Bill Arnold's concept of solid state photosynthesis and his discoveries

Bill Arnold's concept of photosynthesis as a solid state, as opposed to solution, phenomenon led him to an amazing series of fundamental discoveries.     

Experiments

In this article, I have provided a brief history of my life. After tracing my family background and my early interest in physical sciences, I discuss how I entered biology under the influence of Robert Emerson. I have always enjoyed doing experiments and this led to new measurements and analyses of chlorophyll unit, efficiency of photosynthesis, excitation energy transfer, delayed light emission, thermoluminescence and electroluminescence in photosynthetic organisms. It is my view that discoveries are made because we follow our scientific curiosities.This article was written at the invitation of Govindjee.     

Halcyon days with Bill Arnold

The circumstances that led to the discovery that plants luminesce after they are illuminated are described, as are other discoveries that would not have been possible were it not for the fortuitous association I had with my dear and most admirable friend, W.A. Arnold, to whom this special issue is dedicated.     

Bill Arnold: Scientist, philosopher, friend

Bill Arnold is a great scientist in whose laboratory I was privileged to work during the years he discovered the thermoluminescence and the electroluminescence of green plants. I reminisce about Bill's influences on me, scientific and otherwise, during those years.     

Electronic excitation transfer in the photosynthetic unit: Reflections on work of William Arnold

Among his many contributions to photosynthesis, William Arnold made critical suggestions about the mechanism of the initial stages of excitation energy transfer and its measurement. Thus he helped found not only the general concept of the photosynthetic unit but also the key idea behind the detailed functional aspects of its chlorophyll antenna. We review the development of these ideas and the modern form in which they have emerged.Abbreviations Chl chlorophyll - Pc phycocyanin - PSU photosynthetic unit - RC reaction center     

W.A. Arnold's inspiring experiments

The impact is discussed here of some experiments by W.A. Aronld and coworkers on photosynthetic research, specifically on that at the Leiden Biophysics Department. These experiments involved the following topics: photosynthetic unit and electronic excitation transfer to a reaction center, chlorophyll luminescence and photosynthesis, and unexpected experimental results as a source of discoveries.     

The importance of being ordered: improving NMR structures using residual dipolar couplings

Residual dipolar couplings arise from small degrees of alignment of molecules in a magnetic field. Most biomolecules lack sufficient intrinsic magnetic susceptibility anisotropies for practical purposes; however, alignment can be achieved using dilute aqueous phospholipid mixtures, colloidal suspensions of rod-shaped viruses, complex phases of surfactant systems and strained gels. The stability of the liquid crystalline phases varies with respect to temperature range, pH variation and time and is critically dependent on sample composition and experimental conditions. The magnitude of the residual dipolar couplings depends upon the degree of ordering and allows the determination of the corresponding inter-nuclear vectors with respect to the molecule's alignment frame. Inclusion of dipolar constraints into NMR structure calculations leads to improved precision and accuracy of the resulting structures, especially in cases where the information content provided by traditional NOE constraints is limited. In addition, rapid evaluation of backbone protein folds and determination of the relative orientations of individual components in multi-molecular complexes have become feasible. Dipolar coupling based strategies may well emerge as the most critical developments, in establishing NMR as a valuable and competitive methodology in the structural genomics initiative.     

Transients of delayed fluorescence induction signal and photosynthetic antennas: A possible relationship. Mathematical modeling approach

A mathematical model was developed for resolved temporal transients of experimentally recorded delayed fluorescence (DF) induction signal. During an intermittent light regime, antennas of the photosynthetic apparatus were treated as targets, repeatedly hit by potentially absorbable photons within a series of consecutive light flashes. Formulas were derived for the number of antennas, cumulatively hit by a specific number of photons, as a function of the flash serial number (time). Model parameters included number of absorbable photons in one flash, antenna sizes, and their number. A series of induction curves were analyzed, obtained from a Zea mays leaf segment and differing in the previous dark period (t _d). Each curve, consisting of the two most prominent DF transients (C and D), was fitted with several model types, differing in the number of absorbed photons. For both transients, the best fitting result was achieved when DF induction was linked to the second absorbed photon. As expected, model parameters related to antenna sizes showed weaker dependence on t _d than those referring to antenna number. With restrictions applied to this model, the two DF induction transients may be related to two classes of photosynthetic antennas. Their different sizes may have a predominant influence on the efficiency of photon absorption and possibly time-dependent appearance of DF transients. Published in Russian in Fiziologiya Rastenii, 2006, Vol. 53, No. 3, pp. 325–335. The text was submitted by the authors in English.     

Within-canopy variation in the rate of development of photosynthetic capacity is proportional to integrated quantum flux density in temperate deciduous trees

The present study was undertaken to test for the hypothesis that the rate of development in the capacity for photosynthetic electron transport per unit area (J_max;A), and maximum carboxylase activity of Rubisco (V_cmax;A) is proportional to average integrated daily quantum flux density (Q_int) in a mixed deciduous forest dominated by the shade‐intolerant species Populus tremula L., and the shade‐tolerant species Tilia cordata Mill. We distinguished between the age‐dependent changes in net assimilation rates due to modifications in leaf dry mass per unit area (M_A), foliar nitrogen content per unit dry mass (N_M), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport (F_B), Rubisco (F_R) and in light‐harvesting chlorophyll‐protein complexes (V_cmax;A ∝ M_AN_MF_R; J_max;A ∝ M_AN_MF_B). In both species, increases in J_max;A and V_cmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in M_A. Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in M_A. There was only small within‐canopy variation in the initial rate of biomass accumulation per unit Q_int (slope of M_A versus leaf age relationship per unit Q_int), suggesting that canopy differences in the rate of development of J_max;A and V_cmax;A are directly proportional to Q_int. Nevertheless, M_A, nitrogen, J_max;A and V_cmax;A of mature leaves were not proportional to Q_int because of a finite M_A in leaves immediately after bud‐burst (light‐independent component of M_A). M_A, leaf chlorophyll contents and chlorophyll : N ratio of mature leaves were best correlated with the integrated average quantum flux density during leaf development, suggesting that foliar photosynthetic apparatus, once developed, is not affected by day‐to‐day fluctuations in Q_int. However, for the upper canopy leaves of P. tremula and for the entire canopy of T. cordata, there was a continuous decline in N contents per unit dry mass in mature non‐senescent leaves on the order of 15–20% for a change of leaf age from 40 to 120 d, possibly manifesting nitrogen reallocation to bud formation. The decline in N contents led to similar decreases in leaf photosynthetic capacity and foliar chlorophyll contents. These data demonstrate that light‐dependent variation in the rate of developmental changes in M_A determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.     

The importance of being regular: Caenorhabditis elegans and Pristionchus pacificus defecation mutants are hypersusceptible to bacterial pathogens

Bacterial pathogens have shaped the evolution and survival of organisms throughout history, but little is known about the evolution of virulence mechanisms and the counteracting defence strategies of host species. The nematode model organisms, Caenorhabditis elegans and Pristionchus pacificus, feed on a wealth of bacteria in their natural soil environment, some of which can cause mortality. Previously, we have shown that these nematodes differ in their susceptibility to a range of human and insect pathogenic bacteria, with P. pacificus showing extreme resistance compared with C. elegans. Here, we isolated 400 strains of Bacillus from soil samples and fed their spores to both nematodes. Spores of six Bacillus strains were found to kill C. elegans but not P. pacificus. While the majority of Bacillus strains are benign to nematodes, observed pathogenicity is restricted to either the spore or the vegetative stage. We used the rapid C. elegans killer strain (Bacillus sp. 142) to conduct a screen for hypersusceptible P. pacificus mutants. Two P. pacificus mutants with severe muscle defects and an extended defecation cycle that die rapidly on Bacillus spores were isolated. These genes were identified to be homologous to C. elegans, unc-22 and unc-13. To test whether a similar relationship between defecation and bacterial pathogenesis exists in C. elegans, we used five known defecation mutants. Quantification of the defecation cycle in mutants also revealed a severe effect on survival in C. elegans. Thus, intestinal peristalsis is critical to nematode health and contributes significantly to survival when fed Gram-positive bacteria.     

The rate of transport through a phosphate translocator affects delayed luminescence induction: an experiment and a theoretical model

Delayed luminescence (DL) induction curves were studied in leaves from a mutant pea line containing mutations at both the r and rb loci, compared with leaves from wild type plants. Genes at the r and rb loci encode starch branching enzyme and ADP ‐ glucose pyrophosphorylase, respectively. The presence of mutations at both loci, previously known to reduce the starch content in the dry mature seed by 75%, have been shown to lower the starch level in leaves by at least 20%. During induction, the half‐time for the DL intensity decrease from maximum to steady state in the mutant was 1.5 ± 0.2 times longer than for the wild type. It is proposed that the prolongation of the induction period in leaves from the mutant plants is caused by a lack of inorganic phosphate (Pi) restricting the rate of ATP synthesis at the beginning of induction. The reduced Pi would be compensated by triose flow from the chloroplast, via the triose phosphate translocator, being exchanged for Pi from the cytosol. Analysis of our theoretical photosynthesis model confirmed that a decrease in the rate of Pi released from the Calvin cycle could lead to a prolongation of the induction period.     

闽楠幼树光合特性及生物量分配对光环境的响应

设置100%光环境(L100)、40%光环境(L40)、8%光环境(L8)3种光照梯度,分析万木林闽楠(Phoebe bournei)幼树的光合特性及生物量分配特征对不同光环境的响应。结果表明:(1)闽楠幼树在不同光环境下的最大净光合速率(Pnmax),表观量子效率(AQY),光饱和点(LSP),光补偿点(LCP),暗呼吸速率(Rd)均有显著差异(P<0.05),且随着光照强度的降低,Pnmax,LSP,LCP,Rd随之降低,而AQY却呈现升高的趋势。(2)总生物量、茎生物量、叶生物量、根生物量均表现为L100最大,根冠比、叶生物量比、茎生物量比及根生物量比在3种光环境下无显著差异。(3)闽楠幼树通过改变光合特性,生物量积累来适应光环境的变化,其中光照强度的降低限制了生物量的积累,但并未显著改变生物量地上地下分配比例。闽楠幼树在3种光环境下生物量分配比例并无显著的改变,生物量分配可塑性极低可能是闽楠零散分布的一个重要非人为干扰因素。(4)闽楠幼树所采取的生存策略以地上部分生长为主,光照强度降低时则采取保守策略进行缓慢的资源获取和消耗,全光照条件下采取快速的资源获取和消耗策略。未来造林时可以将闽楠与毛竹(Phyllostachys pubescens)混交来减少郁闭度,促进闽楠幼树的生长。     

Long term delayed luminescence: A possible fast and convenient assay for nutrition deficiencies and environmental pollution damages in plants

Long Term Delayed Luminescence (LDL) of green plants ranging from 0.3 s up to several minutes after irradiation has been found to exhibit complex decay kinetics which are highly dependent on nutritional deficiencies and environmental pollutive components. As a model organism we utilized the unicellular green algaScenedesmus obliquus with fairly well understood properties ofLDL probably involving both photosynthetic reaction centers PS I and PS II. It is shown thatLDL is strongly affected both by depletion of the growth medium of various essential elements such as N, Fe, Ca, Mg or K, as well as by growth in the presence of environmental stress factors such as Cu, Hg, Cd or NO ₂ ^– . Therefore,LDL offers itself as a convenient, highly sensitive and specific assay for a number of stress factors in photosynthesizing plantsin vivo and in the field.     

四种热带雨林树种幼苗比叶重,光合特性和暗呼吸对生长光环境的适应

雾凉季研究了西双版纳热带雨林4种植物幼苗对生长光环境的适应,其中两个树种幼苗喜光（团花和滇南插柚紫）,两个树种幼苗耐荫（滇南红厚壳和玉蕊）发现弱光环境中生长的4种植物比叶重、光合能力、光饱和点、光补偿点暗呼吸速度、叶绿素a/b比较低,叶绿素含量较高。玉蕊和滇南红厚壳幼苗的光合能力和呼吸速度 于团花树和滇南插柚紫。团花树和滇南插机紫的比叶重和光合作用的可塑性大于玉蕊和滇南红厚壳。高光强下生长的团花树和滇南插机紫增加叶氮分配给羧化酶的比较。减少分配给叶绿素的比例。滇南红厚壳和玉蕊适应弱光环境的能力略强于团花树和滇南插机紫,但适应强光的能力较差。研究结果支持树种的生理生态特性决定了其演替状况和生境选择的假说,单位干重叶的光合能力和呼吸速率并未表现出利于光适应的可塑性,表明4种植物生理适应能力较差,形态学上的适应在4种热带雨林树种幼苗光适应方面起到了重要的作用,叶氮分配也是它们光适应的策略之一。