A Simple Alternative Approach to Assessing the Fate of Absorbed Light Energy Using Chlorophyll Fluorescence

We propose a simplified alternative method for quantifying the partitioning of excitation energy between photochemistry, fluorescence and thermal dissipation. This alternative technique uses existing well-defined quantum efficiencies such as Phi(PS II), leaving no 'excess' efficiency unaccounted for, effectively separates regulated and constitutive thermal dissipation processes, does not require the use of F(o) and F'(o) measurements and gives very similar results to the method proposed by Kramer et al. [(2004) Photosynth Res 79: 209-218]. We demonstrate the use of the technique using chlorophyll fluorescence measurements in grapevine leaves and observe a high dependence on thermal dissipation processes (up to 75%) at both high light and low temperature.     

Changes in the Non-Photochemical Quenching of Chlorophyll Fluorescence During Aging of Wheat Flag Leaves

Non-photochemical quenching of chlorophyll fluorescence (q_N) and its three components (q_Nf, q_Nm, and q_Ns) in the flag leaves of wheat grown in the field were studied by a fluorometer PAM-2000 on clear days. The diurnal variation patterns of q_N in just fully extended (JFEL) and aging leaves (AL) were similar, but q_Nm declined markedly in JFEL while it remained at a relatively high level in AL under strong sunlight at noon. Furthermore, at midday q_Nf was higher than q_Ns in JFEL, but much lower in AL. The results show the relative contributions of different mechanisms in preventing the photosynthetic apparatus from photodamage change during leaf development. This revised version was published online in June 2006 with corrections to the Cover Date.     

持续低温弱光对黄瓜叶片气体交换、叶绿素荧光猝灭和吸收光能分配的影响

持续常温弱光(25℃/18℃,l00umol m-2 s-1)、低温弱光(12℃/12℃,100 umol m-2 s-1和7℃/7℃,l00μmolm-2s-1)均导致黄瓜生长减慢或停滞、叶绿素含量、气孔导度和净光合速率、光合电子传递速率下降以及胞间CO2浓度上升.常温弱光和12℃弱光处理对光系统II的最大光化学效率Fv/Fm无显著影响,而7℃弱光处理导致Fv/Fm的可逆性下降.常温弱光和7℃、12℃弱光处理均导致了光化学反应速率的降低以及天线热耗散和反应中心过剩能量的增加.在胁迫后,12℃弱光0比7℃弱光更有利于植株光合功能的恢复.     

Cytogenetic Analysis of Cotton Monosomics

Eleven monosomics in cotton that were obtained in the progenies of three disomic desynaptic plants were cytologically characterized. The transmission of the monosomes in progeny was shown in the 26 monosomic plants. In 23 plants the frequency of monosomics was ranged between 14.29 and 41.67 %. Three monosomics usually occurred in much lower frequencies (from 3.03 to 5.00 %). Various transmission rates indirectly pointed out different monosomes as a specific chromosomes of cotton genome. Three telochromosomes and one isochromosome were isolated from the progenies of the four monosomics. Using translocation test it was recovered that seven monosomes of different monosomics are homologous to one of the chromosomes of six translocation lines of our collection.     

Dissipation of Excitation Energy During Development of Photosystem 2 Photochemistry in Helianthus annuus

Etiolated sunflower cotyledons developed in complete darkness and lacking photosystem (PS) 2 were exposed to continuous 200 µmol(photon) m^–2 s^–1 white light for 1, 3, 6, 12, and 18 h prior to evaluations of excitation-energy dissipation using modulated chlorophyll a fluorescence. Photochemical potential of PS2, measured as the dark-adapted quantum efficiency of PS2 (F_V(M)/F_M), and thermal dissipation from the antenna pigment-protein complex, measured as the Stern-Volmer non-photochemical quenching coefficient (NPQ), increased to 12 h of irradiation. Following 12 h of irradiation, thermal dissipation from the antennae pigment-protein complex decreased while the efficiency of excitation capture by PS2 centers (F_V/F_M) and light-adapted quantum efficiency of PS2 (_PS2) continued to increase to 18 h of irradiation. The fraction of the oxidized state of Q_A, measured by the photochemical quenching coefficient (q_P), remained near optimal and was not changed significantly by irradiation time. Hence during the development of maximum photochemical potential of PS2 in sunflower etioplasts, which initially lacked PS2, enhanced thermal dissipation helps limit excitation energy reaching PS2 centers. Changes of the magnitude of thermal dissipation help maintain an optimum fraction of the oxidized state of Q_A during the development of PS2 photochemistry.     

Decrease of Fluorescence Intensity After the K Step in Chlorophyll a Fluorescence Induction is Suppressed by Electron Acceptors and Donors to Photosystem 2

Chlorophyll a fluorescence induction measured by a fluorometer with a high temperature stressed plant material shows a new K step which is a clear peak due to fast fluorescence rise and subsequent decrease of fluorescence intensity. We focused on an explanation of the decrease of fluorescence after the K step using artificial electron acceptors and donors to photosystem 2 (PS2). Addition of the artificial electron acceptors or donors suppressed the decrease of fluorescence after the K step. We suggest that the decrease mainly reflects (by more than 81 %) an energy loss process in the reaction centre of PS2 which is most probably a nonradiative charge recombination between P680⁺ (oxidised primary electron donor in PS2) and a negative charge stored on either Pheo⁻ or Q_A ⁻ (reduced primary electron acceptor of PS2 and reduced primary quinone electron acceptor of PS2, respectively). We suggest that the energy loss process is only possible when the inhibition of both the donor and the acceptor sides of PS2 occurs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Changes in the Non-Photochemical Quenching of Chlorophyll Fluorescence During Aging of Wheat Flag Leaves

Ultrastructural changes in chloroplasts of primary leaves of 15-d-old bean plants (Phaseolus vulgaris L. cv. Cheren Starozagorski) in response to a single stress (increasing water deficit, WD) as well as to combined stress (WD plus high temperature, WD+HT) were investigated under the possible protective or reparatory effects of the carbamide cytokinin 4-PU-30 [N-(2-chloro-4-pyridyl)-N-phenylurea] applied before or after the stress. Essential structural changes in chloroplast ultrastructure occurred mainly in plants that had experienced WD+HT: the thylakoids were swollen, the envelope was destroyed, and the spatial orientation of inner membrane system was not typical. Changed starch accumulation was also observed. 4-PU-30 protected chloroplast ultrastructure under WD+HT.     

Gradual Disassembly of Photosystem 2 In Vivo Induced by Excess Irradiance. A Hypothesis Based on Changes in 77 K Fluorescence Spectra of Chlorophyll a in Barley Leaves

Effects of short-term exposure to different irradiances on the function of photosystem 2 (PS2) were studied for barley grown at low (LI; 50 μmol m⁻² s⁻¹) and high (HI; 1 100 μmol m⁻² s⁻¹) irradiances. HI barley revealed higher ability to down-regulate the light-harvesting within PS2 after exposure to high irradiance as compared to LI plants. This ability was estimated from the light-induced decreases of F685/F742 and E476/E436 in emission and excitation spectra of 77 K chlorophyll (Chl) a fluorescence in vivo which was 65 and 10 % for HI plants as compared to 30 and 2 % for LI plants, respectively. For LI plants this protective down-regulation of the light-harvesting of PS2 was saturated at 430 μmol m⁻² s⁻¹, and progressive PS2 photodamage was induced at higher irradiances. After exposure of LI segments to 2 200 μmol m⁻² s⁻¹ a pronounced maximum at 700 nm appeared in emission spectrum of 77 K Chl a fluorescence. Based on complementary analysis of 77 K excitation spectra measured at the emission wavelength 685 nm we suggest that this emission maximum may be attributed to the formation of aggregates of light-harvesting complexes of PS2 (LHC2) with part of PS2 core during progressive PS2 photodamage. Our results can be explained assuming different contributions of LHC2 and PS2 core to the total nonradiative dissipation of absorbed excitation energy for the LI and HI barley. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inter-Specific Differences in Cotton for Nutrient Partitioning from Subtending Leaves to Reproductive Parts at Various Developmental Stages: Consequences for Fruit Growth and Yield

A field study was carried out to unravel the inter-specific differences in cotton for the partitioning of N, P, K, S, Ca, Mg, Na and Cl from the subtending leaves to the reproductive parts of Gossypium hirsutum, G. barbadense and G. arboreum at various developmental stages. Results revealed significant differences among the species for the various parameters studied. Overall there was a greater fresh and dry matter yield of various reproductive parts and subtending leaves of G. hirsutum and G. barbadense than G. arboreum, although the leaf photosynthetic rate was similar. Age-dependent increase in leaf area/leaf mass ratio indicated a greater partitioning of earlier acquired assimilates to the growth of reproductive parts. Results indicated greater partitioning of N, P, S and Ca during later reproductive growth (from boll production to its opening) in G. hirsutum and G. barbadense but during earlier reproductive growth in G. arboreum (from bud up to flower formation) as was evident by decreased subtending leaf/reproductive parts ratio. It is concluded that better N, P, S and Ca partitioning ability of G. hirsutum and G. barbadense at the onset of boll development played a major role in the better yield and good quality fiber characteristics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Physiologic Response of Cotton to the Insecticide Imidacloprid under High-Temperature Stress

The insecticide imidacloprid (tradename Trimax™) has been shown to increase cotton (Gossypium hirsutum L.) yield in the absence of insects, but the explanation for this is not clear. Growth room studies were designed to investigate changes in the physiology and biochemistry of imidacloprid-treated cotton plants and provide information on the mode of action of yield enhancement. Imidacloprid was applied at the pinhead square growth stage at the rate of 52.3 g ai/ha and plants were exposed to day temperatures of 30, 33, 36, and 39°C. Increased levels of photosynthesis and higher values of chlorophyll fluorescence yield, measured two days after imidacloprid application, showed an advantage of imidacloprid-treated over untreated plants. The effect of imidacloprid was greater at the higher temperatures of the growth chamber studies. The results suggested that the imidacloprid-treated plants suffered less temperature stress. This suggestion was supported by findings of reduced glutathione reductase in the imidacloprid-treated plants in the growth chamber, indicating that the untreated plants were experiencing more stress, necessitating the activation of this defense mechanism.     

A Rapid and Simple Method for in vitro Plant Regeneration from Split Embryo Axes of Six Cultivars of Cotton

Plant regeneration was achieved from the seed derived decotyledonated split embryo axes of six Indian cultivars of cotton (Gossypium hirsutum L.). Explants were cultured on Murashige and Skoog's basal medium supplemented with 2 % sucrose and 0.65 % agar. Incorporation of 0.25 % charcoal in the medium and incubation of the cultures at 30 ± 2 °C had synergistic effect on the frequency of shoot and root formation. The method employed is genotype independent, simple and rapid.     

Gene Loci in Maize Influencing Susceptibility to Chilling Dependent Photoinhibition of Photosynthesis

Variation in tolerance in chilling-dependent photoinhibition has been associated with a wide range of traits in comparative physiological studies. A sweet corn (Zea mays L.) population of 214 F_2:3 families previously mapped to near-saturation with 93 RFLP DNA markers were subjected to low temperature and high-light events prior to measurement of the maximum dark-adapted quantum efficiency of PS II (F_v/F_m), to identify loci associated with variation in chilling-dependent photoinhibition. In the first assay with ten families varying in seedling growth and germination, significant differences were observed among families in their response to and recovery from exposure to high light at low temperature. All the 214 F_2:3 families from this population were then evaluated for tolerance of chilling-dependent photoinhibition in a controlled environment and then in three replicated trials in the field, each following naturally occurring chilling events during spring. The measured effects on F_v/F_m were analyzed with software that mapped segregating loci that regulate trait expression and linked to genetic markers (PLABQTL). QTL 3.096 (i.e. 96 cM on chromosome three) was consistently identified in both controlled environment and in the mean of the three field trails. Another QTL at 8.025, described the greatest percentage of total phenotypic variance (ca. 10%) for the mean reduction in F_v/F_m of all three periods of measurement in the field. A third QTL (4.136) showed a highly significant association in the third field trial. These three QTLs were closely associated with genes that have been mechanistically related to photoinhibition tolerance and repair. The results suggest that the ratio of F_v/F_m is an approach that may be used in establishing marker-assisted breeding for improved tolerance to chilling of maize in the light and in turn better early-season growth in cool temperate climates.     

Morphometric Analysis of Chloroplasts of Cotton Leaf and Fruiting Organs

We examined morphological and ultrastructural differences in chloroplasts of cotton leaves and the fruiting organs, bract, and capsule wall to advance our understanding of their commonly observed differences in photosynthetic efficiency. Chloroplasts from leaves were large (7.1 μm long in cross section), lens shaped with a well developed membrane system differentiated into grana and stroma lamellae that occupied the large cross-sectional area (12.3 μm²) of the organelle. A few small plastoglobuli and starch grains were scattered in the stroma region. The bract chloroplasts were correlative of leaf chloroplasts in size (6.8 μm in length) and shape with the exception that the bract chloroplasts exhibited greater thylakoid number per granum (15.8) than the leaf chloroplasts (10.5). In contrast to leaf and bract, the capsule wall chloroplasts were smaller in size (4.3 μm) and cross sectional area (6.8 μm²) than either the leaf or bract. The most intriguing feature of the capsule wall chloroplasts was its domination by large starch granules (5.3 μm²) in the stroma which filled the whole chloroplast coercing the membrane system to move towards the periphery of the organelle. Grana number and thylakoids per granum were lowest in the capsule wall chloroplasts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of Potassium Deficiency on Photosynthesis,Chlorophyll Content,and Chloroplast Ultrastructure of Cotton Plants

In cotton (Gossypium hirsutum L.) grown in controlled-environment growth chamber the effects of K deficiency during floral bud development on leaf photosynthesis, contents of chlorophyll (Chl) and nonstructural saccharides, leaf anatomy, chloroplast ultrastructure, and plant dry matter accumulation were studied. After cotton plants received 35-d K-free nutrient solution at the early square stage, net photosynthetic rate (P _N) of the uppermost fully expanded main-stem leaves was only 23 % of the control plants receiving a full K supply. Decreased leaf P _N of K-deficient cotton was mainly associated with dramatically low Chl content, poor chloroplast ultrastructure, and restricted saccharide translocation, rather than limited stomata conductance in K-deficient leaves. Accumulation of sucrose in leaves of K-deficient plants might be associated with reduced entry of sucrose into the transport pool or decreased phloem loading. K deficiency during squaring also dramatically reduced leaf area and dry matter accumulation, and affected assimilate partitioning among plant tissues.     

Effect of Growth Regulators on Photosynthesis, Transpiration and Related Parameters in Water Stressed Cotton

Gas exchange in Gossypium hirsutum L. cv. H-777 as affected by water deficit and growth regulators (IAA, GA₃, BAP, ABA, ethrel) was examined. Sixty days after sowing, growth regulators in concentration 50 µM were applied as foliar spray and irrigation was withheld to get desired (moderate and severe) water deficit. All the parameters were measured on the third leaf from the top between 10:00 and 11:00. Net photosynthetic rate (P_N), transpiration rate (E), stomatal conductance (g_s), carboxylation efficiency (CE), and water potential (_w) decreased significantly with the increasing water stress, however, water use efficiency (WUE) was unaffected. Foliar spray with IAA, GA₃ and BAP partially counteracted the effect of water deficit on the above parameters except _w, which became more negative. ABA and up to some extent ethrel increased WUE and maintained higher _w, however, caused further decrease in P_N, E, and g_s.     

Thermotolerance of Photosystem 2 of Three Alpine Plant Species Under Field Conditions

The species specific response of photosystem 2 (PS2) efficiency and its thermotolerance to diurnal and seasonal alterations in leaf temperature, irradiance, and water relations were investigated under alpine field conditions (1 950 m) and in response to an in situ long-term heat treatment (+3 K). Three plant species were compared using the naturally occurring microstratification of alpine environments, i.e. under contrasting leaf temperatures but under similar macroclimatic conditions. Thermotolerance of PS2 showed a high variability in all three species of up to 9.6 K. Diumal changes (increases or even decreases) in PS2 thermotolerance occurred frequently with a maximum increase of +4.8 K in Loiseleuria procumbens. Increasing leaf temperatures and photosynthetic photon flux density influenced thermotolerance adjustments. Under long-term heating (+3 K) of L. procumbens canopies with infra-red lamps, the maxima of the critical (T_c) and the lethal (T_p) temperature of PS2 increased by at least 1 K. Thermotolerance of the leaf tissue (LT₅₀) increased significantly by +0.6 K. The effects of slight water stress on thermotolerance of PS2 were species specific. High temperature thresholds for photoinhibition were significantly different between species and increased by 9 K from the species in the coldest microhabitat to the species in the warmest. Experimental heating of L. procumbens canopies by +3 K caused a significant (p>0.01) upward shift of the high temperature threshold for photoinhibition by +3 K. Each species appeared to be very well adapted to the thermal conditions of its microhabitat as under the most frequently experienced daytime leaf temperatures no photoinhibition occurred. The observed fine scale thermal adjustment of PS2 in response to increased leaf temperatures shows the potential to optimise photosynthesis under varying environmental conditions as long as the upper thermal limits are not exceeded.     

Characteristics of Chlorophyll Fluorescence and Membrane-Lipid Peroxidation During Senescence of Flag Leaf in Different Cultivars of Rice

With japonica rice 98-08, indica hybrids Shanyou 63, Gangyou 881, and X07S/Zihui 100, and sub-species hybrid Peiai 64S/9311 as materials, chlorophyll (Chl) content, Chl a fluorescence parameters, and membrane lipid peroxidation in flag leaf were measured at late developmental stages under natural conditions. F_v/F_m, q_P, _PS2, and electron transport rate gradually decreased while q_N increased conversely. Excessive photon energy led to the accumulation of active oxygen (O₂ ^–), H₂O, malonyldialdehyde, and products of membrane lipid peroxidation, and resulted in reduced Chl content and early ageing subsequent to the photooxidation during flag leaf senescence. There was obvious diversification of these parameters among rice cultivars. In comparison with japonica cv. 98-08 (tolerant to photooxidation), F_v/F_m decreased in indica cv. Shanyou 63 (susceptible to photooxidation) with greater accumulation of active oxygen and a sharp drop in Chl content, which resulted in yellowish early ageing, and affected the filling and setting of rice grains. The mechanism for premature ageing in indica rice was related to irradiance and temperature at filling stages. On a sunny day at above 25 °C, the reaction centre of photosystem 2 (PS2) exhibited a dynamic change on reversible inactivation. Under the intense irradiance at noon, PS2 function in indica rice exhibited obvious down-regulation and photoinhibition. Under intense irradiance with lowered temperatures, PS2 resulted in photo-damage and early ageing, related to the degradation of PS2-D1 protein and the inhibition of endogenous protection systems such as the xanthophyll cycle and enzymes scavenging active oxygen. Hence for high-yield breeding, based on a good plant-type and utilising heterosis and tolerance of photooxidation, the selection of japonica rice or a sterile line with the japonica genotype as female is a strategy worthy of consideration.     

The Influence of Leaf Windows on the Utilization and Absorption of Radiant Energy in Seven Desert Succulents

Four fluorescence parameters [F_v/F_m = the intrinsic efficiency of energy conversion via photosystem 2 (PS2); F_v/F_m= the efficiency of energy conversion via PS2 in the light; P = fraction of absorbed radiant energy utilized for photosynthesis; and D = fraction of absorbed radiant energy dissipated as heat] were measured on leaves of seven species of succulents having epidermal windows. While the function of leaf windows has reportedly been to increase absorption of radiant energy and, hence, the rate of photosynthesis in these species, recent evidence indicates that this translucent portion of epidermal tissue, lacking chlorophyll, may also result in photoinhibition in these species, especially for those with growth habits aboveground. Species with aboveground and belowground growth habits were compared with their leaf windows covered with reflective tape and with windows unobstructed. Results showed no increase in photoinhibition for these species resulting from the radiant energy penetrating the window tissue. Although the efficiency of the photosynthetic mechanism was not significantly influenced by the additional radiant energy provided by the window for individual species, there were significant differences in the efficiencies of radiant energy capture (F_v/F_m) and utilization (P) between the two growth habits. Species with an aboveground growth habit were less efficient in radiant energy utilization compared with the species having a belowground growth habit.     

Resistance of Cotton to the Root-Knot Nematode, Meloidogyne incognita

Cotton plants resistant to Meloidogyne incognita had roots characterized by fewer and smaller galls, and females that produced fewer egg masses containing fewer eggs than did susceptible plants. Many galls on resistant roots contained no nematodes at the time of examination. Penetration of the resistant cultivar was equal to that of the susceptible cultivar and independent of the number of nematodes in the inoculum. Fewer nematodes penetrated resistant or susceptible plants with eight leaves than those with fewer leaves. Reciprocal grafts of resistant and susceptible plants failed to confer resistance or susceptibility to the rootstock.