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.     

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.     

田间大豆叶片成长过程中的光合特性及光破坏防御机制

田间大豆叶片在成长进程中光饱和光合速率持续提高,但气孔导度的增加明显滞后.尽管叶片在成长初期就具有较高的最大光化学效率,但是仍略低于发育成熟的叶片.随着叶片的成长,光下叶片光系统Ⅱ实际效率增加;非光化学猝灭下降.幼叶叶黄素总量与叶绿素之比较高,随着叶面积的增加该比值下降,在光下,幼叶的脱环氧化程度较高.因此认为大豆叶片成长初期就能够有效地进行光化学调节;在叶片生长过程中依赖叶黄素循环的热耗散机制迅速建立起来有效抵御强光的破坏.     

Photoinhibition and the xanthophyll cycle are not enhanced in the salt-acclimated halophyte Artimisia anethifolia

The effects of high salinity (up to 400 m M NaCl) on photosystem II (PSII) photochemistry, photoinhibition and the xanthophyll cycle were investigated in the halophyte Artimisia anethifolia grown under outdoor conditions. In order to examine the changes in PSII photochemistry, photoinhibition, thermal dissipation associated with the xanthophyll cycle in salt-acclimated plants, the experiments were conducted at midday on a clear day (maximal irradiance 1500 μmol m⁻¹ s⁻¹) and on a cloudy day (maximal irradiance 700 μmol m⁻¹ s⁻¹), respectively. With increasing salt concentration, the accumulation of sodium and chloride in leaves increased considerably while the relative growth rate and CO₂ assimilation rate decreased significantly. Salinity induced no effects on PSII photochemistry, thermal energy dissipation, and the contents of the xanthophyll cycle pigments either on a clear day or on a cloudy day. However, when compared with those on a cloudy day, PSII photochemistry decreased and thermal energy dissipation increased significantly in both control and salt-acclimated plants on a clear day. The levels of zeaxanthin and antheraxanthin at the expense of violaxanthin were higher on a clear day than on a cloudy day. The results suggest that photoinhibition and the xanthophyll cycle were not induced by high salinity but by high light only in A. anethifolia plants. The results also suggest that A. anethifolia showed high resistance not only to high salinity, but also to photoinhibition even when it was treated with high salinity and exposed to full sunlight.     

Changes in activity of energy dissipating mechanisms in wheat flag leaves during senescence

Abstract: Excitation energy dissipation, including the xanthophyll cycle, during senescence in wheat flag leaves grown in the field was investigated at midday and in the morning. With progress of senescence, photosynthesis (Pn) and actual PSII photochemical efficiency (ΦPSII) decreased markedly at midday. The decrease in extent of Pn was greater than that of ΦPSII. However, there was no significant decline in Pn and ΦPSII observed in the morning, except in leaves 60 days after anthesis. The kinetics of xanthophyll cycle activity, thermal dissipation (NPQ), and q_f observed at midday during senescence exhibited two distinct phases. The first phase was characterized by an increase of xanthophyll cycle activity, NPQ, and q_f during the first 45 days after anthesis. The second phase took place 45 days after anthesis, characterized by a dramatic decline in the above parameters. However, the qI, observed both at midday and in the morning, always increased along with senescence. A larger proportion of NPQ insensitive to DTT (an inhibitor of the de-epoxidation of V to Z) was also observed in severely senescent leaves. In the morning, only severely senescent leaves showed higher xanthophyll cycle activity, NPQ, q_f, and qI. It was demonstrated that, at the beginning of senescence or under low light, wheat leaves were able to dissipate excess light energy via NPQ, depending on the xanthophyll cycle. However, the xanthophyll cycle was insufficient to protect leaves against photodamage under high light, when leaves became severely senescent. The ratio of (Fj - Fo)/(Fp - Fo) increased gradually during the first 45 days after anthesis, but dramatically increased 45 days after anthesis. We propose that another photoprotection mechanism might exist around reaction centres, activated in severely senescent leaves to protect leaves from photodamage.     

PSII photochemistry, thermal energy dissipation, and the xanthophyll cycle in Kalanchoë daigremontiana exposed to a combination of water stress and high light

Kalanchoë daigremontiana, a CAM plant grown in a greenhouse, was subjected to severe water stress. The changes in photosystem II (PSII) photochemistry were investigated in water‐stressed leaves. To separate water stress effects from photoinhibition, water stress was imposed at low irradiance (daily peak PFD 150 μmol m^?2 s^?1). There were no significant changes in the maximal efficiency of PSII photochemistry (F_v/F_m), the traditional fluorescence induction kinetics (OIP) and the polyphasic fluorescence induction kinetics (OJIP), suggesting that water stress had no direct effects on the primary PSII photochemistry in dark‐adapted leaves. However, PSII photochemistry in light‐adapted leaves was modified in water‐stressed plants. This was shown by the decrease in the actual PSII efficiency (Φ_PSII), the efficiency of excitation energy capture by open PSII centres (F_v′/F_m′), and photochemical quenching (q_P), as well as a significant increase in non‐photochemical quenching (NPQ) in particular at high PFDs. In addition, photoinhibition and the xanthophyll cycle were investigated in water‐stressed leaves when exposed to 50% full sunlight and full sunlight. At midday, water stress induced a substantial decrease in F_v/F_m which was reversible. Such a decrease was greater at higher irradiance. Similar results were observed in Φ_PSII, q_P, and F_v′/F_m′. On the other hand, water stress induced a significant increase in NPQ and the level of zeaxanthin via the de‐epoxidation of violaxanthin and their increases were greater at higher irradiance. The results suggest that water stress led to increased susceptibility to photoinhibition which was attributed to a photoprotective process but not to a photodamage process. Such a photoprotection was associated with the enhanced formation of zeaxanthin via de‐epoxidation of violaxanthin. The results also suggest that thermal dissipation of excess energy associated with the xanthophyll cycle may be an important adaptive mechanism to help protect the photosynthetic apparatus from photoinhibitory damage for CAM plants normally growing in arid and semi‐arid areas where they are subjected to a combination of water stress and high light.     

The xanthophyll cycle is induced by light irrespective of water status in field-grown lavender (Lavandula stoechas) plants

The water relations, the photosynthetic capacity and the pigment content of leaves, i.e. chlorophylls, carotenes and xanthophylls, were analysed during the summer drought and recovery after autumn rainfalls in lavender ( Lavandula stoechas L.) plants grown in Mediterranean field conditions. Summer drought caused photoinhibition of photosynthesis and significant decreases in chlorophylls (by ca 75%), β -carotene (by ca 65%), and lutein and neoxanthin (by ca 50%), although their contents remained unaltered between predawn and midday, suggesting a progressive decrease in response to drought. In contrast, the levels of violaxanthin decreased from predawn to midday, giving rise to enhanced formation of zeaxanthin and antheraxanthin in high light. Zeaxanthin and antheraxanthin formation was not induced by water deficit. Although the levels of photosynthetic pigments were severely affected by water deficit, carotenoids decreased less than chlorophylls, which resulted in increased levels of carotenoids per unit of chlorophyll. We conclude that the enhanced formation of zeaxanthin in high light and the increased levels of carotenoids per unit of chlorophyll observed in water-stressed plants may help to avoid photoinhibitory damage to the photosynthetic apparatus.     

Midday photoinhibition of two newly developed super-rice hybrids

Super-rice hybrids are two-line hybrid rice cultivars with 15 to 20 % higher yields than the raditional three-line hybrid rice cultivars. Response of photosynthetic functions to midday photoinhibition was compared between seedlings of the traditional hybrid rice (Oryza sativa L.) Shanyou63 and two super-rice hybrids, Hua-an3 and Liangyoupeijiu. Under strong midday sunlight, in comparison with Shanyou63, the two super-rice hybrids were less photoinhibited, as indicated by the lower loss of the net photosynthetic rate (P _N), the quantum yield of photosystem 2 (Φ_PS2), and the maximum and effective quantum yield of PS2 photochemistry (F_v/F_m and F_v′/F_m′). They also had a much higher transpiration rate. Hence the super-rice hybrids could protect themselves against midday photoinhibition at the cost of water. The photoprotective de-epoxidized xanthophyll cycle components, antheraxanthin (A) and zeaxanthin (Z), were accumulated more in Hua-an3 and Liangyoupeijiu than in Shanyou63, but the size of xanthophyll cycle pool of the seedlings was not affected by midday photoinhibition. Compared to Shanyou63, the super-rice hybrids were better photoprotected under natural high irradiance stress and the accumulation of Z and A, not the size of the xanthophyll pool protected the rice hybrids against photoinhibition.     

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.     

Xanthophyll cycle and light stress in nature: uniform response to excess direct sunlight among higher plant species

Photosystem II (PS II) efficiency, nonphotochemical fluorescence quenching, and xanthophyll cycle composition were determined in situ in the natural environment at midday in (i) a range of differently angled sun leaves ofEuonymus kiautschovicus Loesener and (ii) in sun leaves of a wide range of different plant species, including trees, shrubs, and herbs. Very different degrees of light stress were experienced by these leaves (i) in response to different levels of incident photon flux densities at similar photosynthetic capacities amongEuonymus leaves and (ii) as a result of very different photosynthetic capacities among species at similar incident photon flux densities (that were equivalent to full sunlight). ForEuonymus as well as the interspecific comparison all data fell on one single, close relationship for changes in intrinsic PSII efficiency, nonphotochemical fluorescence quenching, or the levels of zeaxanthin + antheraxanthin in leaves, respectively, as a function of the actual level of light stress. Thus, the same conversion state of the xanthophyll cycle and the same level of energy dissipation were observed for a given degree of light stress independent of species or conditions causing the light stress. Since all increases in thermal energy dissipation were associated with increases in the levels of zeaxanthin + antheraxanthin in these leaves, there was thus no indication of any form of xanthophyll cycle-independent energy dissipation in any of the twenty-four species or varieties of plants examined in their natural environment. It is also concluded that transient diurnal changes in intrinsic PSII efficiency in nature are caused by changes in the efficiency with which excitation energy is delivered from the antennae to PSII centers, and are thus likely to be purely photoprotective. Consequently, the possibility of quantifying the allocation of absorbed light into PSII photochemistry versus energy dissipation in the antennae from changes in intrinsic PSII efficiency is explored.Abbreviations A antheraxanthin - F actual level of fluorescence - F_a, F _o minimal fluorescence in the absence, presence of thylakoid energization - F_m, F _m maximal fluorescence in the absence, presence of thylakoid energization - F_m, - F)/F _m actual PSII efficiency ( = percent of absorbed light utilized in PSII photochemistry) - F_v/F_m, F _v /F_m/ PSII efficiency of open centers in the absence, presence of thylakoid energization - NPQ nonphotochemical fluorescence quenching - F_m/F _m - 1; q_p quenching coefficient for photochemical quenching - V violaxanthin - Z zeaxanthin     

Photosynthesis, non-photochemical pathways and activities of antioxidant enzymes in a resilient evergreen oak under different climatic conditions from a valley-savanna in Southwest China

Plants in the savanna‐valleys in Southwest China are annually exposed to different combinations of multiple stresses from the hot‐rainy, to chill‐dry, and to warm‐dry seasons. This study monitored seasonal changes in photosynthesis and photoprotection in an evergreen oak (Cyclobalanopsis helferiana) from one of these valleys for four years during which usual and abnormal drought occurred. In general, during the study period with decreasing xylem water potential (Ψx), photosynthetic gas exchange, quantum yield of photosystem II (PSII) photochemistry and activities of most of the measured antioxidant enzymes decreased, while activities of the xanthophyll cycle and associated non‐photochemical energy dissipation and glutathione peroxidase (GP) (EC 1.11.1.9) increased. In a fairly severe chill period, high concentration of reactive oxygen species induced high activities of most of the antioxidant enzymes and relatively stronger decrease in gas exchange. In the most severe dry period, even when predawn Ψx decreased down to ?4 MPa, considerable Pn (maximum photosynthetic rate) (4 μmol m^–2 s^–1) was still maintained in midmorning. At this time, most of the antioxidant enzyme activities decreased to the lowest values, whereas the xanthophyll cycle and associated non‐photochemical energy dissipation and GP activities increased to their highest levels. High predawn antheraxanthin and zeaxanthin contents were observed in the severe and very severe drought periods. Superoxide dismutase maintained high and fairly constant activity (1500–1800 U mg^?1 protein) and predawn maximum photochemistry efficiency of PSII was always above 0.8 throughout the whole study period. These results indicated that the photosynthetic apparatus of the oak leaves was highly capable of maintaining its function under the multiple stresses in different seasons in the present valley‐savanna.     

Leaf orientation and the response of the xanthophyll cycle to incident light

Summary Leaves from two species, Euonymus kiautschovicus and Arctostaphylos uva-ursi, with a variety of different orientations and exposures, were examined in the field with regard to the xanthophyll cycle (the interconversion of three carotenoids in the chloroplast thylakoid membranes). East-, south-, and west-facing leaves of E. kiautschovicus were sampled throughout the day and all exhibited a pronounced and progressive conversion of violaxanthin to zeaxanthin, followed by a reconversion of zeaxanthin to violaxanthin later in the day. Maximal levels of zeaxanthin and minimal levels of violaxanthin were observed at the time when each leaf (orientation) received the maximum incident light, which was in the morning in east-facing, midday in southfacing, and in the afternoon in west-facing leaves. A very slight degree of hysteresis in the removal of zeaxanthin compared to its formation with regard to incident light was observed. Leaves with a broader range of orientations were sampled from A. uva-ursi prior to sunrise and at midday. All of the examined pigments (carotenoids and chlorophylls) increased somewhat per unit leaf area with increasing total daily photon receipt. The sum of the carotenoids involved in the xanthophyll cycle, violaxanthin + antheraxanthin + zeaxanthin, increased more strongly with increasing growth light than any other pigment. In addition, the amounts of zeaxanthin present at midday also increased markedly with increasing total daily photon receipt. The percentage of the xanthophyll cycle that was converted to zeaxanthin (and antheraxanthin) at peak irradiance was very large (approximately 80%) in the leaves of both E. kiautschovicus and A. uva-ursi. The daily changes in the components of the xanthophyll cycle that paralleled the daily changes in incident light in the leaves of E. kiautschovicus, and the increasing levels of the xanthophyll cycle components with total daily photon receipt in the leaves of A. uva-ursi, are both consistent with the involvement of zeaxanthin (i.e. the xanthophyll cycle) in the photoprotection of the photosynthetic apparatus against damage due to excessive light.Abbreviations A antheraxanthin - EPS epoxidation state of the xanthophyll cycle=(V+0.5A)/(V+A+Z) - PFD photon flux density (400–700 nm) - PFD_i photon flux density incident upon the upper leaf surface - T_air air temperature - T_L leaf temperature - V violaxanthin - Z zeaxanthin     

Photosynthetic pigment composition and photosystem II photochemistry of wheat ears.

The characteristics of pigment composition and photosystem II (PSII) photochemistry in the flag leaf and ear parts of wheat (Triticum aestivum L.) grown in the field was compared. At the early stage of flowering, awns and the flag leaf showed the highest values in the maximal efficiency of PSII photochemistry (Fv/Fm), actual PSII efficiency (phi(PSII)), photochemical quenching (qP), and the efficiency of excitation capture by open PSII centres (Fv/F'm), followed by glumes, lemmas, and paleae, respectively except that no differences in F'v/F'm were observed among glumes, leamms, and paleae. With progressing grain filling, there was a change in the photosynthetic pigment stoichiometry. In the ear parts, neoxanthin and antheraxanthin decreased equally with chlorophyll levels. Lutein and zeaxanthin decreased less than chlorophyll levels while beta-carotene and violaxanthin decreased faster than chlorophyll levels. No big differences in pigment composition were observed among different ear parts. For the flag leaf, neoxanthin and beta-carotene decreased concomitantly with chlorophyll, whereas lutein and xanthophyll cycle pigment were less affected, leading to increases in lutein/chlorophyll and xanthophyll cycle pigment/chlorophyll ratios. Fv/Fm, phi(PSII), qP, and F'v/F'm decreased gradually in the flag leaf and ear parts but to different extents. The largest changes were observed in awns, followed by the lemmas of floret 2, the lemmas of floret 1, glumes, and the flag leaf, respectively. The results suggest that during grain filling, a down-regulation of PSII associated with an increase of the de-epoxidation state of the xanthophyll cycle carotenoids occurred in the flag leaf but not in the ear parts.     

ABA对水稻幼苗叶片光抑制的光保护作用

经ABA处理的水稻幼苗叶片和对照相比 ,PSII光化学效率 (Fv/Fm)和非光化学猝灭系数 (qN)显著受抑制。经高光处理 1h后 ,ABA处理的水稻幼苗叶片光抑制程度比对照小 ,这暗示ABA对高光光抑制具有一定的光保护作用 ,且间接表明ABA提高水稻幼苗抗光抑制的能力与叶黄素循环密切相关。     

Photosystem II photochemistry and photosynthetic pigment composition in salt-adapted halophyte Artimisia anethifolia grown under outdoor conditions

The effects of high salinity (0-400 mmol/L NaCl) on photosystem II (PSII) photochemistry and photosynthetic pigment composition were investigated in the halophyte Artimisia anethifolia grown under outdoor conditions and exposed to full sunlight. High salinity resulted in an inhibition in plant growth and a significant accumulation of sodium and chloride in leaves. However, high salinity induced no effects on the actual PSII efficiency, the efficiency of excitation energy capture by open PSII reaction centres, photochemical quenching, and non-photochemical quenching at midday. High salinity also induced neither changes in the maximum efficiency of PSII photochemistry, the efficiency with which a trapped exciton can move an electron into the electron transport chain further than QA and the quantum yield of electron transport beyond QA, nor changes in absorption, trapping and electron transport fluxes per PSII reaction centre. No significant changes were observed in the levels of neoxanthin, lutein, beta-carotene, violaxanthin, antheraxanthin, and zeaxanthin expressed on a total chlorophyll basis in salt-adapted plants. Our results suggest that Artimisia anethifolia showed high resistance not only to high salinity, but also to photoinhibition even if it was treated with high salinity as high as 400 mmol/L NaCl and exposed to full sunlight. The results indicate that tolerance of PSII to high salinity and photoinhibition can be viewed as an important strategy for Artimisia anethifolia, a halophyte plant, to grow in very high saline soil.     

Positive correlation between levels of retained zeaxanthin + antheraxanthin and degree of photoinhibition in shade leaves of Schefflera arboricola (Hayata) Merrill

Attached intact leaves of Schefflera arboricola grown at three different photon flux densities (PFDs) were subjected to 24-h exposures to a high PFD and subsequent recovery at a low PFD. While sun leaves showed virtually no sustained effects on photosystem II (PSII), shade-grown leaves exhibited pronounced photoinhibition of PSII that required several days at low PFD to recover. Upon transfer to high PFD, levels of nonphotochemical quenching in PSII as well as levels of zeaxanthin were initially low in shade leaves but continued to increase gradually during the 24-h exposure. The xanthophyll cycle pool size rose gradually during and also subsequent to the photoinhibitory treatment in shade leaves. Upon return to low PFD, a marked and extremely long-lasting retention of zeaxanthin and antheraxanthin was observed in shade but not sun leaves. During recovery, changes in the conversion state of the xanthophyll cycle therefore closely mirrored the slow increases in PSII efficiency. This novel report of a close association between zeaxanthin retention and lasting PSII depressions in these shade leaves clearly suggests a role for zeaxanthin in photoinhibition of shade leaves. In addition, there was a decrease in β-carotene levels, some decrease in chlorophyll, but no change in lutein and neoxanthin (all per leaf area) in the shade leaves during and subsequent to the photoinhibitory treatment. These data may be consistent with a degradation of a portion of core complexes but not of peripheral light-harvesting complexes. A possible conversion of β-carotene to form additional zeaxanthin is discussed. Received: 24 October 1997 / Accepted: 12 November 1997     

The xanthophyll cycle and sustained thermal energy dissipation activity in Vinca minor and Euonymus kiautschovicus in winter

The influence of low temperature on the operation of the xanthophyll cycle and energy dissipation activity, as ascertained through measurements of chlorophyll fluorescence, was examined in two broad-leaved evergreen species, Vinca minor L. and Euonymus kiautschovicus Loessner. In leaves examined under laboratory conditions, energy dissipation activity developed more slowly at lower leaf temperatures, but the final, steady-state level of such activity was greater at lower temperatures where the rate of energy utilization (through photosynthetic electron transport) was much lower. The rate at which energy dissipation activity increased was similar to that of the de-epoxidation of violaxanthin to antheraxanthin and zea-xanthin at different temperatures. However, leaves in the field examined prior to sunrise on mornings following cold days and nights exhibited a retention of antheraxanthin and zeaxanthin that was associated with sustained decreases in photosystem II efficiency. We therefore suggest that this phenomenon of ‘photoinhibition’ in response to light and cold temperatures during the winter results from sustained photoprotective thermal energy dissipation associated with the xanthophyll cycle. Such retention of the de-epoxidized components of the xanthophyll cycle responded to day-to-day changes in temperature, being greatest on the coldest mornings (when photoprotective energy dissipation might be most required) and less on warmer mornings when photosynthesis could presumably proceed at higher rates.     

Enhanced Employment of the Xanthophyll Cycle and Thermal Energy Dissipation in Spinach Exposed to High Light and N Stress

The involvement of the xanthophyll cycle in photoprotection of N-deficient spinach (Spinacia oleracea L. cv Nobel) was investigated. Spinach plants were fertilized with 14 mM nitrate (control, high N) versus 0.5 mM (low N) fertilizer, and grown under both high- and low-light conditions. Plants were characterized from measurements of photosynthetic oxygen exchange and chlorophyll fluorescence, as well as carotenoid and cholorophyll analysis. Compared with the high-N plants, the low-N plants showed a lower capacity for photosynthesis and a lower chlorophyll content, as well as a lower rate of photosystem II photosynthetic electron transport and a corresponding increase in thermal energy dissipation activity measured as nonphotochemical fluorescence quenching. The low-N plants displayed a greater fraction of the total xanthophyll cycle pool as zeaxanthin and antheraxanthin at midday, and an increase in the ratio of xanthophyll cycle pigments to total chlorophyll. These results indicate that under N limitation both the light-collecting system and the photosynthetic rate decrease. However, the increased dissipation of excess energy shows that there is excess light absorbed at midday. We conclude that spinach responds to N limitation by a combination of decreased light collection and increased thermal dissipation involving the xanthophyll cycle.     

Relationship between photosynthetic radiation-use efficiency of barley canopies and the photochemical reflectance index (PRI)

Some processes of excess radiation dissipation have been associated with changes in leaf reflectance near 531 nm. We aimed to study the relations between the photochemical reflectance index (PRI) derived from this signal, and photosynthetic radiation-use efficiency (defined as net CO₂ assimilation rate/incident photon flux density) in a cereal canopy. Measurements of reflectance, fluorescence, gas exchange and xanthophyll cycle pigments were made in the morning, midday and afternoon in barley canopies with two levels of nitrogen fertilization. The photosynthetic radiation-use efficiency decreased at midday, mainly in the third leaf, in both treatments, with lower values for the nitrogen deficient leaves. The zeaxanthin content showed the inverse pattern, increasing at midday and in the nitrogen deficient treatment. The photosynthetic radiation-use efficiency was well correlated with the epoxidation state, EPS (violaxanthin + 0.5 antheraxanthin)/(violaxanthin + antheraxanthin + zeaxanthin). The PRI [here defined as (R₅₃₉ - R₅₇₀)/(R₅₃₉+ R₅₇₀)] was significantly correlated with epoxidation state and zeaxanthin and with photosynthetic radiation-use efficiency. These results validate the utility of PRI in the assessment of radiation-use efficiency at canopy level.     

The xanthophyll cycle and energy dissipation in differently oriented faces of the cactus Opuntia macrorhiza

  Diurnal changes in titratable acidity, photosynthesis, energy dissipation activity, and the carotenoid composition of differently oriented cladodes of the cactus Opuntia macrorhiza were characterized during exposure to full sunlight in the field. Four cladode faces were chosen such that each was exposed to maximum photon flux densities (PFD) at different times of the day in addition to receiving different daily integrated PFDs. The sum of all carotenoids per chlorophyll was found to increase with increasing exposure to PFD, with the carotenoids of the xanthophyll cycle present in the most exposed face at more than twice the concentration found in the least exposed face. All faces exhibited large increases in xanthophyll cycle-dependent energy dissipation as the sun rose in the morning, even those receiving only minimal levels of diffuse radiation. The transient high levels of energy dissipation in those faces that did not receive direct sunlight in the morning may have been due to low temperature inhibition of photosynthesis (predawn low of 2°C). For the two faces receiving peak PFDs in the morning hours (north and east faces), the level of energy dissipation activity increased rapidly during exposure to direct sunlight in the early morning, gradually declining in the late morning under warm temperatures, and was negligible during the afternoon low light conditions. Changes in the xanthophyll cycle paralleled the changes in energy dissipation with the majority of the cycle present as violaxanthin (V) prior to sunrise, largely de-epoxidized to zeaxanthin (Z) and antheraxanthin (A) during exposure to direct sunlight, and reconverted to V during the afternoon. For the two faces receiving peak PFDs in the afternoon (south and west faces), energy dissipation activity increased dramatically during the early morning low light period, subsequently decreasing during midday as decarboxylation of malic acid proceeded maximally (providing a high concentration of CO₂ for photosynthesis), and then increased to the highest level in the late afternoon as the supply of malic acid was depleted and rates of photosynthetic electron transport declined. The xanthophyll cycle, largely present as Z and A prior to sunrise in the south and west faces, was de-epoxidized to the greatest extent in the late afternoon, followed by epoxidation back to the predawn level by sunset. In all cladode faces high levels of energy dissipation activity were accompanied by decreases in the intrinsic efficiency of photosystem II (PSII), indicative of a regulatory process that diverted the excess energy away from the reaction centers during periods of excess light. Furthermore, the overnight retention of Z and A by the south and west faces was accompanied by a sustained reduction in PSII efficiency (i.e., “photoinhibition”). We suggest that this “photoinhibition” represents the sustained engagement of nocturnally retained Z and A in the photoprotective down-regulation of PSII. Received: 8 May 1996 / Accepted: 9 September 1996