RESUMEN

BACKGROUND AND AIMS: The photoprotective role of foliar anthocyanins has long been ambiguous: exacerbating, being indifferent to or ameliorating the photoinhibition of photosynthesis. The photoinhibitory light spectrum and failure to separate photo-resistance from repair, as well as the different methods used to quantify the photo-susceptibility of the photosystems, could lead to such a discrepancy. METHODS: We selected two congeneric deciduous shrubs, Prunus cerasifera with anthocyanic leaves and Prunus triloba with green leaves, grown under identical growth conditions in an open field. The photo-susceptibilities of photosystem II (PSII) and photosystem I (PSI) to red light and blue light, in the presence of lincomycin (to block the repair), of exposed leaves were quantified by a non-intrusive P700+ signal from PSI. Leaf absorption, pigments, gas exchange and Chl a fluorescence were also measured. KEY RESULTS: The content of anthocyanins in red leaves (P. cerasifera) was >13 times greater than that in green leaves (P. triloba). With no difference in maximum quantum efficiency of PSII photochemistry (Fv/Fm) and apparent CO2 quantum yield (AQY) in red light, anthocyanic leaves (P. cerasifera) showed some shade-acclimated suites, including lower Chl a/b ratio, lower photosynthesis rate, lower stomatal conductance and lower PSII/PSI ratio (on an arbitrary scale), compared with green leaves (P. triloba). In the absence of repair of PSII, anthocyanic leaves (P. cerasifera) showed a rate coefficient of PSII photoinactivation (ki) that was 1.8 times higher than that of green leaves (P. triloba) under red light, but significantly lower (-18 %) under blue light. PSI of both types of leaves was not photoinactivated under blue or red light. CONCLUSIONS: In the absence of repair, anthocyanic leaves exhibited an exacerbation of PSII photoinactivation under red light and a mitigation under blue light, which can partially reconcile the existing controversy in terms of the photoprotection by anthocyanins. Overall, the results demonstrate that appropriate methodology applied to test the photoprotection hypothesis of anthocyanins is critical.

Asunto(s)

RESUMEN

Understanding crop responses to the light spectrum is critical for optimal indoor production. Far-red light is of particular interest, because it can accelerate growth through both physiological and morphological mechanisms. However, the optimal amount of supplemental far-red light for indoor lettuce production is yet to be quantified. Lettuce 'Cherokee', 'Green SaladBowl', and 'Little Gem' were grown under 204 µmol·m-2·s-1 warm-white light-emitting diodes (LEDs) with supplemental far-red ranging from 5.3 to 75.9 µmol·m-2·s-1. Supplemental far-red light increased canopy light interception 5 days after the start of far-red light treatment (DAT) for 'Green SaladBowl' and 'Little Gem' and 7 DAT for 'Cherokee'. The increase in light interception was no longer evident after 12 and 16 DAT for 'Green SaladBowl' and 'Little Gem', respectively. We did not find evidence that supplemental far-red light increased leaf-level photosynthesis. At the final harvest, shoot dry weights of 'Cherokee' and 'Little Gem' increased by 39.4% and 19.0%, respectively, while 'Green SaladBowl' was not affected. In conclusion, adding far-red light in indoor production increased light interception during early growth and likely increased whole plant photosynthesis thus growth, but those effects were cultivar-specific; the increase in dry weight was linear up to 75.9 µmol·m-2·s-1 far-red light.

RESUMEN

BACKGROUND: Coastal wetlands are threatened by the increased salinity that may result from sea level rise. Salinity stress alters species zonation patterns through changes in competitive outcome between species differing in salinity tolerance. This study therefore aimed to understand how salinity and light affect two dominant and competing coastal wetland grasses that differ in salt tolerance, height and photosynthetic metabolism. METHODS: The C4 species Spartina anglica and the C3 species Phragmites australis were grown at five salinity levels (0, 7, 14, 21 and 28 ppt) and two light fluxes (100 % and 50 % of natural daylight) in an outdoor experimental setup for 102 d with full access to nutrients. KEY RESULTS: Salinity reduced the biomass, height and shoot density of P. australis from 81.7 g dry weight (DW), 0.73 m and 37 shoots per pot at a salinity of 0 ppt to 16.8 gDW, 0.3 m and 14 shoots per pot at a salinity of 28 ppt. Biomass, height and shoot density of S. anglica did not respond or were only slightly reduced at the highest salinity of 28 ppt. High salinity also resulted in a higher tissue concentration of N and P in P. australis. Both species had low ability to acclimate to the lower light flux. Shade acclimation in S. anglica occurred via modest changes in specific leaf area, pigment content and biomass allocation. CONCLUSIONS: High salinity reduced traits important for light competition and increased the nutrient concentration in P. australis leaf and root biomass, while this was overall unaffected in S. anglica. This is likely to reduce the competitive ability of P. australis over S. anglica for light because at high salinities the former cannot effectively shade the lower-growing S. anglica. Neither species effectively acclimates to shade, which could explain why S. anglica does not occur in the understorey of P. australis at low salinities.

Asunto(s)

RESUMEN

(1) Background: A central subject in clonal plant ecology is to elucidate the mechanism by which clones forage resources in heterogeneous environments. Compared with studies conducted in laboratories or experimental gardens, studies on light foraging of forest woody clonal plants in their natural habitats are limited. (2) Methods: We investigated wild populations of an evergreen clonal understory shrub, Japanese pachysandra (Pachysandra terminalis Siebold & Zucc.), in two cool-temperate forests in Japan. (3) Results: Similar to the results of herbaceous clonal species, this species formed a dense stand in a relatively well-lit place, and a sparse stand in a shaded place. Higher specific rhizome length (i.e., length per unit mass) in shade resulted in lower ramet population density in shade. The individual leaf area, whole-ramet leaf area, or ramet height did not increase with increased light availability. The number of flower buds per flowering ramet increased as the canopy openness or population density increased. (4) Conclusions: Our results provide the first empirical evidence of shade avoidance and light foraging with morphological plasticity for a clonal woody species.

RESUMEN

Zea mays and Miscanthus × giganteus use NADP-ME subtype C4 photosynthesis and are important food and biomass crops, respectively. Both crops are grown in dense stands where shaded leaves can contribute a significant proportion of overall canopy productivity. This is because shaded leaves, despite intercepting little light, typically process light energy very efficiently for photosynthesis, when compared to light-saturated leaves at the top of the canopy. However, an apparently maladaptive loss in photosynthetic light-use efficiency as leaves become shaded has been shown to reduce productivity in these two species. It is unclear whether this is due to leaf aging or progressive shading from leaves forming above. This was resolved here by analysing photosynthesis in leaves of the same chronological age in the centre and exposed southern edge of field plots of these crops. Photosynthetic light-response curves were used to assess maximum quantum yield of photosynthesis; the key measure of photosynthetic capacity of a leaf in shade. Compared to the upper canopy, maximum quantum yield of photosynthesis of lower canopy leaves was significantly reduced in the plot centre; but increased slightly at the plot edge. This indicates loss of efficiency of shaded leaves is due not to aging, but to the altered light environment of the lower canopy, i.e., reduced light intensity and/or altered spectral composition. This work expands knowledge of the cause of this maladaptive shade response, which limits productivity of some of the world's most important crops.

RESUMEN

Previous studies of heat tolerance of tropical trees have focused on canopy leaves exposed to full sunlight and high temperatures. However, in lowland tropical forests with leaf area indices of 5-6, the vast majority of leaves experience varying degrees of shade and a reduced heat load compared to sun leaves. Here we tested whether heat tolerance is lower in shade than in sun leaves. For three tropical tree species, Calophyllum inophyllum, Inga spectabilis, and Ormosia macrocalyx, disks of fully developed shade and sun leaves were subjected to 15-min heat treatments, followed by measurement of chlorophyll a fluorescence after 48 h of recovery. In two of the three species, the temperature causing a 50% decrease of the fluorescence ratio Fv/Fm (T50) was significantly lower (by ~ 1.0 °C) in shade than in sun leaves, indicating a moderately decreased heat tolerance of shade leaves. In shade leaves of these two species, the rise in initial fluorescence, F0, also occurred at lower temperatures. In the third species, there was no shade-sun difference in T50. In situ measurements of photosynthetic CO2 assimilation showed that the optimum temperature for photosynthesis tended to be lower in shade leaves, although differences were not significant. At supra-optimal temperatures, photosynthesis was largely constrained by stomatal conductance, and the high-temperature CO2 compensation point, TMax, occurred at considerably lower temperatures than T50. Our study demonstrates that the temperature response of shade leaves of tropical trees differs only marginally from that of sun leaves, both in terms of heat tolerance and photosynthetic performance.

Asunto(s)

RESUMEN

The wild progenitors of major C4 crops grew as individuals subjected to little shading. Today they are grown in dense stands where most leaves are shaded. Do they maintain photosynthetic efficiency in these low light conditions produced by modern cultivation? The apparent maximum quantum yield of CO2 assimilation (ΦCO2max,app), a key determinant of light-limited photosynthesis, has not been systematically studied in field stands of C4 crops. ΦCO2max,app was derived from the initial slope of the response of leaf CO2 uptake (A) to photon flux (Q). Leaf fractional light absorptance (α) was measured to determine the absolute maximum quantum yield of CO2 assimilation on an absorbed light basis (ΦCO2max,abs). Light response curves were determined on sun and shade leaves of 49 field plants of Miscanthus × giganteus and Zea mays following canopy closure. ΦCO2max,app and ΦCO2max,abs declined significantly by 15-27% (P<0.05) with canopy depth. Experimentally, leaf age was shown unlikely to cause this loss. Modeling canopy CO2 assimilation over diurnal courses suggested that the observed decline in ΦCO2max,app with canopy depth costs 10% of potential carbon gain. Overcoming this limitation could substantially increase the productivity of major C4 crops.

Asunto(s)

RESUMEN

Stomatal anatomical traits and rapid responses to several components of visible light were measured in Tilia cordata Mill. seedlings grown in an open, fully sunlit field (C-set), or under different kinds of shade. The main questions were: (i) stomatal responses to which visible light spectrum regions are modified by growth-environment shade and (ii) which separate component of vegetational shade is most effective in eliciting the acclimation effects of the full vegetational shade. We found that stomatal opening in response to red or green light did not differ between the plants grown in the different environments. Stomatal response to blue light was increased (in comparison with that of C-set) in the leaves grown in full vegetational shade (IABW-set), in attenuated UVAB irradiance (AB-set) or in decreased light intensity (neutral shade) plus attenuated UVAB irradiance (IAB-set). In all sets, the addition of green light-two or four times stronger-into induction light barely changed the rate of the blue-light-stimulated stomatal opening. In the AB-set, stomatal response to blue light equalled the strong IABW-set response. In attenuated UVB-grown leaves, stomatal response fell midway between IABW- and C-set results. Blue light response by neutral shade-grown leaves did not differ from that of the C-set, and the response by the IAB-set did not differ from that of the AB-set. Stomatal size was not modified by growth environments. Stomatal density and index were remarkably decreased only in the IABW- and IAB-sets. It was concluded that differences in white light responses between T. cordata leaves grown in different light environments are caused only by their different blue light response. Differences in stomatal sensitivity are not dependent on altered stomatal anatomy. Attenuated UVAB irradiance is the most efficient component of vegetational shade in stimulating acclimation of stomata, whereas decreased light intensity plays a minor role.

Asunto(s)

RESUMEN

Light gradients within tree canopies play a major role in the distribution of plant resources that define the photosynthetic capacity of sun and shade leaves. However, the biochemical and diffusional constraints on gas exchange in sun and shade leaves in response to light remain poorly quantified, but critical for predicting canopy carbon and water exchange. To investigate the CO2 diffusion pathway of sun and shade leaves, leaf gas exchange was coupled with concurrent measurements of carbon isotope discrimination to measure net leaf photosynthesis (An ), stomatal conductance (gs ) and mesophyll conductance (gm ) in Eucalyptus tereticornis trees grown in climate controlled whole-tree chambers. Compared to sun leaves, shade leaves had lower An , gm , leaf nitrogen and photosynthetic capacity (Amax ) but gs was similar. When light intensity was temporarily increased for shade leaves to match that of sun leaves, both gs and gm increased, and An increased to values greater than sun leaves. We show that dynamic physiological responses of shade leaves to altered light environments have implications for up-scaling leaf level measurements and predicting whole canopy carbon gain. Despite exhibiting reduced photosynthetic capacity, the rapid up-regulation of gm with increased light enables shade leaves to respond quickly to sunflecks.

Asunto(s)

RESUMEN

Increasing rice yield potential is essential to secure world food supply. The quantitative trait locus qTSN4 was reported to achieve yield increases by enhancing both source and sink capacity. Three greenhouse experiments and one field experiment in the Philippines were conducted to study near-isogenic lines (NILs) in two genetic backgrounds, subjected to treatments with restricted light resources through shading (greenhouse) or population density (field and greenhouse). A consistent promotion of flag leaf width, leaf area and panicle size in terms of spikelet number was observed in the presence of qTSN4, regardless of environment. However, grain production per plant was enhanced only in one greenhouse experiment. An in-depth study demonstrated that increased flag leaf size in the presence of qTSN4 was associated with increased photosynthetic rates, along with lower SLA and greater N content per leaf weight and per area. This was emphasized under low light situation as the qTSN4-NILs did not express shade acclimation traits in contrast with the recipient varieties. The authors conclude that qTSN4 is a promising subject for further physiological studies, particularly under limited radiation. However, the QTL alone may not be a reliable source of increased yield potential because its effects at the plant and population scale are prone to genotype × environment interactions and the increased panicle size is compensated by the adaptive plasticity of other morphological traits.

RESUMEN

The photosynthetic induction response under constant and fluctuating light was examined in naturally occurring saplings (about 0.5-2 m in height) of three shade-tolerant tree species, Pourouma bicolor spp digitata, Dicorynia guianensis, and Vouacapoua americana, growing in bright gaps and in the shaded understorey in a Neotropical rain forest. Light availability to saplings was estimated by hemispherical photography. Photosynthetic induction was measured in the morning on leaves that had not yet experienced direct sunlight. In Dicorynia, the maximum net photosynthesis rate (A max) was similar between forest environments (ca 4 µmol m-2 s-1), whereas for the two other species, it was twice as high in gaps (ca 7.5) as in the understorey (ca 4.5). However, the time required to reach 90% of A max did not differ among species, and was short, 7-11 min. Biochemical induction was fast in leaves of Pourouma, as about 3 min were needed to reach 75% of maximum carboxylation capacity (V cmax); the two other species needed 4-5 min. When induction continued after reaching 75% of V cmax, stomatal conductance increased in Pourouma only (ca 80%), causing a further increase in its net photosynthesis rate. When fully induced leaves were shaded for 20 min, loss of induction was moderate in all species. However, gap saplings of Dicorynia had a rapid induction loss (ca 80%), which was mainly due to biochemical limitation as stomatal conductance decreased only slowly. When leaves were exposed to a series of lightflecks separated by short periods of low light, photosynthetic induction increased substantially and to a similar extent in all species. Although A max was much lower in old than in young leaves as measured in Dicorynia and Vouacapoua, variables of the dynamic response of photosynthesis to a change in light tended to be similar between young and old leaves. Old leaves, therefore, might remain important for whole-plant carbon gain, especially in understorey environments. The three shade-tolerant species show that, particularly in low light, they are capable of efficient sunfleck utilization.

RESUMEN

Photosynthetic capacities and respiration rates of Alocasia macrorrhiza leaves were measured for 4 weeks following reciprocal transfers between high (20% of full sun) and low (1% of full sun) light environments. Photosynthetic capacities and respiration rates of mature, high-light leaves were 1.7 and 4.5 times those of low-light leaves, respectively. Following transfer, respiration rates adjusted within 1 week to those characteristic of plants grown in the new environment. By contrast, photosynthetic capacities either did not adjust or changed only slowly following transfer. Most of the difference in respiration between high- and low-light leaves was related to the carbohydrate status as determined by the daily PFD and little was directly related to the maintenance costs of the photosynthetic apparatus. Leaf construction cost was directly proportional to maximum photosynthetic capacity. Consequently, although daily carbon gain per unit leaf area was the same for low-light and high to low-light transferred plants within a week after transfer, the carbon return per unit of carbon investment in the leaves remained lower in the high to low transfer plants throughout the 4 week measurement period. Conversely, in high-light, the low leaf construction cost of the low to high-light transferred plants resulted in carbon gain per unit investment just as high as that of the high-light plants.