Light-induced spectral absorbance changes in relation to photosynthesis and the epoxidation state of xanthophyll cycle components in cotton leaves

When cotton (Gossypium hirsutum L., cv Acaia SJC-1) leaves kept in weak light were suddenly exposed to strong red actinic light a spectral absorbance change took place having the following prominent characteristics. (a) It was irreversible within the first four minute period after darkening. (b) The difference in leaf absorbance between illuminated and predarkened leaves had a major peak at 505 nanometers, a minor peak at 465 nanometers, a shoulder around 515 nanometers, and minor troughs at 455 and 480 nanometers. (c) On the basis of its spectral and kinetic characteristics this absorbance change can be readily distinguished from the much faster electrochromic shift which has a peak at 515 nanometers, from the slow, so-called light-scattering change which has a broad peak centered around 535 nanometers and is reversed upon darkening, and from absorbance changes associated with light-induced chloroplast rearrangements. (d) The extent and time course of this absorbance change closely matched that of the deepoxidation of violaxanthin to zeaxanthin in the same leaves. (e) Both the absorbance change and the ability to form zeaxanthin were completely blocked in leaves to which dithiothreitol (DTT) had been provided through the cut petlole. DTT treatment also caused strong inhibition of that component of the 535-nanometer absorbance change which is reversed in less than 4 minutes upon darkening and considered to be caused by increased light scattering. Moreover, DTT inhibited a large part of nonphotochemical quenching of chlorophyll fluorescence in the presence of excessive light. However, DTT had no detectable effect on the photon yield of photosynthesis measured under strictly rate-limiting photon flux densities or on the light-saturated photosynthetic capacity, at least in the short term. We conclude that it is possible to monitor light-induced violaxanthin de-epoxidation in green intact leaves by measurement of the absorbance change at 505 nanometers. Determination of absorbance changes in conjunction with measurements of photosynthesis in the presence and absence of DTT provide a system well suited for future studies of meachanisms of dissipation of excessive excitation energy in intact leaves.     

Nitrogen Assimilating Enzyme Activities and Enzyme Protein during Development and Senescence of Effective and Plant Gene-Controlled Ineffective Alfalfa Nodules

Effective (N₂-fixing) alfalfa (Medicago sativa L.) and plant-controlled ineffective (non-N₂-fixing) alfalfa recessive for the in₁ gene were compared to determine the effects of the in₁ gene on nodule development, acetylene reduction activity (ARA), and nodule enzymes associated with N assimilation and disease resistance. Effective nodule ARA reached a maximum before activities of glutamine synthetase (GS), glutamate synthase (GOGAT), aspartate aminotransferase (AAT), asparagine synthetase (AS), and phosphoenolpyruvate carboxylase (PEPC) peaked. Ineffective nodule ARA was only 5% of effective nodule ARA. Developmental profiles of GS, GOGAT, AAT, and PEPC activities were similar for effective and ineffective nodules, but activities in ineffective nodules were lower and declined earlier. Little AS activity was detected in developing ineffective nodules. Changes in GS, GOGAT, AAT, and PEPC activities in developing and senescent effective and ineffective nodules generally paralleled amounts of immunologically detectable enzyme polypeptides. Effective nodule GS, GOGAT, AAT, AS, and PEPC activities declined after defoliation. Activities of glutamate dehydrogenase, malate dehydrogenase, phenylalanine ammonia lyase, and caffeic acid-o-methyltransferase were unrelated to nodule effectiveness. Maximum expression of nodule N-assimilating enzymes appeared to require the continued presence of a product associated with effective bacteroids that was lacking in in₁ effective nodules.     

Cryptic dioecy in flowering plants

In some dioecious plant species, mates and/or females have large and presumably costly opposite-sex structures that are sterile. This is termed 'cryptic dioecy'. Several new cases of cryptic dioecy have recently been studied. They may give information about the minimal requirements for the evolution of separate sexes from hermaphroditism, because the most important differences contributing to the initial advantage of the breeding system have not been obscured by further developments. Reviewed in this light, cryptic dioecy can provide evidence on the role of reallocation of reproductive resources in the evolution of dioecy.     

Anomalous nonstoichiometry detected by dual label analysis of ribulose 1,5-bisphosphate carboxylase

When ribulose-1,5-bisphosphate carboxylase is assayed under N₂ using [³H]ribulose 1,5-bisphosphate and ¹⁴CO₂, [³H]3-phosphoglycerate and [¹⁴C]3-phosphoglycerate are produced in nonstoichiometric amounts in a ratio which approaches 7 at low concentrations of CO₂ (2 micromolar) assuming a 1:1 ratio at V_max (280 micromolar). The log of the molar ratio varies as a linear function of log[CO₂]. Nonstoichiometry could be explained by CO₂ contaminatio of the reactants or tritium contamination of the products. However, the magnitude of CO₂ contamination required (18 ± 4 micromolar) is far in excess of controlled CO₂ (<0.1 micromolar), and the required tritium contaminant would have to vary from 30 to 85% of the purified 3-phosphoglycerate at the 58 and 2 micromolar CO₂ assay levels, respectively. This contrasts with detectable tritium contamination which is only 1 to 4% and correctable. Nonstoichiometry is evident using either 1 or 5 labeled [³H]ribulose 1,5-bisphosphate. When 3-phosphoglycerate is reisolated as glycerate the ³H/¹⁴C ratio remains unchanged.     

运用电教媒体演示实验改进生物课堂教学

生物课堂教学有其本身特点和内涵,通过观察生命活动现象,了解生命活动规律,去认识生命活动本质。《生理卫生》是向中学生讲授人的生命活动规律及卫生保健的一门学科。人体是个复杂、奇妙的机体,如何去向学生介绍,观察人的生命活动,特別是观察人的心脏活动,了解心脏的生理知识,沒有比较复杂的医学仪器是难以实现的。在中学的课堂里也不可能利用临床医学上的方法去进行研究,或解剖大型哺乳动物去演示。两栖类动物(青蛙、蟾蜍)是实验的好材料,也适于幻灯投影演示。通过电化演示放大,产生由“静”变“动”的效果,给学生以直观的感性知识,逐步理解教材中的理论概念。