探究非绿叶植物的光合作用

光合作用是高中生物学中重要的一部分,现行各个版本的高中教材均将绿叶中色素的提取作为一个重要的实验。自然界中非绿叶植物虽然只占很小的比例,但是由于叶片的颜色与常见的绿色叶片不同,使其光合作用很容易引起学生的注意,作为学生课外探究实验非常具有可行性。本实验通过提取、分离非绿叶植物叶片中的色素及测定其色素的作用光谱来探究非绿叶植物的光合作用,并以此为契机引入探究实验的一般模式．引导学生掌握科学探究的过程。     

Maximal steady state versus state of conditioning.

Criteria for the identification of maximal steady state as related to state of conditioning were evaluated. 13 volunteers walker and/or ran during a series of 15 min tests on a treadmill. The speeds ranged from mild to exhaustive. Heart rate was monitored continuously; VO2 was determined from 6 min to 9 min; and venous blood was obtained at 10 min and 15 min for lactate analyses. Max VO2 was established for each subject. Subjects were classified on level of conditioning according to the quantity and quality of their activity record for the previous 6 months. The 10 min heart rate associated with a blood lactate level of 2.2 mM/L (MSSHR) was the best predictor of conditioning. The relative VO2 (% of max VO2) found with a 10 min blood lactate concentration of 2.2 mM/L (RMSSVO2) was almost as accurate as MSSHR in predicting state of conditioning. Changes in blood lactate levels between 10 min and 15 min were not significantly related to conditioning.     

Imaging photosynthesis in wounded leaves of Arabidopsis thaliana

Chlorophyll fluorescence imaging provides a non-invasive and non-destructive means with which to measure photosynthesis. This technique has been used, in combination with 14CO2 feeding, to study the spatial and temporal changes in source-sink relationships which occur in mechanically wounded leaves of Arabidopsis thaliana. Twenty-four hours after wounding, cells proximal to the wound margin showed a rapid induction of PhiII upon illumination (a measure of the efficiency of photosystem II photochemistry) whilst cells more distal to the wound margin exhibited a much slower induction of PhiII and a large, transient increase in NPQ (a measure of the rate constant for non-photochemical energy dissipation within the light-harvesting antenna). These results are indicative of an increase in sink strength in the vicinity of the wound and this was confirmed by the retention of 14C photosynthate in this region. It has been hypothesized that wound-induced cell wall (apoplastic) invertase (cwINV) activity plays a central role in generating localized increases in sink strength in stressed plant tissue and that hexose sugars generated by the sucrolytic activity of cwINV may act as a signal regulating gene expression. Enzyme activity measurements, quantitative RT-PCR, and T-DNA insertional mutagenesis have been used to determine that expression of AtcwINV1 is responsible for all induced cwINV activity in mechanically wounded leaves. Whilst inactivation of this gene abolished wound-induced cwINV activity, it did not affect localized alterations in source-sink relationships of wounded leaves or wound-regulated gene expression. The signals that may regulate source-sink relationships and signalling in wounded leaves are discussed.     

Regulation of photosynthesis by sugars in sugarcane leaves

In sugarcane, increased sink demand has previously been shown to result in increased photosynthetic rates that are correlated with a reduction in leaf hexose concentrations. To establish whether sink limitation of photosynthesis is a result of sugar accumulation in the leaf, excision and cold-girdling techniques were used to modify leaf sugar concentrations in pot-grown sugarcane. In excised leaves that were preincubated in darkness for 3h, sucrose accumulation was reduced but accumulated again upon transfer to the light, while hexose concentrations remained lower than in controls (7.7 micromol mg(-1)FW versus 18.6 micromol mg(-1)FW hexose in controls). These results were associated with a 66% and 59% increase in photosynthetic assimilation (A) and electron transport rate (ETR), respectively, compared to controls maintained in the light. Similar increases in photosynthesis were observed when dark-treated leaves were supplied with 5mM sorbitol, but not when supplied with 5mM sucrose. Further analyses of (14)C-labeled sugars indicated rapid turnover between sucrose and hexose. Cold-girdling (5 degrees C) increased sucrose and hexose levels and resulted in a decline of photosynthetic rates over 5d (48% and 35% decline in assimilation rate and ETR, respectively). These sugar-induced changes in photosynthesis were independent of changes in stomatal conductance. This study demonstrates that the down-regulation of photosynthesis in response to culm sugar accumulation reported previously could be due to the knock-on effect of accumulation of sugar in leaf tissue, and supports the contention that hexose, rather than sucrose, is responsible for the modulation of photosynthetic activity.     

Inhibition of photosynthesis by carbohydrates in wheat leaves

The rate of net CO₂ assimilation of mature wheat (Triticum aestivum L.) leaves in ambient air (21% O₂, 340 microbars CO₂) declined with time of illumination at temperatures lower than 25°C, but not at higher temperatures, and the rate of decline increased when maintained in air with higher CO₂ concentration (700-825 microbars). In this latter case, the decline in the rate of net CO₂ assimilation also occurred at high temperatures. Stomatal conductance also declined with time in some cases and stomata became more sensitive to CO₂, but this was not the primary cause of the decrease in CO₂ assimilation because internal partial pressure of CO₂ remained constant. Treatments which reduced the rate of translocation (e.g. lower temperatures, chilling the base of the leaf) produced a marked decline in CO₂ assimilation of leaves in atmospheric and high CO₂ concentrations. The decreased net CO₂ assimilation was correlated with carbohydrate accumulation in each case, suggesting end product inhibition of photosynthesis. Analysis of CO₂ assimilation in high carbohydrate leaves as a function of intercellular CO₂ partial pressure showed reduction in the upper part of the curve. The initial slope of this curve, however, was not affected. Photosynthetic rates in the upper part of this curve generally recovered after a short period in darkness in which carbohydrates were removed from the leaf. The stimulation of net CO₂ assimilation by 2% O₂ (Warburg effect), and the apparent quantum yield, decreased after several hours of light.     

Lipids in alfalfa leaves in relation to cold hardiness

The lipid composition of the leaves of hardy Vernal and cold-sensitive Caliverde alfalfa plants, grown at different temperatures, was determined. Phosphatidyl glycerol, phosphatidyl inositol, and the sulfolipid content were directly related to growth temperature. Mono- and digalactose diglyceride and phosphatidyl choline and ethanolamine were inversely related to temperature. At corresponding growth temperatures Vernal plants showed higher percentages of mono- and digalactose diglyceride and phosphatidyl choline and ethanolamine than Caliverde plants, while the opposite was true for phosphatidyl glycerol and inositol and sulfolipid. Differences in fatty acid composition of corresponding leaf lipid fractions of plants grown at different temperatures or differences in fatty acid composition between lipid fractions of plants of different varieties in general were negligible.     

旱作条件下不同苜蓿品种光合作用的日变化

晴天,利用LI-6400光合仪研究了旱作条件下4年生紫花苜蓿新疆大叶、巨人201、牧歌401和路宝再生草初花期（2004年6月23日）的光合作用日变化特征.结果表明：①4个苜蓿品种Pn、Tr和Gs的日变化曲线均呈“双峰”型,12：00左右存在明显的光合＂午休＂现象,但不同品种Pn、Tr和Gs的高峰和低谷出现的时刻和高低不同.WUE日进程,除路宝呈“单峰”型外,新疆大叶、巨人201和牧歌401呈“双峰”型,4个品种在8：00左右的WUE均达到全天的最高值,14：00左右的峰值不明显.②根据Pn、Ci、Ls的变化方向,推测4个品种的光合“午休”主要受气孔因素限制.③国外苜蓿品种巨人201、牧歌401和路宝日平均Pn、Tr和WUE均优于国内地方品种新疆大叶,尤其是巨人201是一种高光合、高蒸腾、高水分利用率的品种.④相关分析结果表明,对Pn影响最显著的因子是PAR,其次是Gs、RH、Ta和VPD;对蒸腾速率影响最显著的因子为PAR,其次是RH、Ta、Gs和VPD.     

Steady-state photosynthesis in alfalfa leaflets: effects of carbon dioxide concentration

When the CO₂ concentration to which Medicago sativa L. var. El Unico leaflets were exposed was increased from half-saturation to saturation (doubled rate of photosynthesis), glycolate and glycine production apparently decreased due to inhibition of a portion of the glycolate pathway. Serine and glycerate production was not inhibited. We conclude that serine and glycerate were made from 3-phosphoglycerate and not from glycolate and that the conversion of glycine to serine may not be the major source of photorespiratory CO₂ in alfalfa. In investigations of glycolate and photorespiratory metabolism, separate labeling data should be obtained for glycine and serine as those two amino acids may be produced from different precursors and respond differently to environmental perturbations. The increased photosynthetic rate (at saturating CO₂) resulted in greater labeling of both soluble and insoluble products. Sucrose labeling increased sharply, but there was no major shift of tracer carbon flow into sucrose relative to other metabolites. The flow of carbon from the reductive pentose phosphate cycle into the production of tricarboxylic acid cycle intermediates and amino acids increased. Only small absolute increases occurred in steady-state pool sizes of metabolites of the reductive pentose phosphate cycle at elevated CO₂, providing further evidence that the cycle is well regulated.     

Interaction between photosynthesis and respiration in illuminated leaves

Plants are sessile organisms that often receive excessive amounts of light energy. This excess energy can be exported from the chloroplasts and dissipated by the mitochondrial respiratory chain. The inner membrane of plant mitochondria possesses unique non-phosphorylating pathways, involving alternative oxidase and type II NAD(P)H dehydrogenases. There are accumulating amounts of evidence showing that these energy-wasteful pathways are up-regulated under excess light conditions, suggesting that they play key roles in efficient photosynthesis. Based on recent advances in our understanding about the metabolic interaction between chloroplasts and mitochondria, we discuss the importance of the respiratory chain for stabilizing the photosynthetic system.     

Longevity and factors influencing photosynthesis in tea leaves

Quadratic relationship between the age of a tea leaf and the net photosynthetic rate (PN) has been found. A progressive increase in PN was recorded for four months. Then the PN slowly declined, yet even seven-month-old tea leaves sustained a low PN. In a tea shoot, the PN increased from the first leaf onwards. Besides the physiological maturity and proximity, photon flux density (PFD) played an important role in reducing the PN. The tea leaf PN was influenced by cultivation procedures which in turn disrupted the quantum of PFD transmitted through the canopy.     

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.     

The diurnal time course of net photosynthesis of soybean leaves: Analysis with a physiologically based steady-state photosynthesis model

Summary A physiologically based steady-state model of whole leaf photosynthesis (WHOLEPHOT) is used to analyze observed net photosynthesis daily time courses of soybean, Glycine max (L.) Merr., leaves. Observations during two time periods of the 1978 growing season are analyzed and compared. After adjustment of the model for soybean, net photosynthesis rates are calculated with the model in response to measured incident light intensity, leaf temperature, air carbon dioxide concentration, and leaf diffusion resistance. The steady-state calculations closely approximate observed net photosynthesis. Results of the comparison reveal a decrease in photosynthetic capacity in leaves sampled during the second time period, which is associated with decreasing ability of leaves to respond to light intensity and internal air space carbon dioxide concentration, increasing mesophyll resistance, and increasing stomatal resistance.     

Effect of inorganic phosphate on photosynthesis in bean leaves

It was shown that raising pod seedlings by the hydroponics method on KH2PO4 solutions at concentrations between 10(-7) and 10(-5) M leads to an increase in the rate of oxygen release (delta O2/delta t), with the chlorophyll content in leaves being unchanged. The values of the parameters FM/FT of slow fluorescence induction and B/A of photoinduced changes in ESR1 signals from pod leaves correlate with the delta O2/delta t value.     

Photoinhibition of photosynthesis in willow leaves under field conditions

Chlorophyll fluorescence of leaves of a willow (Salix sp.) stand grown in the field in northern Sweden was measured on several occasions during the growing season of 1987. For leaves that received mostly full daylight, the F _V/F _P ratio declined roughtly 15% in the afternoon on cloudless days in July (F _P is the fluorescence at the peak of the induction curve obtained at the prevailing air temperature after 45 min of dark adaptation, and F _V is variable fluoresence, F _V=F _P-F _O, where F _O is minimal fluorescence). There was no decrease in the F _V/F _P ratio on cloudy days, while the effect was intermediate on changeable days. In view of this light dependence, together with the fact that the decline in the F _V/F _P ratio was paralleled with an equal decline in the corresponding fluorescence ratio F _V/F _M at 77K, and a similar decline in the maximum quantum yield of O₂ evolution, it is suggested that the decline in the F _V/F _P ratio represents a damage in photosyntem II attributable to photoinhibition. Recovery of the F _V/F _P ratio in dim light following a decline on a cloudless day took 7–16 h to go to completion; the F _V/F _P ratio was fully restored the following morning. When all active leaves of a peripheral shoot were compared, the F _V/F _P ratio in the afternoon of a day of bright light varied greatly from leaf to leaf, though the majority of leaves showed a decline. This variation was matched by a pronounced variation in intercepted photon flux density. When leaves developed in the shade were exposed to full sunlight by trimming of the stand an increased sensitivity to photoinhibition was observed as compared to peripheral leaves. The present study indicates that peripheral willow shoots experienced in the order of 10–20% photoinhibition during an appreciable part of their life. This occurred even though the environmental conditions were within the optimal range of photosynthesis and growth.Abbreviations and symbols F _O minimum fluorescence - F _P fluorescence at the peak of the induction curve obtained at normal ambient temperatures - F _V variable fluorescence - F _M maximum fluorescence obtained at 77K - PPFD photosynthetic photon flux density