The role of nitrogen in a simple scheme to scale up photosynthesis from leaf to canopy

A simple analytical scheme, involving the distribution of nitrogen, to scale up photosynthesis from leaf to canopy is proposed. The scheme is based on the assumption that there are two pools of nitrogen in leaves: nitrogen in photosynthetic, degradable structures (N_p) and nitrogen in non-photosynthetic and non-degradable structures (N_s). The rate of photon-saturated photosynthesis, F_m, is assumed to be proportional to N_p and is distributed inside the canopy similarly to photon flux density (PFD). Prior assumptions of an optimum distribution of nitrogen are not a prerequisite. Calculations made with the scheme lead to development of the hypothesis that the canopy can be treated as a ‘big leaf’ on the time scales involved in acclimation of photosynthesis to PFD. Simulations using parameters for tree species with different requirements for PFD show that shade-tolerant species may have denser canopies than sun-demanding species because of smaller amounts of non-photosynthetic structural nitrogen and/or supporting tissue in their leaves.     

Within-canopy variation in the rate of development of photosynthetic capacity is proportional to integrated quantum flux density in temperate deciduous trees

The present study was undertaken to test for the hypothesis that the rate of development in the capacity for photosynthetic electron transport per unit area (J_max;A), and maximum carboxylase activity of Rubisco (V_cmax;A) is proportional to average integrated daily quantum flux density (Q_int) in a mixed deciduous forest dominated by the shade‐intolerant species Populus tremula L., and the shade‐tolerant species Tilia cordata Mill. We distinguished between the age‐dependent changes in net assimilation rates due to modifications in leaf dry mass per unit area (M_A), foliar nitrogen content per unit dry mass (N_M), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport (F_B), Rubisco (F_R) and in light‐harvesting chlorophyll‐protein complexes (V_cmax;A ∝ M_AN_MF_R; J_max;A ∝ M_AN_MF_B). In both species, increases in J_max;A and V_cmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in M_A. Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in M_A. There was only small within‐canopy variation in the initial rate of biomass accumulation per unit Q_int (slope of M_A versus leaf age relationship per unit Q_int), suggesting that canopy differences in the rate of development of J_max;A and V_cmax;A are directly proportional to Q_int. Nevertheless, M_A, nitrogen, J_max;A and V_cmax;A of mature leaves were not proportional to Q_int because of a finite M_A in leaves immediately after bud‐burst (light‐independent component of M_A). M_A, leaf chlorophyll contents and chlorophyll : N ratio of mature leaves were best correlated with the integrated average quantum flux density during leaf development, suggesting that foliar photosynthetic apparatus, once developed, is not affected by day‐to‐day fluctuations in Q_int. However, for the upper canopy leaves of P. tremula and for the entire canopy of T. cordata, there was a continuous decline in N contents per unit dry mass in mature non‐senescent leaves on the order of 15–20% for a change of leaf age from 40 to 120 d, possibly manifesting nitrogen reallocation to bud formation. The decline in N contents led to similar decreases in leaf photosynthetic capacity and foliar chlorophyll contents. These data demonstrate that light‐dependent variation in the rate of developmental changes in M_A determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.     

Fruit-set and recruitment in populations of Cypripedium calceolus L. in Estonia

The overall mean percentage of fruiting for over 3500 flowers observed in eight Estonian populations of a self-compatible clonal orchid Cypripedium calceolus over 11 years was 10.5%. The larger clones set relatively fewer fruits, otherwise the general pattern of fruiting was close to random, despite several significant local deviations from random pollination. There is no cost associated with the fruit-set at the clonal level. The fruit-set which appears pollinator limited was not correlated with the frequency of seedlings in a population. Data presented provide information on the relationship between fruit and seedling production in orchids. The sites suitable for seedling establishment are characterized as having relatively more extensive moss cover, less vascular plant cover, more moisture and better light conditions. Recruitment is concluded to be microsite limited, and the fruit-set to be without significant influence on the fitness of populations.     

Reduced light availability and increased competition diminish the reproductive success of wet forest sedge Carex loliacea L.

Wet forest ecosystems in temperate regions have been heavily drained and logged, often with significant negative consequences for biodiversity in these habitats. Our research focused on population maintenance mechanisms of a declining wet forest sedge Carex loliacea L. We studied germination under different light regimes and seedling survival under different vegetation densities using an in situ removal experiment. For successful germination, seeds of C. loliacea need light; germination in reduced light conditions is depressed. The seeds of C. loliacea are able to accumulate a seed bank and exhibit seasonal dormancy cycles. Survival of seedlings strongly depends on competition with other plant species. Our results imply that changes in habitat conditions (draining, forest cutting) affect the successful generative reproduction of C. loliacea primarily via a change in light conditions, which is a strong factor both at the stage of germination and seedling growth. However, adult plants are able to persist over a much broader range of habitat conditions without detectable vitality loss.     

Adjustment of leaf photosynthesis to shade in a natural canopy: rate parameters

The present study was performed to investigate the adjustment of the rate parameters of the light and dark reactions of photosynthesis to the natural growth light in leaves of an overstorey species, Betula pendula Roth, a subcanopy species, Tilia cordata P. Mill., and a herb, Solidago virgaurea L., growing in a natural plant community in Järvselja, Estonia. Shoots were collected from the site and individual leaves were measured in a laboratory applying a standardized routine of kinetic gas exchange, Chl fluorescence and 820 nm transmittance measurements. These measurements enabled the calculations of the quantum yield of photosynthesis and rate constants of excitation capture by photochemical and non-photochemical quenchers, rate constant for P700⁺ reduction via the cytochrome b₆f complex with and without photosynthetic control, actual maximum and potential (uncoupled) electron transport rate, stomatal and mesophyll resistances for CO₂ transport, K_m(CO₂) and V_m of ribulose-bisphosphate carboxylase-oxygenase (Rubisco) in vivo. In parallel, N, Chl and Rubisco contents were measured from the same leaves. No adjustment toward higher quantum yield in shade compared with sun leaves was observed, although relatively more N was partitioned to the light-harvesting machinery in shade leaves ( H. Eichelmann et al., 2004 ). The electron transport rate through the Cyt b₆f complex was strongly down-regulated under saturating light compared with darkness, and this was observed under atmospheric, as well as saturating CO₂ concentration. In vivo V_m measurements of Rubisco were lower than corresponding reported measurements in vitro, and the k_cat per reaction site varied widely between leaves and growth sites. The correlation between Rubisco V_m and the photosystem I density was stronger than between V_m and the density of Rubisco active sites. The results showed that the capacity of the photosynthetic machinery decreases in shade-adjusted leaves, but it still remains in excess of the actual photosynthetic rate. The photosynthetic control systems that are targeted to adjust the photosynthetic rate to meet the plant's needs and to balance the partial reactions of photosynthesis, down-regulate partial processes of photosynthesis: excess harvested light is quenched non-photochemically; excess electron transport capacity of Cyt b₆f is down-regulated by ΔpH-dependent photosynthetic control; Rubisco is synthesized in excess, and the number of activated Rubisco molecules is controlled by photosystem I-related processes. Consequently, the nitrogen contained in the components of the photosynthetic machinery is not used at full efficiency. The strong correlation between leaf nitrogen and photosynthetic performance is not due to the nitrogen requirements of the photosynthetic apparatus, but because a certain amount of energy must be captured through photosynthesis to maintain this nitrogen within a leaf.