Growth Light Levels Research Articles

Leaf photosynthetic plasticity does not predict biomass responses to growth irradiance in rice

Phenotypic plasticity, the capacity of an organism to generate alternative phenotypes in response to different environments, is a particularly important characteristic to enable sessile plants to adapt to rapid changes in their surroundings. Leaf anatomical and physiological traits exhibit plasticity in response to growth irradiances, but it is relatively unclear if the plasticity varies among genotypes for a species. Equally importantly, empirical results on how leaf-level plasticity influences whole-plant growth are largely absent. We conducted an integrated investigation into the light-introduced plasticity by measuring 48 traits involving plant growth, leaf anatomy, leaf biochemistry, and leaf physiology of five rice genotypes grown under two irradiances. More than half of the estimated traits were significantly affected by growth light intensities, and the sizes of the cumulative effect of growth light ranged from -25.04% (stomatal conductance at high measurement light) to 135.2% (tiller number). Growth irradiance levels dramatically shifted the relationship between photosynthetic rate and stomatal conductance. However, the relationship between photosynthetic rate and mesophyll conductance was rarely influenced by growth light levels. Importantly, the present study highlights the significant variation in trait plasticity across rice genotypes and that the light-introduced biomass changes were rarely predicted by leaf photosynthetic plasticity. Our findings imply that the genotypes with high productivity at the low growth light conditions do not necessarily have high productivity under high light conditions.

Elevated pCO2 enhances under light but reduces in darkness the growth rate of a diatom, with implications for the fate of phytoplankton below the photic zone

AbstractExperimentally elevated pCO2 and the associated pH drop are known to differentially affect many aspects of the physiology of diatoms under different environmental conditions or in different regions. However, contrasting responses to elevated pCO2 in the dark and light periods of a diel cycle have not been documented. By growing the model diatom Phaeodactylum tricornutum under 3 light levels and 2 different CO2 concentrations, we found that the elevated pCO2/pH drop projected for future ocean acidification reduced the diatom's growth rate by 8–25% during the night period but increased it by up to 9–21% in the light period, resulting in insignificant changes in growth over the diel cycle under the three different light levels. The elevated pCO2 increased the respiration rates irrespective of growth light levels and light or dark periods and enhanced its photosynthetic performance during daytime. With prolonged exposure to complete darkness, simulating the sinking process in the dark zones of the ocean, the growth rates decreased faster under elevated pCO2, along with a faster decline in quantum yield and cell size. Our results suggest that elevated pCO2 enhances the diatom's respiratory energy supplies to cope with acidic stress during the night period but enhances its death rate when the cells sink to dark regions of the oceans below the photic zone, with implications for a possible acidification‐induced reduction in vertical transport of organic carbon.

Carbon Isotopic Fractionation of Alkenones and Gephyrocapsa Coccoliths Over the Late Quaternary (Marine Isotope Stages 12–9) Glacial‐Interglacial Cycles at the Western Tropical Atlantic

AbstractThe sensitivity of coccolithophores to changing CO2 and its role modulating cellular photosynthetic carbon isotopic fractionation (εp) is crucial to understand the future adaptation of these organisms to higher CO2 world and to assess the reliability of εp for past CO2 estimation. Here, we present εp measured on natural fossil samples across the glacial‐interglacial (G‐I) CO2 variations of marine isotope stages 12 to 9 interval (454–334 ka) at the western tropical Atlantic Ocean Drilling Program Site 925 together with a set of organic and inorganic geochemical, micropaleontological and morphometrical data from Gephyrocapsa coccoliths in the same samples. The ∼2‰ variation in εp is significantly correlated with the CO2[aq] concentrations calculated from assumption of air‐sea equilibrium with measured ice core pCO2 concentrations. The sensitivity of εp to CO2[aq] is similar to that derived from a multiple regression model of culture observations and is not well simulated with the classical purely diffusive model of algal CO2 acquisition. The measured range of Gephyrocapsa cell sizes is insufficient to explain the non‐CO2 effects on εp at this location, via either direct size effect or growth rate correlated to cell size. Primary productivity, potentially triggered by shifting growth rates and light levels, may also affect εp. Proposed productivity proxies % Florisphaera profunda and the ratio between the C37 to C38.et alkenone (C37/C38.et ratio) both correlates modestly with the non‐CO2 effects on εp. When the observed G‐I εp to CO2 sensitivity at this site is used to estimate pCO2 from εp since the Miocene, the inferred pCO2 declines are larger in amplitude compared to that calculated from a theoretical εp diffusive model. We find that oxygen and carbon stable isotope vital effects in the near monogeneric‐separated Gephyrocapsa coccoliths (respectively Δδ18OGephyrocapsa–Trilobatus sacculifer and εcoccolith) are coupled through the time series, but the origins of these vital effects are not readily explained by existing models.

Photosynthesis and calcification of the coccolithophore Emiliania huxleyi are more sensitive to changed levels of light and CO2 under nutrient limitation

Photophysiological responses of phytoplankton to changing multiple environmental drivers are essential in understanding and predicting ecological consequences of ocean climate changes. In this study, we investigated the combined effects of two CO2 levels (410 and 925 μatm) and five light intensities (80 to 480 μmol photons m−2 s−1) on cellular pigments contents, photosynthesis and calcification of the coccolithophore Emiliania huxleyi grown under nutrient replete and limited conditions, respectively. Our results showed that high light intensity, high CO2 level and nitrate limitation acted synergistically to reduce cellular chlorophyll a and carotenoid contents. Nitrate limitation predominantly enhanced calcification rate; phosphate limitation predominantly reduced photosynthetic carbon fixation rate, with larger extent of the reduction under higher levels of CO2 and light. Reduced availability of both nitrate and phosphate under the elevated CO2 concentration decreased saturating light levels for the cells to achieve the maximal relative electron transport rate (rETRmax). Light-saturating levels for rETRmax were lower than that for photosynthetic and calcification rates under the nutrient limitation. Regardless of the culture conditions, rETR under growth light levels correlated linearly and positively with measured photosynthetic and calcification rates. Our findings imply that E. huxleyi cells acclimated to macro-nutrient limitation and elevated CO2 concentration decreased their light requirement to achieve the maximal electron transport, photosynthetic and calcification rates, indicating a photophysiological strategy to cope with CO2 rise/pH drop in shoaled upper mixing layer above the thermocline where the microalgal cells are exposed to increased levels of light and decreased levels of nutrients.

Post‐fire changes in plant growth form composition and diversity in Andean páramo grassland

AbstractQuestionsFire suppression policies have been widely adopted in the páramo grasslands of the northern Andes to protect their biodiversity and ecosystem services. Páramos have been regularly burned for many years, and it is not clear how páramo vegetation will respond to significant changes in their fire regimes. This study investigates differences in plant growth form composition, light levels and soil temperatures in páramo plots representing a range of recovery times since the last fire.LocationReserva Ecológica El Ángel and La Bretaña Nature Reserve, Carchi, Northern Ecuador.MethodsWe assessed the frequency of ten páramo growth forms, vegetation height, soil temperature, and light intensity in fifteen fire sites with historical records of fire (<1–15 years since fire), and one recently unburned site (at least 40 years since fire). A chronosequence of sites was used to assess potential changes in plant community composition in post‐fire succession of páramo.ResultsThe recovery of páramo vegetation after fire comprised three phases: initial recruitment with high growth form diversity, followed by reduced diversity, light and soil temperatures in dense tussock vegetation, and ultimately canopy height stratification with a return of diversity. All but one of the plant growth forms were represented in each of the three phases, and the changes reflected differences in relative abundance.ConclusionsPost‐fire páramo succession is characterised by clear shifts in the relative abundance of plant growth forms, ending with (co‐)dominance of upright shrubs. The long‐term consequences of such shifts for biodiversity and ecosystem function, given the widespread adoption of fire suppression policies in the páramo, need careful, evidence‐based consideration.

Contrasting nonphotochemical quenching patterns under high light and darkness aligns with light niche occupancy in Arctic diatoms

AbstractOver the seasons, Arctic diatom species occupy shifting habitats defined by contrasting light climates, constrained by snow and ice cover dynamics interacting with extreme photoperiod and solar angle variations. How Arctic diatom photoadaptation strategies differ across their heterogeneous light niches remains a poorly documented but crucial missing link to anticipate Arctic Ocean responses to shrinking sea‐ice and increasing light. To address this question, we selected five Arctic diatom species with diverse life traits, representative of distinct light niches across the seasonal light environment continuum: from snow‐covered dimly lit bottom ice to summer stratified waters. We studied their photoacclimation plasticity to two growth light levels and the subsequent responses of their nonphotochemical quenching (NPQ) and xanthophyll cycle to both dark incubations and light shifts. We deciphered NPQ and xanthophyll cycle tuning in darkness and their light‐dependent induction kinetics, which aligned with species' light niche occupancy. In ice‐related species, NPQ was sustained in darkness and its induction was more reactive to moderate light shifts. Open‐water species triggered strong NPQ induction in darkness and reached higher maximal NPQ under high light. Marginal ice zone species showed strong adaptation to light fluctuations with a dark response fine‐tuned depending upon light history. We argue these traits are anchored in diverging photoadaption strategies fostering Arctic diatom success in their respective light niches.

Natural disasters and regional development – the case of earthquakes

AbstractWe analyze the impact of earthquakes on nighttime lights at a sub-national level, i.e., on grids of different size. We argue that existing studies on the impact of natural disasters on economic development have several important limitations, both at the level of the outcome variable as well as at the level of the independent variable, e.g., the timing of an event and the measuring of its intensity. We aim to overcome these limitations by using geophysical event data on earthquakes together with satellite nighttime lights. Using panel fixed effects regressions covering the entire world for the period 1992–2013, we find that earthquakes reduce both light growth rates and light levels significantly. The effects persist for approximately 5 years, but we find no long-run effects. Effects are stronger the smaller the area of a unit of observation. National institutions and economic conditions are relevant moderating factors.

High antioxidant capability interacts with respiration to mediate two Alexandrium species growth exploitation of photoperiods and light intensities

Light drives phytoplankton photosynthesis, so phytoplankton in their living habitats must exploit variable light levels and exposure durations, depending upon seasons, latitudes, depths and mixing events. Comparative growth, physiology and biochemical compositions were explored for the small Alexnadrium minutum (˜40 μm3 biovolume) and large Alexandrium catenella (˜9300 μm3 biovolume), globally wide spread coastal toxic red tide dinoflagellates, responding to a matrix of photoperiods (Light:Dark, 8:16, 16:8 and 24:0) and growth light irradiances. Smaller A. minutum grew faster under shorter photoperiods across growth light levels, while larger A. catenella grew fastest under longer photoperiods at the lowest applied light level. Photosystem II function responded largely to the instantaneous growth light level across photoperiod lengths, while the cell biovolume-based respiration, antioxidant capacity as well as cell composition responded more to photoperiod duration than to light level. These complex photophysiological responses resolved into linear correlations between growth rate versus cellular antioxidant activity and versus dark respiration, indicating that respiration energizes cellular antioxidant systems to benefit the growth of the cells. These results show the growth responses of Alexandrium species to light levels across photoperiods vary with species, and possibly with cell size. Together with previous results this puts a note of caution on meta-analytical extrapolations of physiological responses to light intensity derived from studies applying different photoperiods to different taxa, because different taxa show differential, even opposite growth responses to photoperiods and light intensities.

Natural Disasters and Regional Development - the Case of Earthquakes

We analyze the impact of earthquakes on nighttime lights at a sub-national level, i.e. on grids of different size. We argue that existing studies on the impact of natural disasters on economic development have several important limitations, both at the level of the outcome variable – usually national income or growth – as well as on the level of the independent variable, e.g. the timing of an event and the measuring of its intensity. We aim to overcome these limitations by using geophysical event data on earthquakes together with satellite nighttime lights. Using panel fixed effects regressions covering the entire world for the period 1992-2013 we find that earthquakes reduce both light growth rates and light levels significantly. The effects are persistent for approximately 5 years, but we find no long run effects. The effects are strong and robust in a small grid and gets weaker the larger the unit of observation. National institutions and economic conditions are relevant mediating factors.

Reconstruction of annual growth in relation to summer temperatures and translocation of nutrients in the aquatic moss Drepanocladus trifarius from West Greenland

Mosses often dominate the submerged vegetation in Arctic lakes and ponds, making them essential contributors to the primary production in these habitats. However, little is known about the factors controlling annual growth of Arctic mosses and their sensitivity to climatic changes. It has been suggested that nutrient translocation occurs in mosses, and that annual growth of mosses therefore depends strongly on weather conditions and less on local environmental conditions. In this study, we examined annual growth of Drepanocladus trifarius ((F. Weber and D. Mohr) Broth.) from two ponds in West Greenland in relation to weather conditions. A reconstruction of annual growth increments from 2009 to 2014 was made in 200 individual mosses, and biomass and length were related to different weather parameters. In addition, we examined whether there would be an indication of nutrient translocation across annual growth segments. We found a positive relationship between mean summer temperature and growth segment length, which indicates the importance of temperature during seasons with sufficient light levels for growth of the D. trifarius. Weather parameters associated with light conditions had no significant effect on growth, which probably reflect that D. trifarius in two shallow ponds were not light limited. The nutrient stoichiometry showed that phosphorus (P) contents in the tissue were low (0.04–0.11% DW), and nutrient resorption efficiencies of P amounted to 11–29%. This suggests that D. trifarius was P limited during its growth season, but appears capable of nutrient translocation across annual segments, possibly to maintain growth in oligotrophic environments. Despite low nitrogen (N) contents (0.94–2.09%), no resorption of N was found, which indicates that D. trifarius was not N-limited in order to sustain growth. In conclusion, this study shows that growth of D. trifarius in small high Arctic ponds are mainly controlled by summer temperatures.

Photoinactivation of Photosystem II in Prochlorococcus and Synechococcus.

The marine picocyanobacteria Synechococcus and Prochlorococcus numerically dominate open ocean phytoplankton. Although evolutionarily related they are ecologically distinct, with different strategies to harvest, manage and exploit light. We grew representative strains of Synechococcus and Prochlorococcus and tracked their susceptibility to photoinactivation of Photosystem II under a range of light levels. As expected blue light provoked more rapid photoinactivation than did an equivalent level of red light. The previous growth light level altered the susceptibility of Synechococcus, but not Prochlorococcus, to this photoinactivation. We resolved a simple linear pattern when we expressed the yield of photoinactivation on the basis of photons delivered to Photosystem II photochemistry, plotted versus excitation pressure upon Photosystem II, the balance between excitation and downstream metabolism. A high excitation pressure increases the generation of reactive oxygen species, and thus increases the yield of photoinactivation of Photosystem II. Blue photons, however, retained a higher baseline photoinactivation across a wide range of excitation pressures. Our experiments thus uncovered the relative influences of the direct photoinactivation of Photosystem II by blue photons which dominates under low to moderate blue light, and photoinactivation as a side effect of reactive oxygen species which dominates under higher excitation pressure. Synechococcus enjoyed a positive metabolic return upon the repair or the synthesis of a Photosystem II, across the range of light levels we tested. In contrast Prochlorococcus only enjoyed a positive return upon synthesis of a Photosystem II up to 400 μmol photons m-2 s-1. These differential cost-benefits probably underlie the distinct photoacclimation strategies of the species.

A Hard Day's Night: Diatoms Continue Recycling Photosystem II in the Dark

Marine diatoms are photosynthetic, and thrive in environments where light fluctuates. Like all oxygenic photosynthetic organisms diatoms face a light-dependent inactivation of the Photosystem II complexes that photooxidize water to generate biosynthetic reductant. To maintain photosynthesis this photoinactivation must be countered by slow and metabolically expensive protein turnover, which is light dependent in cyanobacteria and in plants. We tracked daily cycles of the content, synthesis and degradation of Photosystem II, in a small and in a large marine diatom, under low and high growth light levels. We show that, unlike plants, diatoms maintain extensive cycling of Photosystem II proteins even in the dark. Photosystem II protein cycling saturates at low light, and continued cycling in dark periods, using energy from respiration, allows the diatoms to catch up to excess photoinactivation accumulated over the preceding illuminated period. The large diatom suffers only limited photoinactivation of Photosystem II, but cycling of Photosystem II protein exceeds Photosystem II inactivation, so the large diatom recycles functional Photosystem II units before they are inactivated. Through the diel cycle the contents of active Photosystem II centers and Photosystem II proteins change predictably, but are not correlated, generating large changes in the fraction of total PSII that is active at a given time or growth condition. We propose that dark and steady cycling of Photosystem II proteins is driven by the tight integration of chloroplastic and mitochondrial metabolism in diatoms. This ability for baseline, continuous Photosystem II repair could contribute to the success of diatoms in mixed water environments that carry them from illumination to darkness and back.

Interactive effects of nitrogen and light on growth rates and RUBISCO content of small and large centric diatoms

Among marine phytoplankton groups, diatoms span the widest range of cell size, with resulting effects upon their nitrogen uptake, photosynthesis and growth responses to light. We grew two strains of marine centric diatoms differing by ~4 orders of magnitude in cell biovolume in high (enriched artificial seawater with ~500 µmol L−1 µmol L−1 NO3−) and lower-nitrogen (enriched artificial seawater with <10 µmol L−1 NO3−) media, across a range of growth light levels. Nitrogen and total protein per cell decreased with increasing growth light in both species when grown under the lower-nitrogen media. Cells growing under lower-nitrogen media increased their cellular allocation to RUBISCO and their rate of electron transport away from PSII, for the smaller diatom under low growth light and for the larger diatom across the range of growth lights. The smaller coastal diatom Thalassiosira pseudonana is able to exploit high nitrogen in growth media by up-regulating growth rate, but the same high-nitrogen growth media inhibits growth of the larger diatom species.

Rate Constants of PSII Photoinhibition and its Repair, and PSII Fluorescence Parameters in Field Plants in Relation to their Growth Light Environments.

The extent of photoinhibition of PSII is determined by a balance between the rate of photodamage to PSII and that of repair of the damaged PSII. It has already been indicated that the rate constants of photodamage (kpi) and repair (krec) of the leaves differ depending on their growth light environment. However, there are no studies using plants in the field. We examined these rate constants and fluorescence parameters of several field-grown plants to determine inter-relationships between these values and the growth environment. The kpi values were strongly related to the excess energy, EY, of the puddle model and non-regulated energy dissipation, Y(NO), of the lake model, both multiplied by the photosynthetically active photon flux density (PPFD) level during the photoinhibitory treatment. In contrast, the krec values corrected against in situ air temperature were very strongly related to the daily PPFD level. The plants from the fields showed higher NPQ than the chamber-grown plants, probably because these field plants acclimated to stronger lightflecks than the averaged growth PPFD. Comparing chamber-grown plants and the field plants, we showed that kpi is determined by the incident light level and the photosynthetic capacities such as in situ rate of PSII electron transport and non-photochemical quenching (NPQ) [e.g. Y(NO)×PPFD] and that krec is mostly determined by the growth light and temperature levels.

Faster recovery of a diatom from UV damage under ocean acidification

Diatoms are the most important group of primary producers in marine ecosystems. As oceanic pH declines and increased stratification leads to the upper mixing layer becoming shallower, diatoms are interactively affected by both lower pH and higher average exposures to solar ultraviolet radiation. The photochemical yields of a model diatom, Phaeodactylum tricornutum, were inhibited by ultraviolet radiation under both growth and excess light levels, while the functional absorbance cross sections of the remaining photosystem II increased. Cells grown under ocean acidification (OA) were less affected during UV exposure. The recovery of PSII under low photosynthetically active radiation was much faster than in the dark, indicating that photosynthetic processes were essential for the full recovery of photosystem II. This light dependent recovery required de novo synthesized protein. Cells grown under ocean acidification recovered faster, possibly attributable to higher CO2 availability for the Calvin cycle producing more resources for repair. The lower UV inhibition combined with higher recovery rate under ocean acidification could benefit species such as P.tricornutum, and change their competitiveness in the future ocean.

The response of rbcL, rbcS and rca genes in cucumber (Cucumis sativus L.) to growth and induction light intensity

Low light is one of the most important factors affecting cucumber growth in winter solar greenhouses in northern China. Whether induction light intensity has an effect on expressions of rbcL, rbcS and rca genes in cucumber leaves grown under low light for long periods of time remains unclear. Here, two genotypes of cucumber were used to determine the effects of growth and induction light levels on photosynthetic induction. It was found that regardless of growth and induction light intensity, the transcript levels of rbcL, rbcS and rca increased quickly once the plant was exposed to light. The variations of the photosynthesis (P N), stomatal conductance (g s), actual photosystem II efficiency (ΦPS II) and Rubisco activity were similar to transcript levels. Plants grown at a photon flux density (PFD) of 100 μmol m−2 s−1 had lower P N, and initial and total Rubisco activity than those grown at 600 μmol m−2 s−1, regardless of induction PFD; for plants grown at the same PFD, P N, g s and Rubisco activation rate induced under PFD of 100 μmol m−2 s−1 were lower than those under 600 μmol m−2 s−1. Rubisco, especially RCA, played a great role on photosynthesis during photosynthetic induction. Compared with Jinyan No. 2, Deltastar may possess potential low-light tolerance with a lower decrease of P N, and initial and total Rubisco activities when grown and induced at 100 μmol m−2 s−1 than 600 μmol m−2 s−1. Moreover, the mechanism of Rubisco activase in regulating Rubisco during photosynthetic induction is also discussed.

Differential responses of growth and photosynthesis in the marine diatom Chaetoceros muelleri to CO2 and light availability

Ihnken S., Roberts S. and Beardall J. 2011. Differential responses of growth and photosynthesis in the marine diatom Chaetoceros muelleri to CO2 and light availability. Phycologia 50: 182–193. DOI: 10.2216/10-11.1This study investigated the impact of photon flux and elevated CO2 concentrations on growth and photosynthetic electron transport on the marine diatom Chaetoceros muelleri and looked for evidence for the presence of a CO2-concentrating mechanism (CCM). pH drift experiments clearly showed that C. muelleri has the capacity to use bicarbonate to acquire inorganic carbon through one or multiple CCMs. The final pH achieved in unbuffered cultures was not changed by light intensity, even under very low photon flux, implying a low energy demand of bicarbonate use via a CCM. In short-term pH drift experiments, only treatment with the carbonic anhydrase inhibitor ethoxyzolamide (EZ) slowed down the rise in pH considerably. EZ was also the only inhibitor that altered the final pH attained, although marginally. In growth experiments, CO2 availability was manipulated by changing the pH in closed flasks at a fixed dissolved inorganic carbon (DIC) concentration. Low-light-treated samples showed lower growth rates in elevated CO2 conditions. No CO2 effect was recorded under high light exposure. The maximal photosynthetic capacity, however, increased with CO2 concentration in saturating, but not in subsaturating, light intensities. Growth and photosynthetic capacity therefore responded in opposite ways to increasing CO2 availability. The capacity to photoacclimate to high and low photon flux appeared not to be affected by CO2 treatments. However, photoacclimation was restricted to growth photon fluxes between 30 and 300 µmol photons m−2 s−1. The light saturation points for photosynthetic electron transport and for growth coincided at 100 µmol photons m−2 s−1. Below 100 µmol photons m−2 s−1 the light saturation point for photosynthesis was higher than the growth photon flux (i.e. photosynthesis was not light saturated under growth conditions), whereas at higher growth photon flux, photosynthesis was saturated below growth light levels.

Effects of clonal integration and light availability on the growth and physiology of two invasive herbs

Summary 1. Clonal plants benefit from the ability to translocate resources among interconnected ramets to colonize heterogeneous habitats. Clonal integration affects the resource level and morphological traits of ramets, and thus may influence their physiology and general performance. Although leaf gas exchange and its associated physiological adjustments are key traits to assess plant fitness, the effect of clonal integration on these functional traits is insufficiently understood. 2. In a glasshouse experiment, we addressed how clonal integration affects gas exchange properties, leaf characters and growth of ramets in two invasive plants, Alternanthera philoxeroides and Phyla canescens, under full sun and 85% shade. We also used stable‐isotope labelling to assess the maternal subsidy to daughter ramets. 3. Similar effects of connection were observed in both species for most gas exchange parameters and leaf characters. Clonal integration did not affect photosynthetic capacity and respiratory rates of ramets. When grown in shade, ramets connected with an unshaded mother plant displayed higher area‐based leaf nitrogen and chlorophyll content than severed ramets, but the additional nitrogen and chlorophyll was not translated to increased photosynthetic capability. Overall, severed ramets displayed significant light response for leaf nitrogen (area‐based), photosynthetic nitrogen use efficiency, chlorophyll to nitrogen ratio, and nitrogen stable‐isotope signature, but the light response was eliminated by clonal integration in connected ramets. 4. Both species displayed substantial maternal carbohydrate subsidy that benefited the growth of daughter ramets, but species‐specific patterns were observed in the growth of daughter ramets and the amount of subsidy. The amount of subsidy was independent of ramet growth light levels for P. canescens, but shaded, connected ramets of A. philoxeroides received more subsidy than unshaded controls, facilitating a larger growth improvement relative to severed counterparts than P. canescens. 5. Synthesis. We observed increased leaf nitrogen and chlorophyll in shaded, connected ramets of two clonal invasive plants. Clonal integration may facilitate nitrogen assimilation and allow pre‐acclimation to high‐light conditions for shaded, connected ramets, thus promoting the opportunistic expansion of these invaders on site scale. The species‐specific maternal subsidy pattern demonstrated that clonal plants possess different strategies to subsidize ramets under light‐limited conditions.

Photosynthetic acclimation to different light levels in the brown marine macroalga, Hizikia fusiformis (Sargassaceae, Phaeophyta)

Hizikia fusiformis thalli experience dynamic incident light conditions during the period of growth. The present study was designed to examine how changing photon irradiance affects the photosynthesis both in the short and long terms by culturing H. fusiformis under three different light levels: 35 μmol photons m-2 s-1 (low light, LL), 85 μmol photons m-2 s-1 (intermediate light, IL), and 165 μmol photons m-2 s-1 (high light, HL). A similar relative growth rate was observed between IL- and HL-grown algae, but the growth rate was significantly reduced in LL-grown algae. The photosynthetic rates (Pn) measured at their respective growth light levels were found to be lowest in the thalli grown at LL and highest at HL. However, LL-grown algae exhibited much higher Pn in comparison with IL- and the HL-grown thalli at the same measuring photosynthetic photon flux density, indicating the photosynthetic acclimation to low growth light in H. fusiformis. The photosynthesis–light curves showed that LL-grown algae had a highest light-saturating maximum Pn (Pmax) in comparison with IL- or HL-grown algae when the photosynthetic rates were expressed on the biomass basis. However, Pmax was highest in HL-grown algae compared to IL- or LL-grown algae when the rates were normalized to chlorophyll a. The photosynthesis–inorganic carbon (Ci) response curves were also significantly affected by the growth light conditions. The highest value of apparent photosynthetic conductance occurred in LL-grown algae while the lowest value in HL-grown algae. Additionally, the activity of external carbonic anhydrase (CA) tended to increase while the total CA activity inclined to decrease in H. fusiformis thalli when the growth light level altered from 35 to 165 μmol photons per square meter per second. The external CA inhibitors showed a higher inhibition in HL-grown algae compared with LL-grown algae. It was proposed that photosynthetic acclimation to low light condition in H. fusiformis was achieved through an increase in the number of reaction centers and increased capacities of electron transport and of Ci transport within cells. The ability of photosynthetic acclimation to low light confers H. fusiformis thalli to overcome the environmental low light condition as a result of the attenuation of seawater or self-shading through enhancing its photosynthetic performance and carbon assimilation necessary for growth.

Determinants of whole‐plant light requirements in Bornean rain forest tree saplings

1 Shade tolerance, defined as the ability to survive and grow under low light, varies markedly among tree species. However, the role of low-light growth responses in determining shade tolerance is unclear, as are the effects of non-light resources such as soil nutrients. 2 A conceptually simple field measure of shade tolerance is the whole-plant light compensation point (WPLCP), evaluated as the x-intercept of the relationship between growth and incident light integrated over a long time interval. Here we compare WPLCP for growth and survivorship of saplings of Bornean tree species differing in shade tolerance, and evaluate the importance of various physiological and morphological traits in predicting WPLCP. We also examine both phenotypic and evolved differences in WPLCP between tree saplings growing on two distinct soil types at Sepilok Forest Reserve, Sabah, Malaysia. 3 Growth-based estimates of WPLCP showed essentially a 1 : 1 correspondence to threshold light levels for survivorship. At higher light, more light-demanding species showed higher growth, resulting in a steeper slope of the relationship between relative growth rate (RGR) and light availability than in more shade-tolerant species. This resulted in significant crossovers in the RGR–light relationship among species. 4 Dark respiration (Rd) was the single best predictor of WPLCP; other leaf traits such as leaf nitrogen and photosynthetic capacity were correlated with, but excluded as predictors of, WPLCP in multiple regression analyses. 5 Although soil type had no consistent phenotypic effect on WPLCP, evolved responses among species were pronounced: species associated with the nutrient-poor, drought-prone, sandstone-derived soils had higher WPLCP values than alluvial soil specialists in phylogenetically controlled comparisons. 6 Our results indicate that minimum light levels for growth do not diverge from those for survivorship, and do not support the view that low-light survivorship solely determines shade tolerance. Our analyses also suggest that Rd is the strongest determinant of whole-plant light requirements in tropical tree saplings, and thus may be an easily measured surrogate of WPLCP and shade tolerance. 7 Prediction of tree species resource requirements is crucial for understanding forest dynamics and promoting ecology-based forest management and restoration, particularly in diverse tropical forests where data on the resource requirements of most species are not available. Easily measured surrogates of resource requirements (e.g. Rdas a predictor of shade tolerance) will contribute to this goal, as will an improved understanding of the interactive effects of multiple resources on tree performance.