Growth Light Environment Research Articles

Recovery kinetics of winter stressed conifers: The effects of growth light environment, extent of the season, and species.

Recovery kinetics of winter stressed conifers: The effects of growth light environment, extent of the season, and species.

Reconciling species‐level vs plastic responses of evergreen leaf structure to light gradients: shade leaves punch above their weight

*When grown in a common light environment, the leaves of shade-tolerant evergreen trees have a larger leaf mass per unit area (LMA) than their light-demanding counterparts, associated with differences in lifespan. Yet plastic responses of LMA run counter to this pattern: shade leaves have smaller LMA than sun leaves, despite often living longer. *We measured LMA and cell wall content, and conducted punch and shear tests, on sun and shade leaves of 13 rainforest evergreens of differing shade tolerance, in order to understand adaptation vs plastic responses of leaf structure and biomechanics to shade. *Species shade tolerance and leaf mechanical properties correlated better with cell wall mass per unit area than with LMA. Growth light environment had less effect on leaf mechanics than on LMA: shade leaves had, on average, 40% lower LMA than sun leaves, but differences in work-to-shear, and especially force-to-punch, were smaller. This was associated with a slightly larger cell wall fraction in shade leaves. *The persistence of shade leaves might reflect unattractiveness to herbivores because they yield smaller benefits (cell contents per area) per unit fracture force than sun leaves. In forest trees, cell wall fraction and force-to-punch are more robust correlates of species light requirements than LMA.

Effects of sudden exposure to high light levels on two tree fern species Dicksonia antarctica (Dicksoniaceae) and Cyathea australis (Cyatheaceae) acclimated to different light intensities

We examined the responses of two tree fern species (Dicksonia antarctica and Cyathea australis) growing under shade or variable light (intermittent shade) to sudden exposure to high light levels. Steady-state gas exchange as well as dynamic responses of plants to artificial sunflecks indicated that difference in growth light environment had very little effect on the tree ferns’ capacities to utilise and acclimate to prevailing light conditions. Two weeks of exposure to high light levels (short-term acclimation) led to decreases in all photosynthetic parameters and more negative predawn frond water potentials, mostly irrespective of previous growth light environments. After 3months in high light levels (long-term acclimation), D. antarctica fully recovered, while C. australis previously grown under variable light, recovered only partially, suggesting high light level stress effects under the variable light environments for this species.

Photosynthetic responses to understory shade and elevated carbon dioxide concentration in four northern hardwood tree species

Seedling responses to elevated atmospheric CO(2) concentration ([CO(2)]) and solar irradiance were measured over two growing seasons in shade-tolerant Acer saccharum Marsh. and Fagus grandifolia J.F. Ehrh. and shade-intolerant Prunus serotina, a J.F. Ehrh. and Betula papyrifera Marsh. Seedlings were exposed to a factorial combination of [CO2] (ambient and elevated (658 micromol mol-1)) and understory shade (deep and moderate) in open-top chambers placed in a forest understory. The elevated [CO(2)] treatment increased mean light-saturated net photosynthetic rate by 63% in the shade-tolerant species and 67% in the shade-intolerant species. However, when measured at the elevated [CO(2)], long-term enhancement of photosynthesis was 10% lower than the instantaneous enhancement seen in ambient-[CO(2)]-grown plants (P < 0.021). Overall, growth light environment affected long-term photosynthetic enhancement by elevated [CO(2)]: as the growth irradiance increased, proportional enhancement due to elevated [CO(2)] decreased from 97% for plants grown in deep shade to 47% for plants grown in moderate shade. Results suggest that in N-limited northern temperate forests, trees grown in deep shade may display greater photosynthetic gains from a CO(2)-enriched atmosphere than trees growing in more moderate shade, because of greater downregulation in the latter environment. If photosynthetic gains by deep-shade-grown plants in response to elevated [CO(2)] translate into improved growth and survival of shade-intolerant species, it could alter the future composition and dynamics of successional forest communities.

Modulation of PsbS and flexible vs sustained energy dissipation by light environment in different species

Contrasting acclimation strategies of photosynthesis and photoprotection were identified for annual mesophytes (spinach, pumpkin, and Arabidopsis) vs the tropical evergreen Monstera deliciosa. The annual species utilized full sunlight for photosynthesis to a much greater extent than the evergreen species. Conversely, the evergreen species exhibited a greater capacity for photoprotective thermal energy dissipation as well as a greater expression of the PsbS protein in full sun than the annual species. In all species, the majority of thermal energy dissipation [assessed as non‐photochemical fluorescence quenching (NPQ)] was the flexible, ΔpH‐dependent form of NPQ over the entire range of growth light environments. However, in response to a transfer of shade‐grown plants to high light, the evergreen species exhibited a high level of sustained thermal dissipation (qI), but the annual species did not. This sustained energy dissipation in the evergreen species was not ΔpH‐dependent nor did the low level of PsbS in shade leaves increase upon transfer to high light for several days. Sustained ΔpH‐independent NPQ was correlated (a) initially, with sustained D1 protein phosphorylation and xanthophyll cycle arrest and (b) subsequently, with an accumulation over several days of PsbS‐related one‐helix proteins and newly synthesized zeaxanthin and lutein.

Reexamining the empirical relation between plant growth and leaf photosynthesis.

Technological advances during the past several decades have greatly enhanced our ability to measure leaf photosynthesis virtually anywhere and under any condition. Associated with the resulting proliferation of gas-exchange data is a lingering uncertainty regarding the importance of such measurements when it comes to explaining intrinsic causes of plant growth variation. Accordingly, in this paper we rely on a compilation of data to address the following questions: from both statistical and mechanistic standpoints, how closely does plant growth correlate with measures of leaf photosynthesis? Moreover, in this context, does the importance of leaf photosynthesis as an explanatory variable differ among growth light environments? Across a wide array of species and environments, relative growth rate (RGR) was positively correlated with daily integrals of photosynthesis expressed per unit leaf area (Aarea), leaf mass (Amass), and plant mass (Aplant). The amount of RGR variation explained by these relationships increased from 36% for the former to 93% for the latter. Notably, there was close agreement between observed RGR and that estimated from Aplant after adjustment for theoretical costs of tissue construction. Overall, based on an analysis of growth response coefficients (GRCs), gross assimilation rate (GAR), a photosynthesis-based estimate of biomass gain per unit leaf area, explained about as much growth variation as did leaf mass ratio (LMR) and specific leaf area (SLA). Further analysis of GRCs indicated that the importance of GAR in explaining growth variation increased with increasing light intensity. Clearly, when considered in combination with other key determinants, appropriate measures of leaf gas exchange effectively capture the fundamental role of leaf photosynthesis in plant growth variation.

Seasonal variation in the leaf gas exchange of tropical forest trees in the rain forest–savanna transition of the southern Amazon Basin

The photosynthetic light response of Amazonian semi-deciduous forest trees of the rain forest–savanna transition near Sinop Mato Grosso, Brazil was measured between July 2000 and September 2003 to test the hypothesis that the photosynthetic capacity of trees acclimated to different growth light environments will decline during the dry season. Maximum photosynthesis (Amax) and stomatal conductance (gmax) were significantly higher during the wet season; however, the physiological response to drought was not a clear function of growth light environment. For some species, such as Psychotria sp. growing in the mid-canopy, internal leaf CO2 concentration (Ci) was &gt;30% lower during the dry season suggesting that declines in Amax were caused in part by stomatal limitations to CO2 diffusion. For other species, such as Brosimum lactescens growing at the top of the canopy, Tovomita schomburgkii growing in the mid-canopy, and Dinizia excelsa growing in the understorey, dry season Ci declined by &lt;20% suggesting that factors independent of CO2 diffusion were more important in limiting Amax. Dry-season declines in gmax appeared to be important for maintaining a more consistent leaf water potential for some species (T. schomburgkii and D. excelsa) but not others (Psychotria sp.). These results indicate that while seasonal drought exerts an important limitation on the physiological capacity of semi-deciduous Amazonian forest trees, the mechanism of this limitation may differ between species.

The seasonal photosynthetic responses of seedlings of the endangered plant Cathaya argyrophylla to different growth light environments

The seasonal photosynthetic responses of seedlings of the endangered plant Cathaya argyrophylla to different growth light environments

Photosynthetic Acclimation to Different Growth Light Environments in Seedlings of Three Tropical Rainforest Syzygium Species

Light is one of the most important factors that affect the regeneration and succession of tropical rain forests. Tropical trees with different successional status have different acclimation capacities to different growth light environments. In the present study, gas exchange and diurnal variation of chlorophyll fluorescence parameters were measured in the seedlings of three tropical rainforest Syzygium species, which are early (S. szemaoense), middle (S. cumini) and late (S. latilimbum) successional species. The seedlings were cultivated under three different light regimes (fully open site, 42% and 14% daylight). Their leaf mass per unit area (LMA) and chlorophyll content were also determined. The early successional species had the highest light- saturated photosynthetic rate (P max) and photosynthetic acclimation capacity to the variation of growth light environments, the next was the middle successional species. The late successional species had the lowest P max values and its photosynthesis was strongly suppressed when grown in full light. Nevertheless, grown under both the full light and 42% daylight, all the three species did not suffer from irreversible photoinhibition, as indicated by their dawn values of maximal photochemical efficiency of PS Ⅱ (F v/F m) larger than 0.8, and their φPSⅡ was able to recover on the same day. Dynamic (reversible) photoinhibition occurred in all species, the degree of which increased with growth light level. Among species, the degrees of dynamic photoinhibition were similar. The non-photochemical fluorescence-quenching rate (NPQ) increased with the growth light level for all species. However, in the fully open site, the NPQ rates in the morning were higher in the late and middle successional species than in the early successional species. This compensated for the lower photosynthetic rates of the former two species, and probably plays a role in preventing photodamage to these species under full light. Leaf dark respiration rate per unit area (R d) and leaf mass per area (LMA) tended to increase with the growth light level. Under the same light regimes, R d was higher in early than in middle and late successional species. The LMA values of late successional species were higher than for the other two species. This could be favorable for this shade-tolerant species to prevent herbivory. For all of the three species grown in 14% daylight, area-based P max, photosynthetic light saturation point, light compensation point, R d and LMA were reduced, while chlorophyll content per unit dry weight were increased compared to those in the other two light regimes. This showed their favorable acclimation to low light. Nevertheless, under 14% daylight the seedlings of both the early and middle species died, only the seedlings of the late successional species were still alive. This indicates that this late successional species has high capacity to acclimate to low light.

SEASONAL ACCLIMATION OF PHOTOSYNTHESIS IN EASTERN HEMLOCK AND PARTRIDGEBERRY IN DIFFERENT LIGHT ENVIRONMENTS

The effects of growth light environment on the seasonal acclimation of photosynthetic capacity as well as rates of photosynthesis in the field were investigated in the angiosperm Mitchella repens L. (partridgeberry) and the gymnosperm Tsuga canadensis (L.) Carr. (eastern hemlock). Irrespective of growth light environment, photosynthetic capacity decreased in winter in partridgeberry but exhibited no seasonal change in eastern hemlock. These acclimatory patterns contrast with previously published results that report strong effects of growth light intensity on seasonal acclimation in another understory angiosperm, Mahonia repens (Lindley) Don., and strong winter-time decreases in photosynthetic capacity in a number of gymnosperms. With the exception of eastern hemlock in the exposed site, actual rates of photosynthesis were similar on summer days and on mild days in winter, whereas rates of photosynthesis measured on extremely cold days were much lower. Thus, even within the winter season, daily fluctuations in temperature profoundly affect photosynthesis. The carotenoid pigments that make up the xanthophyll cycle acclimated to winter in a manner that is consistent with their role in the protective process of energy dissipation.

Identification of mutants of Arabidopsis defective in acclimation of photosynthesis to the light environment.

In common with many other higher plant species, Arabidopsis undergoes photosynthetic acclimation, altering the composition of the photosynthetic apparatus in response to fluctuations in its growth environment. The changes in photosynthetic function that result from acclimation can be detected in a noninvasive manner by monitoring chlorophyll (Chl) fluorescence. This technique has been used to develop a screen that enables the rapid identification of plants defective at ACCLIMATION OF PHOTOSYNTHESIS TO THE ENVIRONMENT (APE) loci. The application of this screen to a population of T-DNA-transformed Arabidopsis has successfully led to the identification of a number of mutant lines with altered Chl fluorescence characteristics. Analysis of photosynthesis and pigment composition in leaves from three such mutants showed that they had altered acclimation responses to the growth light environment, each having a distinct acclimation-defective phenotype, demonstrating that screening for mutants using Chl fluorescence is a viable strategy for the investigation of acclimation. Sequencing of the genomic DNA flanking the T-DNA elements showed that in the ape1 mutant, a gene was disrupted that encodes a protein of unknown function but that appears to be specific to photosynthetic organisms, whereas the ape2 mutant carries an insertion in the region of the TPT gene encoding the chloroplast inner envelope triose phosphate/phosphate translocator.

Light gradient partitioning by tropical tree seedlings in the absence of canopy gaps.

To explore the importance of light availability for seedling growth in low light environments, we examined light-dependent growth, biomass allocation and mortality of tree seedlings growing in sites with 0.2-6.5% full sun, the range of light commonly encountered in the understory of closed canopy, lowland tropical forests. We transplanted seedlings of the canopy tree species, Dipteryx panamensis, Virola koschnyii, and Brosimum alicastrum into second-growth forest and native tree plantations at La Selva Biological Station, Costa Rica. We assessed seedling survival, growth, and seedling light environments bimonthly for 14months. Plants were harvested at the end of the study to assess leaf area, total biomass, biomass partitioning and root growth. Survivorship of all species exceeded 60% across all microsites, although both D. panamensis and B. alicastrum had lower probabilities of survival in the darkest microsites. All species showed a strong positive relationship between light availability and growth, increasing in total biomass as light increased. However, the strength of the growth response differed among species causing a change in the rank order of species growth rates as light availability increased. Although D. panamensis showed the lowest growth rates in the darkest microsites, a strong response to increasing light led to a cross-over in performance, such that D. panamensis had the highest growth rate at the highest light levels studied. These data suggest that resource gradient partitioning could occur even in low light environments (0.2-6.5%). Given the limited range of light regimes sampled (i.e., non-gap microsites), our data demonstrate that growth of tropical tree seedlings beneath closed canopies is highly sensitive to light availability and that shade-tolerant species vary in these responses. Our results show that understory light heterogeneity, in the absence of canopy gaps, can significantly affect recruitment processes for shade-tolerant tree species.

Dependence of photosynthesis and energy dissipation activity upon growth form and light environment during the winter.

Two very distinctive responses of photosynthesis to winter conditions have been identified. Mesophytic species that continue to exhibit growth during the winter typically exhibit higher maximal rates of photosynthesis during the winter or when grown at lower temperatures compared to individuals examined during the summer or when grown at warmer temperatures. In contrast, sclerophytic evergreen species growing in sun-exposed sites typically exhibit lower maximal rates of photosynthesis in the winter compared to the summer. On the other hand, shaded individuals of those same sclerophytic evergreen species exhibit similar or higher maximal rates of photosynthesis in the winter compared to the summer. Employment of the xanthophyll cycle in photoprotective energy dissipation exhibits similar characteristics in the two groups of plants (mesophytes and shade leaves of sclerophytic evergreens) that exhibit upregulation of photosynthesis during the winter. In both, zeaxanthin + antheraxanthin (Z + A) are retained and PS II remains primed for energy dissipation only on nights with subfreezing temperatures, and this becomes rapidly reversed upon exposure to increased temperatures. In contrast, Z + A are retained and PS II remains primed for energy dissipation over prolonged periods during the winter in sun leaves of sclerophytic evergreen species, and requires days of warming to become fully reversed. The rapid disengagement of this energy dissipation process in the mesophytes and shade sclerophytes apparently permits a rapid return to efficient photosynthesis and increased activity on warmer days during the winter. This may be associated with a decreasing opportunity for photosynthesis in source leaves relative to the demand for photosynthesis in the plant's sinks. In contrast, the sun-exposed sclerophytes - with a relatively high source to sink ratio - maintain PS II in a state primed for high levels of energy dissipation activity throughout much of the winter. Independent of whether photosynthesis was up- or downregulated, all species under all conditions exhibited higher levels of soluble carbohydrates during the winter compared to the summer. Thus downregulation of photosynthesis and of Photosystem II do not appear to limit carbohydrate accumulation under winter conditions. A possible signal communicating an altered source/sink balance, or that may be influencing the engagement of Z + A in energy dissipation, is phosphorylation of thylakoid proteins such as D1.

Assessing leaf pigment content and activity with a reflectometer

This study explored reflectance indices sampled with a ‘leaf reflectometer’ as measures of pigment content for leaves of contrasting light history, developmental stage and functional type (herbaceous annual versus sclerophyllous evergreen). We employed three reflectance indices: a modified normalized difference vegetation index (NDVI), an index of chlorophyll content; the red/green reflectance ratio (RRED∶RGREEN), an index of anthocyanin content; and the change in photochemical reflectance index upon dark–light conversions (ΔPRI), an index of xanthophyll cycle pigment activity. In Helianthus annuus (sunflower), xanthophyll cycle pigment amounts were linearly related to growth light environment; leaves in full sun contained approximately twice the amount of xanthophyll cycle pigments as leaves in deep shade, and at midday a larger proportion of these pigments were in the photoprotective, de‐epoxidized forms relative to shade leaves. Reflectance indices also revealed contrasting patterns of pigment development in leaves of contrasting structural types (annual versus evergreen). In H. annuus sun leaves, there was a remarkably rapid increase in amounts of both chlorophyll and xanthophyll cycle pigments along a leaf developmental sequence. This pattern contrasted with that of Quercus agrifolia (coast live oak, a sclerophyllous evergreen), which exhibited a gradual development of both chlorophyll and xanthophyll cycle pigments along with a pronounced peak of anthocyanin pigment content in newly expanding leaves. These temporal patterns of pigment development in Q. agrifolia leaves suggest that anthocyanins and xanthophyll cycle pigments serve complementary photoprotective roles during early leaf development. The results illustrate the use of reflectance indices for distinguishing divergent patterns of pigment activity in leaves of contrasting light history and functional type.

Growth history of a Neotropical tree inferred from the spacing of leaf scars

ABSTRACTThe Neotropical tree,Pourouma aspera(Cecropiaceae), supports large lobed leaves on an erect stem which seldom branches below a height of 5 m. The prominent leaf scars are visible on trunks of mature trees and the distance between scars is highly correlated with the height growth rate when the leaves were produced. Relationships between height growth rate, internode length and light environment were determined from markedP. asperasaplings in a tropical wet lowland forest and used to infer the past growth histories of larger saplings and trees. Inferred growth rates were quite variable, both within and between individuals. Estimated passage times to grow from 0.7 to 2.2 m tall ranged from 0.6–23 y, with a mean of 4.4 y (N = 61). The study illustrates the importance of examining the distribution of growth rate over the length of the stem in addition to the more usual distribution of numbers of individuals per growth rate class. Species such asPourouma, which respond strongly to light, complete much of their total juvenile growth during intervals of high light, even though most saplings are growing slowly in shade at any given time.

Correlations Between Biomass Allocation, Relative Growth Rate and Light Environment in Tropical Forest Saplings

The growth rates and patterns of aboveground biomass allocation were determined for saplings of nine species of a wet tropical forest in Costa Rica. Relative growth rate and allocation were calculated by remeasuring the dimensions of forest-grown saplings and using biomass-dimension relationships derived from harvested saplings to convert linear dimensions to biomass estimates. The light environment of each sapling was characterized in terms of direct and indirect site factors calculated from hemispherical 'fisheye' photographs of the canopy. Relative growth rate (RGR) was significantly correlated with both direct and indirect site factors. A significant negative correlation was found between ln(% leaf allocation) and RGR for six species, but no such correlation occurred for the three species most highly associated with canopy gaps. Thus, shade-tolerant saplings shift most of their above-ground growth to leaf replacement when heavily shaded, while gap-associated species allocate about 2/3 of aboveground growth to the stem and branches under all measured conditions. This differential plasticity in leaf vs wood allocation may be an important factor in determining the shade tolerance and successional status of woody species. Key-words: Allocation, growth, light, sapling, tropical

Comparative analysis of photosynthetic light environments within the crowns of juvenile rain forest trees.

Irradiances within the crowns of saplings of two tropical tree species were simultaneously compared in primary rain forest in Costa Rica. The species examined, Minquartia guianensis Aubl., a relatively slow-growing, canopy species, and Pithecellobium pedicellare (DC) Benth., a less-tolerant, emergent species, have different crown and leaf display patterns. Crown light environments were assessed by placing arrays of quantum sensors among leaves and recording at 5-s intervals for seven days with microloggers. Median total daily quantum flux densities for saplings of both species were less than 2% of full sun and did not differ significantly. More than 90% of the measurements within the crowns of these saplings were less than 25 micromol m(-2) s(-1). Spatial variability of photon flux densities within sapling crowns was similar for the two species despite differences in leaf display patterns. In saplings of both species, photon flux densities varied significantly over the relatively short distances within crowns and from day to day. Height growth of both species was significantly correlated with total daily photon flux densities and with percentage of full sun. However, only the tolerant species, Minquartia, showed a significant correlation between diameter growth and crown light environment.