Photosynthesis and Chlorophyll Fluorescence Characteristics in Relationship to Changes in Pigment and Element Composition of Leaves of Platanus occidentalis L. during Autumnal Leaf Senescence

The loss of chlorophyll and total leaf nitrogen during autumnal senescence of leaves from the deciduous tree Platanus occidentalis L. was accompanied by a marked decline in the photosynthetic capacity of O₂ evolution on a leaf area basis. When expressed on a chlorophyll basis, however, the capacity for light-and CO₂-saturated O₂ evolution did not decline, but rather increased as leaf chlorophyll content decreased. The photon yield of O₂ evolution in white light (400-700 nanometers) declined markedly with decreases in leaf chlorophyll content below 150 milligrams of chlorophyll per square meter on both an incident and an absorbed basis, due largely to the absorption of light by nonphotosynthetic pigments which were not degraded as rapidly as the chlorophylls. Photon yields measured in, and corrected for the absorptance of, red light (630-700 nanometers) exhibited little change with the loss of chlorophyll. Furthermore, PSII photochemical efficiency, as determined from chlorophyll fluorescence, remained high, and the chlorophyll a/b ratio exhibited no decline except in leaves with extremely low chlorophyll contents. These data indicate that the efficiency for photochemical energy conversion of the remaining functional components was maintained at a high level during the natural course of autumnal senescence, and are consistent with previous studies which have characterized leaf senescence as being a controlled process. The loss of chlorophyll during senescence was also accompanied by a decline in fluorescence emanating from PSI, whereas there was little change in PSII fluorescence (measured at 77 Kelvin), presumably due to decreased reabsorption of PSII fluorescence by chlorophyll. Nitrogen was the only element examined to exhibit a decline with senescence on a dry weight basis. However, on a leaf area basis, all elements (C, Ca, K, Mg, N, P, S) declined in senescent leaves, although the contents of sulfur and calcium, which are not easily retranslocated, decreased to the smallest extent.     

Regreening of senescent Nicotiana leaves. I. Reappearance of NADPH-protochlorophyllide oxidoreductase and light-harvesting chlorophyll a/b-binding protein

Decapitation of Nicotiana rustica L. plants above a single senescent leaf induced regreening, which was promoted by cytokinin treatment. Regreening required low light. The decline in leaf protein content and increase in protease activity seen during senescence were reversed on regreening. Western blotting showed that light-harvesting chlorophyll a/b-binding protein declined considerably during senescence, but on regreening it increased back to the levels seen in green leaves. NADPH-protochlorophyllide oxidoreductase (POR) was found by Western blotting at high levels in etiolated cotyledons, but at low levels in green leaves and not at all in senescent leaves. However, POR reappeared in regreening leaves, and cytokinin accelerated its increase.     

Leaf movement,stress avoidance and photosynthesis in Vitis californica

Summary Gas exchange and chlorophyll fluorescence techniques were used to evaluate the hypothesis that leaf movement in Vitis californica Benth. (California wild grape) allows a compromise between sunlight interception and stress damage in order to maximize photosynthetic carbon gain over the life of the leaf. Leaves that were restrained horizontally tolerated their increased radiation loads if critical temperatures were not exceeded. Reductions in photosynthetic capacity and the F _V/F _M fluorescence ratio only occurred in leaves that attained high temperatures. Leaf orientation and canopy position were important determinants of leaf temperature. These results indicate that excessive leaf temperature, not high PFD, can be a principle cause of reduced carbon gain and senescence in this species in the wild. Leaf movement appears to protect photosynthetic components in midsummer.     

Effects of ozone on the photosynthetic apparatus and leaf proteins during leaf development in wheat

Leaves of Triticum aestivum cv. Avalon were grown in an atmosphere that contained 150 nmole mol^-1 ozone for 7h each day. After leaves had reached maximum size, the leaf blade was divided into three sections to provide tissue of different age, the youngest at the base of the blade and the oldest at the leaf tip. The ozone treatment was found to decrease significantly the light-saturated rate and quantum yield of CO₂ assimilation and the maximum quantum yield of photosystem II photochemistry in the oldest leaf section. No effects were found on the basal and middle sections of the leaf. These ozone-induced decreases in the photosynthetic parameters were associated with decreases in the efficiency of utilization of light for CO₂ assimilation at the photon flux density under which the leaves were grown. The depression in photosynthetic performance of tissue near the leaf tip was accompanied by large decreases in the contents of total, soluble and thylakoid proteins and chlorophyll. There was also found to be a preferential loss of ribulose-1,5-carboxylase-oxygenase. These ozone-induced changes in chlorophyll and protein contents and the photosynthetic activities of the leaf tissue were similar to changes normally associated with leaf senescence. Two-dimensional polyacrylamide gel analyses of leaf proteins demonstrated the loss of some minor, and unidentified, proteins, whilst another group of minor proteins appeared. It is concluded that daily exposure of the leaf to 150 nmol mol^-1 ozone for 7h had no effect on the development of the photosynthetic apparatus and its activities during leaf expansion, but it did promote the onset of premature senescence in fully expanded tissue that resulted in a loss of pigments, proteins and photosynthetic capacity and efficiency.     

Seasonal changes in light and temperature affect the balance between light harvesting and light utilisation components of photosynthesis in an evergreen understory shrub

In a temperate climate, evergreen species in the understory are exposed to large changes in photosynthetic photon flux density (PPFD) and temperature over the year. We determined the photosynthetic traits of leaves of an evergreen understory shrub Aucuba japonica at three sites at monthly intervals: understorys of a deciduous forest; an evergreen forest; and a gap in a mixed forest. This set up enabled us to separate the effects of seasonal change in PPFD and temperature on photosynthetic acclimation under natural conditions. The effects of PPFD and temperature were analysed by simple and multiple regression analyses. The amounts of light utilisation components (LU), represented by nitrogen and rubisco contents per area, were higher in winter, when temperature was low and PPFD was high. The LU relative to the amount of light harvesting components (LH), represented by chlorophyll a/b and rubisco/chlorophyll ratios, and the inverse of chlorophyll/nitrogen ratio were also higher in winter. We quantified the effects of PPFD and temperature on the LU and LH components. Across sites PPFD had stronger effects than air temperature, while within a site temperature had stronger effects on photosynthetic acclimation. We concluded that the photosynthetic apparatus is strongly affected by the prevailing PPFD at the time of leaf development. Within a given light regime, however, plants acclimated by increasing LU relative to LH primarily in response to temperature and to a lesser extent to PPFD.     

Changes in ribulosebisphosphate carboxylase/oxygenase and ribulose 5-phosphate kinase abundances and photosynthetic capacity during leaf senescence

The abundances of ribulose-1,5-bisphosphate carboxylate/oxygenase (Rubisco) and ribulose-5-phosphate (Ru5P) kinase in field-grown soybean (Glycine max L. Merr.) leaves were quantified by a Western blot technique and related to changes in chlorophyll and photosynthetic capacity during senescence. Even though the leaf content of Rubisco was approximately 80-fold greater than that of Ru5P kinase, the decline in the levels of these two Calvin cycle enzymes occurred in parallel during the senescence of the leaves. Moreover, the decrease in the content of Rubisco was accompanied by parallel decreases of both the large and small subunits of this enzyme but not by an accumulation of altered large or small subunit isoforms. With increasing senescence, decreases in abundances of Rubisco, Ru5P kinase and chlorophyll were closely correlated with the decline in photosynthetic capacity; thus, the specific photosynthetic capacity when expressed per abundance of any of these parameters was rather constant despite an 8-fold decrease in photosynthetic capacity. These results suggest that during senescence of soybean leaves the chloroplast is subject to autolysis by mechanisms causing an approximately 80-fold greater rate of loss of Rubisco than Ru5P kinase.Jointly supported by the United States Department of Agricultural Research Service and the Kentucky Agricultural Experiment Station, Lexington (paper No. 88 3 286).Mention of a commercial product does not constitute endorsement by the United States Department of Agriculture.     

Isolation,characterization, and mapping of the stay green mutant in rice

Leaf color turns yellow during senescence due to the degradation of chlorophylls and photosynthetic proteins. A stay green mutant was isolated from the glutinous japonica rice Hwacheong-wx through N-methyl-N-nitrosourea mutagenesis. Leaves of the mutant remained green, while turning yellow in those of the wild-type rice during senescence. The stay green phenotype was controlled by a single recessive nuclear gene, tentatively symbolized as sgr(t). All the phenotypic characteristics of the mutant were the same as those of the wild-type lines except for the stay green trait. The leaf chlorophyll concentration of the mutant was similar to that of the wild-type before heading, but decreased steeply in the wild-type during grain filling, while very slowly in the mutant. However, no difference in photosynthetic activity was observed between the stay green mutant and the yellowing wild-type leaves, indicating that senescence is proceeding normally in the mutant leaves and that the mutation affects the rate of chlorophyll degradation during the leaf senescence. Using phenotypic and molecular markers, we mapped the sgr(t) locus to the long arm of chromosome 9 between RFLP markers RG662 and C985 at 1.8- and 2.1-cM intervals, respectively. Received: 29 April 2001 / Accepted: 17 July 2001     

Photosynthetic Characteristics of Rice Leaves Aged under Different Irradiances from Full Expansion through Senescence

Effects of irradiance on photosynthetic characteristics were examined in senescent leaves of rice (Oryza sativa L.). Two irradiance treatments (100 and 20% natural sunlight) were imposed after the full expansion of the 13th leaf through senescence. The photosynthetic rate was measured as a function of intercellular CO₂ pressure with a gas-exchange system. The amounts of cytochrome f, coupling factor 1, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and chlorophyll were determined. The coupling factor 1 and cytochrome f contents decreased rapidly during senescence, and their rates of decrease were much faster from the 20% sunlight treatment than from the full sunlight treatment. These changes were well correlated with those in the photosynthetic rate at CO₂ pressure = 600 microbars, but not with those under the ambient air condition (350 microbars CO₂) and 200 microbars CO₂. This suggested that the amounts of coupling factor 1 and cytochrome f from the full sunlight treatment cannot be limiting factors for the photosynthetic rate at ambient air conditions. The Rubisco content also decreased during senescence, but its decrease from the 20% sunlight treatment was appreciably retarded. However, this difference was not reflected in the photosynthetic rates at the ambient and 200 microbars CO₂. This implied that in vivo Rubisco activity may be regulated in the senescent leaves from the 20% sunlight treatment. The chlorophyll content decreased most slowly. In the 20% sunlight treatment, it remained apparently constant with a decline in chlorophyll a/b ratio. These photosynthetic characteristics of the senescent rice leaves under low irradiance were discussed in relation to acclimation of shade plants.     

Effect of Leaf Age and Shading on Photosynthesis in Potatoes (Solanum tuberosum)

The effects of leaf age and of shading on photosynthetic rateand on other leaf parameters of potato (Solanum tuberosum L.)were studied using a portable gas exchange system. A rapid decreasein the rate of photosynthesis during leaf senescence was observed.This was accompanied by an increase in stomatal resistance,and as a result a fairly constant level of sub-stomatal CO₂concentration was maintained at all leaf ages. The reductionin the photosynthetic rate in older leaves was therefore assumedto be essentially mesophyllic in origin, whereas the stomatalresponse was probably secondary. Canopy density significantly affected the rate of photosyntheticreduction with leaf age. Leaves maintained under high radiationintensities manifested a slower decline in their photosyntheticrate, especially in the early stages of their senescence, thanleaves kept under shade conditions. The latter leaves were foundto be more adapted to low radiation intensities, as indicatedby changes in their chlorophyll a:b ratio and specific leafweight Solanum tuberosum L, potato, photosynthetic rate, mesophyll, stomata, leaf age, radiation intensity, chlorophyll a:b ratio     

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.     

Effects of Shading on the Senescence and Photosynthetic Physiology of the Early-Flowering Rice Mutant FTL10 at Noon

FTL10 is an early-flowering mutant of rice (Oryza sativa L.) with a premature senescent phenotype. Early leaf senescence can cause negative effects on rice yield. Moreover, rice leaves are damaged under high-light conditions, which promote rice senescence. Artificial shading can reduce the amount of light absorbed by rice leaves. The aim of this study was to investigate the effects of shading at noon (11:30–14:00) on the senescence and yield of FTL10. The results showed that shading improved the total antioxidant capacity of rice leaves, reduced the accumulation of reactive oxygen species (ROS) and reduced the expression of genes related to senescence. In the shaded group, the degradation rate of chlorophyll and Rubisco proteins, which are related to photosynthesis, was relatively slow, and the photosynthetic rate was relatively high. Compared with those under the natural growth conditions, the proportion of photosynthetic electron allocated to photorespiration in the shaded group rice leaves was lower, and the proportion allocated to carbon fixation was higher. The yield data showed that the single-spike weight and yield per plant of rice significantly increased after shading. Therefore, our research shows that shading at noon could delay FTL10 senescence and increase yields.

     

Changes of Photosynthetic Characteristics in Relation to Leaf Senescence in Two Maize Hybrids with Different Senescent Appearance

A field experiment was conducted to investigate the changes in chlorophyll (Chl) and nitrogen (N) contents, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and phosphoenolpyruvate carboxylase (PEPC) contents and PEPC activity, and the photon-saturated net photosynthetic rate (P _Nsat), and their relationships with leaf senescence in two maize hybrids with different senescent appearance. One stay-green (cv. P3845) and one earlier senescent (cv. Hokkou 55) hybrid were used in this study, and we found that Chl and N contents and the P _Nsat in individual leaves of P3845 were greater than those in corresponding leaves of Hokkou 55 at the successive growth stages. In addition, larger contents of RuBPCO and PEPC, and a greater activity of PEPC were observed in P3845. Due to the lower rates of decrease of Chl, RuBPCO, and PEPC amounts per unit of N, and the lower net C translocation rate per unit of N in the stay-green hybrid, leaf senescence was delayed in comparison to the earlier senescent hybrid.     

Chlorophyllase activities and chlorophyll degradation during leaf senescence in non-yellowing mutant and wild type of Phaseolus vulgaris L

The activities of chlorophyllase, contents of pigments including chlorophyll a and b, chlorophyllide a and b, and phaeophorbide a during leaf senescence under low oxygen (0.5% O2) and control (air) were investigated in a non-yellowing mutant and wild-type leaves of snap beans (Phaseolus vulgaris L.). Chlorophyllase from leaf tissues had maximum activity when incubated at 40C in a mixture containing 50% acetone. In both mutant and wild type, chlorophyllase activity was the highest in freshly harvested non-senescent leaves and decreased sharply in the course of senescence, indicating that the loss of chlorophylls in senescing leaves is not directly related to the activity of chlorophyllase and that chlorophyllase activity is not altered in the mutant. The wild type had higher ratios of chlorophyll a to chlorophyll b than the mutant and chlorophyll a : b ratios increased during senescence in both types. In the senescent mutant leaves, accumulations of chlorophyllide a and chlorophyllide b were detected, but no phaeophorbide a was found. Chlorophyllide b had a greater accumulation than chlorophyllide a in the early stage of senescence. Low oxygen treatment not only delayed chlorophyll degradation but also enhanced the accumulations of chlorophyllide a and b and lowered the ratios of chlorophyll a to chlorophyll b.     

Biochemical Changes that Occur during Senescence of Wheat Leaves : I. Basis for the Reduction of Photosynthesis

Changes in activities of photosynthetic enzymes and photochemical processes were followed with aging of vegetative and flag leaves of wheat (Triticum aestivum L. cv Roy). Activities of stromal enzymes began to decline prior to photochemical activities. In general, total soluble protein and the activities of ribulose-1,5-bisphosphate carboxylase and NADP-triose-phosphate dehydrogenase declined in parallel and at an earlier age than leaf chlorophyll (Chl), leaf photosynthesis, and photosynthetic electron transport activity. Leaves appeared to lose whole chloroplasts as opposed to a general degradation of all chloroplasts based on three lines of evidence: (a) electron transport activity calculated on an area basis declined much earlier than the same data expressed on a Chl basis; (b) Chl content per chloroplast was similar for mature and senescent tissue; and (c) the absorbance at 550 nanometers (light scattering) per unit of Chl remained essentially constant until the end of senescence. Chloroplasts did, however, undergo some modifications before they were lost (e.g. loss of stromal enzyme activities), but the reduction in leaf photosynthesis was apparently caused by a loss of whole chloroplasts.     

Photoinhibition and photoprotection in senescent leaves of field-grown wheat plants

Photosynthesis, photosystem II (PSII) photochemistry, photoinhibition and the xanthophyll cycle in the senescent flag leaves of wheat (Triticum aestivum L.) plants grown in the field were investigated. Compared to the non-senescent leaves, photosynthetic capacity was significantly reduced in senescent flag leaves. The light intensity at which photosynthesis was saturated also declined significantly. The light response curves of PSII photochemistry indicate that a down-regulation of PSII photochemistry occurred in senescent leaves in particular at high light. The maximal efficiency of PSII photochemistry in senescent flag leaves decreased slightly when measured at predawn but substantially at midday, suggesting that PSII function was largely maintained and photoinhibition occurred in senescent leaves when exposed to high light. At midday, PSII efficiency, photochemical quenching and the efficiency of excitation capture by open PSII centers decreased considerably, while non-photochemical quenching increased significantly. Moreover, compared with the values at early morning, a greater decrease in CO₂ assimilation rate was observed at midday in senescent leaves than in control leaves. The levels of antheraxanthin and zeaxanthin via the de-epoxidation of violaxanthin increased in senescent flag leaves from predawn to midday. An increase in the xanthophyll cycle pigments relative to chlorophyll was observed in senescent flag leaves. The results suggest that the xanthophyll cycle was activated in senescent leaves due to the decrease in CO₂ assimilation capacity and the light intensity for saturation of photosynthesis and that the enhanced formation of antheraxanthin and zeaxanthin at high light may play an important role in the dissipation of excess light energy and help to protect photosynthetic apparatus from photodamage. Our results suggest that the well-known function of the xanthophyll cycle to safely dissipate excess excitation energy is also important for maintaining photosynthetic function during leaf senescence.     

Modifications in photosystem II photochemistry in senescent leaves of maize plants

Analyses of CO2 exchange and chlorophyll fluorescence were carried out to assess photosynthetic performance during senescence of maize leaves. Senescent leaves displayed a significant decrease in CO2 assimilatory capacity accompanied by a decrease in stomatal conductance and an increase in intercellular CO2 concentration. The analyses of fluorescence quenching under steady-state photosynthesis showed that senescence resulted in an increase in non-photochemical quenching and a decrease in photo-chemical quenching. It also resulted in a decrease in the efficiency of excitation energy capture by open PSII reaction centres and the quantum yield of PSII electron transport, but had very little effect on the maximal efficiency of PSII photochemistry. The results determined from the fast fluorescence induction kinetics indicated an increase in the proportion of QB-non-reducing PSII reaction centres and a decrease in the rate of QA reduction in senescent leaves. Theoretical analyses of fluorescence parameters under steady-state photosynthesis suggest that the increase in the non-photochemical quenching was due to an increase in the rate constant to thermal dissipation of excitation energy by PSII and that the decrease in the quantum yield of PSII electron transport was associated with a decrease in the rate constant of PSII photochemistry. Based on these results, it is suggested that the decrease in the quantum yield of PSII electron transport in senescent leaves was down-regulated by an increase in the proportion of QB-non-reducing PSII reaction centres and in the non-photochemical quenching. The photosynthetic electron transport would thus match the decreased demand for ATP and NADPH in carbon assimilation which was inhibited significantly in senescent leaves.Key words: Chlorophyll fluorescence, gas exchange, maize (Zea mays L.), photochemical and non-photochemical quenching, photosystem II photochemistry.      

Leaf expansion and carbon assimilation in cotton leaves grown at two photosynthetic photon flux densities

Increasing photosynthetic photon flux density (PPFD) received during development from 5.5 to 31.2 mol m^-2 d^-1 resulted in greater leaf and mesophyll cell surface areas in cotton (Gossypium hirsutum L.). The relationships between the amounts of these surface areas and potential CO₂ assimilation by these leaves were evaluated. Leaf area (epidermal surface area of one side of a leaf), mesophyll cell surface area, and net rate of CO₂ uptake (P_n) were measured from the time leaves first unfolded until P., was substantially reduced. At the higher PPFD, leaf and mesophyll surface areas increased more rapidly during expansion, and P_n per unit leaf area was greater than at the lower PPFD. Although leaves at the higher PPFD reached the maximum P., per unit mesophyll cell surface area 4 to 5 days earlier than leaves at the lower PPFD, the maxima for these P., were similar. Leaves grown at the higher PPFD had the potential to assimilate 2.2, 3.5, or 5.8 times the amount of CO₂ as leaves from the lower PPFD when P., was expressed per unit mesophyll surface, per unit leaf surface, or per whole leaf, respectively. Greater and earlier development of both P., and mesophyll cell surface area at higher PPFD apparently had a compounding effect on the potential for carbon assimilation by a leaf.     

Carbon metabolism and gas exchange in leaves of Zea mays L.

The aim of this work was to investigate the mechanism of formation of triose phosphates and 3-phosphoglycerate during photosynthetic induction in leaves of Zea mays. Simultaneous measurements of gas exchange, chlorophyll a fluorescence and metabolite contents of maize leaves were made. Leaves illuminated in the absence of CO₂ showed a build-up of triose phosphates during the first 2 min of illumination which was comparable to the build-up observed in the presence of CO₂. Isolated mesophyll protoplasts, which lack the Calvin cycle, also showed a build-up of triose phosphates upon illumination. Leaves contained amounts of phosphoglycerate mutase and enolase adequate to account for the formation of triose phosphates and 3-phosphoglycerate from intermediates of the C₄ cycle and their precursors.