Jim R. Ehleringer

Implications of quantum yield differences on the distributions of C3 and C4 grasses

SummaryThe implications of a reduced quantum yield (initial slope of the photosynthetic light response curve) in C4 plants and temperature dependence of quantum yield in C3 plants on total canopy primary production were investigated using computer simulations. Since reduced quantum yield represents the only known disadvantage of the C4 photosynthetic pathway, simulations were conducted with grass canopies (high LAI and hence photosynthesis in most leaves will be light-limited) to see if quantum yield is a significant factor in limiting the primary production and thus distributions of C4 grasses. Simulations were performed for three biogeographical or environmental conditions: the Great Plains region of North America, the Sonoran Desert of North America, and shade habitats. For all three cases, the simulations predicted either spatial or temporal gradients in the abundances of C4 grasses identical to the abundance patterns of C4 grasses observed in the field. It is thus concluded that while the C4 photosynthetic mechanism may be highly advantageous in specific environments, it may be disadvantageous in others.

Leaf hairs: Effects on physiological activity and adaptive value to a desert shrub

SummaryThe effects of leaf hairs on photosynthesis, transpiration, and leaf energy balance were measured on the desert shrub Encelia farinosa in order to determine the adaptive significance of the hairs. The pubescence reduces leaf absorptance resulting in a reduced heat load, and as a consequence lower leaf temperatures and lower transpiration rates. In its native habitat where air temperatures often exceed 40° C, the optimum temperature for photosynthesis in E. farinosa occurs at 25° C, and at leaf temperatures above 35° C net photosynthesis declines precipitously. An advantage of leaf pubescence is that it allows a leaf temperature much lower than air temperature. As a result, leaf temperatures are near the temperature optimum for photosynthesis and high, potentially lethal leaf temperatures are avoided. However, there is a disadvantage associated with leaf pubescence. By reflecting quanta that might otherwise be used in photosynthesis, the presence of leaf hairs reduces the rate of photosynthesis. A tradeoff model was used to assess the overall advantage of possessing leaf hairs. In terms of the carbon gaining capacity of the leaf, the model predicted that for different environmental conditions different levels of leaf pubescence were optimal. In other words, under aird conditions and/or high air temperatures, leaves of E. farinosa would have a higher rate of photosynthesis by being pubescent than by not being pubescent. The predictions from this model agreed closely with observed patterns of leaf pubescence in the field.

Leaf Pubescence: Effects on Absorptance and Photosynthesis in a Desert Shrub

The presence of leaf pubescence (leaf hairs) in Encelia farinosa, a desert species of the Composite family, reduces the absorptance of photosynthetically active radiation (400 to 700 nanometers) by as much as 56 percent more than a closely related but nonpubescent species, E. californica, a native of the relatively moist southern California coast. Pubescence in E. farinosa, which increases through the growing season, modifies the leaf energy balance and dramatically reduces the photosynthetic rate. The reduction in the photosynthetic rate is caused by decreased light absorption rather than decreased carbon dioxide conductance through the boundary layer.

Differential 18O enrichment of leaf cellulose in C3 versus C4 grasses

We show that differences in the oxygen isotope ratio of leaf water between C3 and C4 grasses (five species of each photosynthetic type) become less distinct as relative humidity increases, and that 18O leaf water differences translate directly to the oxygen isotope ratio of leaf cellulose. A conceptual model is presented that is based on grass blade growth characteristics and observed patterns of progressive enrichment in grasses. The Barbour and Farquhar (2000) model was capable of explaining the oxygen isotope ratio of bulk leaf cellulose of C3 and C4 grasses grown under a variety of growth conditions.

Carbon Isotope Ratios of Plants of a Tropical Dry Forest in Mexico

Carbon isotope ratios were measured on leaves of the dominant species within a tropical dry forest in Mexico, which was characterized as being strongly drought deciduous. A variety of succulent epiphytes from diverse microsites had carbon isotope ratios indicating obligate crassulacean acid metabolism. All woody species examined had isotope ratios indicative of the C3 photosynthetic pathway, although there was a wide range of isotopic values. Two of the woody species that retained leaves during the dry season showed a large variation in carbon isotope ratios (up to 6 %o) that was dependent on microsite location. Key-words: Epiphytes, isotope ratios, crassulacean acid metabolism, water-use-efficiency, succulents