Characterization of photosynthetic pigment composition, photosystem II photochemistry and thermal energy dissipation during leaf senescence of wheat plants grown in the field

Photosynthetic pigment composition and photosystem II (PSII) photochemistry were characterized during the flag leaf senescence of wheat plants grown in the field. During leaf senescence, neoxanthin and beta-carotene decreased concomitantly with chlorophyll, whereas lutein and xanthophyll cycle pigments were less affected, leading to increases in lutein/chlorophyll and xanthophyll cycle pigments/chlorophyll ratios. The chlorophyll a/b ratio also increased. With the progression of senescence, the maximal efficiency of PSII photochemistry decreased only slightly in the early morning (low light conditions), but substantially at midday (high light conditions). Actual PSII efficiency, photochemical quenching and the efficiency of excitation capture by open PSII centres decreased significantly both early in the morning and at midday and such decreases were much greater at midday than in the early morning. At the same time, non-photochemical quenching, zeaxanthin and antheraxanthin contents at the expense of violaxanthin increased both early in the morning and at midday, with a greater increase at midday. The results in the present study suggest that a down-regulation of PSII occurred in senescent leaves and that the xanthophyll cycle plays a role in the protection of PSII from photoinhibitory damage in senescent leaves by dissipating excess excitation energy, particularly when exposed to high light.     

RuBP carboxylase activity in wheat following ear emergence and evaluation of the role of ear

Ribulose bisphosphate carboxylase activity and chlorophyll content were measured in the leaves and reproductive parts of two wheat cultivars after ear emergence. The chlorophyll content of the flag leaf was mostly higher than that of the awns and glumes. Awns had the highest chlorophyll content among reproductive parts. Light transmission to the lower leaves was higher in the dwarf cultivar Moti than in the medium tall cultivar Kalyansona. RuBP carboxylase activity in Kalyansona leaves was higher than in Moti leaves. In postanthesis stages there was no difference in RuBP carboxylase activity in the flag leaf and lower leaves between cultivars. Awns had the maximum activity of RuBP carboxylase followed by glumes and grains among ear parts. The relative capacity for photosynthesis in the ear parts was several times higher than in the flag leaf on a unit chlorophyll basis. It is suggested that in a crop canopy in the field, the spike(ear) may have a greater importance in grain development than has been previously estimated.     

Relationships between carotenoid composition and growth habit in British plant species

The pigment composition of leaves from a number of different plant species collected from field sites in the region of Sheffield, UK, have been compared using high-performance liquid chromatography. Expression of pigment content per unit leaf area was dominated by variation in the total leaf chlorophyll. Neither chlorophyll per unit area nor the chlorophyll a/b ratio were found to be correlated with the habitat from which the plants originated. When the amounts of different carotenoids were expressed relative to the total carotenoid pool, it was found that whilst neither total carotene (α- +β-carotene) nor neoxanthin correlated with ability to grow in shade, the leaf content of both lutein and the total xanthophyll cycle carotenoids (zeaxanthin, anther-axanthin and violaxanthin) did, with lutein content being high in shade species and xanthophyll cycle intermediates low. There was a strong negative correlation between the relative amounts of each of these groups of carotenoids. The ratio of lutein to xanthophyll cycle carotenoids was strongly correlated to an index of shade tolerance.     

Carbon isotope ratios in ear parts of triticale : influence of grain filling

Four triticale (×Triticosecale Wittmack) genotypes were grown under rainfed conditions with limited irrigation support in Lleida in northeast Spain. For each variety, samples consisting of 10 tillers with half-sterilized spikes were taken three times from anthesis to maturity. Carbon isotope ratios (δ¹³C) were then determined in water extracts from ear bracts (glumes, paleas, and lemmas), awns and flag leaves, and in powdered kernels. For the half-sterilized spikes, carbon isotope analysis was carried out separately in bracts and awns from fertile and nonfertile spikelets. The δ¹³C in the water-soluble fraction of awns, glumes, and glumells from fruitless spikelets was significantly higher than that from fertile spikelets sampled at mid-grain filling. Differences in δ¹³C among sterile and fertile spikelets were not significant in samples taken a few days after anthesis or at maturity. These results are in accordance with some degree of refixation by awns and ear bracts of the CO₂ respired by grains during grain filling. There was progressively higher δ¹³C from flag leaf blades to awns, glumes, and glumells. This variation in δ¹³C along plant parts may be caused by differences in the ratio of assimilation rate to CO₂-diffusive conductance. Values of δ¹³C of mature kernels were between the values at anthesis and mid-grain filling for the water-soluble fraction of flag leaves and inner bracts and were fairly similar to those of glumes and awns.     

Photosynthesis and Transpiration in Leaves and Ears of Wheat and Barley Varieties

Carbon exchange rate (CER) and transpiration were measured inflag leaves, whole ears, glumes (referring to the total areaof glumes and lemmas) and awns, in six hexaploid spring wheats(Triticum aestivum L.), three cultivated tetraploid spring wheats(T. turgidum L.), four wild tetraploid wheats (T. dicoccoides),eight six-rowed barleys (Hordeum vulgare L.) and five two-rowedbarleys (H. vulgare L.). Differences between varieties and between species in total earCER and transpiration were associated largely with differencesin ear surface area rather than with rates per unit area. Ratesof CER and transpiration per unit area of ears were 40–80%of those of flag leaves, depending on the species. However, since ear surface area was greater than flag leaf areaby a factor of 1.1, 3.9, 5.5 and 4.4, in hexaploid wheat, tetraploidwheat, six-rowed barley, and two-rowed barley, respectively,total ear CER reached up to 90% of that of the flag leaf. The contribution of awns to total ear CER depended largely ontotal awn surface area per ear, rather than on CER per unitawn area. Awns contributed about 40–80% of total spikeCER, depending on the species, but only 10–20% of spiketranspiration. The disproportionately small contribution ofawns to ear transpiration was caused by the very low rate oftranspiration per unit area of awns. Thus, while transpirationratio (CER/transpiration) was about the same in flag leavesand glumes, it was higher by several orders of magnitude inthe awns. A large amount of awns in the ear is therefore a drought adaptiveattribute in these cereals, for which tetraploid wheat exceededhexaploid wheat and six-rowed barley exceeded two-rowed barley. Key words: Carbon exchange rate, Transpiration, Barley, Wheat     

Light-dependent photosynthetic characteristics indicated by chlorophyll fluorescence in five mangrove species native to Pohnpei Island,Micronesia

To investigate the adaptive capacity to strong light among mangrove species, we examined light-dependent properties of photosynthesis in relation to photoinhibition using chlorophyll fluorescence for sun-leaves of five mangrove tree species; Sonneratia alba, Rhizophora stylosa, Rhizophora apiculata, Bruguiera gymnorrhiza and Xylocarpus granatum. These species were classified into three groups; pioneer -S. alba and R. stylosa, intermediate -R. apiculata and B. gymnorrhiza and climax species -X. granatum. A further distinction can be made between the two intermediate species, the less shade-tolerant R. apiculata and the shade-tolerant B. gymnorrhiza. There was a clear association between successional status and light-saturated electron transport rate (ETR) where ETR was higher in S. alba and R. stylosa > R. apiculata and B. gymnorrhiza > X. granatum. Based on its lower initial slope of light-dependent ETR and lower ratio of variable to maximum fluorescence (Fv/Fm) after a dark-adaptation, X. granatum appeared to suffer greater photoinhibition than other species. The response of qP, which represents PSII openness, to incident light indicates a ranking of tolerant capacity to photoinhibition of S. alba and R. stylosa > R. apiculata > B. gymnorrhiza and X. granatum. The difference in qP response between R. apiculata and B. gymnorrhiza might be attributed to the higher leaf absorptance and the lower degree of thermal dissipation indicated by the higher F'v/F'm in B. gymnorrhiza. It appears that the co-ordination of leaf absorptance, thermal dissipation and consumption of absorbed light energy through electron transport is important in the acclimation of mangrove species to exposed habitats.     

Photosynthesis and chlorophyllafluorescence during flag leaf senescence of field-grown wheat plants

The characteristics of photosynthetic gas exchange, chlorophyll a fluorescence, and xanthophyll cycle pigments during flag leaf senescence of field-grown wheat plants were investigated. With senescence progressing, the light-saturated net CO₂ assimilation rate expressed either on a basis of leaf area or chlorophyll decreased significantly. The apparent quantum yield of net photosynthesis decreased when expressed on a leaf area basis but increased when expressed on a chlorophyll basis. The maximal efficiency of PSII photochemistry decreased very little while actual PSII efficiency, photochemical quenching, and the efficiency of excitation capture by open PSII centers decreased considerably. At the same time, non-photochemical quenching increased significantly. A substantial decrease in the contents of violaxanthin and zeaxanthin, but a slight decrease in the content of antheraxanthin were observed. However, the de-epoxidation status of the xanthophyll cycle was positively correlated with progressive senescence. This increase was due mainly to a smaller decrease in zeaxanthin than in violaxanthin. Our results suggest that PSII apparatus remained functional, but a down-regulation of PSII occurred under the steady state of photosynthesis in senescent flag leaves. Such a down-regulation was associated with the closure of PSII centers and an enhanced xanthophyll cycle-related thermal dissipation in the PSII antennae.     

Xanthophyll cycle pool size and composition in relation to the nitrogen content of apple leaves

The objective of this study was to determine xanthophyll cycle pool size and composition in response to N status and their relationships to non-photochemical quenching in apple leaves. Bench-grafted Fuji/M.26 trees were fertilized with different N concentrations (0-20 mM) in a modified Hoagland's solution for 6 weeks to create a wide range of leaf N status (1-4.4 g m(-2)). Chlorophyll content, xanthophyll cycle pool size, lutein, total carotene, and neoxanthin on a leaf area basis all increased linearly with increasing leaf N. However, only the ratios of the xanthophyll cycle pool and of lutein to chlorophyll were higher in low N leaves than in high N leaves. Under high light at midday, both zeaxanthin (Z), expressed on a chlorophyll basis, and the percentage of the xanthophyll cycle pool present as Z, increased as leaf N decreased. Thermal dissipation of excitation energy, measured as non-photochemical quenching of chlorophyll fluorescence, was positively related to, whereas efficiency of excitation transfer and photosystem II quantum efficiency were negatively related to, Z, expressed on a chlorophyll basis or on a xanthophyll cycle pool basis. It is concluded that both xanthophyll cycle pool size (on a chlorophyll basis) and conversion of violaxanthin to zeaxanthin are enhanced in response to N limitation to dissipate excessive absorbed light under high irradiance.     

Characterization of the Xanthophyll Cycle and Other Photosynthetic Pigment Changes Induced by Iron Deficiency in Sugar Beet (Beta vulgaris L.)

In this work we characterize the changes induced by iron deficiency in the pigment composition of sugar beet (Beta vulgaris L.) leaves. When sugar beet plants were grown hydroponically under limited iron supply, neoxanthin and β-carotene decreased concomitantly with chlorophyll a, whereas lutein and the carotenoids within the xanthophyll cycle were less affected. Iron deficiency caused major increases in the lutein/chlorophyll a and xanthophyll cycle pigments/chlorophyll a molar ratios. Xanthophyll cycle carotenoids in Fe-deficient plants underwent epoxidations and de-epoxidations in response to ambient light conditions. In dark adapted Fe-deficient plants most of the xanthophyll cycle pigment pool was in the epoxidated form violaxanthin. We show, both by HPLC and by in vivo 505 nanometers absorbance changes, that in Fe deficient plants and in response to light, the de-epoxidated forms antheraxanthin and zeaxanthin were rapidly formed at the expense of violaxanthin. Several hours after returning to dark, the xanthophyll cycle was shifted again toward violaxanthin. The ratio of variable to maximum chlorophyll fluorescence from intact leaves was decreased by iron deficiency. However, in iron deficient leaves this ratio was little affected by light conditions which displace the xanthophyll cycle toward epoxidation or de-epoxidation. This suggests that the functioning of the xanthophyll cycle is not necessarily linked to protection against excess light input.     

Iron deficiency-induced changes in the photosynthetic pigment composition of field-grown pear (Pyrus communis L) leaves

In this work we characterize the changes induced by iron deficiency in the pigment composition of pear (Pyrus communis L.) leaves grown under high light intensities in field conditions in Spain. Iron deficiency induced decreases in neoxanthin and β-carotene concomitantly with decreases in chlorophyll a, whereas lutein and carotenoids within the xanthophyll cycle were less affected. Iron deficiency caused major increases in the lutein/chlorophyll a and xanthophyll cycle pigments/chlorophyll a molar ratios. The chlorophyll a/chlorophyll b ratio increased in response to iron deficiency. The carotenoids within the xanthophyll cycle in iron-deficient and in iron-sufficient (control) leaves underwent epoxidations and de-epoxidations in response to ambient light conditions. In control leaves dark-adapted for several hours, most of the xanthophyll cycle pigment pool was in the epoxidated form vio-laxanthin, whereas iron-deficient leaves had significant amounts of zeaxanthin. Iron-deficient leaves also exhibited an increased non-photochemical quenching, supporting the possibility of a role for pigments within the xanthophyll cycle in photoprotection.     

Grain filling in relation to monocarpic senescence of wheat in varying source-sink ratios

Flag leaf removal at any stage of grain growth hastened senescence (reduction in chlorophyll content) of the sterile glumes whereas a removal of the latter did not alter senescence of the former. Kernel mass, grain mass per ear, harvest index and sink activity reduced more conspicuously by the removal of glumes than by the flag leaf removal. Removal of grains hastened senescence of the glumes only, although protein content increased at a later grain removal in the majority of source organs. Moisture contents of the developing grains were mostly reduced by the removal of either the flag leaf or the glumes, though flag leaf removal at anthesis affected it the most. Protein levels of the developing grains reduced at harvest only when the glumes were removed at anthesis but sugar levels in later phases always decreased irrespective of the time of removal and nature of the source organs.     

铝胁迫对蓼科植物生长和光合、蒸腾特性的影响

采用水培试验,设置5种铝处理浓度,研究了铝对3种蓼科植物酸模叶蓼、杠板归和辣蓼叶片光合、蒸腾和叶绿素荧光参数的影响。结果表明,高铝处理(400μmol.L-1)显著抑制3种蓼科植物地上部和根系生长,并且导致3种蓼科植物叶片叶绿素含量、Chla/Chlb、净光合速率(Pn)、水分利用效率(WUE)、PSII光合电子传递量子效率(φPSII)和光化学猝灭系数(qP)显著下降。中低铝处理(25~100μmol.L-1)时,与对照相比,酸模叶蓼生物量显著增加,杠板归显著减少,辣蓼先增加后减少。其中,酸模叶蓼和辣蓼叶绿素含量、Chla/Chlb、Pn、蒸腾速率(Tr)、胞间CO2浓度(Ci)、PSII最大光化学效率(Fv/Fm)、qP均未发生显著变化,但辣蓼WUE、φPSII和非光化学猝灭系数(NPQ)显著下降,酸模叶蓼无显著变化;而杠板归除Ci、Fv/Fm外,其余叶片光合、蒸腾及叶绿素荧光参数均出现显著下降。上述结果表明,酸模叶蓼在中低铝处理条件下可通过保持较高的叶绿素含量、Chla/Chlb、WUE、Pn、PSII反应中心光化学反应效率以及提高非辐射能量耗散来增强其对铝的耐性。     

Photoprotective Effects of D1 Protein Turnover and the Lutein Cycle on Three Ephemeral Plants under Heat Stress

To clarify the characteristics of photoinhibition and the primary defense mechanisms of ephemeral plant leaves against photodestruction under high temperature stress, inhibitors and the technology to determine chlorophyll fluorescence were used to explore the protective effects of D1 protein turnover and the lutein cycle in the high temperature stress of the leaves of three ephemeral plants. The results showed that the maximum light conversion efficiency (Fv/Fm) of the ephemeral plant leaves decreased, and the initial fluorescence (Fo) increased under 35°C ± 1°C heat stress for 1–4 h or on sunny days in the summer. Both Fv/Fm and Fo could be recovered after 8 h of darkness or afternoon weakening of the external temperature. Streptomycin sulfate (SM) or dithiothreitol (DTT) accelerated the decrease of Fv/Fm and the photochemical quenching coefficient (qP) in the leaves of three ephemeral plants at high temperature, and the decrease was greater in the SM than in the DTT treatment. When the high temperature stress was prolonged, the Y(II) values of light energy distribution parameters of PSII decreased, and the Y(NPQ) and Y(NO) values increased gradually in all the treatment groups of the three ephemeral plants. The results showed that the leaves of the three ephemeral plants had their own highly advanced mechanisms to protect against photodamage, which inhibited the turnover of D1 protein and xanthophyll cycle. This can damage the PSII reaction center in the leaves of the three ephemeral plants under high temperature. The protective effect of D1 protein turnover on heat stress in Erodium oxyrrhynchum and Senecio subdentatus was greater than that of the lutein cycle, while the protective effect of lutein cycle was greater than that of D1 protein turnover in Heliotropium acutiflorum subjected to heat damage.     

追施氮肥时期对冬小麦旗叶叶绿素荧光特性的影响

在大田条件下,研究了不同追氮时期对小麦旗叶叶绿素荧光特性、光合速率及籽粒产量的影响.结果表明,拔节期追肥较起身期或挑旗期追肥,改善了小麦旗叶PSⅡ的活性(F_v/F_o)、光化学最大效率(F_v/F_m)、光化学猝灭系数(qP)、实际量子产量(ΦPSⅡ)及光合速率,降低了籽粒灌浆中前期非辐射能量耗散,有利于叶片所吸收的光能较充分地用于光合作用,提高了籽粒灌浆后期非辐射能量的耗散,减缓了叶片光抑制程度和衰老进程.拔节期追肥可显著增加穗粒数和千粒重,提高产量.     

Effect of Heat Stress on Photosynthetic Characteristics of Different Green Organs of Winter Wheat During Grain-filling Stage

Four winter wheat ( Triticum aestivum L.) varieties (“JD 8”, “Jing 411”,“Centurk” and “Tam 202”) were used to study the effect of heat stress on photosynthetic characteristics of flag leaf blade, flag leaf sheath, peduncle, glume, lemma and awn during grain-filling stage. The results showed that heat acclimation during grain-filling stage increased thermotolerance of wheat with significant differences among different green organs. During heat stress, the decreases of the efficiency of primary light energy conversion ( Fv/Fm ) of PS and pigment (chlorophyll and carotenoid) content were much slower in peduncle, flag leaf sheath and glume than in flag leaf blade, lemma and awn; and the percentage of decrease in net photosynthetic rate ( Pn ) of ear was lower than that of the flag leaf blade. The measured photosynthetic parameters ( Fv/Fm , Pn and pigment content) of “JD 8”, a relatively heat tolerant variety, declined more slowly than those of the other three varieties during the whole heat stress period.      

超级高产小麦旗叶荧光动力学研究

探明超级小麦品种的旗叶光合作用与荧光动力学特性,为超级小麦品种选育利用提供理论依据。以超级小麦临麦4号为试验材料,应用CI-301PS型便携式光合作用测定系统和FMS-2便携式荧光测定仪(英国Hansatech公司)在田间试验中测定旗叶光合作用与荧光动力学参数。结果表明,与普通高产对照品种皖麦52和烟农19相比,超级小麦临麦4号的光合作用参数光合速率、光饱和点和CO2饱和点、羧化效率高,光补偿点和CO2补偿点低;光合机构系统工作参数PSII实际的光化学效率(ΦPSII)、光化学猝灭系数(qP)、PSII反应中心的激发能捕获效率(Fv/Fm)、PSⅡ潜在活性Fv/Fo和电子传递速率(ETR)值高,非光化学猝灭系数(NPQ)值低。这表明超级小麦临麦4号的光合机构系统工作能力强和工作效率高,保证旗叶光合作用的高效运行,为子粒灌浆提供充足的能量和碳水化合物。     

Monocarpic senescence in wheat: Influence of sterile glumes and ear

The senescence of sterile glumes, flag leaf and the other two leaves below the ear of wheat ( Triticum aestivum L. cv. Sonalika) was studied in relation to grain development and surgical manipulation. The senescence of sterile glumes was faster than that of the leaves in terms of chlorophyll and protein degradation. The flag leaf senesced later than the other two leaves below it. Removal of sterile glumes markedly reduced the harvest index (crop: straw ratio) and average dry weight per grain as compared to removal of the flag leaf. Maximum grain weight was achieved after the glumes had senesced completely. Removal of the ear delayed senescence of all the three leaves. It is concluded that sterile glumes are important suppliers of assimilate for grain filling and that nutrient drainage is the primary cause of the monocarpic senescence in wheat.     

Effects of groundwater depth variation on photosynthesis and photoprotection of <Emphasis Type="Italic">Elaeagnus angustifolia</Emphasis> L.

Physiological and photosynthetic responses were investigated at three different depths of groundwater (DGW: 1.4, 2.4, and 3.4 m) in Elaeagnus angustifolia L., a locally adapted tree to the arid region in northwest China. Predawn leaf water potential and chlorophyll content declined gradually with the increasing DGW, whereas there was little effect on predawn variable-to-maximum chlorophyll fluorescence ratio F _v/F _m and leaf carotenoid compositions (xanthophyll cycle pool, neoxanthin, lutein, and β-carotene). Net photosynthetic rate (P _n), quantum yield of PSII electron transport (Φ_PSII), stomatal conductance (Gs), and intercellular CO₂ concentration (Ci) declined obviously; however, P _n decreased more than Φ_PSII at deeper DGW. The photoinhibition of PSII at all three DGW occurred at midday in summer and increased as DGW increased. The ΔpH-dependent thermal dissipation and the level of de-epoxidation of the xanthophyll cycle at all three DGW reached their maxima at midday with the increase of light intensity. However, the fraction of functional PSII and light intensity at deeper DGW (2.4, 3.4 m) showed a negative correlation. This correlation suggested that most of violaxanthin was converted into zeaxanthin at midday, and the reversible inactivation of partial PSII reaction centers took place at deeper DGW. These results together suggest that both the xanthophyll cycle-dependent thermal dissipation and the reversible inactivation of partial PSII might have played important roles in avoiding the excess light-induced energy damage in leaves of this tree species at deeper DGW.     

Photo- and antioxidative protection during summer leaf senescence in Pistacia lentiscus L. grown under Mediterranean field conditions

Summer leaf senescence in Pistacia lentiscus L. plants serves to remobilize nutrients from the oldest leaves to the youngest ones, and therefore contributes to plant survival during the adverse climatic conditions typical of Mediterranean summers, i.e. water deficit superimposed on high solar radiation and high temperatures. To evaluate the extent of photo- and antioxidative protection during leaf senescence of this species, changes in carotenoids, including xanthophyll cycle pigments, and in the levels of ascorbate and alpha-tocopherol were measured prior to and during summer leaf senescence in 3-year-old plants grown under Mediterranean field conditions. Although a chlorophyll loss of approx. 20% was observed during the first stages of leaf senescence, no damage to the photosynthetic apparatus occurred as indicated by constant maximum efficiencies of photosystem II photochemistry. During this period the de-epoxidation state of the xanthophyll cycle, and lutein, neoxanthin and ascorbate levels were kept constant. At the same time beta-carotene and alpha-tocopherol levels increased by approx. 9 and 70%, respectively, presumably conferring photo- and antioxidative protection to the photosynthetic apparatus. By contrast, during the later stages of leaf senescence, characterized by severe chlorophyll loss, carotenoids were moderately degraded (neoxanthin by approx. 20%, and both lutein and beta-carotene by approx. 35%), ascorbate decreased by approx. 80% and alpha-tocopherol was not detected in senescing leaves. This study demonstrates that mechanisms of photo- and antioxidative protection may play a major role in maintaining chloroplast function during the first stages of leaf senescence, while antioxidant defences are lost during the latest stages of senescence.     

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.