Optimal use of leaf nitrogen explains seasonal changes in leaf nitrogen content of an understorey evergreen shrub

Evidence that spatial scale and environment factors explain grassland community assembly in woodland-grassland ecotones

How communities of living organisms assemble has long been a central question in ecology. The impact of habitat filtering and limiting similarity on plant community structures is well known, as both processes are influenced by individual responses to environmental fluctuations. Yet, the precise identifications and quantifications of the potential abiotic and biotic factors that shape community structures at a fine scale remains a challenge. Here, we applied null model approaches to assess the importance of habitat filtering and limiting similarity at two spatial scales. We used 63 natural vegetation plots, each measuring 5 × 5 m, with three nested subplots measuring 1 × 1 m, from the 2021 field survey, to examine the alpha diversity as well as beta diversity of plots and subplots. Linear mixed-effects models were employed to determine the impact of environmental variables on assembly rules. Our results demonstrate that habitat filtering is the dominant assembly rules at both the plot and subplot levels, although limiting similarity assumes stronger at the subplot level. Plot-level limiting similarity exhibited a positive association with fine-scale partitioning, suggesting that trait divergence originated from a combination of limiting similarity and spatial partitioning. Our findings also reveal that the community assembly varies more strongly with the mean annual temperature gradient than the mean annual precipitation. This investigation provides a pertinent illustration of non-random assembly rules from spatial scale and environmental factors in plant communities in the loess hilly region. It underscores the critical influence of spatial and environmental constraints in understanding the assembly of plant communities.

Multi-Omics Analysis Reveals Synergistic Enhancement of Nitrogen Assimilation Efficiency via Coordinated Regulation of Nitrogen and Carbon Metabolism by Co-Application of Brassinolide and Pyraclostrobin in Arabidopsis thaliana

Improving nitrogen (N) assimilation efficiency without yield penalties is important to sustainable food security. The chemical regulation approach of N assimilation efficiency is still less explored. We previously found that the co-application of brassinolide (BL) and pyraclostrobin (Pyr) synergistically boosted biomass and yield via regulating photosynthesis in Arabidopsis thaliana. However, the synergistic effect of BL and Pyr on N metabolism remains unclear. In this work, we examined the N and protein contents, key N assimilatory enzyme activities, and transcriptomic and metabolomic changes in the four treatments (untreated, BL, Pyr, and BL + Pyr). Our results showed that BL + Pyr treatment synergistically improved N and protein contents by 56.2% and 58.0%, exceeding the effects of individual BL (no increase) or Pyr treatment (36.4% and 36.1%). Besides synergistically increasing the activity of NR (354%), NiR (42%), GS (62%), and GOGAT (62%), the BL + Pyr treatment uniquely coordinated N metabolism, carbon utilization, and photosynthesis at the transcriptional and metabolic levels, outperforming the effects of individual BL or Pyr treatments. These results revealed that BL + Pyr treatments could synergistically improve N assimilation efficiency through improving N assimilatory enzyme activities and coordinated regulation of N and carbon metabolism. The identified genes and metabolites also informed potential targets and agrochemical combinations to enhance N assimilation efficiency.

Trade-off of leaf-scale resource-use efficiencies along the vertical canopy of the subtropical forest

Leaf resource-use efficiencies are key indicators of plant adaptability to climate change, as they depend on both photosynthetic carbon assimilation and available resources. However, accurately quantifying the response of the coupled carbon and water cycles is challenging due to the canopy vertical variability in resource-use efficiencies, which introduces greater uncertainty into the calculations. Here we experimented to ascertain the vertical variations of leaf resource-use efficiencies along three canopy gradients of coniferous (Pinus elliottii Engelmann.) and broad-leaved (Schima Superba Gardn & Champ.) forests over one year in the subtropical region of China. The efficiency of water (WUE), and nitrogen (NUE) showed higher values in the top canopy level for the two species. The maximum efficiency of light (LUE) occurred in the bottom canopy level for both species. The impact of photosynthetic photon flux density (PPFD), leaf temperature (T_leaf), and vapor pressure deficit (VPD) on leaf resource-use efficiencies varied with canopy gradients in slash pine and schima superba. We also observed a trade-off between NUE and LUE for slash pine and between NUE and WUE for schima superba. Moreover, the variation in the correlation between LUE and WUE indicated a change in resource-use strategies for slash pine. These results emphasize the significance of vertical variations in resource-use efficiencies to enhance the prediction of future carbon-water dynamics in the subtropical forest.

The role of climate, vegetation, and soil factors on carbon fluxes in Chinese drylands

Drylands dominate the trend and variability of the land carbon (C) sink. A better understanding of the implications of climate-induced changes in the drylands for C sink-source dynamics is urgently needed. The effect of climate on ecosystem C fluxes (gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem productivity (NEP)) in drylands has been extensively explored, but the roles of other concurrently changing factors, such as vegetation conditions and nutrient availability, remain unclear. We used eddy-covariance C-flux measurements from 45 ecosystems with concurrent information on climate (mean annual temperature (MAT) and mean annual precipitation (MAP)), soil (soil moisture (SM) and soil total nitrogen content (soil N)), and vegetation (leaf area index (LAI) and leaf nitrogen content (LNC)) factors to assess their roles in C fluxes. The results showed that the drylands in China were weak C sinks. GPP and ER were positively correlated with MAP, while they were negatively correlated with MAT. NEP first decreased and then increased with increasing MAT and MAP, and 6.6 °C and 207 mm were the boundaries for the NEP response to MAT and MAP, respectively. SM, soil N, LAI, and MAP were the main factors affecting GPP and ER. However, SM and LNC had the most important influence on NEP. Compared with climate and vegetation factors, soil factors (SM and soil N) had a greater impact on C fluxes in the drylands. Climate factors mainly affected C fluxes by regulating vegetation and soil factors. To accurately estimate the global C balance and predict the response of ecosystems to environmental change, it is necessary to fully consider the discrepant effects of climate, vegetation, and soil factors on C fluxes, as well as the cascade relationships between different factors.

Changes in the leaf nutrient and pigment contents of Berberis microphylla G. Forst. in relation to irradiance and fertilization

Berberis microphylla G. Forst. commonly named calafate, is a Patagonian shrub that grows in humid areas of the steppe, coastal thickets, edges and gaps of Nothofagus forests or along streams and rivers, with small purple berries. The objective of this study was to evaluate the changes in leaf nutrient (carbon, nitrogen, carbon:nitrogen, phosphorus and potassium) and pigment contents (chlorophyll a and b, chlorophyll a:b ratio and carotenoids) of B. microphylla plants growing under different irradiances (low = 24%, medium = 57%, and high = 100% of the natural irradiance) and fertilization levels (0 = 0.00 g, 1 = 3.36 g, and 2 = 6.72 g per plant) during two growing seasons (2008-2009, 2009-2010). Also, we explored the relationships of these variables with anthocyanin, as well as with total phenol fruit contents. The fertilization has been highlighted, particularly in the content of foliar nutrients, where nitrogen, phosphorus and potassium contents were highest with fertilization level 2 (2.0%, 0.1%, and 0.6%, respectively), while carbon:nitrogen ratio (37.5) was maximum on fertilization level 0. Irradiance has greatly affected the content of foliar pigments. Thus, chlorophyll a, b, and carotenoids were highest under low irradiance (0.4, 0.1 and 0.2 mmol/m2, respectively), while chlorophyll a:b ratio was maximum under medium and high irradiance conditions (3.1). In addition, the quantity of fruit secondary metabolite (anthocyanin and phenol) could be estimated using carbon and potassium leaf contents and chlorophyll a and b contents. On the other hand, the annual climatic variability between 2008-2009 and 2009-2010 mainly affected the variables on nutrient and pigment contents, likely evidencing the influence of two distinct climate periods, El Niño/La Niña phenomena, respectively. The changes observed in the leaf nutrient and pigment contents of B. microphylla could be related to the acclimation capacity of B. microphylla shrubs to changes in environmental conditions via arrangements in leaf composition.

Functional differences in seasonally absorbed nitrogen in a winter-green perennial herb

Nitrogen (N) uptake in response to its availability and effective N-use are important for determining plant fitness, as N is a major limiting resource and its availability changes both seasonally and annually. Storage organs such as bulbs are considered an adaptive trait with respect to plant N-use strategies. It is well known that N is remobilized from storage organs to satisfy the high demand for new growth that is not completely satisfied by external uptake alone. However, little is known about how this N absorbed during different seasons contributes to plant performance. By manipulating seasonal N availability in potted Lycoris radiata var. radiata (Amaryllidaceae), a winter-green perennial, we found that the N absorbed during different seasons had different effects on leaf growth and leaf N concentrations, effectively increasing the growth and survival of the plants. N absorbed during the summer (leafless period; N was thus stored in the bulb) enhanced plant growth by increasing leaf growth. Compared with the plants supplied with N during autumn (leaf flush period), the leafy plants also showed greater growth per unit leaf area despite the lower area-based photosynthetic capacity of the latter. By contrast, N absorbed during the autumn increased the leaf N concentration and thus the photosynthetic capacity, which was considered to enhance survival and growth of the plant during winter by reducing the potentially fatal risk caused by the absorption of photons under low temperature. Our findings have important implications for estimating plant responses to environmental changes. We predict that changes in seasonal N availability impact the performance of plants, even that of perennials that have large storage organs, via an altered relative investment of N into different functions.

Late growing season carbon subsidy in native gymnosperms in a northern temperate forest

Evergreen tree species that maintain positive carbon balance during the late growing season may subsidize extra carbon in a mixed forest. To test this concept of 'carbon subsidy', leaf gas exchange characteristics and related leaf traits were measured for three gymnosperm evergreen species (Chamaecyparis thyoides, Tsuga canadensis and Pinus strobus) native to the oak-hickory deciduous forest in northeast USA from March (early Spring) to October (late Autumn) in a single year. All three species were photosynthetically active in Autumn. During the Summer-Autumn transition, photosynthetic capacity (Amax) of T. canadensis and P. strobus increased (T-test, P < 0.001) and was maintained in C. thyoides (T-test, P = 0.49), while dark respiration at 20 °C (Rn) and its thermal sensitivity were generally unchanged for all species (one-way ANOVA, P > 0.05). In Autumn, reductions in mitochondrial respiration rate in the daylight (RL) and the ratio of RL to Rn (RL/Rn) were observed in P. strobus (46.3% and 44.0% compared to Summer, respectively). Collectively, these physiological adjustments resulted in higher ratios of photosynthesis to respiration (A/Rnand A/RL) in Autumn for all species. Across season, photosynthetic biochemistry and respiratory variables were not correlated with prevailing growth temperature. Physiological adjustments allowed all three gymnosperm species to maintain positive carbon balance into late Autumn, suggesting that gymnosperm evergreens may benefit from Autumn warming trends relative to deciduous trees that have already lost their leaves.

Developmental and reproductive performance of a specialist herbivore depend on seasonality of, and light conditions experienced by, the host plant

Host plant phenology (as influenced by seasonality) and light-mediated changes in the phenotypic and phytochemical properties of leaves have been hypothesised to equivocally influence insect herbivore performance. Here, we examined the effects of seasonality, through host plant phenology (late growth-season = autumn vs flowering-season = winter) and light environment (shade vs full-sun habitat) on the leaf characteristics of the invasive alien plant, Chromolaena odorata. In addition, the performance of a specialist folivore, Pareuchaetes insulata, feeding on leaves obtained from both shaded and full-sun habitats during autumn and winter, was evaluated over two generations. Foliar nitrogen and magnesium contents were generally higher in shaded plants with much higher levels during winter. Leaf water content was higher in shaded and in autumn plants. Total non-structural carbohydrate (TNC) and phosphorus contents did not differ as a function of season, but were higher in shaded foliage compared to full-sun leaves. Leaf toughness was noticeably higher on plants growing in full-sun during winter. With the exception of shaded leaves in autumn that supported the best performance [fastest development, heaviest pupal mass, and highest growth rate and Host Suitability Index (HSI) score], full-sun foliage in autumn surprisingly also supported an improved performance of the moth compared to shaded or full-sun leaves in winter. Our findings suggest that shaded and autumn foliage are nutritionally more suitable for the growth and reproduction of P. insulata. However, the heavier pupal mass, increased number of eggs and higher HSI score in individuals that fed on full-sun foliage in autumn compared to their counterparts that fed on shaded or full-sun foliage in winter suggest that full-sun foliage during autumn is also a suitable food source for larvae of the moth. In sum, our study demonstrates that seasonal and light-modulated changes in leaf characteristics can affect insect folivore performance in ways that are not linear.

Biological limits on nitrogen use for plant photosynthesis: a quantitative revision comparing cultivated and wild species

The relationship between leaf photosynthesis and nitrogen is a critical production function for ecosystem functioning. Cultivated species have been studied in terms of this relationship, focusing on improving nitrogen (N) use, while wild species have been studied to evaluate leaf evolutionary patterns. A comprehensive comparison of cultivated vs wild species for this relevant function is currently lacking. We hypothesize that cultivated species show increased carbon assimilation per unit leaf N area compared with wild species as associated with artificial selection for resource-acquisition traits. We compiled published data on light-saturated photosynthesis (A_max ) and leaf nitrogen (LN_area ) for cultivated and wild species. The relationship between A_max and LN_area was evaluated using a frontier analysis (90^th percentile) to benchmark the biological limit of nitrogen use for photosynthesis. Carbon assimilation in relation to leaf N was not consistently higher in cultivated species; out of 14 cultivated species, only wheat, rice, maize and sorghum showed higher ability to use N for photosynthesis compared with wild species. Results indicate that cultivated species have not surpassed the biological limit on nitrogen use observed for wild species. Future increases in photosynthesis based on natural variation need to be assisted by bioengineering of key enzymes to increase crop productivity.

Effects of changes in micro-weather conditions on structural features, total protein and carbohydrate content in leaves of the Atlantic rain forest tree golden trumpet (Tabebuia chrysotricha)

Golden trumpet, Tabebuia chrysotricha, is a native tree from the Brazilian Atlantic rain forest, with a broad latitudinal distribution. In this study, we investigated the potential effects of short-term changes in micro-weather conditions on structural features, and total protein and carbohydrate content of golden trumpet leaves, using structural and histochemical approaches. Leaves were harvested in four different micro-weather conditions: 1. Afternoon, after a hot, sunny day; 2. at dawn, after a previously hot, sunny day; 3. at noon, of a hot, sunny day; and 4. at noon, of a cold, cloudy day. Leaflets exposed to low light irradiance showed flattened chloroplasts, uniformly distributed within the cells, throughout the palisade parenchyma. Conversely, leaflets exposed to high light irradiance presented flattened and rounded chloroplasts, in the upper and lower palisade parenchyma cells, respectively. The strongest protein staining was found for leaves harvested at the coldest period, whereas the weakest protein staining was found for leaves harvested after a hot, sunny day. The largest and most numerous starch grains were found for leaves harvested in the afternoon, after a hot, sunny day. Conversely, the smallest and less numerous starch grains were found for leaves harvested at dawn. Analysis of the data reported herein suggests that the leaflet responses to transient changes in micro-weather conditions are likely to contribute to the golden trumpet successful establishment in the broad latitudinal distribution in which the species is found.

Optimality of nitrogen distribution among leaves in plant canopies

The vertical gradient of the leaf nitrogen content in a plant canopy is one of the determinants of vegetation productivity. The ecological significance of the nitrogen distribution in plant canopies has been discussed in relation to its optimality; nitrogen distribution in actual plant canopies is close to but always less steep than the optimal distribution that maximizes canopy photosynthesis. In this paper, I review the optimality of nitrogen distribution within canopies focusing on recent advancements. Although the optimal nitrogen distribution has been believed to be proportional to the light gradient in the canopy, this rule holds only when diffuse light is considered; the optimal distribution is steeper when the direct light is considered. A recent meta-analysis has shown that the nitrogen gradient is similar between herbaceous and tree canopies when it is expressed as the function of the light gradient. Various hypotheses have been proposed to explain why nitrogen distribution is suboptimal. However, hypotheses explain patterns observed in some specific stands but not in others; there seems to be no general hypothesis that can explain the nitrogen distributions under different conditions. Therefore, how the nitrogen distribution in canopies is determined remains open for future studies; its understanding should contribute to the correct prediction and improvement of plant productivity under changing environments.

Ecophysiology of four co-occurring lycophyte species: an investigation of functional convergence

Lycophytes are the most early divergent extant lineage of vascular land plants. The group has a broad global distribution ranging from tundra to tropical forests and can make up an important component of temperate northeast US forests. We know very little about the in situ ecophysiology of this group and apparently no study has evaluated if lycophytes conform to functional patterns expected by the leaf economics spectrum hypothesis. To determine factors influencing photosynthetic capacity (Amax), we analysed several physiological traits related to photosynthesis to include stomatal, nutrient, vascular traits, and patterns of biomass distribution in four coexisting temperate lycophyte species: Lycopodium clavatum, Spinulum annotinum, Diphasiastrum digitatum and Dendrolycopodium dendroideum. We found no difference in maximum photosynthetic rates across species, yet wide variation in other traits. We also found that Amax was not related to leaf nitrogen concentration and is more tied to stomatal conductance, suggestive of a fundamentally different sets of constraints on photosynthesis in these lycophyte taxa compared with ferns and seed plants. These findings complement the hydropassive model of stomatal control in lycophytes and may reflect canalization of function in this group. Our data also demonstrate functional ecological similarities: De. dendroideum and D. digitatum are species that have substantial belowground biomass investment and are consistently more similar to each other across multiple traits than either is to the more surficial S. annotinum and L. clavatum. Such differences may partition environments in ways that allow for the close coexistence of these species.

Seasonal changes in temperature response of photosynthesis and its contribution to annual carbon gain in Daphniphyllum humile, an evergreen understorey shrub

We evaluated seasonal variation in photosynthetic temperature dependence and its contribution to annual carbon gain in an evergreen understorey shrub, Daphniphyllum humile Maxim, growing at the forest border and in the understorey of a deciduous forest. Plants at both sites exhibited similar optimal temperatures for photosynthesis (T(opt)). The activation energy for ribulose-1,5-bisphosphate (RuBP) carboxylation (HaV) at both sites tended to be higher in summer than in spring or autumn, suggesting that HaV may be the controlling factor in the T(opt) shift in D. humile. In contrast to the seasonal changes in T(opt ), the maximum photosynthetic rate at the optimal temperature (P(opt)) differed between the two sites: it was lower in autumn than in summer at the forest border, but was the same in summer and autumn in the understorey. In the understorey plants, nitrogen content (Narea) increased in autumn, but this was not the case for forest border plants. In addition, Rubisco content increased significantly in autumn in the understorey leaves but decreased distinctly in forest border leaves. Increased Narea and Rubisco in understorey leaves resulted in increased in photosynthesis in autumn. Annual carbon gain was 30.8 mol · m(-2) in forest border leaves and 5.8 mol · m(-2) in understorey leaves. Carbon gain in understorey leaves during the short period after overstorey leaf fall and before snow accumulation was approximately 49% of annual carbon gain. Furthermore, autumn carbon gain calculated using activation energy of summer with autumn photosynthetic parameters underestimated the autumn carbon gain by as much as 31%. In conclusion, photosynthetic temperature acclimation may be a key factor in increasing annual carbon gain in understorey D. humile.

Low temperature acclimation of photosynthetic capacity and leaf morphology in the context of phloem loading type

Carbon export from leaf mesophyll to sugar-transporting phloem occurs via either an apoplastic (across the cell membrane) or symplastic (through plasmodesmatal cell wall openings) pathway. Herbaceous apoplastic loaders generally exhibit an up-regulation of photosynthetic capacity in response to growth at lower temperature. However, acclimation of photosynthesis to temperature by symplastically loading species, whose geographic distribution is particularly strong in tropical and subtropical areas, has not been characterized. Photosynthetic and leaf anatomical acclimation to lower temperature was explored in two symplastic (Verbascum phoeniceum, Cucurbita pepo) and two apoplastic (Helianthus annuus, Spinacia oleracea) loaders, representing summer- and winter-active life histories for each loading type. Regardless of phloem loading type, the two summer-active species, C. pepo and H. annuus, exhibited neither foliar anatomical nor photosynthetic acclimation when grown under low temperature compared to moderate temperature. In contrast, and again irrespective of phloem loading type, the two winter-active mesophytes, V. phoeniceum and S. oleracea, exhibited both a greater number of palisade cell layers (and thus thicker leaves) and significantly higher maximal capacities of photosynthetic electron transport, as well as, in the case of V. phoeniceum, a greater foliar vein density in response to cool temperatures compared to growth at moderate temperature. It is therefore noteworthy that symplastic phloem loading per se does not prevent acclimation of intrinsic photosynthetic capacity to cooler growth temperatures. Given the vagaries of weather and climate, understanding the basis of plant acclimation to, and tolerance of, low temperature is critical to maintaining and increasing plant productivity for food, fuel, and fiber to meet the growing demands of a burgeoning human population.