高山植物叶绿体与线粒体位置相关性研究

利用透射电镜对生长于青藏高原东北部达坂山(海拔3900m)的4种高山植物叶肉细胞进行了超微结构观察。首次在蒲公英(Taraxacum mongolicum)和乳白香青(Anaphalis lacten)的叶肉细胞中发现了叶绿体“吞噬”线粒体的现象。在所研究的4种高山植物中,线粒体的数量均较多,线粒体在细胞中的分布表现出不均一,且常分布在叶绿体附近,二者靠的很紧,常常可以观察到5～6个线粒体将叶绿体包围起来的现象。研究表明,高山植物叶肉细胞中叶绿体和线粒体在位置上的这种变化是对逆境的一种适应,是青藏高原特殊生态条件长期胁迫的结果。     

达坂山蚤缀和裸茎金腰叶绿体超微结构的研究

对高山植物达坂山蚤缀和裸茎金腰叶绿体超微结构的研究表明,叶绿体和线粒体镶嵌很紧密;基粒和基质类囊体膨大,膨大的类囊体呈梭形或圆形,没有发现脂质小球的存在;基粒叠垛程度很低,平均每个基粒类囊体的片层数为5．4。这2种高山植物叶绿体超微结构上的特征是青藏高原特殊的生态条件,包括低温、低气压、强辐射影响的结果。     

青藏高原高山植物的形态和解剖结构及其对环境的适应性研究进展

高山植物是一类生长于树线以上至雪线的山地植物。揭示高山植物适应环境的形态和结构特征及其内在机制,对研究全球气候变化下,植物对环境的响应和适应具有重要的理论意义。然而,国内在高山植物功能生态学的研究上相对薄弱,已有研究主要集中在对青藏高原高山植物的报道上。结合国外高山植物的相关研究报道,从形态和解剖结构两个方面对青藏高原高山植物的研究进展进行了综述,重点阐述了高山植物的形态、解剖结构及其与环境的适应性关系。植株矮小（有的呈垫状）、叶片小而厚、具有通气组织、栅栏组织多层、机械组织发达、虫媒花性状、线粒体数量多和叶绿体基粒片层少等是这一地区高山植物普遍具有的形态和结构特征。高山植物形成上述结构的特异性是高山特殊综合生态环境长期作用的结果。同时,也是高山植物对高山环境的高度适应。最后,对这一领域存在的问题以及未来研究的重点和方向进行了探讨。目的是引起国内研究者的关注,促进我国高山植物功能生态学的研究与发展。     

基于GIS的青藏高原地区四种高山植物的空间遗传格局

青藏高原地区景观特征影响着高山植物的现代遗传格局。基于叶绿体DNA(cpDNA)变异数据,运用GIS空间插值方法和Monmonier算法,分析了青藏高原地区4种高山植物长花马先蒿(Pedicularis longiflora)、西川红景天(Rhodiola alsia)、窄叶鲜卑花(Sibiraea angustata)和西藏沙棘(Hippophae tibetana)的遗传多样性和遗传分化景观格局。结果表明:在4种高山植物的遗传景观格局中,高值区域主要位于东喜马拉雅-横断山脉地区,每种植物的地理隔离也位于这个地区;Partial Mantel test检测表明,地理隔离均显著影响4种植物种群的基因交流,验证了长期地理隔离造成的种群分化是产生该地区遗传格局的主要原因。     

高CO_2浓度对大豆不同叶位叶片叶绿体淀粉粒积累的效应

对高CO_2浓度下生长的大豆(Glycine max(L.)Merr.)不同叶位的叶片进行了电镜观察,揭示出大豆不同叶位叶片的叶绿体对倍增的CO_2浓度反应不一。其显著的超微结构差异特征是:1.叶位居中的叶片叶绿体积累的淀粉粒不仅很大,而且最多,有的叶绿体中的淀粉粒可达20个,几乎充满着叶绿体的基质空间。2.下位叶叶绿体的淀粉粒积累较多,通常为2～5个;3.上位叶叶绿体所含淀粉粒既小又少,虽然有的叶绿体中也积累有3～4个淀粉粒,但大多数叶绿体中所含淀粉粒仅有1～2个。以上结果联系到大豆中位叶的光合作用速率较高及对籽粒产量起作用最大来讨论是很有意义的。     

沙冬青淀粉粒及其与叶绿体发育的关系

沙冬青叶绿体中的淀粉粒一般为１－３个,主要有４种类型。第１种外周部分电子密度较高,中央部分较低,但每个部分的电子密度十分均匀。它们近似椭圆形,附近的类囊体形态正常,结构清晰。第２种电子密度由外向内逐渐变低,多为椭圆形,附近的类囊体较清晰。第３种外周部分的电子密度很高,中央部分不均匀,形状多种多样,附近的类囊体有的不清晰。第４种电子密度很低,十分均匀,形状不规则,附近的类囊体已经解体或正在解体。分析表明,淀粉的形态变化明显与叶绿体发育有关。     

锌胁迫对不知火杂柑和椪柑叶片光合特性及超微结构的影响

采用砂基培养法,运用CI－340便携式光合测定仪、H-7650透射电子显微镜研究了不同锌浓度如0mg／L（锌缺乏）、0．05mg／L（对照）和0．5mg／L（锌过量）处理下不知火杂柑（简称不知火）和槿柑叶片叶绿体色素含量、净光合速率及超微结构的变化。结果表明：（1）锌缺乏处理的不知火及锌过量处理的槛柑叶片叶绿素（a＋b）和类胡萝卜素含量、锌缺乏与锌过量处理的两者叶片净光合速率均相对低于对照。（2）锌缺乏处理下,两者叶片叶绿体出现变形或空室化,其中碰柑叶绿体内淀粉粒和质体小球增多,细胞核及线粒体正常,而不知火淀粉粒少,细胞核中出现许多黑色小颗粒物质。锌过量处理下,两者叶片叶绿体出现空室化或叶绿体膜模糊,线粒体解体,其中槿柑叶绿体内质体小球增多,淀粉粒少;而不知火淀粉粒明显增多、增大,基粒片层膨胀、松弛。可见,锌胁迫对两者叶片叶绿体色素、净光合速率及超微结构有着明显的影响,其影响程度因品种而异。     

青藏高原及其周边地区高山植物谱系地理学研究进展

青藏高原及其周边地区是世界上高山植物最丰富的地区,通过谱系地理学研究可以探讨高山植物演化历史与高原隆升和第四纪冰期的关系。根据对已经报道的36种高山植物的谱系地理分析,其谱系地理模式主要表现为:一、冰期退却到高原边缘的避难所,冰后期回迁到高原面;二、地理隔离造成冰期存在多处避难所(含微型避难所),冰后期发生局域性扩张。青藏高原在晚第三纪的快速隆升促进了物种的分化和成种,而第四纪冰期更是加剧了物种的快速分化,高原隆升和第四纪周期性气候波动是形成青藏高原高山植物现代谱系地理格局的主要原因。横断山脉地区作为第四纪冰期高山植物的主要避难所,在进化生物学和保护生物学方面具有重要启示。最后在物种选择、采样策略、基因片段选择和研究方法等4个方面提出青藏高原地区谱系地理学的研究方向。     

盐胁迫下芦苇叶肉细胞超微结构的研究

对青藏高原柴达木盆地柯柯盐湖边盐碱地上生长的芦苇叶肉细胞的超微结构进行了研究,并以西宁地区非盐碱地上生长的芦苇作对照。结果表明：西宁地区的芦苇叶肉细胞的叶绿体呈椭圆形,其膜系统完整,基粒片层和基质片层发育良好。在盐碱地上生长的芦苇叶肉细胞的叶绿体呈圆形,叶绿体内出现较大的淀粉粒,并发现有线粒体嵌入叶绿体的现象。叶绿体的类囊体膨大,线粒体的嵴也有膨大的现象。在盐湖水中生长的芦苇叶肉细胞,叶绿体的类囊体排列紊乱、扭曲、松散。类囊体膜局部被破坏,部分类囊体膜解体,空泡化,甚至消失,一些溶解了的类囊体流进细胞质中。综上所述,芦苇叶肉细胞超微结构的变化是该植物适应柯柯盐湖地区盐渍、低温、低气压、强辐射等环境因子的结果。     

高CO2浓度对大平不同叶位叶片叶绿体淀粉粒积累的效应

对高ＣＯ２浓度下生长的大豆不同叶位的叶片进行电镜观察,揭示出大豆不同叶位叶片的叶绿体对倍增的ＣＯ２浓度反应不一。其显的超微结构差异特征是：１．叶位居中的叶片叶绿体积累的淀粉粒不仅很大,而且最多,有的叶绿体中的分粒可达２０个,几种充满着叶绿体的基质空间。２．下位叶叶绿体的淀粉粒积累较多,通常为２￣５个;３．上位叶叶绿体所含淀粉粒既小又少,虽然有的叶绿体中也积累有３￣４个淀粉粒,但大多叶绿体中所含淀     

Effect of Doubled-CO2 Concentration on the Ultrastructure of Chloroplasts from Medicago sativa and Setaria italica

It has been reported in quite a number of literatures that doubled CO2 concentration increased the photosynthetic rate and dry matter production of C3 plants, but substantially affected C4 plants little. However, why may CO2 enrichment promote growth and either no change or decrease reproductive allocation of the C3 species, but havinag no effects on growth characteristics of the C4 plants? So far, there has been no satisfactory explanation on that mentioned above, except the differences in their CO2 compensatory points. In the past, although some studies on ultrastructure of the chloroplasts under doubled CO2 concentration were limitedly conducted. Almost all the relevant experimental materials were only from C3 plants not from C4 plants, and even though the results were of inconsistancy. Thereby, it needs to verify whether the differences in photosynthesis of C3 and C4 plants at doubled CO2 level is caused by the difference in their chloroplast deterioration. Experiments to this subject were conducted at the Botanical Garden of Institute of Botany, Academia Sinica in 1993 and 1994. Both experimental materials from C3 plant alfalfa (Medicago sativa) and C4 plant foxtail millet (Setaria italica) were cultivated in the cylindrical open-top chambers (2.2 m in diameter × 2.4 m in height) with aluminum frames covered by polyethylene film. Natural air or air with 350× 10-6 CO2 were blown from the bottom of the chamber space with constant temperature between inside and outside of the chamber 〈0.2℃〉. Electron microscopic observation revealed that the ultrastructure of the chloroplasts from C3 plant Medicago sativa and C4 plant Seteria italica growing under the same doubled CO2 concentration were quite different from each other. The differential characteristics in ultrastructure of chloro plasts displayed mainly in the configuration of thylakoid membrances and the accumulation of starch grains. They were as follows: 1. The most striking feature was the building up of starch grains in the chloroplasts of the bundle sheath cells (BSCs) and the mesophyll cells (MCs) at doubled CO2 concentra tion. The starch grains appeared centrifugally first in the BSCs and then in the chloroplast of the other MCs. It was worthy to note that the starch grains in the chloroplasts of C4 plant Setaria ira/ica were much more than those of the C3 plant Medicago sativa . The decline of photosynthesis in the doubled CO2-grown C4 plants might be caused by an over accumulation of starch grains, that deformed the chloroplast even demaged the stroma thylakoids and grana. There might exsist a correlation between the comformation of thylakoid system and starch grain accumulation, namely conversion and transfer of starch need energy from ATP, and coupling factor (CF) for ATP formation distributed mainly on protoplastic surface (PSu) of stroma thylakoid membranes, as well as end and margin membranes of grana thylakoids. Thereby, these results could provide a conclusive evidence for the reason of non effectiveness on growth characteristics of C4 plant. 2. Under normal condition , the mature chlolroplats of higher plants usually develop complete and regularly arranged photosynthetic membrane systems . Chloroplasts from the C4 plant Setaria italica, however, exerted significant changes on stacking degree, grana width and stroma thylakoid length under doubled CO2 concentration; In these changes, the grana stacks were smaller and more numerous, and the number of thylakoids per granum was greatly increased, and the stroma thylakoid was greatly lengthened as compared to those of the control chloroplasts. But the grana were mutually intertwined by stroma thylakoid. The integrity of some of the grana were damaged due to the augmentation of the intrathylakoid space . Similarly, the stroma thylakoids were also expanded. In case. the plant was seriously effected by doubled CO2 concentration as observed in C4 plant Setaria italica , its chloroplasts contained merely the stroma (matrix) with abundant starch grains, while grana and stroma thylakoid membranes were unrecognizable, or occasionally a few residuous pieces of thylakoid membranes could be visualized, leaving a situation which appeared likely to be chloroplast deterioration. However, under the same condition the C3 plant Medicago sativa possessed normally developed chloroplasts, with intact grana and stroma thylakoid membranes. Its chloroplasts contained grana intertwined with stroma thylakoid membranes, and increased in stacking degree and granum width, in spite of more accumulated starch grains within the chloroplasts. These configuration changes of the thylakoid system were in consistant with the results of the authors another study on chloroplast function, viz. the increased capacity of chloroplasts for light absorption and efficiency of PSⅡ.     

弱光胁迫对花生功能叶片RuBP羧化酶活性及叶绿体超微结构的影响

间作套种是我国主要的花生(Arachis hypogaea)种植方式之一。然而, 与单作相比, 在间作套种体系中, 花生截获的光能较少, 生长发育差, 产量低, 研究不同品种耐阴机理对选择适宜间作套种的花生品种具有重要意义。该研究用耐阴性不同的两个花生品种‘花育22号’ (强耐阴性)和‘白沙1016’ (弱耐阴性)为材料, 在大田条件下采用不同透光率遮阴网设置50%自然光强(中度弱光胁迫)和15%自然光强(严重弱光胁迫) 2个弱光处理, 从出苗期开始遮阴40天, 以自然光强为对照, 研究了弱光胁迫对花生功能叶片RuBP羧化酶活性和叶绿体超微结构的影响。结果表明: 光强为自然光照50%和15%的处理, ‘花育22号’ RuBP羧化酶活性与对照相比虽有降低, 但差异不显著, 而‘白沙1016’分别比对照低40.1%和59.4%, 显著低于对照。与对照相比, 50%自然光强下‘花育22号’叶绿体数不变, 叶绿体基粒数和基粒片层数显著增多, 叶绿体变长且发育完好, 15%自然光强下, 叶绿体数、基粒数和淀粉粒数显著减少, 叶绿体膜和基粒片层出现破损, 但叶绿体变长, 基粒片层数增加; ‘白沙1016’在50%自然光强下, 叶绿体数目和超微结构变化同‘花育22号’相似, 在15%自然光强下叶绿体变圆, 基粒数的降幅和基粒片层破损程度大于‘花育22号’且基粒片层数减少, 淀粉粒数增多。因此, 弱光胁迫特别是严重弱光胁迫条件下, 功能叶RuBP羧化酶活性降低幅度小、叶绿体超微结构受损程度低是‘花育22号’耐阴的光合生理基础。     

Effects of low light stress on rubisco activity and the ultrastructure of chloroplast in functional leaves of peanut

Aims In recent years, intercropping system has become one of the major practice of peanut (Arachis hypogaea) cultivation in northern China because of the high land and energy utilization efficiency, to some extent compensating for the production loss caused by decreasing area of cultivation land. Intercropped peanut plants often have a lower pod yield compared with monoculture due to constraint on light availability. This study was conducted to explore the shade-tolerance mechanism in two peanut cultivars, ‘Huayu 22’ and ‘Baisha 1016’, that grew in an intercropping system, by studying chloroplast ultrastructure and rubisco activity under different levels of shading.
Methods A field experiment was conducted with three levels of light treatments, including full natural light (CK), 50% natural light indensity (NLI), and 15% NLI. The ‘Huayu 22’ was used as a shade-tolerant cultivar and the ‘Baisha 1016’ as a shade-susceptible cultivar based on previous studies. Experimental plants of both cultivars were shaded for 40 days from emergency in 2006. Rubisco activity, the number and shapes of chloroplasts and starch grains, and number of grana and granum lamella were investigated in functional leaves of plants in all treatments.
Important findings The functional leaves of peanut plants in the 50% and 15% NLI treatments had a lower rubisco activity than those in the CK treatment. In the ‘Baisha 1016’, the reduction in rubisco activity was 40.1% in the 50% NLI treatment and 59.4% in the 15% NLI treatment, respectively, compared to the CK treatment;whereas no significant differences were found among treatments in the ‘Huayu 22’ in the rubisco activity. Compared with the CK, the number of chloroplasts remained unchanged, the number of grana and lamella in grana increased, and the individual chloroplast was longer and in perfect development in the functional leaves of plants of the ‘Huayu 22’ grown in the 50% NLI treatment. In contrast, the number of chloroplasts, grana and starch grains of the ‘Huayu 22’ plants decreased significantly, the chloroplast membrane and grana lamella were damaged, the number of granum lamella increased, and the individual chloroplast became longer in the 15% NLI treatment. The number and ultrastructure of chloroplasts in the ‘Baisha 1016’ plants followed similar patterns of changes as those of the ‘Huayu 22’ in the 50% NLI treatment. For plants of the ‘Baisha 1016’ in the 15% NLI treatment, their chloroplasts became more roundly shaped, with decreasing number of grana lamella and increasing number of starch grains, compared with the CK. There were a greater decrease in the grana number and more damage in the grana lamella in plants of the ‘Baisha 1016’ than those of the ‘Huayu 22’. In conclusion, the shade tolerance of the ‘Huayu 22’ resulted from lack of changes in rubisco activity and less damage in the ultrastructure of chloroplasts when under low light stress compared with the ‘Baisha 1016’.     

Ultrastructural Development of Chloroplasts in Sacred Lotus Embryo Bud under Invisible Light

The present paper reports that the development ultrastructural observations of chloroplasts from sacred lotus (Nelumbo nucifera) embryo buds under invisible light. Embryo bud of sacred lotus is enclosed by three layers of thick integument (pericap, seed coat and thick fleshy cotyledons). During the period of the formation of embryo bud, it remained in dark condition, but turned from pale yellow to bluish-green. It was noteworthy that chloroplasts of the embryo bud had well developed giant grana under invisible light. Their developmental pathway in sacred lotus, however, was different from those of other higher plants grown under sunlight, intermittent light, or even in dark conditions (Fig. 1). The chloroplast development of embryo buds in Sacred lotus seeds in invisible light underwent only in the following three stages: (1) In the first stage the development was similar to that from other higher plants, the inner envelope membranes of the proplastids were invaginating. (2) In the second stage, a proplastid centre composed of prolamellar bodies (PLB)with semicrystalline structure was formed, and was accompanied by one or two huge starch grains in almost each proplastid. In the meantime, prothylakoid membranes extended parallelly from the plastid centre in three forms: (a) One plastid centre extending parallelly prothylakoid membranes from itself in one direction; (b) The same to (a), but extending in two directions; (c) Two plastid centres extending parallelly prothylakoid membranes between the centres. (3) In the third stage, grana and stroma thylakoid membranes of chloroplasts were formed. It is to be noted that most of chloroplasts had only one or two giant grana which often extended across the entire chloroplast body, and the length of the grana thylakoid membranes of the chloroplasts from embryo bud in Sacred lotus is 3 to 5 times as many as that in other higher plants. However, their stromatic thylakoid membranes were rather rare and very short. The giant grana were squeezed to the margin of the chloroplast envelope by one or two huge starch grains.     

Analyzing photosynthetic activity and growth of Solanum lycopersicum seedlings exposed to different light qualities

To investigate how light quality influences tomato (Solanum lycopersicum L) seedlings, we examined changes in plant growth, chloroplast ultrastructure, photosynthetic parameters and some photosynthesis-related genes expression levels. For this, tomato plants were grown under different light qualities with the same photosynthetic photon flux density: red (R), blue (B), yellow (Y), green (G) and white (W) lights. Our results revealed that, compared with plants grown under W light, the growth of plants grown under monochromatic lights was inhibited with the growth reduction being more significant in the plants grown under Y and G lights. However, the monochromatic lights had their own effects on the growth and photosynthetic function of tomato seedlings. The plant height was reduced under blue light, but expression of rbcS, rbcL, psbA, psbB genes was up-regulated, and the ΦPSII and electron transport rate (ETR) values were enhanced. More starch grains were accumulated in chloroplasts. The root elongation, net photosynthetic rate (Pn), NPQ and rbcS and psbA genes expression were promoted under red light. Yellow light- and green light-illuminated plants grew badly with their lower Rubisco content and Pn value observed, and less starch grains accumulated in chloroplast. However, less influence was noted of light quality on chloroplast structure. Compared with yellow light, the values of ΦPSII, ETR, qP and NPQ of plants exposed to green light were significantly increased, suggesting that green light was beneficial to both the development of photosynthetic apparatus to some extent.     

Chloroplast Structure and Starch Grain Accumulation in Leaves That Received Different Red and Far-Red Levels during Development

An important step in understanding influence of growth environment on carbon metabolism in plants is to gain a better understanding of effects of light quality on the photosynthetic system. Electron microscopy was used to study chloroplast ultrastructure in developing and fully expanded leaves of tobacco (Nicotiana tabacum L. cv Burley 21). Brief exposures to red or far-red light at the end of each day during growth under controlled environments influenced granum size, granum number and starch grain accumulation in chloroplasts, and the concentration of sugars in leaf lamina. Far-red-treated leaves had chloroplasts with more but smaller grana than did red-treated leaves. Red light at the end of the photosynthetic period resulted in more and larger starch grains in the chloroplasts and a lower concentration of sugars in leaves. Chloroplast ultrastructure and starch grain accumulation patterns that were initiated in the expanding leaves were also evident in the fully expanded leaves that received the treatment during development. It appears that the phytochrome system in the developing leaves sensed the light environment and initiated events which influenced chloroplast development and partitioning of photosynthate to adapt the plant for better survival under those environmental conditions.     

Effects of Quantum Flux Density on Photosynthesis and Chloroplast Ultrastructure in Tissue-Cultured Plantlets and Seedlings of Liquidambar styraciflua L. towards Improved Acclimatization and Field Survival

Liquidambar styraciflua L. seedlings and tissue-cultured plantlets were grown under high, medium, or low (315, 155, or 50 microeinsteins per square meter per second photosynthetically active radiation) quantum flux densities. Net photosynthesis, chlorophyll content, and chloroplast ultrastructure of leaves differentiated from these conditions were investigated. Seedling photosynthetic rates at light saturation were positively related to light pretreatments, being 6.44, 4.73, and 2.75 milligrams CO₂ per square decimeter per hour for high, medium, and low light, respectively. Cultured plantlets under all light conditions had appreciably higher photosynthetic rates than noncultured seedlings; corresponding rates were 12.14, 13.55, and 11.36 milligrams CO₂ per square decimeter per hour. Chlorophyll in seedlings and plantlets was significantly higher in low light-treated plants. Seedling leaves had chloroplasts with abundant starch regardless of light pretreatment. In high light, starch granules were predominant and associated with disrupted granal structure. Low light seedling chloroplasts had smaller starch grains and well-formed grana. In contrast, tissue culture-differentiated leaves were devoid of starch; grana were well organized in higher quantum flux density treatments, but disorganized at low flux densities.     

LIGHT AND ELECTRON MICROSCOPY OF PYRENOIDS AND SPECIES DELIMITATION IN VOLVULINA (CHLOROPHYTA,VOLVOCACEAE)1

The appearances of pyrenoids in the vegetative cells of Volvulina steinii Playfair and V. pringsheimii Starr were observed in detail by light and electron microscopy in relation to the culture age to clarify the taxonomic relationship between the two species. In V. pringsheimii, the pyrenoids were always present in the bottom of the cupshaped chloroplasts and their gross morphology did not vary in relation to the culture age, while those of V. steinii appeared de novo and developed as the culture aged. In 24-h cultures of V. steinii, pyrenoids were not observed in the chloroplasts. In 48-h cultures, a pyrenoid matrix developed apparently de novo in the brim of the cupshaped chloroplast. Subsequently, starch grains appeared around the pyrenoid matrix in 72-h cultures. The volume of the matrix and the associated starch grains increased and tubular channels entered into the pyrenoid matrix in 96-h cultures. In addition, the pyrenoid in the parental chloroplast of V. pringsheimii divided and was distributed to each daughter cell during cell divisions in daughter colony formation, while the parental pyrenoid of V. steinii did not divide and went to one of the daughter cells. Therefore, these two species can be clearly distinguished by the differences in the position of pyrenoids in the cupshaped chloroplasts and stability of pyrenoid appearance in relation to the culture age, as well as in the fate of parental pyrenoids during daughter colony formation.     

Chloroplast ultrastructure changes in Pisum sativum associated with supplementary ultraviolet (UV-B) radiation

Pea plants (Pisum sativum L. cv. Greenfeast) were grown and exposed to supplementary UV-B radiation from day 17 after planting under growth cabinet conditions. The effects of this exposure on the ultrastructure of chloroplasts and the total soluble sugar and starch concentrations were estimated. Supplementary UV-B radiation was shown to damage the structure of chloroplasts, as manifested by dilation of thylakoid membranes, a progressive disruption of the thylakoid structure and disintegration of the double membrane envelope surrounding the chloroplast, accompanied by the accumulation of large starch grains. Diurnal changes observed in starch concentration suggest that the higher concentration of starch in supplementary UV-B-treated leaves is due to its immobilization, rather than to any increase in starch synthesis: soluble sugars accumulated and remained at a higher level and then later declined.