Nonrecirculating hydroponic system suitable for uptake studies at very low nutrient concentrations

Seedlings of maize (Zea mays L. cv WF9 × Mo 17) growing at low water potentials in vermiculite contained greatly increased proline concentrations in the primary root growth zone. Proline levels were particularly high toward the apex, where elongation rates have been shown to be completely maintained over a wide range of water potentials. Proline concentration increased even in quite mild treatments and reached 120 millimolal in the apical millimeter of roots growing at a water potential of −1.6 megapascal. This accounted for almost half of the osmotic adjustment in this region. Increases in concentration of other amino acids and glycinebetaine were comparatively small. We have assessed the relative contributions of increased rates of proline deposition and decreased tissue volume expansion to the increases in proline concentration. Proline content profiles were combined with published growth velocity distributions to calculate net proline deposition rate profiles using the continuity equation. At low water potential, proline deposition per unit length increased by up to 10-fold in the apical region of the growth zone compared to roots at high water potential. This response accounted for most of the increase in proline concentration in this region. The results suggest that osmotic adjustment due to increased proline deposition plays an important role in the maintenance of root elongation at low water potentials.     

Nutrient-limited growth rates: quantitative benefits of stress responses and some aspects of regulation

Young sunflower plants (Helianthus annuus L.) under stress oflow nitrate or phosphate availability exhibited increases inroot: shoot ratio and in kinetic parameters for uptake. Theyshowed no significant changes in photosynthetic utilizationof either nutrient. Increases in root: shoot ratio were achievedby early and persistent suppression of shoot growth, but notroot growth. Affinity for phosphate uptake, 1/K_m(P), increasedwith phosphate stress, as did affinity for nitrate uptake, 1/K_m(N),with nitrate stress. Maximal uptake rate, V_max, for phosphateuptake increased with phosphorus stress; V_max for nitrate didnot increase with nitrogen stress. Phosphate V_max was relatedstrongly to root nutrient status. Decreases in V_max with plantage were not well explained by changes in age structure of roots.Estimated benefits of acclimatory changes in root: shoot ratioand uptake kinetics ranged up to 2-fold increases in relativegrowth rate, RGR. The relation of RGR to uptake physiology followedpredictions of functional balance moderately well, with somesystematic deviations. Analyses of RGR using growth models implyno significant growth benefit from regulating Vmax, specifically,not from down-regulating it at high nutrient availability. Quantitativebenefits of increases in root: shoot ratio and uptake parametersare predicted to be quite small under common conditions whereinnutrient concentrations are significantly depleted by uptake.The root: shoot response is estimated to confer the smallestbenefit under non-depleting conditions and the largest benefitunder depleting conditions. Even then, the absolute benefitis predicted to be small, possibly excepting the case of heterogeneoussoils. Depleting and non-depleting conditions are addressedwith very different experimental techniques. We note that atheoretical framework is lacking that spans both these cases,other than purely numerical formulations that are not readilyinterpreted. Key words: Nutrient stress, nutrient uptake, nutrient use efficiency, relative growth rate, Helianthus annuus     

Bifungites, trace fossils from Devonian-Mississippian rocks of Pennsylvania and Montana,U.S.A

The trace fossil Bifungites Desio 1940, first recognized in Late Devonian rocks of Libya and later in Algeria, is common in Late Devonian rocks of Montana, Pennsylvania, Ohio, and Michigan, and Early Mississippian rocks of Pennsylvania. Its manifestations are given for seven stratigraphic units from forty localities in Pennsylvania, Montana, and Michigan.Bifungites commonly occur on bedding planes and consist of a horizontal shaft with doubly terminating arrow- or dumb-bell-shaped projections. Less apparent are paired vertical cylindrical sediment-filled tubes connected at their base to the bi-arrow or dumb-bell shaft. The combination represents the mud or silt casting of an inverted pi-shaped, μ, infaunal tubular burrow (domichnia). Siltstone slabs have been transversely sectioned to reveal this π-shaped, double-arrow burrow pattern with undisturbed stratification between the vertical tubes.Three new ichnospecies are proposed based on markedly different size and shape of the fossil traces in three stratigraphic zones. A Late Devonian Girard Shale type is the smallest with wide, short arrows on a narrow shaft whose median overall length is 12.5 mm. The latest Devonian Sappington type has well-formed, shortly barbed arrow terminations and median length of 28 mm. An Early Mississippian Meadville type is the largest with median length of 36 mm and prominently barbed broad arrows.The Bifungites organism is unrecognized but it was probably a sedentary, soft-bodied, infaunal suspension-feeding animal inhabiting shallow nearshore marine and brackish water environments. The identified biota directly or indirectly associated with Bifungites is extremely limited to a few brachiopods, clams, and other trace fossils.     

Root system adjustments: regulation of plant nutrient uptake and growth responses to elevated CO2

Nutrients such as nitrogen (N) and phosphorus (P) often limit plant growth rate and production in natural and agricultural ecosystems. Limited availability of these nutrients is also a major factor influencing long-term plant and ecosystem responses to rising atmospheric CO₂ levels, i.e., the commonly observed short-term increase in plant biomass may not be sustained over the long-term. Therefore, it is critical to obtain a mechanistic understanding of whether elevated CO₂ can elicit compensatory adjustments such that acquisition capacity for minerals increases in concert with carbon (C) uptake. Compensatory adjustments such as increases in (a) root mycorrhizal infection, (b) root-to-shoot ratio and changes in root morphology and architecture, (c) root nutrient absorption capacity, and (d) nutrient-use efficiency can enable plants to meet an increased nutrient demand under high CO₂. Here we examine the literature to assess the extent to which these mechanisms have been shown to respond to high CO₂. The literature survey reveals no consistent pattern either in direction or magnitude of responses of these mechanisms to high CO₂. This apparent lack of a pattern may represent variations in experimental protocol and/or interspecific differences. We found that in addressing nutrient uptake responses to high CO₂ most investigators have examined these mechanisms in isolation. Because such mechanisms can potentially counterbalance one another, a more reliable prediction of elevated CO₂ responses requires experimental designs that integrate all mechanisms simultaneously. Finally, we present a functional balance (FB) model as an example of how root system adjustments and nitrogen-use efficiency can be integrated to assess growth responses to high CO₂. The FB model suggests that the mechanisms of increased N uptake highlighted here have different weights in determining overall plant responses to high CO₂. For example, while changes in root-to-shoot biomass allocation, r, have a small effect on growth, adjustments in uptake rate per unit root mass, [`(n)]\bar \nu , and photosynthetic N use efficiency, p*, have a significantly greater leverage on growth responses to elevated CO₂ except when relative growth rate (RGR) reaches its developmental limit, maximum RGR (RGR_max).     

Compensatory Roles of Nitrogen Uptake and Photosynthetic N-use Efficiency in Determining Plant Growth Response to Elevated CO2: Evaluation Using a Functional Balance Model

We used a modified functional balance (FB) model to predictgrowth response of Helianthus annuus L. to elevated CO₂. Modelpredictions were evaluated against measurements obtained twiceduring the experiment. There was a good agreement between modelpredictions of relative growth rate (RGR) responses to elevatedCO₂and observations, particularly at the second harvest. Themodel was then used to compare the relative effects of biomassallocation to roots, nitrogen (N) uptake and photosyntheticN-use efficiency (PNUE) in determining plant growth responseto elevated CO₂. The model predicted that a rather substantialincrease in biomass allocation to root growth had little effecton whole plant growth response to elevated CO₂, suggesting thatplasticity in root allocation is relatively unimportant in determininggrowth response. Average N uptake rate at elevated comparedto ambient CO₂was decreased by 21–29%. In contrast, elevatedCO₂increased PNUE by approx. 50% due to a corresponding risein the CO₂-saturation factor for carboxylation at elevated CO₂.The model predicted that the decreased N uptake rate at elevatedCO₂lowered RGR modestly, but this effect was counterbalancedby an increase in PNUE resulting in a positive CO₂effect ongrowth. Increased PNUE may also explain why in many experimentselevated CO₂enhances biomass accumulation despite a significantdrop in tissue nitrogen concentration. The formulation of theFB model as presented here successfully predicted plant growthresponses to elevated CO₂. It also proved effective in resolvingwhich plant properties had the greatest leverage on such responses.Copyright 2000 Annals of Botany Company Elevated CO₂, functional balance model, Helianthus annuus L., N uptake, photosynthetic nitrogen use efficiency, root:shoot ratio     

Concentration quenching in chlorophyll-a and relation to functional charge transferin vivo

Chlorophyll-a in ordinary solvents exhibits concentration quenching. Dimeric chlorophyll is reasonably well confirmed as the quenching species, by a critical reanalysis of available data on concentration dependence and on spectral features, in ordinary solvents, and in several analogous quenching environments. This quenching in the dimer in vitro is somewhat less firmly analyzed as due to a new fast internal conversion. Much peripheral evidence supports transient charge transfer as the cause of internal conversion. The same evidence points to a strong similarity to functional charge transfer in vivo. I suggest that inability to extract P680 may be due to its conversion to a form resembling P700 by addition of water.A number of straightforward experiments are suggested to test these proposals. In particular, it is desirable to test for the existence of a vibronic perturbation (from a highern* state) in the dimer, as an alternative to charge transfer for explaining the observed internal conversion. Such a vibronic cause would raise interesting problems for phototrap function in vivo.     

Mechanisms of competition: thermal inhibition of tree seedling growth by grass

The relative importance of thermal interference and competition for below-ground resources in the inhibition of tree seedling growth by grass was determined under field conditions. Snow gum (Eucalyptus pauciflora) seedlings were grown in bare soil or soil covered with either live grass or straw. Covering soil with straw produced thermal conditions in soil and air that were indistinguishable from those associated with live grass. In contrast, seedlings grown in bare soil experienced more rapid increase in soil temperatures during late winter and spring, less frequent and less severe frosts, and temperature maxima that more closely followed those of the atmosphere than seedlings growing in live grass or straw. After 1 year, seedlings in bare soil had four times the biomass of those grown in grass or straw. Inhibition of seedling growth by grass was attributed to alteration of the thermal environment which caused (1) seedlings to have a short growing season largely restricted to summer, (2) temporal separation in competition for resources with consumption of below-ground resources by grass in spring reducing availability of resources to support tree seedling growth in early summer, and (3) seedlings to be more subject to stress from temperature extremes. These results show that thermal interference plays a major role in interactions between plants.     

Modelling stomatal conductanceof field-grown sunflower under varying soil water content and leafenvironment: comparison of three models of stomatal response toleaf environment and coupling with an abscisic acid-based modelof stomatal response to soil drying

Stomatal conductance, g_s, responds both tothe immediate or local environment of the leaf, such as CO₂ partialpressure and irradiance, and to root‐sourced signals of water stress,particularly abscisic acid (ABA). Two models for the combined controlof g_s were formulated and tested in sunflower(Helianthus annuus). First, several empirical models weretested for the local control, demonstrating that the Ball–Berrymodel [Ball, Woodrow & Berry (in Progress in PhotosynthesisResearch Vol. 4, pp. 5.221–5.224: M. Nijhoff,Dordrecht, The Netherlands) 1987] is consistently amongthe most accurate. A problem of statistical non‐independence inthis model is shown to be minor. The model offers regularity ofparameter values among most species and, despite an oversimplicationin representing known humidity‐response mechanisms, it incorporates othersignalling loops from CO₂ and assimilation. In the firstcombined model, ABA as its concentration in xylem sap, [ABA]_xy,down‐regulates the slope, m, in the Ball–Berry modelby the factor g_fac = exp(– β[ABA]_xy).The ABA‐induced reduction in g_s decreases CO₂ assimilation andsurface humidity, thus appearing to induce the local‐control mechanismto amplify the ABA‐induced stomatal closure. In the second combinedmodel, g_s is estimated as the minimum of the local(Ball–Berry) response and the product g_fac g_s,max,with g_s,max as a maximal unstressed conductance.Both models can predict g_s from the external environmentalvariables with good accuracy (r² near 0·8 over20‐fold variations in g_s). Further analyses showthat g_s responds to humidity almost quadraticallyrather than linearly. It also responds to assimilation as a powerlaw with an exponent that is significantly less than 1. These limitations,shared by other models, suggest more research into biochemical signalling.