Growth and dry-mass partitioning in tomato as affected by phosphorus nutrition and light

We studied the effects of phosphorus (P) and light on the physiological and morphological components of growth of young tomato plants (Lycopersicon esculentum Mill. cv. Capita). The importance of dry‐mass partitioning and starch accumulation in explaining the effects of P limitation on growth was examined more closely. Plants were grown at a wide range of exponential P supply rates (between 70 and 320 mg g^?1 d^?1) and one free‐access treatment (1 mm ). Two light levels (70 and 300 µmol m^?2 s^?1) were applied. Growth response coefficients (GRCs) were calculated to address the importance of different growth parameters in explaining relative growth rate (RGR). At both light levels, net assimilation rate (NAR) was more important than leaf area ratio (LAR) in explaining the effects of P on growth as indicated by GRCs. At less severe P limitation, LAR became more important and NAR less important. Dry‐mass partitioning to both roots and leaves played a minor role in determining the effects of P limitation on growth as indicated by low GRCs. The increase in starch at mild P limitation showed that the assimilate supply was not limiting. At severe P limitation, the rate of photosynthesis was decreased, as suggested by the decrease in starch accumulation.     

Control of Leaf Growth and its Role in Determining Variation in Plant Growth Rate from an Ecological Perspective

Abstract: Plants vary widely in their relative growth rate (RGR), be it dependent on environmental conditions or due to their genetic background. In a comparison of the RGR of grasses growing under different environmental conditions, variation in RGR tends to correlate with that in the leaf elongation rate (LER). When different species or genotypes thereof are compared under identical growing conditions, variation in LER may or may not correlate with that in RGR, depending on the comparison. However, since RGR is described by an exponential equation, whereas LER is mainly a linear process, we conclude that any correlation between RGR and LER must be fortuitous. That is, exponential growth must be due to increases with time in plant traits such as 1) leaf dry mass per unit leaf length invested per unit time, and/or 2), i.e., the total LER of all the growing leaves at one point in time. The latter can be achieved as follows: 1) each subsequent leaf has a higher LER than the preceding one; 2) leaves appear at an increasing rate; 3) the duration of the process of leaf elongation increases for subsequent leaves. In this review, we only explore possible factors that account for changes in with time, in different genotypes and under different environmental conditions. Inherent variation in LER of individual leaves and variation due to environmental factors may reflect variation in the rate of cell division and/or in cell elongation.     

Respiratory energy requirements and rate of protein turnover in vivo determined by the use of an inhibitor of protein synthesis and a probe to assess its effect

Protein turnover is generally regarded as a major maintenance process, but experimental evidence to support this contention is scarce. Here we quantify the component of dark respiration rate associated with overall protein turnover of tissues in vivo. The effect of an inhibitor of cytosolic protein synthesis (cycloheximide, CHM) on dark respiration was tested on a cell suspension from potato ( Solanum tuberosum L.) and quantified on leaf discs of expanding and full-grown primary leaves of bean ( Phaseolus vulgaris L.). The in vivo effect of CHM on protein biosynthesis was assessed by monitoring the inhibition of the induction of the ethylene-forming enzyme (EFE) activity. The present method yields the energy costs of turnover of the total pool of proteins irrespective of their individual turnover rates. Average turnover rates were derived from the respiratory costs and the specific costs for turnover.
Inhibition of respiration by CHM was readily detectable in growing-cell suspensions and discs of expanding leaves, The derived respiratory costs of protein turnover in expanding leaves were maximally 17–37% of total respiration. Turnover costs in full-grown primary leaves of bean amounted to 17–21% of total dark respiration. The maximum degradation constants (i.e. K_d-values) derived for growing and full-grown leaves were up to 2.42 × 10⁻⁶ and 1.12 × l0⁻⁶ s⁻¹, respectively.     

Faster accumulation and greater contribution of glomalin to the soil organic carbon pool than amino sugars do under tropical coastal forest restoration

Microbial metabolic products play a vital role in maintaining ecosystem multifunctionality, such as soil physical structure and soil organic carbon (SOC) preservation. Afforestation is an effective strategy to restore degraded land. Glomalin-related soil proteins (GRSP) and amino sugars are regarded as stable microbial-derived C, and their distribution within soil aggregates affects soil structure stability and SOC sequestration. However, the information about how afforestation affects the microbial contribution to SOC pools within aggregates is poorly understood. We assessed the accumulation and contribution of GRSP and amino sugars within soil aggregates along a restoration chronosequence (Bare land, Eucalyptus exserta plantation, native species mixed forest, and native forest) in tropical coastal terraces. Amino sugars and GRSP concentrations increased, whereas their contributions to the SOC pool decreased along the restoration chronosequence. Although microaggregates harbored greater microbial abundances, amino sugars and GRSP concentrations were not significantly affected by aggregate sizes. Interestingly, the contributions of amino sugars and GRSP to SOC pools decreased with decreasing aggregate size which might be associated with increased accumulation of plant-derived C. However, the relative change rate of GRSP was consistently greater in all restoration chronosequences than that of amino sugars. The accumulation of GRSP and amino sugars in SOC pools was closely associated with the dynamics of soil fertility and the microbial community. Our findings suggest that GRSP accumulates faster and contributes more to SOC pools during restoration than amino sugars did which was greatly affected by aggregate sizes. Afforestation substantially enhanced soil quality with native forest comprising species sequestering more SOC than the monoculture plantation did. Such information is invaluable for improving our mechanistic understanding of microbial control over SOC preservation during degraded ecosystem restoration. Our findings also show that plantations using arbuscular mycorrhizal plants can be an effective practice to sequester more soil carbon during restoration.     

Phenotypic plasticity in response to nitrate supply of an inherently fast-growing species from a fertile habitat and an inherently slow-growing species from an infertile habitat

The aim of the present study was to investigate possible differences in plasticity between a potentially fast-growing and a potentially slow-growing grass species. To this end, Holcus lanatus (L.) and Deschampsia flexuosa (L.) Trin., associated with fertile and infertile habitats, respectively, were grown in sand at eight nitrate concentrations. When plants obtained a fresh weight of approximately 5 g, biomass allocation, specific leaf area, the rate of net photosynthesis, the organic nitrogen concentration of various plant parts and the root weight at different soil depths were determined. There were linear relationships between the morphological and physiological features studied and the In-transformed nitrate concentration supplied, except for the specific leaf area and root nitrogen concentration of H. lanatus, which did not respond to the nitrate concentration. The root biomass of H. lanatus was invariably distributed over the soil layers than that of D. flexuosa. However, D. flexuosa allocated more root biomass to lower soil depths with decreasing nitrate concentration, in contrast to H. lanatus, which did not respond. The relative response to nitrate supply, i.e. the value of a character at a certain nitrate level relative to the value of that character at the highest nitrate supply, was used as a measure for plasticity. For a number of parameters (leaf area ratio, root weight ratio, root nitrogen concentration, vertical root biomass distribution and rate of net photosynthesis per unit leaf weight) the potentially slow-growing D. flexuosa exhibited a higher phenotypic plasticity than the potentially fast-growing H. lanatus. These findings are in disagreement with current literature. Possible explanations for this discrepancy are discussed in terms of differences in experimental approach as well as fundamental differences in specific traits between fast- and slow-growing grasses.     

Studies on the transposition rates of mobile genetic elements in a natural population of Drosophila melanogaster

In an isolated population of Drosophila melanogaster on Ishigaki Island the chromosomal distribution of several retrotransposons, including copia, 412, 297, 17.6, I, and jockey elements, was examined by in situ hybridization. In this population the cosmopolitan inversion, In(2L)t, is known to exist in high frequency. One major haplotype concerning the occupied sites of the transposable elements was identified in the In(2L)t-carrying chromosomes. This haplotype is suggested to be the ancestral one. The age of the inversion in this local population was estimated to be 1,400 generations. The transposition rates of these elements were estimated based on the age of the inversion and the number of the elements lost and gained. The excision rates were in the range from 9.13 x 10(-5) to 2.25 x 10(-4) per site per generation. They were similar each other in the copia-like elements as well as in the LINE-like elements. The rate was higher in the copia-like elements than in the LINE-like elements. Insertions occurred in the range from 6.79 x 10(-4) to 9.05 x 10(-4) per element per generation. It is herein shown that both insertions and excisions occurred at a significantly higher rate in this population than in the laboratory.      

Partitioning of Electrons between the Cytochrome and Alternative Pathways in Intact Roots

To test the hypothesis that the cytochrome pathway is not invariably saturated when the alternative pathway is engaged, we titrated root respiration of several species with KCN (an inhibitor of the cytochrome pathway), both in the absence and presence of an inhibitor of the alternative pathway (salicylhydroxamic acid, SHAM). The slopes of the resultant KCN [rho] plots ([rho]cyt) were then used to determine whether the cytochrome pathway was saturated in each species. The species used were Festuca ovina ssp. ovina L., Phaseolus vulgaris L., and six Poa species (Poa pratensis L., Poa compressa L., Poa trivialis L., Poa alpina L., Poa costiniana Vick., and Poa fawcettiae Vick.). Although the cytochrome pathway was saturated in a number of species (i.e. [rho]cyt values were 1.0), several others exhibited [rho]cyt values of less than 0.5. Alternative pathway capacity correlated negatively with [rho]cyt, with [rho]cyt values of less than 1.0 occurring in tissues in which the alternative pathway capacity was greater than 25 to 30% of total respiration. The species that did not show full engagement of the cytochrome pathway rarely exhibited SHAM inhibition in the absence of KCN. We conclude that this lack of SHAM inhibition is not due to a lack of alternative pathway engagement but rather to the diversion of electrons from the alternative pathway to the unsaturated cytochrome path following the addition of SHAM.     

Two soluble glycosyltransferases glycosylate less efficiently in vivo than their membrane bound counterparts

Many Golgi glycosyltransferases are type II membrane proteins which are cleaved to produce soluble forms that are released from cells. Cho and Cummings recently reported that a soluble form of alpha1, 3- galactosyltransferase was comparable to its membrane bound counterpart in its ability to galactosylate newly synthesized glycoproteins (Cho,S.K. and Cummings,R.D. (1997) J. Biol. Chem., 272, 13622-13628). To test the generality of their findings, we compared the activities of the full length and soluble forms of two such glycosyltransferases, ss1,4 N-Acetylgalactosaminyltransferase (GM2/GD2/ GA2 synthase; GalNAcT) and beta galactoside alpha2,6 sialyltransferase (alpha2,6-ST; ST6Gal I), for production of their glycoconjugate products in vivo . Unlike the full length form of GalNAcT which produced ganglioside GM2 in transfected cells, soluble GalNAcT did not produce detectable GM2 in vivo even though it possessed in vitro GalNAcT activity comparable to that of full length GalNAcT. When compared with cells expressing full length alpha2,6-ST, cells expressing a soluble form of alpha2,6-ST contained 3-fold higher alpha2,6-ST mRNA levels and secreted 7-fold greater alpha2,6-ST activity as measured in vitro , but in striking contrast contained 2- to 4-fold less of the alpha2,6-linked sialic acid moiety in cellular glycoproteins in vivo . In summary these results suggest that unlike alpha1,3-galactosyltransferase the soluble forms of these two glycosyltransferases are less efficient at glycosylation of membrane proteins and lipids in vivo than their membrane bound counterparts.      

Effect of photosynthesis and carbohydrate status on respiratory rates and the involvement of the alternative pathway in leaf respiration

In spinach (Spinacia oleracea Hybrid 102 [New World seeds]) and wheat (Triticum aestivum L. cv Gabo) leaves, O₂ uptake rates in the dark were faster after the plants had been allowed to photosynthesize for a period of several hours. Alternative path activity also increased following a period of photosynthesis in these leaves. No such effects were observed with isolated mitochondria. In spinach and wheat leaves, the level of fructose plus glucose decreased during a period of darkness. In pea (Pisum sativum cv Alaska) leaves, the level of these sugars did not vary significantly during the day, and respiratory rates were also constant. In slices cut from wheat leaves harvested at the end of the night, addition of sugars increased the rate of respiration and engaged the previously latent alternative oxidase. In pea leaves, O₂ uptake in the first few minutes following illumination was faster than that observed before illumination, but declined during the next 15 to 20 minutes. Adding the alternative oxidase inhibitor salicylhydroxamic acid, or imposing high bicarbonate concentrations during the period of photosynthesis, prevented the rise in O₂ uptake rate during the immediate post illumination period.

We conclude that the level of respiratory substrate in leaves determines their rate of O₂ uptake, and the degree to which the alternative path contributes to that O₂ uptake.