ALBERT

Notes: The present study was performed to investigate the adjustment of the constituents of the light and dark reactions of photosynthesis to the natural growth irradiance in the 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 properties of individual leaves were measured in a laboratory, by applying a routine of kinetic gas exchange and optical measurements that revealed photosystem II (PSII), photosystem I (PSI), and cytochrome b6f densities per leaf area and the distribution of excitation (or chlorophyll, Chl) between the two photosystems. In parallel, N, Chl and ribulose-bisphosphate carboxylase-oxygenase (Rubisco) content was measured from the same leaves. The amount of N in photosynthetic proteins was calculated from the measured contents of the components of the photosynthetic machinery. Non-photosynthetic N was found as the residual of the budget. Growth in shade resulted in the decrease of leaf dry mass to a half of the DW in sun leaves in each species, but the total variation, from the top to the bottom of the canopy, was larger. Through the whole cross-section of the canopy, leaf dry weight (DW) and Rubisco content per area decreased by a factor of four, N content by a factor of three, but Chl content only by a factor of 1.7. PSII density decreased by a factor of 1.9, but PSI density by a factor of 3.2. The density of PSI adjusted to shade to a greater extent than the density of PSII. In shade, the distribution of N between the components of the photosynthetic machinery was shifted toward light-harvesting proteins at the expense of Rubisco. Non-photosynthetic N decreased the most substantially, from 54% in the sun leaves of B. pendula to 11% in the shade leaves of T. cordata. It is concluded that the redistribution of N toward light-harvesting Chl proteins in shade is not sufficient to keep the excitation rate of a PSII centre invariant. Contrary to PSII, the density of PSI – the photosystem that is in immediate contact with the carbon assimilation system – shade-adjusts almost proportionally with the latter, whereas its Chl antenna correspondingly increases. Even under N deficiency, a likely condition in the natural plant community, a substantial part of N is stored in non-photosynthetic proteins under abundant irradiation, but much less under limiting irradiation. At least in trees the general sequence of down-regulation due to shade adjustment is the following: (1) non-protein cell structures and non-photosynthetic proteins; (2) carbon assimilation proteins; (3) light reaction centre proteins, first PSI; and (4) chlorophyll-binding proteins.

Notes: 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 (Jmax;A), and maximum carboxylase activity of Rubisco (Vcmax;A) is proportional to average integrated daily quantum flux density (Qint) 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 (MA), foliar nitrogen content per unit dry mass (NM), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport (FB), Rubisco (FR) and in light-harvesting chlorophyll-protein complexes (Vcmax;A ∝ MANMFR; Jmax;A ∝ MANMFB). In both species, increases in Jmax;A and Vcmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in MA. Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in MA. There was only small within-canopy variation in the initial rate of biomass accumulation per unit Qint (slope of MA versus leaf age relationship per unit Qint), suggesting that canopy differences in the rate of development of Jmax;A and Vcmax;A are directly proportional to Qint. Nevertheless, MA, nitrogen, Jmax;A and Vcmax;A of mature leaves were not proportional to Qint because of a finite MA in leaves immediately after bud-burst (light-independent component of MA). MA, 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 Qint. 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 MA determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.

Notes: 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, Km(CO2) and Vm 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 Vm measurements of Rubisco were lower than corresponding reported measurements in vitro, and the kcat per reaction site varied widely between leaves and growth sites. The correlation between Rubisco Vm and the photosystem I density was stronger than between Vm 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.

Notes: Responses of foliar light-saturated net assimilation rate (Amax), capacity for photosynthetic electron transport (Jmax) and mitochondrial respiration rate (Rd) to long-term canopy light and temperature environment were investigated in a temperate deciduous canopy composed of Populus tremula L. in the upper (17–28 m) and of Tilia cordata Mill. in the lower canopy layer (4–17 m). Climatic measurements indicated that seasonal average daily maximum air temperature (Tmax) was 5·5 °C (range 0·7–10·5 °C) higher in the top than in the bottom of the canopy, and strong positive correlations were observed between Tmax and seasonal average integrated quantum flux density (Qint), as well as between seasonal average daily mean temperature and Qint. Because of changes in leaf dry mass and nitrogen per unit area, Amax, Jmax, and Rd scaled positively with Qint in both species at a common leaf temperature (T). According to Jmax versus T response curves and dark chlorophyll fluorescence transients, photosynthetic electron transport was less heat resistant in P. tremula with optimum temperature of Jmax, Topt, of 33·5 ± 0·6 °C than in T. cordata with Topt of 40·7 ± 0·6 °C. This difference was suggested to manifest evolutionary adaptation of photosynthetic electron transport to cooler environments in P. tremula, the range of which extends farther north than that in T. cordata. Possibly because of acclimation to long-term canopy temperature environment, Topt was positively related to Qint in P. tremula, foliage of which was also exposed to higher irradiances and temperatures, but not in T. cordata, in the canopy of which quantum flux densities and temperatures were lower, and gradients in the environmental factors less pronounced. Parallel to changes in Topt, the activation energy for photosynthetic electron transport decreased with increasing Qint in P. tremula, indicating that Jmax of leaves acclimated to colder environment was more responsive to T in lower temperatures than that of high T acclimated leaves. Similar alterations in the activation energy for mitochondrial respiration rate were also observed, indicating that acclimation to temperature of mitochondrial and chloroplastic electron transport proceeds in a co-ordinated manner, and possibly involves long-term changes in membrane fluidity properties. We conclude that, because of correlations between temperature and light, the shapes of Jmax versus T, and Rd versus T response curves vary within tree canopies, and this needs to be taken account in modelling whole canopy photosynthesis.