Assessing photosynthetic downregulation in sunflower stands with an optically-based model.

Using a simple light-use efficiency model based on optical measurements, we explored spatial patterns of photosynthetic activity in fertilized and unfertilized sunflower stands. The model had two components: (1) absorbed photosynthetically active radiation (APAR), and (2) radiation-use efficiency. APAR was the product of photosynthetic photon flux density (PPFD) and leaf absorptance, which was derived from leaf reflectance. Radiation-use efficiency was either assumed to be constant or allowed to vary linearly with the photochemical reflectance index (PRI), a measure of xanthophyll cycle pigment activity. When efficiency was assumed to be constant, the model overestimated photosynthetic rates in upper canopy layers exposed to direct PPFD, particularly in the unfertilized canopy due to the greater photosynthetic downregulation associated with higher levels of photoprotective (de-epoxidized) xanthophyll cycle pigments in these conditions. When efficiency was allowed to vary according to the PRI, modeled photosynthetic rates closely matched measured rates for all canopy layers in both treatments. These results illustrate the importance of considering reduced radiation-use efficiency due to photosynthetic downregulation when modeling photosynthesis from reflectance, and illustrate the potential for detecting radiation-use efficiency through leaf optical properties. At least under the conditions of this study, these results also suggest that xanthophyll cycle pigment activity and net carbon uptake are coordinately regulated, allowing assays of Photosystem II activity to reveal changing rates of net assimilation. Because the optical methods in this study are adaptable to multiple spatial scales (leaf to landscape), this approach may provide a scalable model for estimating photosynthetic rates independently from flux measurements.

Estimation of leaf area with an integrating sphere.

Relative absorptance of intact branches measured with an integrating sphere was compared to leaf area estimated by conventional methods (volume displacement and scanning area meter) for three conifer species: Picea mariana (Mill.) BSP, Pinus banksiana (Lamb.) and Pseudotsuga menziesii (Mirb.) Franco. A consistent relationship between relative absorptance and surface area emerged for the three species. The ability to predict leaf area from absorptance was further explored by measuring branches of Pseudotsuga menziesii grown in varying light and nutrient regimes. When a single equation was used to predict leaf area under all growth conditions, errors were as large as 40% primarily because of variation in leaf absorptivity, with the largest errors associated with extremely nutrient-deficient foliage. When separate empirical equations were developed for each growth treatment, predicted leaf surface area agreed to within 5% of the area determined by the volume displacement method. Leaf surface area estimated from theoretical principles was also in good agreement with total surface area estimated independently by conventional methods. With proper accounting for needle absorptivity, which varied with growth conditions, leaf area estimates obtained by the integrating sphere method were of similar accuracy to those obtained by conventional methods, with the added advantage that the method allowed intact foliage to be sampled nondestructively in the field. Because the integrating sphere method preserves branch structure during measurement, it could provide a useful measure of needle area for photosynthetic or developmental studies requiring repeated sampling of the same branch.

The photochemical reflectance index: an optical indicator of photosynthetic radiation use efficiency across species, functional types, and nutrient levels.

The photochemical reflectance index (PRI), derived from narrow-band reflectance at 531 and 570 nm, was explored as an indicator of photosynthetic radiation use efficiency for 20 species representing three functional types: annual, deciduous perennial, and evergreen perennial. Across species, top-canopy leaves in full sun at midday exhibited a strong correlation between PRI and ΔF/Fm', a fluorescence-based index of photosystem II (PSII) photochemical efficiency. PRI was also significantly correlated with both net CO2 uptake and radiation use efficiency measured by gas exchange. When species were examined by functional type, evergreens exhibited significantly reduced midday photosynthetic rates relative to annual and deciduous species. This midday reduction was associated with reduced radiation use efficiency, detectable as reduced net CO2 uptake, PRI, and ΔF/Fm' values, and increased levels of the photoprotective xanthophyll cycle pigment zeaxanthin. For each functional type, nutrient deficiency led to reductions in both PRI and ΔF/Fm' relative to fertilized controls. Laboratory experiments exposing leaves to diurnal courses of radiation and simulated midday stomatal closure demonstrated that PRI changed rapidly with both irradiance and leaf physiological state. In these studies, PRI was closely correlated with both ΔF/Fm' and radiation use efficiency determined from gas exchange at all but the lowest light levels. Examination of the difference spectra upon exposure to increasing light levels revealed that the 531 nm Δ reflectance signal was composed of two spectral components. At low irradiance, this signal was dominated by a 545-nm component, which was not closely related to radiation use efficiency. At progressively higher light levels above 100 µmol m-2 s-1, the 531-nm signal was increasingly dominated by a 526-nm component, which was correlated with light use efficiency and with the conversion of the xanthophyll pigment violaxanthin to antheraxanthin and zeaxanthin. Further consideration of the two components composing the 531-nm signal could lead to an index of photosynthetic function applicable over a wide range of illumination. The results of this study support the use of PRI as an interspecific index of photosynthetic radiation use efficiency for leaves and canopies in full sun, but not across wide ranges in illumination from deep shade to full sun. The discovery of a consistent relationship between PRI and photosynthetic radiation use efficiency for top-canopy leaves across species, functional types, and nutrient treatments suggests that relative photosynthetic rates could be derived with the "view from above" provided by remote reflectance measurements if issues of canopy and stand structure can be resolved.

Photoinhibition in Vitis californica: The Role of Temperature during High-Light Treatment.

Leaves of Vitis californica Benth. (California wild grape) exposed to a photon flux density (PFD) equivalent to full sun exhibited temperature-dependent reductions in the rates or efficiencies of component photosynthetic processes. During high-PFD exposure, net CO(2) uptake, photon yield of oxygen evolution, and photosystem II chlorophyll fluorescence at 77 Kelvin (F(m), F(v), and F(v)/F(m)) were more severely inhibited at high and low temperatures than at intermediate temperatures. Sun leaves tolerated high PFD more than growth chamber-grown leaves but exhibited qualitatively similar temperature-dependent responses to high-PFD exposures. Photosystem II fluorescence and net CO(2) uptake exhibited different sensitivities to PFD and temperature. Fluorescence and gas exchange kinetics during exposure to high PFD suggested an interaction of multiple, temperature-dependent processes, involving both regulation of energy distribution and damage to photosynthetic components. Comparison of F(v)/F(m) to photon yield of oxygen evolution yielded a single, curvilinear relationship, regardless of growth condition or treatment temperature, whereas the relationship between F(m) (or F(v)) and photon yield varied with growth conditions. This indicated that F(v)/F(m) was the most reliable fluorescence indicator of PSII photochemical efficiency for leaves of different growth conditions and treatments.

Remote sensing of the xanthophyll cycle and chlorophyll fluorescence in sunflower leaves and canopies.

Sudden illumination of sunflower (Helianthus annuus L. cv. CGL 208) leaves and canopies led to excess absorbed PFD and induced apparent reflectance changes in the green, red and near-infrared detectable with a remote spectroradiometer. The green shift, centered near 531 nm, was caused by reflectance changes associated with the de-epoxidation of violaxanthin to zeaxanthin via antheraxanthin and with the chloroplast thylakoid pH gradient. The red (685 nm) and near-infrared (738 nm) signals were due to quenching of chlorophyll fluorescence. Remote sensing of shifts in these spectral regions provides non-destructive information on in situ photosynthetic performance and could lead to improved techniques for remote sensing of canopy photosynthesis.

Leaf movement, stress avoidance and photosynthesis in Vitis californica.

Gas exchange and chlorophyll fluorescence techniques were used to evaluate the hypothesis that leaf movement in Vitis californica Benth. (California wild grape) allows a compromise between sunlight interception and stress damage in order to maximize photosynthetic carbon gain over the life of the leaf. Leaves that were restrained horizontally tolerated their increased radiation loads if critical temperatures were not exceeded. Reductions in photosynthetic capacity and the F V/F M fluorescence ratio only occurred in leaves that attained high temperatures. Leaf orientation and canopy position were important determinants of leaf temperature. These results indicate that excessive leaf temperature, not high PFD, can be a principle cause of reduced carbon gain and senescence in this species in the wild. Leaf movement appears to protect photosynthetic components in midsummer.

Perception of vertical dimension.

Ten edentulous adults were investigated to learn more about the preferred vertical dimension of occlusion (PVDO) and the comfortable zone. A screw jack device was employed, using the forced-choice and external approach methods. The dimensional discrimination ability of each subject was assessed at the mean PVDO, and at points 3 mm above and below it. For each subject, both the forced-choice and external approach methods gave a mean PVDO value which was reasonably constant. The comfortable zone widths were less consistent. In all but two of the subjects the mean PVDO values appeared consistent with a clinically acceptable occlusal vertical dimension. No relationship was found between the dimensional discrimination ability of the subjects and their definition of PVDO and comfortable zone.