Leaf size modifies support biomass distribution among stems, petioles and mid-ribs in temperate plants

The implications of extensive variation in leaf size for biomass distribution between physiological and support tissues and for overall leaf physiological activity are poorly understood. Here, we tested the hypotheses that increases in leaf size result in enhanced whole-plant support investments, especially in compound-leaved species, and that accumulation of support tissues reduces average leaf nitrogen (N) content per unit dry mass (N(M)), a proxy for photosynthetic capacity. Leaf biomass partitioning among the lamina, mid-rib and petiole, and whole-plant investments in leaf support (within-leaf and stem) were studied in 33 simple-leaved and 11 compound-leaved species. Support investments in mid-ribs and petioles increased with leaf size similarly in simple leaves and leaflets of compound leaves, but the overall support mass fraction within leaves was larger in compound-leaved species as a result of prominent rachises. Within-leaf and within-plant support mass investments were negatively correlated. Therefore, the total plant support fraction was independent of leaf size and lamina dissection. Because of the lower N(M) of support biomass, the difference in N(M) between the entire leaf and the photosynthetic lamina increased with leaf size. We conclude that whole-plant support costs are weakly size-dependent, but accumulation of support structures within the leaf decreases whole-leaf average N(M), potentially reducing the integrated photosynthetic activity of larger leaves.     

Changes of Photosynthetic Characteristics in Relation to Leaf Senescence in Two Maize Hybrids with Different Senescent Appearance

A field experiment was conducted to investigate the changes in chlorophyll (Chl) and nitrogen (N) contents, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and phosphoenolpyruvate carboxylase (PEPC) contents and PEPC activity, and the photon-saturated net photosynthetic rate (P _Nsat), and their relationships with leaf senescence in two maize hybrids with different senescent appearance. One stay-green (cv. P3845) and one earlier senescent (cv. Hokkou 55) hybrid were used in this study, and we found that Chl and N contents and the P _Nsat in individual leaves of P3845 were greater than those in corresponding leaves of Hokkou 55 at the successive growth stages. In addition, larger contents of RuBPCO and PEPC, and a greater activity of PEPC were observed in P3845. Due to the lower rates of decrease of Chl, RuBPCO, and PEPC amounts per unit of N, and the lower net C translocation rate per unit of N in the stay-green hybrid, leaf senescence was delayed in comparison to the earlier senescent hybrid.     

Photosynthesis and resource distribution through plant canopies

Plant canopies are characterized by dramatic gradients of light between canopy top and bottom, and interactions between light, temperature and water vapour deficits. This review summarizes current knowledge of potentials and limitations of acclimation of foliage photosynthetic capacity (A(max)) and light-harvesting efficiency to complex environmental gradients within the canopies. Acclimation of A(max) to high light availability involves accumulation of rate-limiting photosynthetic proteins per unit leaf area as the result of increases in leaf thickness in broad-leaved species and volume: total area ratio and mesophyll thickness in species with complex geometry of leaf cross-section. Enhancement of light-harvesting efficiency in low light occurs through increased chlorophyll production per unit dry mass, greater leaf area per unit dry mass investment in leaves and shoot architectural modifications that improve leaf exposure and reduce within-shoot shading. All these acclimation responses vary among species, resulting in species-specific use efficiencies of low and high light. In fast-growing canopies and in evergreen species, where foliage developed and acclimated to a certain light environment becomes shaded by newly developing foliage, leaf senescence, age-dependent changes in cell wall characteristics and limited foliage re-acclimation capacity can constrain adjustment of older leaves to modified light availabilities. The review further demonstrates that leaves in different canopy positions respond differently to dynamic fluctuations in light availability and to multiple environmental stresses. Foliage acclimated to high irradiance respond more plastically to rapid changes in leaf light environment, and is more resistant to co-occurring heat and water stress. However, in higher light, co-occurring stresses can more strongly curb the efficiency of foliage photosynthetic machinery through reductions in internal diffusion conductance to CO(2). This review demonstrates strong foliage potential for acclimation to within-canopy environmental gradients, but also highlights complex constraints on acclimation and foliage functioning resulting from light x foliage age interactions, multiple environmental stresses, dynamic light fluctuations and species-specific leaf and shoot structural constraints.     

Photosynthetic Acclimation to Elevated CO2 in Relation to Leaf Saccharide Constituents in Wheat and Sunflower

Wheat (T. durum cvs. HD 4502 and B 449, T. aestivum cvs. Kalyansona and Kundan) and sunflower (Helianthus annuus L. cv. Morden) were grown under atmospheric (360±10 cm³ m^–3, AC) and elevated CO₂ (650±50 cm³ m^–3, EC) concentration in open top chambers for entire period of growth and development till maturity. Leaf net photosynthetic rate (P _N) of EC-grown plants of wheat measured at EC was significantly decreased in comparison with AC-plants of wheat measured at EC. Sunflower, however, showed no significant depression in P _N in EC-plants. There was a decrease in ribulose-1,5-bisphosphate carboxylase (RuBPC) activity, its activation state and amount in EC-plants of wheat, whereas no significant decrease was observed in sunflower. The above different acclimation to EC in wheat and sunflower was related with saccharide constituents accumulated in the leaves. Under EC, sunflower accumulated in the leaves more starch, whereas wheat accumulated more sugars.     

Heterotrophic Soil Respiration in Relation to Environmental Factors and Microbial Biomass in Two Wet Tropical Forests

We examined the correlation between fungal and bacterial biomass, abiotic factors such as soil moisture, carbon in the light soil fraction and soil nitrogen to a depth of 0–25 cm and heterotrophic soil respiration using a trenching technique – in a secondary forest (Myrcia splendens, Miconia prasina and Casearia arborea) and a pine (Pinus caribeae) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil respiration was significantly reduced where roots were excluded for 7 years in both the secondary forest and the pine plantation. Microbial biomass was also significantly reduced in the root exclusion plots. In root exclusion treatment, total fungal biomass was on average 31 and 65% lower than the control plots in the pine plantation and the secondary forest, respectively, but the total bacterial biomass was 24 and 8.3% lower than the control plots in the pine plantation and the secondary forest, respectively. Heterotrophic soil respiration was positively correlated with fungal biomass (R²=0.63, R²=0.39), bacterial biomass (R²=0.16, R²=0.45), soil moisture (R²=0.41, R²=0.56), carbon in light fraction (R²=0.45, R²=0.39) and total nitrogen (R²=0.69, R²=0.67) in the pine plantation and the secondary forest, respectively. The regression analysis suggested that fungal biomass might have a greater influence on heterotrophic soil respiration in the pine plantation, while the bacterial biomass might have a greater influence in the secondary forest. Heterotrophic soil respiration was more sensitive to total N than to carbon in the light fraction, and soil moisture was a major factor influencing heterotrophic soil respiration in these forests where temperature is high and relatively invariable.     

Variation in Photosynthetic Characteristics and Leaf Area Contributes to Spathiphyllum Cultivar Differences in Biomass Production

Three genetically related Spathiphyllum cultivars, Claudia, Double Take, and Petite with similar initial sizes and biomass, were grown in a shaded greenhouse and fertilized with a constant supply of nitrogen at 200 g m^–3 using an ebb-and-flow fertigation system. Seven months later, Claudia and Double Take had plant sizes and biomasses significantly greater than Petite. Stomatal conductances of Claudia and Double Take were 30 % greater, thus net photosynthetic rates (P _N) were significantly higher than in Petite. In addition, the leaf areas (LA) of Claudia and Double Take were 60 % larger than of Petite. Since P _N was expressed per leaf surface area, the greater the LA was, the more CO₂ was fixed. Thus, the differences in plant size and biomass production of Claudia and Double Take compared to Petite are attributed to high P _N and increased LA.     

Persistence during Tasks with Horses in Relation to Social Support,General Self-efficacy and Self-esteem in Adolescents

ABSTRACT

In young people, interactions with horses may contribute to the development of social skills and self-efficacy. Our aim was to identify relationships between psychological variables and young people's experience of activities with horses. Participants in this study were 13–15-year-old adolescents (n = 25), given a 4-month intervention consisting of weekly sessions of stable work and horseback riding. We investigated the relationship between psychological variables from survey data with video-recorded behavioral observations during tasks and contact with horses. From the behavioral data, we calculated the frequency per hour of “trying again” when met with a challenge, as a measure of participants' persistence in horse-related tasks. The number of times per hour that that participant initiated contact with the horse was used as a measure of frequency of physical contact (petting). We examined correlations between the change from early to late stages of the intervention in trying again during horse-related tasks and for physical contact and three psychological variables measured prior to the intervention: perceived social support, self-esteem, and general self-efficacy. We detected an association between lower pretest levels of perceived social support and general self- efficacy, and an increase during the intervention in persistence of trying again when met with challenges during tasks with the horse. In a linear regression analysis, controlling for initial levels of persistence during horse-related tasks, self-esteem, and self-efficacy, social support was the only unique predictor of an increase in persistence during the intervention. No association was found between the psychological variables and change in frequency of petting the horse. The results indicate that adolescents with lower levels of perceived social support at the start of the intervention showed greater increases in their persistence in trying again after being met with challenges during horse-related tasks. Since persistence is a central aspect in the development of mastery of tasks, the possible positive social impact of the horse/stable environment should be given more attention in future intervention research.     

A leaf gas exchange model that accounts for intra-canopy variability by considering leaf nitrogen content and local acclimation to radiation in grapevine (Vitis vinifera L.)

Understanding the distribution of gas exchange within a plant is a prerequisite for scaling up from leaves to canopies. We evaluated whether leaf traits were reliable predictors of the effects of leaf ageing and leaf irradiance on leaf photosynthetic capacity (V(cmax) , J(max) ) in field-grown vines (Vitis vinifera L). Simultaneously, we measured gas exchange, leaf mass per area (LMA) and nitrogen content (N(m) ) of leaves at different positions within the canopy and at different phenological stages. Daily mean leaf irradiance cumulated over 10 d (PPFD(10) ) was obtained by 3D modelling of the canopy structure. N(m) decreased over the season in parallel to leaf ageing while LMA was mainly affected by leaf position. PPFD(10) explained 66, 28 and 73% of the variation of LMA, N(m) and nitrogen content per area (N(a) ), respectively. Nitrogen content per unit area (N(a) = LMA × N(m) ) was the best predictor of the intra-canopy variability of leaf photosynthetic capacity. Finally, we developed a classical photosynthesis-stomatal conductance submodel and by introducing N(a) as an input, the model accurately simulated the daily pattern of gas exchange for leaves at different positions in the canopy and at different phenological stages during the season.     

Fine Root Biomass,Production, Turnover Rates,and Nutrient Contents in Boreal Forest Ecosystems in Relation to Species,Climate, Fertility,and Stand Age: Literature Review and Meta-Analyses

Fine roots <2 mm in diameter play a key role in regulating the biogeochemical cycles of ecosystems and are important to our understanding of ecosystem responses to global climate changes. Given the sensitivity of fine roots, especially in boreal region, to climate changes, it is important to assess whether and to what extent fine roots in this region change with climates. Here, in this synthesis, a data set of 218 root studies were complied to examine fine root patterns in the boreal forest in relation to site and climatic factors. The mean fine root biomass in the boreal forest was 5.28 Mg ha^?1, and the production of fine roots was 2.82 Mg ha^?1 yr^?1, accounting for 32% of annual net primary production of the boreal forest. Fine roots in the boreal forest on average turned over 1.07 times per year. Fine roots contained 50.9 kg ha^?1 of nitrogen (N) and 3.63 kg ha^?1 of phosphorous (P). In total, fine roots in the boreal forest ecosystems contain 6.1 × 10⁷ Mg N and 4.4×10⁶Mg P pools, respectively, about 10% of the global nutrients of fine roots. Fine root biomass, production, and turnover rate generally increased with increasing mean annual temperature and precipitation. Fine root biomass in the boreal forest decreased significantly with soil N and P availability. With increasing stand age, fine root biomass increased until about 100 years old for forest stands and then leveled off or decreased thereafter. These results of meta analysis suggest that environmental factors strongly influence fine root biomass, production, and turnover in boreal forest, and future studies should place a particular emphasis on the root-environment relationships.     

Leaf structural and photosynthetic characteristics,and biomass allocation to foliage in relation to foliar nitrogen content and tree size in three Betula species

Young trees 0.03-1.7 m high of three coexisting Betula species were investigated in four sites of varying soil fertility, but all in full daylight, to separate nutrient and plant size controls on leaf dry mass per unit area (MA), light-saturated foliar photosynthetic electron transport rate (J) and the fraction of plant biomass in foliage (F(L)). Because the site effect was generally non-significant in the analyses of variance with foliar nitrogen content per unit dry mass (N(M)) as a covariate, N(M) was used as an explaining variable of leaf structural and physiological characteristics. Average leaf area (S) and dry mass per leaf scaled positively with N(M) and total tree height (H) in all species. Leaf dry mass per unit area also increased with increasing H, but decreased with increasing N(M), whereas the effects were species-specific. Increases in plant size led to a lower and increases in N(M) to a greater FL and total plant foliar area per unit plant biomass (LAR). Thus, the self-shading probably increased with increasing N(M) and decreased with increasing H. Nevertheless, the whole-plant average M(A), as well as M(A) values of topmost fully exposed leaves, correlated with N(M) and H in a similar manner, indicating that scaling of MA with N(M) and H did not necessarily result from the modified degree of within-plant shading. The rate of photosynthetic electron transport per unit dry mass (J(M)) scaled positively with N(M), but decreased with increasing H and M(A). Thus, increases in M(A) with tree height and decreasing nitrogen content not only resulted in a lower plant foliar area (LAR = F(L)/M(A)), but also led to lower physiological activity of unit foliar biomass. The leaf parameters (J(M), N(M) and M(A)) varied threefold, but the whole-plant characteristic FL varied 20-fold and LAR 30-fold, indicating that the biomass allocation was more plastically adjusted to different plant internal nitrogen contents and to tree height than the foliar variables. Our results demonstrate that: (1) tree height and N(M) may independently control foliar structure and physiology, and have an even greater impact on biomass allocation; and (2) the modified within-plant light availabilities alone do not explain the observed patterns. Although there were interspecific differences with respect to the statistical significance of the relationships, all species generally fit common regressions. However, these differences were consistent, and suggested that more competitive species with inherently larger growth rates also more plastically respond to N and H.