Plant morphogenesis: A geometrical model for the ramification

A geometrical model is proposed that describes the emergence of a primordium at the shoot apex in Dicotyledons. It is based on recent fundamental results on plant morphogenesis, viz.:

the emergence is preceded by the reorganization of the microtubules of the cortical cytoskeleton, leading to a new orientation of the synthesis of the cell wall microfibrils;

the resulting global stress is related to the general orientation of the cell growth;

The model sums up the continuous interactions that link the microtubules, the microfibrils and the cell growth axis. The paper tries to answer three essential questions:

Why does the principal stem shifts its growth direction after each lateral emergence?

Why do the three axes involved in any ramification (namely the old and the new principal stems and the lateral emergence) exhibit a plane configuration whereas this is an essentially three dimensional phenomenon?

Does phyllotaxis exclusively depend upon the local emergence of a primordium?

An interactive procedure between empirical botanical knowledge and the mathematical model leads to an insight of the compatibility mechanisms that link the various microtubules and microfibrils networks, and the apical dome restoration. A geometrical formalism allows a redefinition of both the “generating centre” and the “organizing centre”, and their field effect.     

A biochemically semi-detailed model of auxin-mediated vein formation in plant leaves

We present here a model intended to capture the biochemistry of vein formation in plant leaves. The model consists of three modules. Two of these modules, those describing auxin signaling and transport in plant cells, are biochemically detailed. We couple these modules to a simple model for PIN (auxin efflux carrier) protein localization based on an extracellular auxin sensor. We study the single-cell responses of this combined model in order to verify proper functioning of the modeled biochemical network. We then assemble a multicellular model from the single-cell building blocks. We find that the model can, under some conditions, generate files of polarized cells, but not true veins.     

揭开植物的叶色之谜

植物的叶色是自然界美丽景色的重要组成部分,通过对植物叶色的成因、变化原因和影响变化的因素等几个方面加以阐述,力求揭开植物的叶色之谜。     

Fluorescence emission spectra of plant leaves and plant constituents

Summary The UV-B radiation (e.g. 337 nm) induced blue fluorescence (BF) and red chlorophyll fluorescence spectra (RF) of green leaves from plants with different leaf structure were determined and the possible nature and candidates of the blue fluorescence emission investigated. The blue fluorescence BF is characterized by a main maximum in the 450 nm region and in most cases by a second maximum/shoulder in the 530 nm region. The latter has been termed green fluorescence GF. The red chlorophyll fluorescence RF, in turn, exhibits two maxima in the 690 and 730 nm region. In general, the intensity of BF, GF and RF emission is significantly higher in the lower than the upper leaf side. The ratio of BF to RF emission (F450/F690) seems to vary from plant species to plant species. BF and GF emission spectra appear to be a mixed signal composed of the fluorescence emission of several substances of the plant vacuole and cell wall, which may primarily arise in the epidermis. Leaves with removed epidermis and chlorophyll-free leaves, however, still exhibit a BF and GF emission. Candidates for the blue fluorescence emission ( max near 450 nm) are phenolic substances such as chlorogenic acid, caffeic acid, coumarins (aesculetin, scopoletin), stilbenes (t-stilbene, rhaponticin), the spectra of which are shown. GF emission ( max near 530 nm) seems to be caused by substances like the alkaloid berberine and quercetin. Riboflavine, NADPH and phyllohydroquinoneK ₁ seem to contribute little to the BF and GF emission as compared to the other plant compounds. Purified natural-carotene does not exhibit any blue fluorescence.     

A model of photosystem II for the analysis of fast fluorescence rise in plant leaves

The polyphasic patterns of fluorescence induction rise in pea leaves in vivo and after the treatment with ionophores have been studied using a Plant Efficiency Analyzer. To analyze in detail photosystem II (PS II) electron transfer processes, an extended PS II model was applied, which included the sums of exponential functions to specify explicitly the light-driven formation of the transmembrane electric potential (ΔΨ(t)) as well as pH in the lumen (pH_L(t)) and stroma (pH_S(t)). PS II model parameters and numerical coefficients in ΔΨ(t), pH_L(t), and pH_S(t) were evaluated to fit fluorescence induction data for different experimental conditions: leaf in vivo or after ionophore treatment at low or high light intensity. The model imitated changes in the pattern of fluorescence induction rise due to the elimination of transmembrane potential in the presence of ionophores, when ΔΨ = 0 and pH_L(t), pH_S(t) changed to small extent relative to control values in vivo, with maximum ΔΨ(t) ∼ 90 mV and ΔΨ(t) ∼ 40 mV for the stationary state at ΔpH ≅ 1.8. As the light intensity was increased from 300 to 1200 μmol m⁻² s⁻¹, the heat dissipation rate constants increased threefold for nonradiative recombination of P680⁺Phe⁻ and by ∼30% for P680⁺Q_A⁻. The parameters ΔΨ, pH_S and pH_L were analyzed as factors of PS II redox state populations and fluorescence yield. The kinetic mechanism of fluorescence quenching is discussed, which is related with light-induced lumen acidification, when ⁺Q_A⁻ and P680⁺ recombination probability increases to regulate the Q_A reduction.     

A model of photosystem II for the analysis of a fast fluorescence rise in plant leaves

The polyphasic patterns of fluorescence induction rise in pea leaves in vivo and after the treatment with ionophores have been studied using a plant efficiency analyzer. To analyze in detail photosystem II (PS II) electron transfer processes, an extended PS II model was applied, which included the sums of exponential functions to specify explicitly the light-driven formation of the transmembrane electric potential (delta psi(t)) as well as pH in the lumen (pHL(t)) and stroma (pHs(t)). PS II model parameters and numerical coefficients in delta psi(t), and pHs(t) were evaluated to fit fluorescence induction data for different experimental conditions: leaf in vivo or after ionophore treatment at low or high light intensity. The model imitated changes in the pattern of fluorescence induction rise due to the elimination of transmembrane potential in the presence of ionophores, when delta psi = 0 and pHL(t), pHS(t) altered to small extent relative to control values in vivo, with maximum delta psi(t) approximately 90 MB and delta psi(t) approximately 40 MB, for the stationary state at deltapH aproximately equal to 1.8. As the light intensity was increased from 300 to 1200 micromol x m(-2) x s(-1), the heat dissipation rate constants increased threefold for nonradiative recombination of P680+Phe- and by approximately 30% for P680+Q(A)-. The parameters delta psi, pH(S) and pH(L) were analyzed as factors of PS II redox state populations and fluorescence yield. The kinetic mechanism of qE quenching is discussed, which is related with light induced pH(L) lumen acidification, when Q(A)- and P680+ recombination probability increases to regulate the QA reduction.     

A theory and a model for interpreting the proton nuclear magnetic resonance spectra of water in plant leaves.

Some plant leaves display complex, orientation-dependent, proton nuclear magnetic resonance (1H NMR) spectra. The spectral patterns vary as the angle between the leaf surface and the applied magnetic field is varied. They also vary with temperature and with the quantity of absorbed manganous ions, but they are independent of magnetic field strength. In this paper, we propose a theory to explain the origin of the spectra and a model from which the patterns can be calculated. The theory shows how heterogeneous magnetic susceptibilities and local dipolar magnetic fields in chloroplasts can shift the water-proton resonance field. The model describes a simplified leaf structure in which the chloroplasts are nonrandomly aligned with respect to the leaf surface. Model calculations are tested by comparison with experimental spectra from hawthorn leaves (Crataegus sp.).     

The tensor-based model of plant growth applied to leaves of Arabidopsis thaliana: A two-dimensional computer model

Plant organs grow in coordinated and continuous way. Such growth is of a tensor nature, hence there is an infinite number of different directions of growth rate in each point of the growing organ. Three mutually orthogonal directions of growth can be recognized in which growth achieves extreme values (principal directions of growth [PDGs]). Models based on the growth tensor have already been successfully applied to the root and shoot apex. This paper presents the 2D model of growth applied to the arabidopsis leaf. The model employs the growth tensor method with a non-stationary velocity field. The postulated velocity functions are confirmed by growth measurements with the aid of the replica method.     

Cyanide-induced death of cells in plant leaves

Destruction of guard cell nuclei in epidermis isolated from leaves of pea, maize, sunflower, and haricot bean, as well as destruction of cell nuclei in leaves of the aquatic plants waterweed and eelgrass were induced by cyanide. Destruction of nuclei was strengthened by illumination, prevented by the antioxidant alpha-tocopherol and an electron acceptor N,N,N ,N -tetramethyl-p-phenylenediamine, and removed by quinacrine. Photosynthetic O2 evolution by the leaf slices of a C3 plant (pea), or a C4 plant (maize) was inhibited by CN- inactivating ribulose-1,5-bisphosphate carboxylase, and was renewed by subsequent addition of the electron acceptor p-benzoquinone.     

外源喜树碱对植物叶片气孔导度的影响

前人研究表明,保卫细胞微管系统在气孔运动中起到重要作用:保卫细胞质膜上内向K⁺通道的正常活性有赖于微管的正常解聚/聚合的动态变化,微管系统可能通过调节保卫细胞K⁺通道而控制气孔运动,即微管解聚导致内向K⁺通道关闭,保卫细胞无法因膨压调节吸收水分而抑制气孔开放。有学者认为,喜树碱或其类似物能够与微管蛋白结合,并降低微管结构稳定性,其机制则可能是抑制细胞高分子量微管结合蛋白对微管组装的辅助作用,但这方面的实验证据相对匮乏。因此,为了进一步研究喜树碱的生态生物化学功能,我们采取叶面喷施喜树碱和PEG模拟干旱诱导气孔关闭的方法,研究了外源喜树碱对喜树幼苗气孔导度的影响,同时以没有内源喜树碱的烟草为实验材料进行了对照研究。研究结果表明:0.0115mmol·L^-1的喜树碱水饱和溶液对喜树和烟草幼苗叶片气孔导度的影响规律一致,均表现出明显的抑制气孔开放的效果,这为喜树碱与微管蛋白结合提供了部分证据。     

Plant form based on the pipe model theory II. Quantitative analysis of ramification in morphology

Ramification in tree structure was investigated by the main axis cutting method, which differs from the ordinary stratified clipping method. An axis running from the arbitrary terminal leader of the shoot to the stem base was termed the “main axis”. Cutting the main axis into pieces of constant length gives the “segment layer”, which consists of segments of the main axis and all branches and leaves diverging from the respective segments. There was a linear relationship between the weight of a main axis segment (in the range where leaves exist) of constant length and that of all the parts above the segment. Since plant form is determined by branches diverging regularly from the mother branch or stem, this linear relationship is considered to support the concept of the pipe model theory. It is also suggested that the proportionality constant of the linear relationship may specify the branching structure or ramifications of plant form.     

Optical parameters of leaves of 30 plant species

Optical parameters (absorption coefficient k, infinite reflectance R∞, scattering coefficient 8) are tabulated for seven wavelengths and analyzed for statistical differences for 30 plant species. The wavelengths are: 550 nm (green reflectance peak), 650 nm (chlorophyll absorption band), 850 nm (infrared reflectance plateau), 1450 nm (water absorption band), 1650 nm (reflectance peak following water absorption band at 1450 nm), 1950 nm (water absorption band), and 2200 nm (reflectance peak following water absorption band at 1950 nm).     

Widespread occurrence of cholinesterase activity in plant leaves

In contrast to previous work, the distribution of cholinesterase was found to be ubiquitous in plant leaves. Cholinesterase activity was detected in 91% of the 70 species surveyed from 50 higher plants and three families of ferns. A radiometric assay was used to determine the hydrolysis of acetylcholine by leaf tissue slices in the presence and absence of 29 μ M diisopropyl phosphofluoridate. The results obtained using this inhibitor as a criterion for cholinesterase activity were found to be consistent with previous studies using neostigmine as the inhibitor although there were some quantitative differences between the inhibitors. With some of the tested plants acetyl-β-methylcholine was also hydrolyzed, indicating that acetylcholinesterase rather than pseudocholinesterase was present at least in these cases. These findings demonstrate that the relative activity of cholinesterase in leaves can serve as an indicator of organophosphorous anticholinesterase contamination of the environment.     

Mesophyll conductance and its limiting factors in plant leaves

Mesophyll conductance (g_m) represents the CO₂ diffusion facility from sub-stomatal internal cavities to carboxylation sites in chloroplasts, and the variation of g_m across genotypes as well as environmental conditions is expected to be related to the anatomical structures and biochemical properties of leaves. In recent years, the variation of g_m has attracted wide attention. The limiting factors in photosynthetic rate are no longer divided simply into stomatal limitation and non-stomatal limitation, but splitted in stomatal limitation, mesophyll limitation and carboxylation limitation. In this review, we summarize the potential influences of cell wall, cell membrane, cytoplasm, chloroplast envelope and stroma on g_m, and indicate that cell wall thickness and the surface area of chloroplast exposed to intercellular air space (S_c) are the most important factors influencing the g_m. We also analyze the probable effects of biochemical process related with aquaporins and carbonic anhydrase on g_m. Meanwhile, the regulation mechanisms of long- and short-term environment changes (including temperature, light intensity, drought, and nutrients) on g_m are also summarized. The relationship between g_m and hydraulic conductance (K_leaf) is debated. Finally, we discuss the scientific problems related with g_m.