S. L. Gulmon

Constraints on Leaf Structure and Function in Reference to Herbivory

Leaves vary considerably in their physical and physiological properties. Although there have been advances in our understanding of the significance of their shapes (Gates and Papian 1971, Gilbert 1975, Givnish 1979, Wiens 1978) and appendages of leaves (Gilbert 1971, Ehleringer et al. 1976), we still do not have a comprehensive understanding of the evolutionary significance of variability in such physiological and developmental properties as leaf growth, metabolic rates, or longevity. The variation in a number of leaf properties is related to both environmental resource levels and patterns of herbivory. To place our discussion in an evolutionary context, we consider the benefit a plant derives from a leaf to be the rate of leaf carbon gain, times the carbon gain period, minus the carbon cost of leaf growth and maintenance and the losses due to herbivory. All of these components, with the exception of herbivory, can be evaluated in terms of the rates of carbon dioxide exchange of leaves (Figure 1).

Environmental and Evolutionary Constraints on the Photosynthetic Characteristics of Higher Plants

plants differ in their photosynthetic capacity by over two orders of magnitude (Figure 13.1). Relative carbon-gaining capacity is a critical parameter in plant competitive relationships (Mooney, 1977). It is understood that factors other than carbon gain per se, such as reproductive output or success, may determine long-term fitness, but these in turn cannot be completely divorced from carbon-gaining capacity. In view of this dependence, we explore here a theoretical framework for the physiological and evolutionary basis of this variation in photosynthetic capacity among plant species. We then consider the implications of these constraints on photosynthesis for carbon allocation within the plant.

The effects of light and nitrogen on photosynthesis, leaf characteristics, and dry matter allocation in the chaparral shrub, Diplacus aurantiacus

SummaryPlants of Diplacus aurantiacus, a successional shrub common in California chaparral, were grown under controlled conditions in which either quantum flux density or nitrogen availability was varied. Photosynthesis and leaf nitrogen content were determined on a leaf area and a leaf weight basis, and whole plant growth was monitored.There was a direct relationship between photosynthesis and leaf nitrogen content on both area and weight bases. Reduced light intensity of the growth environment resulted in reductions in light-saturated photosynthesis and nitrogen content on an area basis, but not on a weight basis. With reduced nitrogen availability, photosynthesis and leaf nitrogen content per unit leaf weight decreased.Resource use efficiency increased as the resource became more limiting. The results are consistent with a model of plant growth in which net carbon gain of the leaf is maximized.

Photosynthetic capacity in relation to leaf position in desert versus old-field annuals

SummaryDesert annuals of Death Valley, California have higher average light-saturated photosynthetic capacities and leaf nitrogen contents than do early-successional annuals of Illinois. The leaves of annuals in the light-unlimited Death Valley environment change little in specific weight, nitrogen, or photosynthetic capacity with age. In contrast, these properties decrease markedly with age in the leaves of the Illinois annuals even in leaves not exposed to the usual shading that accompanies canopy development. These results are interpreted in a carbon-gained-per-nitrogen-invested context.

Comparative Photosynthetic Capacities of Intertidal Algae under Exposed and Submerged Conditions

Photosynthetic rates were measured for five species of intertidal marine algae, in the air and submerged. Ulva expansa and Prionitis lanceolata from the lower intertidal show reduced photosynthetic capacity air in air compared to submergedrates. In contrast, species from the middle and upper littoral (Iridaea flaccida, Porphyra perforata, Fucus distichus, and Endocladia muricata) reach maximum photosynthesis after some degree of drying. For these latter species, photosynthetic rates can be 1.6 to 6.6 times greater in air than in water at the same illumination and temperature. Desiccation rates under natural conditions are slow enough that these algae are capable of continuing a high rate of photosynthetic activity for extended periods while exposed and may fix the bulk of their carbon at this time. The capacity of these algae for sustained photosynthesis in air vary according to their intertidal zonation. It is suggested that these relationships may be partially responsible for the vertical distribution of intertidal marine algae.

Further observations on the water relations ofProsopis tamarugo of the northern Atacama desert

SummaryProsopis tamarugo, a tree native to the Atacama desert of Chile apparently has unique water relations. It is proposed that in its native habitat, where there is essentially no precipitation, establishment occurs during the rare flooding periods, with water coming as runoff from the Andes. These plants subsequently exist as phreatophytes tapping the relatively shallow ground water. Although phreatophytic, the plants appears to come under increasing drought stress as the growing season progresses. Because of the very low water potentials of the salty surface soils, water evidently moves from the plant into the soil under certain conditions. This water may be reabsorbed subsequently and used by the plant as the water table capillary fringe is depleted toward the end of the leafy period.

Effects of fertiliser addition and subsequent gopher disturbance on a serpentine annual grassland community

SummaryApplication of slow release fertiliser to small (0.5x1 m) plots within a serpentine annual grassland community led to significant increases in above-ground biomass and a shift in species relative abundances. In fertilised plots the native forb species which usually dominate the grassland were almost totally replaced by grasses. In the years following initial fertiliser application, a heavy mulch formed from the previous years grass growth allowed establishment of grass species such as Bromus mollis but significantly reduced forb establishment. Gopher disturbance of fertilised plots in the second and third years of the experiment effectively removed the grass mulch and allowed re-establishment of forb species.

Patterns of seed germination in Californian serpentine grassland species

SummaryGermination of nine Californian serpentine annual species and one perennial grass was examined as a function of soil and litter cover and seasonal timing of rainfall. Germination responses varied with the date of moisture application, and the patterns of variation differed among species. Germination occurred in waves over time, but in most of the species, virtually all the seeds had germinated by December. These results indicate that yearly climatic variation can affect species composition and competitive relationships among species through the germination process. Soil and litter cover produced little or no effect except for the smallest-seeded species.

Parallel evolution of leaf pubescence in Encelia in coastal deserts of North and South America

SummaryMembers of the shrub genus Encelia occur in the arid coastal regions of western North America and South America along gradients of precipitation extending from less than 50 mm annually to somewhat over 350 mm. At moist ends of the gradient species possess glabrate, green leaves. Proceeding to drier regions, the species possess progressively more pubescent, whiter leaves. Leaf absorptance to solar radiation decreases with precipitation in an identical pattern along gradients in both North and South America.

Photosynthetic characteristic of South African sclerophylls

SummarySclerophyll shrubs of the South African mountain fynbos have leaves similar in structural and physiological properties to leaves from evergreen shrubs of other mediterranean-climate regions. These leaves have relatively low photosynthetic capacities (<14 μmol CO2 m−2 s−1), and are light saturated at relatively low photon irradiances (<1.0 mmol m−2 s−1). They have broad temperature responses of photosynthesis, moderate water-use efficiencies (∼0.003 mol CO2/mol H2O), high specific weights (∼200 g m−2) and low nitrogen contents (∼10 mg g−1).