARA). There was generally a greater reduction in ARA under the low-light treatment compared to the high-light treatment at a particular NH(3) concentration. The NH(3) concentration necessary to decrease P was greater than that needed to decrease ARA, and there was no evidence of a causal relationship between P and ARA in response to NH(3).

Regulation of soybean nitrogen fixation in response to rhizosphere oxygen: I. Role of nodule respiration.

Nitrogen fixation (acetylene reduction) rates of nodules on intact field-grown soybean (Glycine max) subjected to altered oxygen concentration (0.06-0.4 cubic millimeter per cubic millimeter) returned to initial rates during an 8-hour transitory period. Hydroponically grown soybean plants also displayed a transitory (1-4 hours) response to changes in the rhizosphere oxygen concentration after which the fixation rates returned to those observed under ambient oxygen concentrations. It was hypothesized that soybean nodules contain a regulatory mechanism which maintains a stable oxygen concentration inside nodules at a sufficiently low concentration to allow nitrogenase to function. A possible physiological mechanism which could account for this regulation is adjustment in nodule respiration activity such that nodule oxygen concentration and nitrogen fixation are maintained at stable levels. Experiments designed to characterize the non-steady-state oxygen response and to test for the presence of nodule respiratory control are presented. Non-steady-state acetylene reduction and nodule respiration (oxygen uptake) rates measured after alterations in the external oxygen concentration indicated that the regulatory mechanism required 1 to 4 hours to completely adjust to changes in the external oxygen concentration. Steady-state nodule respiration, however, did not respond to alterations in the rhizosphere oxygen concentration. It was concluded that soybean nodules can adjust to a wide range of rhizosphere oxygen concentrations, but the mechanism which controls nitrogen fixation rates does not involve changes in the nodule respiration rate.

Regulation of Soybean Nitrogen Fixation in Response to Rhizosphere Oxygen: II. Quantification of Nodule Gas Permeability.

Nodule nitrogen fixation rates are regulated by a mechanism which is responsive to the rhizosphere oxygen concentration. In some legumes, this oxygen-sensitive mechanism appears to involve changes in the gas permeability of a diffusion barrier in the nodule cortex. In soybean evidence for such a mechanism has not been found. The purpose of this research was to make quantitative measurements of soybean nodule gas permeability to test the hypothesis that soybean nodule gas permeability is under physiological control and responsive to the rhizosphere oxygen concentration. Intact hydroponically grown soybean plants were exposed to altered rhizosphere oxygen concentrations, and the nodule gas permeability, acetylene reduction and nodule respiration rates were repeatedly assayed. After a change in the external oxygen concentration, nitrogenase activity and nodule respiration rates displayed a short-term transient response after which the values returned to rates similar to those observed under ambient oxygen conditions. In contrast to steady-state nitrogenase activity and nodule respiration, nodule gas permeability was dramatically affected by the change in oxygen concentration. Decreasing the external oxygen concentration to 0.1 cubic millimeter per cubic millimeter resulted in a mean increase in nodule gas permeability of 63%. Increasing the rhizosphere oxygen concentration resulted in decreased nodule gas permeability. These data are consistent with the hypothesis that soybean nodules are capable of regulating nitrogen fixation and nodule respiration rates in response to changes in the rhizosphere oxygen concentration and indicate that the regulatory mechanism involves physiological control of the nodule gas permeability.

Leaf ureide degradation and N(2) fixation tolerance to water deficit in soybean.

Accumulation of ureides in leaves is associated with the sensitivity of N(2) fixation in soybean to soil water deficit. Consequently, ureide degradation in leaves may be a key to increasing soybean tolerance to dry soils. Previous research indicated that allantoic acid degradation is catalysed by different enzymes in cultivars Maple Arrow and Williams. The enzyme found in Williams requires manganese as a cofactor. The first objective of this study was to determine if the two degradation pathways were associated with differences in N(2) sensitivity to soil water deficits. N(2) fixation of Williams grown on low-Mn soil was sensitive to stress, but it was relatively tolerant when grown on soil amended with Mn. N(2) fixation in Maple Arrow was relatively tolerant of soil drying regardless of the Mn treatment. The second objective of this study was to expand the study of the degradation pathway to nine additional genotypes. Based on ureide degradation in the presence and absence of Mn, these genotypes also segregated for the two degradation pathways. Those genotypes with the Mn-dependent pathway tended to have drought-sensitive N(2) fixation, but there was one exception. The genotypes not requiring Mn for ureide degradation were drought-tolerant except for one genotype. These results demonstrated the possibility for increasing N(2) fixation tolerance to soil water deficits in soybean by selection of lines with high ureide degradation rates, which were commonly associated with the Mn-independent pathway.

Ureide degradation pathways in intact soybean leaves.

Ureides dramatically accumulate in shoots of N(2)-fixing soybean (Glycine max L. Merr.) under water deficit and this accumulation is higher in cultivars that have N(2) fixation that is sensitive to water deficit. One possible explanation is that ureide accumulation is associated with a feedback inhibition of nitrogenase activity. A critical factor involved in ureide accumulation is likely to be the rate of ureide degradation in the leaves. There exists, however, a controversy concerning the pathway of allantoic acid degradation in soybean. Allantoate amidinohydrolase was reported to be the pathway of degradation in studies using the cultivar Maple Arrow and allantoate amidohydrolase was the pathway that existed in the cultivar Williams. This investigation was undertaken to resolve the existence of these two pathways. An in situ technique was developed to examine the response of ureide degradation in leaf tissue to various treatments. In addition, the response of ureide accumulation and N(2) fixation activity was measured for intact plants in response to treatments that differentially influenced the two degradation pathways. The results from these studies confirmed that Maple Arrow and Williams degraded allantoic acid by different pathways as originally reported. The existence of two degradation pathways within the soybean germplasm opens the possibility of modifying ureide degradation to minimize the influence of soil water deficits on N(2) fixation activity.

Leaf Senescence and Abscisic Acid in Leaves of Field-grown Soybean.

Leaf senescence in field-grown soybean (Merrill) as defined by the period after full expansion, was studied by measuring abscisic acid (ABA), total soluble protein, and chlorophyll in leaves through the later part of the growing season. ABA concentrations increased significantly at the end of the season when leaves had started to turn yellow, well after total soluble protein and chlorophyll had started to decline. The results indicate that events occurring before leaf yellowing are more significant in evaluating leaf senescence since the yellowing condition and rise in ABA are effects of changes in physiological activity beginning when leaves are still green.

Physical and morphological constraints on transport in nodules.

For active nodule nitrogen fixation, O(2), N(2), and carbohydrate must be transported throughout the nodule. No quantitative analysis of these transport processes in the nodules has been presented. By invoking several simplifying assumptions, a second-order differential equation for the various gradients and concentrations in the nodule was solved. Even though the nodule can only be approximated in this analysis, it indicates clearly that intercellular gas spaces must exist in nodules for adequate O(2) distribution. To preserve low O(2) concentrations and protect the nitrogenase, these gas spaces cannot be in direct contact with the ambient atmosphere. It is hypothesized that a gas barrier exists in the cortical region of the nodule to limit O(2) diffusion. This barrier would not substantially inhibit N(2) transport. Carbohydrate transport from the vascular tissue via diffusion in the liquid phase can adequately accommodate the requirements within the nodule.

Response to drought stress of nitrogen fixation (acetylene reduction) rates by field-grown soybeans.

The effects of drought stress on soybean nodule conductance and the maximum rate of acetylene reduction were studied with in situ experiments performed during two seasons and under differing field conditions. In both years drought resulted in decreased nodule conductances which could be detected as early as three days after water was withheld. The maximum rate of acetylene reduction was also decreased by drought and was highly correlated with nodule conductance (r = 0.95). Since nodule conductance is equal to the nodule surface area times the permeability, the relationship of these variables to both whole-plant and unit-nodule nitrogenase activity was explored. Drought stress resulted in a decrease in nodule gas permeability followed by decreases in nodule surface area when drought was prolonged. Under all conditions studied acetylene reduction on a unit-nodule surface area basis was highly correlated with nodule gas permeability (r = 0.92). A short-term oxygen enrichment study demonstrated nodule gas permeability may limit oxygen flux into both drought-stressed and well-watered nodules of these field-grown soybeans.

Soybean seed growth in response to long-term exposures to differing oxygen partial pressures.

Short-term studies have indicated that alterations in the oxygen partial pressure (pO(2)) around developing soybean (Glycine max [L.] Merr.) seeds may alter seed growth characteristics. A 2-year field study was undertaken to determine the effects on seed development of long-term exposures of individual pods to either sub-ambient or supra-ambient pO(2). Pod chambers were used through which fixed pO(2) were continuously flowed throughout seed development. No effects on maturity date were observed from exposures to either sub-ambient or supra-ambient pO(2). On the other hand, seed weight was reduced by 0.10 pO(2) in both years of the study implicating an O(2) limitation on seed growth rate at this fairly high pO(2). In 1 of the 2 years, supra-ambient pO(2) resulted in increased seed weight.

Short photoperiod inhibits winter growth of subtropical grasses.

Grass development is influenced by length of photoperiod, but no direct measurements under natural conditions exist on mass accumulation in response to photoperiod by subtropical grass species. Grasslands of the subtropics are a major resource, but their growth is inhibited substantially during the short-photoperiod months. This research was designed to examine the consequences on grass production under field conditions when the limitation of short photoperiod is artificially removed. Lights, which extended the daylength to 15 h, were placed over plots of four subtropical forage grasses representing three species (Paspalum notatum Flugge; Cynodon dactylon L.; Cynodon nlemfuensis Vanderyst) to measure their mass accumulation in response to extended photoperiod in a 2-year experiment. Forage yields in all grasses at 5-week harvests during the time of shortest daylength were increased up to 6.2-fold more than the yield under the natural daylength. For the 4.5-month period of shortest daylength in each year, forage yields were increased for all grasses with one grass having a yield increase of 3.6-fold under the extended photoperiod as compared to natural daylength. These results demonstrated that selection of grasses that are insensitive to photoperiod could substantially increase forage yield of subtropical grasslands to benefit animal production and enhance carbon sequestration.

Water relations of turgor recovery and restiffening of wilted cabbage leaves in the absence of water uptake.

A novel phenomenon in which wilted cabbage leaves appeared to regain positive turgor pressures without additional water uptake has been previously reported (J Levitt [1986] Plant Physiol 82: 147-153). These experiments were replicated and the biophysical nature of turgor recovery characterized. Leaf water potential and its components were assayed in hydrated, wilted, and desiccated leaves which appeared to regain turgor after wilting. The hypotheses that turgor recovery was due to an increased volumetric elastic modulus (epsilon), or alternatively the result of solute redistribution were tested. Quantitative evidence that turgor recovery occurs in excised leaves was found. Leaf turgor pressure in hydrated leaves ( approximately 0.6 megapascal) decreased to zero upon wilting. After continued desiccation, turgor pressure returned to approximately 0.3 megapascal even though leaf relative water content declined. The epsilon of hydrated leaves was large and there was no evidence of an increased epsilon in the turgor-recovered leaves. Solute mobilization occurred during desiccation. The apoplastic osmotic potential decreased from -0.15 to -0.44 megapascal in hydrated and turgor-recovered leaves, respectively, and solutes were transported from the lamina to the midrib tissue. Solute redistribution coupled with the high epsilon may have resulted in localized turgor recovery in specific cells in the desiccated leaves.

Analysis of acetylene reduction rates of soybean nodules at low acetylene concentrations.

It has been previously proposed that acetylene reduction data at subsaturating acetylene concentrations could be interpreted by use of the Michaelis-Menten equation, based on the acetylene concentration external to the nodules. One difficulty of this view is that the assumption that the system is not diffusion limited is violated when studying intact nodules. The presence of a gas diffusion barrier in the nodule cortex leads to an alternate expression for the gas exchange rates at subsaturating gas concentrations. A theoretical comparison of the ;apparent' Michaelis-Menten model and diffusion model illustrated the difficulties observed in the former model of overestimating the Michaelis-Menten coefficient and yielding a correlation between the Michaelis-Menten coefficient and the maximum rate. On the other hand, use of a diffusion model resulted in (a) estimates of the Michaelis-Menten coefficient consistent with enzyme studies, (b) stability of the estimates of the Michaelis-Menten coefficient independent of treatment, and (c) a sensitivity of the diffusion barrier conductance to plant drought stress. It was concluded that all studies of nodule gas exchange need to consider possible effects caused by the presence of a diffusion barrier.

Water Use Efficiency of Field-grown Maize during Moisture Stress.

Theoretical analysis of the CO(2) assimilation and water loss by single leaves suggests that the water use efficiency of C(4) species decreases as stomatal resistance increases. To confirm this hypothesis for a complete maize crop, results from computer simulations and a field experiment were compiled for varying stomatal resistances. A soil-plant-atmosphere model allowed simulations of the many simultaneous interactions between a crop canopy and its environment. The simulations for varying stomatal resistances clearly indicated that as stomatal resistance increased, water use efficiency of the maize crop decreased. The field experiment data also confirmed that water use efficiency was significantly decreased under water stress conditions when stomatal resistance increased. We concluded that management practices for maize, which induce moisture stress conditions resulting in increased stomatal resistance, reduce both crop photosynthetic productivity and water use efficiency.

Spermatic vein embolization with hot contrast material: fertility results.

Spermatic venography with hot contrast material embolization was undertaken in 81 patients with varicoceles and infertility. Long-term follow-up information was available in 91% of the patients, and there was an overall conception rate of 40.5%. Embolization with hot contrast material was easily performed without special embolization devices and proved to be a safe and effective technique.