Dismantling of Arabidopsis thaliana mesophyll cell chloroplasts during natural leaf senescence

One of the earliest events in the process of leaf senescence is dismantling of chloroplasts. Mesophyll cell chloroplasts from rosette leaves were studied in Arabidopsis thaliana undergoing natural senescence. The number of chloroplasts decreased by only 17% in fully yellow leaves, and chloroplasts were found to undergo progressive photosynthetic and ultrastructural changes as senescence proceeded. In ultrastructural studies, an intact tonoplast could not be visualized, thus, a 35S-GFP::δ-TIP line with a GFP-labeled tonoplast was used to demonstrate that chloroplasts remain outside of the tonoplast even at late stages of senescence. Chloroplast DNA was measured by real-time PCR at four different chloroplast loci, and a fourfold decrease in chloroplast DNA per chloroplast was noted in yellow senescent leaves when compared to green leaves from plants of the same age. Although chloroplast DNA did decrease, the chloroplast/nuclear gene copy ratio was still 31:1 in yellow leaves. Interestingly, mRNA levels for the four loci differed: psbA and ndhB mRNAs remained abundant late into senescence, while rpoC1 and rbcL mRNAs decreased in parallel to chloroplast DNA. Together, these data demonstrate that, during senescence, chloroplasts remain outside of the vacuole as distinct organelles while the thylakoid membranes are dismantled internally. As thylakoids were dismantled, Rubisco large subunit, Lhcb1, and chloroplast DNA levels declined, but variable levels of mRNA persisted.     

A large population of small chloroplasts in tobacco leaf cells allows more effective chloroplast movement than a few enlarged chloroplasts

We generated transgenic tobacco (Nicotiana tabacum cv Xanthi) plants that contained only one to three enlarged chloroplasts per leaf mesophyll cell by introducing NtFtsZ1-2, a cDNA for plastid division. These plants were used to investigate the advantages of having a large population of small chloroplasts rather than a few enlarged chloroplasts in a leaf mesophyll cell. Despite the similarities in photosynthetic components and ultrastructure of photosynthetic machinery between wild-type and transgenic plants, the overall growth of transgenic plants under low- and high-light conditions was retarded. In wild-type plants, the chloroplasts moved toward the face position under low light and toward the profile position under high-light conditions. However, chloroplast rearrangement in transgenic plants in response to light conditions was not evident. In addition, transgenic plant leaves showed greatly diminished changes in leaf transmittance values under both light conditions, indicating that chloroplast rearrangement was severely retarded. Therefore, under low-light conditions the incomplete face position of the enlarged chloroplasts results in decreased absorbance of light energy. This, in turn, reduces plant growth. Under high-light conditions, the amount of absorbed light exceeds the photosynthetic utilization capacity due to the incomplete profile position of the enlarged chloroplasts, resulting in photodamage to the photosynthetic machinery, and decreased growth. The presence of a large number of small and/or rapidly moving chloroplasts in the cells of higher land plants permits more effective chloroplast phototaxis and, hence, allows more efficient utilization of low-incident photon flux densities. The photosynthetic apparatus is, consequently, protected from damage under high-incident photon flux densities.     

Photosynthesis and Other Traits in Relation to Chloroplast Number during Soybean Leaf Senescence

Soon after attaining full expansion, soybean (Glycine max [L.] Merr.) leaves enter a senescence phase marked by decline in photosynthetic rate and the progressive loss of chloroplast activity and composition. Our primary goal was to determine if this loss could be accounted for by sequential degradation of whole chloroplasts or by simultaneous degeneration of all chloroplasts. Total photosynthesis (TPs) measured as ¹⁴CO₂ uptake, chloroplast number, ribulose 1,5-bisphosphate carboxylase activity, uncoupled photosynthetic electron transport activity, soluble protein content, and chlorophyll content declined progressively during the 37 days after full leaf expansion. During this period, chloroplast number per unit leaf area was constant for all genotypes studied. We conclude that leaf senescence may be a two-stage process wherein the first stage chloroplast activity and composition declines, but chloroplast numbers do not change. During a brief terminal stage (11 days in our experiment), whole chloroplasts may be lost as well. As a second objective we wished to determine if variation in single-leaf total photosynthetic rate among soybean cultivars is related to corresponding variation in chloroplast number and/or chloroplast activity/composition. By comparing the means for three cultivars known to have rapid leaf TPs and for the three known to have slow TPs, we found the former group to be superior to the latter for all the previously mentioned leaf physiological traits. This superiority was related primarily to differences in chloroplast number and only secondarily to differences in activity and composition per chloroplast.     

The responses of light interception,photosynthesis and fruit yield of cucumber to LED‐lighting within the canopy

Mathematical models of light attenuation and canopy photosynthesis suggest that crop photosynthesis increases by more uniform vertical irradiance within crops. This would result when a larger proportion of total irradiance is applied within canopies (interlighting) instead of from above (top lighting). These irradiance profiles can be generated by Light Emitting Diodes (LEDs). We investigated the effects of interlighting with LEDs on light interception, on vertical gradients of leaf photosynthetic characteristics and on crop production and development of a greenhouse‐grown Cucumis sativus‘Samona’ crop and analysed the interaction between them. Plants were grown in a greenhouse under low natural irradiance (winter) with supplemental irradiance of 221 µmol photosynthetic photon flux m^?2 s^?1 (20 h per day). In the interlighting treatment, LEDs (80% Red, 20% Blue) supplied 38% of the supplemental irradiance within the canopy with 62% as top lighting by High‐Pressure Sodium (HPS)‐lamps. The control was 100% top lighting (HPS lamps). We measured horizontal and vertical light extinction as well as leaf photosynthetic characteristics at different leaf layers, and determined total plant production. Leaf mass per area and dry mass allocation to leaves were significantly greater but leaf appearance rate and plant length were smaller in the interlighting treatment. Although leaf photosynthetic characteristics were significantly increased in the lower leaf layers, interlighting did not increase total biomass or fruit production, partly because of a significantly reduced vertical and horizontal light interception caused by extreme leaf curling, likely because of the LED‐light spectrum used, and partly because of the relatively low irradiances from above.     

Comparative ecophysiology of leaf and canopy photosynthesis

Leaves and herbaceous leaf canopies photosynthesize efficiently although the distribution of light, the ultimate resource of photosynthesis, is very biased in these systems. As has been suggested in theoretical studies, if a photosynthetic system is organized such that every photosynthetic apparatus photosynthesizes in concert, the system as a whole has the sharpest light response curve and is most adaptive. This condition can be approached by (i) homogenization of the light environment and (ii) acclimation of the photosynthetic properties of leaves or chloroplasts to their local light environments. This review examines these two factors in the herbaceous leaf canopy and in the leaf. Changes in the inclination of leaves in the canopy and differentiation of mesophyll into palisade and spongy tissue contribute to the moderation of the light gradient. Leaf and chloroplast movements in the upper parts of these systems under high irradiances also moderate light gradients. Moreover, acclimation of leaves and chloroplasts to the local light environment is substantial. These factors increase the efficiency of photosynthesis considerably. However, the systems appear to be less efficient than the theoretical optimum. When the systems are optically dense, the light gradients may be too great for leaves or chloroplasts to acclimate. The loss of photosynthetic production attributed to the imperfect adjustment of photosynthetic apparatus to the local light environment is most apparent when the photosynthesis of the system is in the transition between the light-limited and light-saturated phases. Although acclimation of the photosynthetic apparatus and moderation of light gradients are imperfect, these markedly raise the efficiency of photosynthesis. Thus more mechanistic studies on these adaptive attributes are needed. The causes and consequences of imperfect adjustment should also be investigated.     

Nonphotosynthetic retardation of chloroplast senescence by light

Excised apical portions of green wheat leaf sections were treated with aminotriazole to prevent formation of new chloroplasts. Illumination retarded the decline in chlorophyll content per leaf section, the disintegration of chloroplast ultrastructure, and the loss of capacity for photosynthetic carbon fixation. We interpret these 3 effects of illumination as facets of a single light effect in retarding chloroplast senescence. This light effect in retarding chloroplast senescence has features differing from characteristics of photosynthetic carbon fixation. For example, A) application of the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea did not decrease, and may have even slightly increased, the effectiveness of light; B) although the action spectrum contains peaks in the blue and red regions, it differs from the action spectrum for photosynthetic CO₂ assimilation in wheat; C) in nonphotosynthesizing tissue, application of sugars did not retard chloroplast senescence; D) light saturation was achieved by only a few hundred microwatts/cm². Considered together with the well-known light requirement for chloroplast formation, our results indicate that light has a dual, photomorphogenetic control in maintaining the green status of the plant by also exerting a second effect: retarding of senescence of chloroplasts already present.     

A New Model for Leaf Photosynthesis Incorporating the Gradients of Light Environment and of Photosynthetic Properties of Chloroplasts within a Leaf

A leaf photosynthesis model was constructed based upon the notionthat the leaf photosynthesis is a summation of photosynthesisof each chloroplast under in situ micro-environmental conditions.Intra-leaf light environment was calculated using the valuesof transmittance and reflectance of leaf tissues reported previously.Simulations of light response curves of whole leaf photosynthesiswere carried out for the model leaves with different patternsof gradients in light environment and/or in photosynthetic activities.The results indicate that the higher absorption coefficientof chlorophyll in spongy tissue than in palisade tissue andintra-leaf vertical gradient in photosynthetic activity of thechloroplasts as reported for real dorsiventral leaves are bothadvantageous to the productivity of the leaf because these propertiesraise the efficiency of the light utilization. Intra-leaf light environment, leaf photosynthesis, light utilization, palisade tissue, photosynthetic productivity, spongy tissue     

The proteomics of senescence in leaves of white clover,Trifolium repens (L.)

A proteomics approach has been used to study changes in protein abundance during leaf senescence in white clover. Changes in cell ultrastructure were also examined using transmission electron microscopy. The most obvious ultrastructural changes during senescence occurred in chloroplasts, with progressive loss of thylakoid integrity and accumulation of osmiophilic globules in the stroma. Quantitative analysis of 590 leaf protein spots separated by two-dimensional electrophoresis indicated that approximately 40% of the spots showed significant senescence related changes in abundance. Approximately one-third of the protein spots present in mature green leaves were also visible by two-dimensional electrophoresis of an isolated chloroplast fraction, and these spots represented a major proportion of the proteins showing senescence related declines in abundance. Chloroplast proteins that were identified by matrix-assisted laser desorption/ionization-time of flight mass fingerprinting included rubisco large and small subunits, a rubisco activase and the 33 kDa protein of the photosystem II oxygen-evolving complex. These proteins declined in abundance late in senescence, indicating that the photosynthetic apparatus was being degraded. A chloroplast glutamine synthetase showed partial decline in abundance during late senescence but was maintained at levels that may support provision of glutamine for export to other tissues. The results emphasise the importance of proteolysis, chloroplast degradation and remobilisation of nitrogen in leaf senescence.     

Changes in the Number and Composition of Chloroplasts during Senescence of Mesophyll Cells of Attached and Detached Primary Leaves of Wheat (Triticum aestivum L.)

Changes in the number and composition of chloroplasts of mesophyll cells were followed during senescence of the primary leaf of wheat (Triticum aestivum L.). Senescence was due to the natural pattern of leaf ontogeny or was either induced by leaf detachment and incubation in darkness, or incubation of attached leaves in the dark. In each case discrete sections (1 centimeter) of the leaf, representing mesophyll cells of the basal, middle, and tip regions, were examined. For all treatments, senescence was characterized by a loss of chlorophyll and the protein ribulose 1,5-bisphosphate carboxylase (RuBPCase). Chloroplast number per mesophyll cell remained essentially constant during senescence. It was not until more than 80% of the plastid chlorophyll and RuBPCase was degraded that some reduction (22%) in chloroplast number per mesophyll cell was recorded and this was invariably in the mesophyll cells of the leaf tip. We conclude that these data are consistent with the idea that degradation occurs within the chloroplast and that all chloroplasts in a mesophyll cell senesce with a high degree of synchrony rather than each chloroplast senescing sequentially.     

Chloroplast Protein Synthesis During Barley Leaf Growth and Senescence: Effect of Leaf Excision

Chloroplast protein synthesis was measured during the expansion,maturity and senescence of the oldest leaf of barley, Hordeumvulgare L., var. Hassan. A maximum rate of protein synthesisoccurred near the end of the expansion stage 9 d after sowing.Protein synthesis increased again at the beginning of senescenceand reached a new maximum at day 14 after sowing. Detachmentand incubation of leaves in the dark stimulated chioroplastprotein synthesis by fully expanded or by senescent leaves butnot by expanding leaves. If the detached leaves were kept inthe light, chloroplast protein synthesis was stimulated in fullyexpanded but not in senescent leaves. Short treatments (18 h)of leaf segments with growth substances in either light or indarkness, significantly changed the rate of protein synthesisshown by chloroplasts. The relationship between chloroplastprotein synthesis and leaf senescence is discussed. Key words: Hormones, light, maturity     

Profiles of Leaf Nitrogen and Light in Reproductive Canopies of Cotton (Gossypium hirsutum)

During vegetative growth, the vertical profile of leaf nitrogen(N) often parallels the profile of light distribution withinthe canopy. This is more advantageous in terms of canopy photosynthesisthan a uniform distribution of leaf N. We investigated the influenceof both reproductive growth and N supply on the profiles ofN and light in canopies of irrigated cotton crops (Gossypiumhirsutum L.). Regular samplings were made from soon after theonset of reproductive growth until crop maturity. Every 2 weeks,a 1 m²sample of the canopy was cut in four successive verticallayers of equal thickness. Leaf area and N concentration (%)in each layer were measured. The vertical N gradient becamemore marked with ongoing reproductive development. It is hypothesizedthat because of the high rate of growth after the onset of reproductivedevelopment and the long duration of this phase compared toother species, the whole canopy photosynthetic benefit thatwould accrue from maintaining the N gradient is likely to beaccentuated. The rate of decline in leaf N concentration ina layer was not related to either the initial concentrationin the leaves nor the boll load within the layer.Copyright 2001Annals of Botany Company Gossypium hirsutum, leaf nitrogen, light profile, nitrogen, nitrogen distribution, remobilization, reproductive growth     

Effect of Benzyladenine on Diurnal Changes in DNA Content per Chloroplast and Chloroplast Replication in Intact Bean Leaves

The effect of benzyladenine (BA) on the diurnal changes in DNAand Chl contents per chloroplast and chloroplast replicationin primary leaves of bean plants (Phaseolus vulgaris L.) grownunder a 16 h light/8 h dark cycle was studied. Experiments weremade on primary leaves in the early expansion phase, where celldivision had been completed but chloroplasts were replicating.In untreated controls, chloroplast number, Chl content and freshweight per leaf showed daily periodic changes. Chl content perchloroplast increased in the light period every day, and freshweight per leaf increased most rapidly in the early dark period.Chloroplast number per leaf increased rapidly in the early darkperiod on day 9, though the increase began a little earlierand was less sharp on days 8 and 10. During these periods, DNAcontent per chloroplast was decreasing due to chloroplast divisionas chloroplast DNA (ctDNA) per leaf remained unchanged throughoutthe experimental period. BA induced increases in Chi contentper chloroplast, ctDNA content and fresh weight per leaf within6 h of its application, regardless of whether it was appliedat or 10 h after the beginning of the light period. Applicationof BA at 10 h in the light period shifted the start of chloroplastreplication by 6 h compared to that in untreated controls. However,when BA was applied at the beginning of illumination, the startof chloroplast replication showed the same relative change intime as above. 5-Fluorodeoxyuridine (5-FdU) promptly preventedBA-induced increase in Chl content and chloroplast number perleaf as well as ctDNA content per leaf.     

Changes in the Number and Size of Chloroplasts during Senescence of Primary Leaves of Wheat Grown under Different Conditions

Changes in the number and size of chloroplasts in mesophyllcells were investigated in primary leaves of wheat from fullexpansion to yellowing under different growth conditions. Thenumber of chloroplasts per cell decreased slowly, although thedecrease was steady and statistically significant, until thelast stage of leaf senescence, when rapid degradation of chloroplaststook place. Rates of leaf senescence, or the decline in thenumber of chloroplasts, varied greatly among plants grown atdifferent seasons of the year, but about 20% of chloroplastsalways disappeared during the phase when steady loss of chloroplastsoccurred. The area of chloroplast disks also decreased graduallybut significantly, with a rapid decrease late in senescence.Thus, the total quantity of chloroplasts per mesophyll celldecreased substantially during leaf senescence. Yellowed leavescontained numerous structures that resemble oil drops but nochloroplasts. Decreases in rates of photosynthesis that occurduring senescence may, therefore, be largely due to decreasesin the quantity of chloroplasts. However, a better correlationwas found between the decrease in the maximum capacity for photosynthesisand the degradation of RuBP carboxylase. When plants had beengrown with a sufficient supply of nutrients, the number of chloroplastsdecreased steadily but at a reduced rate and the reduction inthe area of chloroplast disks was strongly suppressed. Thus,the quantitative decrease in chloroplasts in senescing leavesappears to be regulated by the requirements for nutrients (nitrogen)of other part of the plant. ³Present address: Department of Biology, Faculty of Science,Toho University, Miyama, Funabashi, Chiba, 274 Japan     

INFLUENCE OF AGE AND MICROCLIMATE ON THE PHOTOCHEMISTRY OF RHODODENDRON MAXIMUM LEAVES II. CHLOROPLAST STRUCTURE AND PHOTOSYNTHETIC LIGHT RESPONSE

The influence of age on chloroplast structure and photosynthetic light response of Rhododendron maximum L. was studied in three different microhabitats. The three microhabitats constituted a gradient of low, intermediate, and high irradiance levels. The most dramatic change in chloroplast structure with increasing age was the proliferation of the number and size of plastoglobuli. The magnitude and age specific rate of chloroplast occlusion by plastoglobuli increased in habitats with higher irradiance. Photosynthetic responses to light differed among the age categories of leaves. Light saturated photosynthesis and quantum yield decreased as leaves aged. However, in high light environments the rate of reduction of quantum yield or light saturated photosynthetic rate was more rapid than in the low light environment. The quantity of plastoglobuli increased in association with reduced light reaction capacity. The presence and abundance of plastoglobuli in R. maximum chloroplasts and their association with reduced photosynthetic performance indicates that the photosynthetic apparatus of the R. maximum chloroplast is sensitive to photodestruction by high irradiance: commonly a winter phenomenon in these environments.     

Leaf dynamics,self-shading and carbon gain in seedlings of a tropical pioneer tree

We examined leaf dynamics and leaf age gradients of photosynthetic capacity and nitrogen concentration in seedlings of the tropical pioneer tree, Heliocarpus appendiculatus, grown in a factorial design under controlled conditions with two levels each of nutrients, ambient light (light levels incident above the canopy), and self-shading (the gradient of light levels from upper to lower leaves on the shoot). Correlations among these parameters were examined in order to determine the influence of self-shading, and the regulation of standing leaf numbers, on leaf longevity and its association with leaf photosynthetic capacity. Leaf longevity and the number of leaves on the main shoot were both reduced in high light, while in the low light environment, they were reduced in the steeper self-shading gradient. In high nutrients, leaf longevity was reduced whereas leaf number increased. Leaf initiation rates were higher in the high nutrient treatment but were not influenced by either light treatment. Maximum-light saturated photosynthetic rate, on an area basis, was greater in the high light and nutrient treatments, while the decline in photosynthetic capacity in realtion to leaf position on the shoot was more rapid in high light and in low nutrients. Leaf longevity was negatively correlated among treatments with initial photosynthetic capacity. The leaf position at which photosynthetic capacity was predicted to reach zero was positively correlated with the number of leaves on the shoot, supporting the hypothesis that leaf numbers are regulated by patterns of self-shading. The negative association of longevity and initial photosynthetic capacity apparently arises from different associations among gradients of photosynthetic capacity, leaf numbers and leaf initiation rates in relation to light and nutrient availability. The simultaneous consideration of age and position of leaves illuminates the role of self-shading as an important factor influencing leaf senescence and canopy structure and dynamics.     

Changes of Anatomical Features, Photosynthesis and Ribulose Bisphosphate Carboxylase-Oxygenase Content of Mango Leaves

Changes in anatomical and physiological features, includingchanges in amount per unit area of anthocyanin and chlorophyll,in leaves of seedling mango (Mangifera indica L. cv. Irwin)trees were determined to understand what controls the rate ofphotosynthesis (Pn) at various stages of development. The youngleaves of seedling trees contained high concentrations of anthocyanin.During enlargement of leaves, the disappearance of anthocyaninand the accumulation of chlorophyll occurred concomitantly;the anthocyanin content began to decrease markedly once theleaf area had reached a maximum. During the early period ofleaf development, the thickness of mesophyll tissue decreasedtemporarily, but when the length of the leaf reached half thatof a mature leaf, the mesophyll began to thicken again. Smallstarch grains appeared in the chloroplasts of the young leavesand chloroplast nucleoids (ct-nuclei) were distributed throughoutthe chloroplasts. When leaves matured, ct-nuclei were displacedto the periphery of chloroplasts because of the accumulationof large starch grains. Compared with young leaves, green andmature leaves contained greater concentrations of ribulose bisphosphatecarboxylase-oxygenase (RuBisCO) protein. The results of immunocytochemicalexamination of RuBisCO under the light microscope reflectedthe results of electrophoresis measurements of RuBisCO. Pn waslow during the chocolate-coloured stage of early leaf development.In green and mature leaves Pn was higher; the average Pn was7·6 mg CO₂ dm^-2 h^-1 under light at intensities above500 µmol m^-2 s^-1.Copyright 1995, 1999 Academic Press Mangifera indica L., mango leaf, chloroplast nucleoids, chloroplast ultrastructure, starch accumulation, anthocyanin, chlorophyll, DAPI staining, SDS-PAGE, immunocytochemical technique     

Release of Nitrogen From Chloroplasts During Leaf Senescence in Rice (Oryza sativa L.)

In order to ascertain the possibility that nitrogen associatedwith chloroplasts serves as a major source of nitrogen redistributedfrom senescent leaves, chloroplasts were isolated from riceleaves and changes with leaf age in total leaf nitrogen andchloroplast nitrogen were examined. Results presented here showthat decrease in total leaf nitrogen during leaf senescencewas closely correlated with decrease of chloroplast nitrogenand roughly 85–95 per cent of leaf nitrogen released fromsenescent leaves during the experimental period could be accountedfor by a loss of chloroplast nitrogen. By dividing chloroplastnitrogen into two fractions, i.e. lamellar and stroma fractions,the question of which fraction was more deeply concerned withthe loss of leaf nitrogen was clarified. Results suggested thatin the vegetative stage of plant growth the stroma was mainlyresponsible for the loss of leaf nitrogen. On the other hand,nitrogen was released from lamellar and stromal fractions atalmost the same rate during the reproductive stage. Oryza sativa L., rice, chloroplasts, nitrogen, leaf senescence     

Increase in DNA Content of Primary Leaves of Phaseolus vulgaris upon Decapitation

Changes in DNA content of bean (Phaseolus vulgaris) primaryleaves after decapitation were investigated. When apical budswere removed at 11 d, DNA content per leaf increased by about20% at 15 d and then decreased in parallel with the controls.The RNA and chlorophyll contents, fresh weight, and leaf areaexpressed on a single leaf basis changed in the same manneras the DNA content in response to decapitation. But when bothapical and lateral buds were removed, all these values continuedincreasing during the test period. Thus, growing lateral budsand apical buds have the same effect on the DNA change in primaryleaves as that due to ageing of the leaves. Cell number perleaf was not increased by any treatment, indicating that theobserved increase in the DNA content of primary leaves is ascribableto an increase in DNA per cell. Next, the whole shoots above the nodes of primary leaves wereremoved at various ages. The response of primary leaves to decapitationvaried according to their age. With age, they lost the abilityto increase their fresh weight, area, and chlorophyll contentbut not their DNA and RNA contents in response to decapitation.Decapitation stimulated chloroplast replication only withinthe period in which chloroplasts were replicating in controlleaves, but it induced chloroplast enlargement at any age. Therefore,the increase in DNA content after decapitation may be partiallydue to an increase in the amount of chloroplast DNA. When stems were heat-girdled above the nodes of the primaryleaves, these leaves showed responses similar to but smallerthan those to decapitation. The senescence of primary leavesseems to be controlled by the distribution of substances whichare transported from the roots.     

Changes in Chlorophyll Content and Organization During Senescence of the Primary Leaves of Phaseolus vulgaris L. in Relation to Photosynthetic Electron Transport

A large decrease was observed in the chlorophyll content ofthe primary leaves of Phaseolus vulgaris during senescence.Chloroplasts isolated from mature and senescent leaves gavevery similar light saturation curves for electron transportreactions involving either PS I or PS II, indicating that theaverage number of chlorophyll molecules associated with eachreaction centre did not change during senescence. It is concludedthat the reaction centres ceased to function at the same timeas, or perhaps before, their antenna chlorophylls were lostfrom the thylakoid membrane, and that the percentage decreasein the number of functional reaction centres per leaf was atleast as great as the percentage decrease in the leaf chlorophyllcontent. The chlorophyll-protein composition of thylakoid membrane preparationswas examined by electrophoresis of samples treated with sodiumdodecyl sulphate. In older leaves a smaller proportion of thechlorophyll applied to polyacrylamide gels was associated withthe P700- chlorophyll a-protein complex. There was also a declinein emission at 734 nm in the 77 °K fluorescence spectrumof intact leaf tissue during senescence. These results indicatethat older leaves contained a smaller proportion of chlorophyllsassociated with PS I, and this is consistent with the decreaseobserved in the leaf chlorophyll a/b ratio during senescence.The effect of these changes in chlorophyll content on the capacityof the chloroplast to carry out photosynthetic electron transportis discussed.     

日光温室光温因子对黄瓜叶绿体超微结构及其功能的影响

在日光温室内,研究了光温因子对黄瓜叶绿体超微结构及其功能的影响．结果表明,因季节之间光、温条件不同,日光温室黄瓜叶片显微结构和叶绿体超微结构有一定差异,1月份光照弱叶肉细胞较大,而5月份光照强叶绿体数较多．在该试验条件下,未发现叶片光合速率与叶绿体超微结构之间有直接或密切的相关性．在各生长季节其光合速率均为第4叶>初展叶>基部叶,与叶龄及各叶位的受光量有关．如果将不同叶位叶放在相同的光照下,则差异明显减少．黄瓜叶片的叶肉细胞、叶绿体和淀粉粒的大小以及叶绿体数、基粒数、基粒厚度、基粒片层数都随叶位的下降而呈增加趋势。不同品种、同品种不同生长时期的叶片显微结构和叶绿体超微结构及其功能也有一定的差异．限制日光温室冬季黄瓜光合作用的主要因素是光照弱、有效光照时数少,而在晴天温度的限制作用相对较小。阴天因光照弱而导致的室内低温则是限制黄瓜生长的关键因素．