Photoinhibition: Then and Now. In: Demmig-adams B, Adams WW, Mattoo AK, editors. Photoprotection, Photoinhibition, Gene Regulation, and Environment. Dordrecht: Springer Netherlands; 2006. pp. 11-22. [DOI: 10.1007/1-4020-3579-9_2]

Genetic variation in high light responses of Theobroma cacao L. accessions

Cacao (Theobroma cacao L.) is a shade-tolerant tree species, but in recent years it has increasingly been cultivated under full sun conditions in an orchard system where photoinhibition is likely. Here we investigate the extent of photoinhibition in 17 cacao accessions from a range of genetic groups, growing under high light conditions. The ability of the photosynthetic systems to respond to high light was assessed using chlorophyll fluorescence parameters (diurnal F _v /F _m and instantaneous light response curves), and differences in photosynthetic pigment content were compared using biochemical assays. Damage due to photoinhibition was assessed using electrolyte leakage, lipid peroxidation, and reactive oxygen species scavenging systems were compared using biochemical assays (for APX, CAT and SOD). There was significant variation between the 17 accessions for photosynthetic parameters, although in all cases the light saturation points were well below the midday light levels. Light acclimation of photosynthetic pigments was evident and variation in the total chlorophyll to total carotenoid ratio was significantly correlated with electrolyte leakage. Significant genetic variation was observed across the 17 accessions in the activities of CAT, APX and SOD. Across all accessions, photoprotection appeared to be restricted by the ability of leaves to generate SOD. Significant negative correlations were observed between SOD activity and both APX activity and electrolyte leakage, while significant positive correlations were observed between electrolyte leakage and both APX and CAT activity. Accessions with higher light saturation points, as well as high carotenoid and high SOD concentrations were able to tolerate the moderately high light, however, none of the accessions were clearly superior to the commonly grown Amelonado accession. The results imply that screening for SOD activity, total carotenoid content and light saturation point can aid in selection of genotypes with better tolerance to high light.

Drought-induced alterations in photosynthetic, ultrastructural and biochemical traits of contrasting sugarcane genotypes

Drought is an important factor which limits growth of sugarcane. To elucidate the physiological and biochemical mechanisms of tolerance, a pot experiment was conducted at Sugarcane Research Institute, Kaiyuan, China. Two genotypes (Yuetang 93-159-sensitive and Yunzhe 05-51-tolerant), were subjected to three treatments; 70±5% (control), 50±5% (moderate drought) and 30±5% (severe drought) of soil field capacity. The results demonstrated that drought induced considerable decline in morpho-physiological, biochemical and anatomical parameters of both genotypes, with more pronounced detrimental effects on Yuetang 93-159 than on Yunzhe 05-51. Yunzhe 05-51 exhibited more tolerance by showing higher dry biomass, photosynthesis and antioxidant enzyme activities. Compared with Yuetang 93-159, Yunzhe 05-51 exhibited higher soluble sugar, soluble protein and proline contents under stress. Yunzhe 05-51 illustrated comparatively well-composed chloroplast structure under drought stress. It is concluded that the tolerance of Yunzhe 05-51 was attributed to improved antioxidant activities, osmolyte accumulation and enhanced photosynthesis. These findings may provide valuable information for future studies on molecular mechanism of tolerance.

Leaf Age and Position Effects on Quantum Yield and Photosynthetic Capacity in Hemp Crowns

We examined the aging of leaves prior to abscission and the consequences for estimating whole-crown primary production in Cannabis sativa L. (hemp). Leaves at three vertical positions in hemp crowns were examined from initial full leaf expansion until 42 days later. Photosynthetic capacity decreased as leaves aged regardless of crown position, light intensity, or photoperiod. Although leaves remained green, the photosynthetic capacity declined logarithmically to values of 50% and 25% of the maximum 9 and 25 days later, respectively. Plants grown under +450 μmol m-2 s-1 supplemental photosynthetically active radiation or enriched diffuse light responded similarly; there was no evidence that photoperiod or enriched diffuse light modified the gas exchange pattern. At approximately 14 days after full leaf expansion, leaf light levels >500 μmol m-2 s-1 decreased photosynthesis, which resulted in ≥10% lower maximum electron transport rate at ≥ 20 days of growth period. Furthermore, leaves were saturated at lower light levels as leaf age progressed (≤500 μmol m-2 s-1). Incorporating leaf age corrections of photosynthetic physiology is needed when estimating hemp primary production.

Leaf area and photosynthesis of newly emerged trifoliolate leaves are regulated by mature leaves in soybean

Leaf anatomy and the stomatal development of developing leaves of plants have been shown to be regulated by the same light environment as that of mature leaves, but no report has yet been written on whether such a long-distance signal from mature leaves regulates the total leaf area of newly emerged leaves. To explore this question, we created an investigation in which we collected data on the leaf area, leaf mass per area (LMA), leaf anatomy, cell size, cell number, gas exchange and soluble sugar content of leaves from three soybean varieties grown under full sunlight (NS), shaded mature leaves (MS) or whole plants grown in shade (WS). Our results show that MS or WS cause a marked decline both in leaf area and LMA in newly developing leaves. Leaf anatomy also showed characteristics of shade leaves with decreased leaf thickness, palisade tissue thickness, sponge tissue thickness, cell size and cell numbers. In addition, in the MS and WS treatments, newly developed leaves exhibited lower net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (E), but higher carbon dioxide (CO 2 ) concentration in the intercellular space (Ci) than plants grown in full sunlight. Moreover, soluble sugar content was significantly decreased in newly developed leaves in MS and WS treatments. These results clearly indicate that (1) leaf area, leaf anatomical structure, and photosynthetic function of newly developing leaves are regulated by a systemic irradiance signal from mature leaves; (2) decreased cell size and cell number are the major cause of smaller and thinner leaves in shade; and (3) sugars could possibly act as candidate signal substances to regulate leaf area systemically.

Effects of periodic photoinhibitory light exposure on physiology and productivity of Arabidopsis plants grown under low light

This work examined the long-term effects of periodic high light stress on photosynthesis, morphology, and productivity of low-light-acclimated Arabidopsis plants. Significant photoinhibition of Arabidopsis seedlings grown under low light (100 μmol photons m-2 s-1) was observed at the beginning of the high light treatment (three times a day for 30 min at 1800 μmol photons m-2 s-1). However, after 2 weeks of treatment, similar photosynthesis yields (Fv/Fm) to those of control plants were attained. The daily levels of photochemical quenching measured in the dark (qPd) indicated that the plants recovered from photoinhibition within several hours once transferred back to low light conditions, with complete recovery being achieved overnight. Acclimation to high light stress resulted in the modification of the number, structure, and position of chloroplasts, and an increase in the average chlorophyll a/b ratio. During ontogenesis, high-light-exposed plants had lower total leaf areas but higher above-ground biomass. This was attributed to the consumption of starch for stem and seed production. Moreover, periodic high light exposure brought forward the reproductive phase and resulted in higher seed yields compared with control plants grown under low light. The responses to periodic high light exposure of mature Arabidopsis plants were similar to those of seedlings but had higher light tolerance.

Our eclectic adventures in the slower eras of photosynthesis: from New England Down Under to biosphere 2 and beyond

This is a tale of a career in plant physiological ecology that enjoyed the freedom to address photosynthetic physiology and biochemistry in leaves of plants from diverse environments. It was supported by block funding (now sadly a thing of the past) for research at the Australian National University, by grants during appointments in the United States and in Germany, and by Columbia University. It became a "career experiment" in which long-term, high-trust support for curiosity-driven plant biology in Australia, and at times in the United States, led to surprisingly innovative results. Although the rich diversity of short-term competitive grant opportunities in the United States sustained ongoing research, it proved difficult to mobilize support for more risky long-term projects. A decade after the closure of the Biosphere 2 Laboratory, this article highlights the achievements of colleagues in experimental climate change research from 1998 to 2003.

Chlorophyll a fluorescence induction: a personal perspective of the thermal phase, the J-I-P rise

The fast (up to 1 s) chlorophyll (Chl) a fluorescence induction (FI) curve, measured under saturating continuous light, has a photochemical phase, the O-J rise, related mainly to the reduction of Q(A), the primary electron acceptor plastoquinone of Photosystem II (PSII); here, the fluorescence rise depends strongly on the number of photons absorbed. This is followed by a thermal phase, the J-I-P rise, which disappears at subfreezing temperatures. According to the mainstream interpretation of the fast FI, the variable fluorescence originates from PSII antenna, and the oxidized Q(A) is the most important quencher influencing the O-J-I-P curve. As the reaction centers of PSII are gradually closed by the photochemical reduction of Q(A), Chl fluorescence, F, rises from the O level (the minimal level) to the P level (the peak); yet, the relationship between F and [Q(A) (-)] is not linear, due to the presence of other quenchers and modifiers. Several alternative theories have been proposed, which give different interpretations of the O-J-I-P transient. The main idea in these alternative theories is that in saturating light, Q(A) is almost completely reduced already at the end of the photochemical phase O-J, but the fluorescence yield is lower than its maximum value due to the presence of either a second quencher besides Q(A), or there is an another process quenching the fluorescence; in the second quencher hypothesis, this quencher is consumed (or the process of quenching the fluorescence is reversed) during the thermal phase J-I-P. In this review, we discuss these theories. Based on our critical examination, that includes pros and cons of each theory, as well mathematical modeling, we conclude that the mainstream interpretation of the O-J-I-P transient is the most credible one, as none of the alternative ideas provide adequate explanation or experimental proof for the almost complete reduction of Q(A) at the end of the O-J phase, and for the origin of the fluorescence rise during the thermal phase. However, we suggest that some of the factors influencing the fluorescence yield that have been proposed in these newer theories, as e.g., the membrane potential ΔΨ, as suggested by Vredenberg and his associates, can potentially contribute to modulate the O-J-I-P transient in parallel with the reduction of Q(A), through changes at the PSII antenna and/or at the reaction center, or, possibly, through the control of the oxidation-reduction of the PQ-pool, including proton transfer into the lumen, as suggested by Rubin and his associates. We present in this review our personal perspective mainly on our understanding of the thermal phase, the J-I-P rise during Chl a FI in plants and algae.

Analysis of non-photochemical energy dissipating processes in wild type Dunaliella salina (green algae) and in zea1, a mutant constitutively accumulating zeaxanthin

Generally there is a correlation between the amount of zeaxanthin accumulated within the chloroplast of oxygenic photosynthetic organisms and the degree of non-photochemical quenching (NPQ). Although constitutive accumulation of zeaxanthin can help protect plants from photo-oxidative stress, organisms with such a phenotype have been reported to have altered rates of NPQ induction. In this study, basic fluorescence principles and the routinely used NPQ analysis technique were employed to investigate excitation energy quenching in the unicellular green alga Dunaliella salina, in both wild type (WT) and a mutant, zea1, constitutively accumulating zeaxanthin under all growth conditions. The results showed that, in D. salina, NPQ is a multi-component process consisting of energy- or DeltapH-dependent quenching (qE), state-transition quenching (qT), and photoinhibition quenching (qI). Despite the vast difference in the amount of zeaxanthin in WT and the zea1 mutant grown under low light, the overall kinetics of NPQ induction were almost the same. Only a slight difference in the relative contribution of each quenching component could be detected. Of all the NPQ subcomponents, qE seemed to be the primary NPQ operating in this alga in response to short-term exposure to excessive irradiance. Whenever qE could not operate, i.e., in the presence of nigericin, or under conditions where the level of photon flux is beyond its quenching power, qT and/or qI could adequately compensate its photoprotective function.

The dynamics of photosynthesis

Despite recent elucidation of the three-dimensional structure of major photosynthetic complexes, our understanding of light energy conversion in plant chloroplasts and microalgae under physiological conditions requires exploring the dynamics of photosynthesis. The photosynthetic apparatus is a flexible molecular machine that can acclimate to metabolic and light fluctuations in a matter of seconds and minutes. On a longer time scale, changes in environmental cues trigger acclimation responses that elicit intracellular signaling between the nucleo-cytosol and chloroplast resulting in modification of the biogenesis of the photosynthetic machinery. Here we attempt to integrate well-established knowledge on the functional flexibility of light-harvesting and electron transfer processes, which has greatly benefited from genetic approaches, with data derived from the wealth of recent transcriptomic and proteomic studies of acclimation responses in photosynthetic eukaroytes.

Fluorescence lifetime imaging microscopy of Chlamydomonas reinhardtii: non-photochemical quenching mutants and the effect of photosynthetic inhibitors on the slow chlorophyll fluorescence transient

Fluorescence lifetime-resolved images of chlorophyll fluorescence were acquired at the maximum P-level and during the slower transient (up to 250 s, including P-S-M-T) in the green photosynthetic alga Chlamydomonas reinhardtii. At the P-level, wild type and the violaxanthin-accumulating mutant npq1 show similar fluorescence intensity and fluorescence lifetime-resolved images. The zeaxanthin-accumulating mutant npq2 displays reduced fluorescence intensity at the P-level (about 25-35% less) and corresponding lifetime-resolved frequency domain phase and modulation values compared to wild type/npq1. A two-component analysis of possible lifetime compositions shows that the reduction of the fluorescence intensity can be interpreted as an increase in the fraction of a short lifetime component. This supports the important photoprotection function of zeaxanthin in photosynthetic samples, and is consistent with the notion of a 'dimmer switch'. Similar, but quantitatively different, behaviour was observed in the intensity and fluorescence lifetime-resolved imaging measurements for cells that were treated with the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, the efficient PSI electron acceptor methyl viologen and the protonophore nigericin and. Lower fluorescence intensities and lifetimes were observed for all npq2 mutant samples at the P-level and during the slow fluorescence transient, compared to wild type and the npq1 mutant. The fluorescence lifetime-resolved measurements during the slow fluorescence changes after the P level up to 250 s for the wild type and the two mutants, in the presence and absence of the above inhibitors, were analyzed with a graphical procedure (polar plots) to determine lifetime compositions. At higher illumination intensity, wild type and npq1 cells show a rise in fluorescence intensity and corresponding rise in the species concentration of the slow lifetime component after the initial decrease following the P level. This reversal is absent in the npq2 mutant, and for all samples in the presence of the inhibitors. Lifetime heterogeneities were observed in experiments averaged over multiple cells as well as within single cells, and these were followed over time. Cells in the resting state (induced by several hours of darkness), instead of the normal swimming state, show shortened lifetimes. The above results are discussed in terms of a superposition of effects on electron transfer and protonation rates, on the so-called 'State Transitions', and on non-photochemical quenching. Our data indicate two major populations of chlorophyll a molecules, defined by two 'lifetime pools' centred on slower and faster fluorescence lifetimes.