Photosynthetic Water Use Efficiency Research Articles

Seasonal leaf phenotypes in the canopy of a tropical dry forest: photosynthetic characteristics and associated traits.

We evaluated the hypothesis that photosynthetic traits differ between leaves produced at the beginning (May) and the end (November-December) of the rainy season in the canopy of a seasonally dry forest in Panama. Leaves produced at the end of the wet season were predicted to have higher photosynthetic capacities and higher water-use efficiencies than leaves produced during the early rainy season. Such seasonal phenotypic differentiation may be adaptive, since leaves produced immediately preceding the dry season are likely to experience greater light availability during their lifetime due to reduced cloud cover during the dry season. We used a construction crane for access to the upper canopy and sampled 1- to 2-month-old leaves marked in monthly censuses for six common tree species with various ecological habits and leaf phenologies. Photosynthetic capacity was quantified as light- and CO2-saturated oxygen evolution rates with a leaf-disk oxygen electrode in the laboratory (O2max) and as light-saturated CO2 assimilation rates of intact leaves under ambient CO2 (Amax). In four species, pre-dry season leaves had significantly higher leaf mass per unit area. In these four species, O2max and Amax per unit area and maximum stomatal conductances were significantly greater in pre-dry season leaves than in early wet season leaves. In two species, Amax for a given stomatal conductance was greater in pre-dry season leaves than in early wet season leaves, suggesting a higher photosynthetic water-use efficiency in the former. Photosynthetic capacity per unit mass was not significantly different between seasons of leaf production in any species. In both early wet season and pre-dry season leaves, mean photosynthetic capacity per unit mass was positively correlated with nitrogen content per unit mass both within and among species. Seasonal phenotypic differentiation observed in canopy tree species is achieved through changes in leaf mass per unit area and increased maximum stomatal conductance rather than by changes in nitrogen allocation patterns.

Variation in Carbon Isotope Discrimination and Photosynthetic Gas Exchange Among Populations of Pseudotsuga menziesii and Pinus ponderosa in Different Environments

Seedlings representing 25 populations of Pseudotsuga menziesii and 26 populations of Pinus ponderosa were grown in a common garden in Moscow, ID, USA. The seeds were collected across the natural distribution of each species, at altitudes ranging from 170 to 2774 m above sea level, latitudes from 33 °N to 53 °N, and longitudes from 105 °W to 124 °W. Lipid-free seeds from mother trees and leaf tissue from the 2-year-old progeny were analysed. The design enabled us not only to measure genetically determined variation in carbon isotope discrimination (Δ) and gas exchange characteristics but also to compare performance in the common garden and in situ. In the common garden, significant population variation in Δ, gas exchange and specific leaf area was detected among seedlings of Pseudotsuga menziesii. Coastal, low-altitude genotypes had significantly lower Δ than interior, high-altitude genotypes. In Pinus ponderosa, populations varied only in specific leaf area. These broadly distributed sympatric species differ in genetic structure with respect to gas-exchange characteristics. In the common garden, high Δ of both species was associated with high stomatal conductance relative to photosynthetic rate. Specific leaf area, although strongly correlated with Δ, varied in the wrong direction to explain variation in Δ. In situ Δ was correlated with both altitude and vapour pressure deficit (VPD) in Pseudotsuga menziesii (r 2 =0.42, P=0002 and r 2 =025, P 0.77 and r 2 =0.06, P>0.21 for altitude and VPD, respectively). Foliage Δ from the progeny grown in the common garden was significantly correlated, in both species, with Δ of the in situ maternal photosynthate in the seeds; however, the correlation was negative in Pseudotsuga menziesii and positive in Pinus ponderosa. The negative correlation indicates a strong acclimatory response, perhaps to VPD, in Pseudotsuga menziesii. Altitudinal decreases in intercellular CO 2 partial pressures inferred from isotopic data were insufficient to compensate for increased VPD. Photosynthetic water-use efficiency (net photosynthesis/transpiration) was estimated to decrease by two- to fourfold from sea level to 2800m altitude within the distribution limits of these species.

Genetic variation of ecophysiological traits in red alder (Alnusrubra Bong.)

We examined the genetic variation of ecophysiological traits within and among 40 red alder (Alnusrubra Bong.) provenances (two families per provenance) in a common-garden experiment in the summer of 1993. The provenances were representative of the entire species range in British Columbia, Canada. We found significant genetic variation among provenances (P < 0.001) in photosynthetic rate (A), mesophyll conductance (gm), transpiration rate (E), stomatal conductance (gsw), stomatal sensitivity to water vapour pressure deficit (SENSVPD), intercellular to ambient CO2 concentration ratio (Ci/Ca), and midday xylem water potential (ψ). Photosynthetic water-use efficiency, however, did not differ significantly among provenances. There were no significant differences between families within provenance for any of these variables. A weak but significant geographic trend was detected in ecophysiological traits: ψ, A, gm, and E increased, and SENSVPD decreased, from southeast to northwest. Photosynthetic rate, E, gm, gsw and ψ were positively related to each other, but negatively correlated with SENSVPD. Ci/Ca was negatively correlated with gsw. These correlations indicate that red alder might have undergone genetic differentiation in drought resistance.

Assessment of the impact of rising carbon dioxide and other potential climate changes on vegetation

The projected doubling of current levels of atmospheric carbon dioxide concentration ([CO 2]) during the next century along with increases in other radiatively active gases have led to predictions of increases in global air temperature and shifts in precipitation patterns. Additionally, stratospheric ozone depletion may result in increased ultraviolet-B (UV-B) radiation incident at the Earth's surface in some areas. Since these changes in the Earth's atmosphere may have profound effects on vegetation, the objectives of this paper are to summarize some of the recent research on plant responses to [CO 2], temperature and UV-B radiation. Elevated [CO 2] increases photosynthesis and usually results in increased biomass, and seed yield. The magnitude of these increases and the specific photosynthetic response depends on the plant species, and are strongly influenced by other environmental factors including temperature, light level, and the availability of water and nutrients. While elevated [CO 2] reduces transpiration and increases photosynthetic water-use efficiency, increasing air temperature can result in greater water use, accelerated plant developmental rate, and shortened growth duration. Experiments on UV-B radiation exposure have demonstrated a wide range of photobiological responses among plants with decreases in photosynthesis and plant growth among more sensitive species. Although a few studies have addressed the interactive effects of [CO 2] and temperature on plants, information on the effects of UV-B radiation at elevated [CO 2] is scarce. Since [CO 2], temperature and UV-B radiation may increase concurrently, more research is needed to determine plant responses to the interactive effects of these environmental variables.

Growth of cotton under continuous salinity stress: influence on allocation pattern, stomatal and non-stomatal components of photosynthesis and dissipation of excess light energy

Cotton (Gossypium hirsutum L.) plants were grown in flowing-culture solutions containing 0%, 26% and 55% natural seawater under controlled and otherwise identical conditions. Leaf Na(+) content rose to 360 mM in 55% seawater, yet the K(+) content was maintained above 100 mM. The K(+)/Na(+) selectivity ratio was much greater in the saline plants than in the control plants. All plants were healthy and able to complete the life cycle but relative growth rate fell by 46% in 26% seawater and by 83% in 55% seawater. Much of this reduction in growth was caused by a decreased allocation of carbon to leaf growth versus root growth. The ratio of leaf area/plant dry weight fell by 32% in 26% seawater and by 50% in 55 % seawater while the rate of carbon gain per unit leaf area fell by only 20% in 26% seawater and by as much as 66% in 55% seawater. Partial stomatal closure accounted for nearly all of the fall in the photosynthesis rate in 26% seawater but in 55% seawater much of the fall also can be attributed to non-stomatal factors. As a result of the greater effect of salinity on stomatal conductance than on CO2-uptake rate, photosynthetic water-use efficiency was markedly improved by salinity. This was also confirmed by stablecarbon-isotope analyses of leaf sugar and of leaf cellulose and starch. - Although non-stomatal photosynthetic capacity at the growth light was reduced by as much as 42% in 55% seawater, no effects were detected on the intrinsic photon yield of photosynthesis nor on the efficiency of photosystem II photochemistry, chlorophyll a/b ratio, carotenoid composition or the operation of the xanthophyll cycle. Whereas salinity caused in increase in mesophyll thickness and content of chloroplast pigments it caused a decrease in total leaf nitrogen content. The results indicate that the salinity-induced reduction in non-stomatal photosynthetic capacity was not caused by any detrimental effect on the photosynthetic apparatus but reflects a decreased allocation to enzymes of carbon fixation. - Rates of energy dissipation via CO2 fixation and photorespiration, calculated from gas-exchange measurements, were insufficient to balance the rate of light-energy absorption at the growth light. Salinity therefore would have been expected to cause the excess excitation energy to rise, leading to an increased nonradiative dissipation in the pigment bed and resulting increases in non-photochemical fluorescence quenching and zeaxanthin formation. However, no such changes could be detected, implying that salinity may have increased energy dissipation via a yet unidentified energy-consuming process. This lack of a response to salinity stress is in contrast to the responses elicited by short-term water stress which caused strong non-photochemical quenching and massive zeaxanthin formation.

Shallow Water Table Effects on Photosynthesis and Corn Yield

The effect of water-table management practices on leaf photosynthesis and corn yield was investigated under two different field conditions in 1989 and 1990. In one field, water-table depths were maintained at 0.3, 0.6, and 0.9 m in field lysimeters during the growing season. In the other field, average water-table depths of 0.2, 0.3, 0.6, 0.9, and 1.1 m were maintained through subirrigation. Photo-synthesis measurements were made regularly during the growing season, and yield data were collected at harvest. In 1989, a relatively dry year, photosynthesis rates were higher at shallow water-table depths than at deep water-table depths. In 1990, a very wet year, photosynthesis rates were not significantly different for water-table depths between 0.3 and 0.9 m, but rates decreased significantly for water-table depths shallower than 0.3 m. Statistical analysis indicates that water-table effects on photosynthesis rates were not consistent. However, effects of various water-table depths on photosynthetic water-use efficiency (PWUE) were highly significant in both dry and wet seasons. Corn yields increased with increasing water-table depths. At water-table depths of 0.2 to 0.3 m, corn yield decreased significantly. In both dry and wet seasons, effects of water-table treatments on grain yield were highly significant and significant relationships were obtained between PWUE values and yield.

Responses of photosynthesis and carbohydrate‐partitioning to limitations in nitrogen and water availability in field‐grown sunflower*

Abstract. Sunflower plants (Helianthus annuus L., cv. CGL 208) were field‐grown in adjacent plots of varying resource availability. Control plants received irrigation (on a 4–5 d interval) and high levels of fertilizer nitrogen. Nutrient‐stress (N‐stress) plants received control levels of irrigation but no nutrient amendments and were determined to be nitrogen‐limited. Water‐stress (H2O‐stress) plants received control levels of fertilizer nitrogen, but no irrigation after approximately 6 weeks of plant growth. Both stress treatments reduced maximum and diurnal net photosynthesis (A) but resulted in different physiological or biochemical adjustments that tended to maintain or increase A per unit of resource (nitrogen or water) in shortest supply while decreasing the ratio of A per unit of abundant resource. Nutrient‐stress reduced total foliar nitrogen, foliar chlorophyll, and initial and total RuBPCase activities, thereby enhancing or preserving photosynthetic nitrogen‐use efficiency (NUE), defined as the maximum A observed per unit of leaf nitrogen, relative to the control and H2O‐stress treatments. In addition, N‐stress reduced photosynthetic water‐use efficiency (WUE), defined as the ratio of A to stomatal conductance to water vapour (g). The slope of A versus g increased with H2O‐stress. In addition, sunflower plants responded to H2O‐stress by accumulating foliar glucose and sucrose and by exhibiting diurnal leaf wilting, which presumably provided additional improvements in photosynthetic WUE through osmoregulation and reduction of midday radiation interception respectively. Photosynthetic NUE was decreased by H2O‐stress in that control levels of total nitrogen, foliar chlorophyll, and RuBPCase activities were maintained even after mean diurnal levels of A had fallen to less than 50% of the control level. We conclude that field‐grown sunflower manages a trade‐off between photosynthetic WUE and NUE, increasing use efficiency of the scarce resource while decreasing use efficiency of the abundant resource.

Experimental Studies of Ponderosa Pine. III. Differences in Photosynthesis, Stomatal Conductance, and Water-Use Efficiency Between Two Genetic Lines

As part of an intensive study of heritable differences among the progeny of Pinus ponderosa parents from two contrasting habitats (coastal vs. interior, continental), we examined the potential for differences in photosynthesis rate, stomatal conductance, and photosynthetic water-use efficiency. Plants from a cross between two coastal parents (ponderosa × ponderosa) exhibited lower photosynthetic water-use efficiencies, relative to plants from a coastal × interior cross (ponderosa × scopulorum). The lower water-use efficiencies in the ponderosa × ponderosa plants were evident as a lower ratio of external to intercellular CO2 concentrations and higher stomatal conductances at any given rate of photosynthesis. The ponderosa × scopulorum plants exhibited lower stomatal conductances over a range of leaf-to-air water vapor concentration differences, which was partially explained by lower stomatal densities. The ponderosa × scopulorum plants also exhibited lower maximum photosynthesis rates and lower needle nitrogen concentrations. Taken together, the results suggest that in adapting to drier habitats, P. ponderosa has acquired improved water-use efficiencies and lower transpiration rates, but at the expense of reduced maximum photosynthesis rates.

Photosynthetic Responses of C 3 and C 4 Wetland Species in a Tropical Swamp

(1) A tropical swamp near Nairobi, Kenya containing monotypic stands of the C4 sedges Cyperuspapyrus, Cyperus latifolius and the C3 species Typha domingensis was investigated. (2) Aerial biomass of C. papyrus was 2050 g m-2, C. latifolius 2170 g m-2 and T. domingensis 1350 g m-2. The LAI of C. latifolius (15 3) was three times that of T. domingensis. The Cyperus canopies intercepted more than 95% and the Typha canopy more than 85% of incident photosynthetically active radiation. (3) Photosynthetic light response curves were constructed for the photosynthetic bracteoles of C. papyrus and leaves of C. latifolius and T. domingensis. Maximum rates of net photosynthesis were 26 8 ,umol m-2 s-' for C. papyrus, 21 8 ,umol m-2 s-' for C. latifolius and 17 8 ,umol m-2 s-' for T. domingensis. The apparent quantum yield of net photosynthesis from incident light was higher in C. papyrus than the other two species. (4) The relationship between net photosynthesis and stomatal conductance was almost linear for all species but the C4 plants achieved their highest rates of net photosynthesis at lower conductances than the C3 T. domingensis. (5) Both photosynthetic water-use efficiency and nitrogen-use efficiency in the C4 sedges were more than double the values for T. domingensis. (6) The adaptive advantage of C4 photosynthesis in wetlands is discussed and it is suggested that the higher nitrogen-use efficiency of the C4 C. papyrus may explain why it is more tolerant of oligotrophic conditions than other (mainly) C3 species.

Photosynthetic acclimation and water-use efficiency of three species of understory herbaceous bamboo (Gramineae) in Panama.

To assess the role of photosynthetic acclimation in the response of tropical understory herbs to treefall light gaps, photosynthetic response curves were determined for three species of herbaceous bamboo growing in treatments of sun and shade at Barro Calorado Island, Panama. Increased maximum photosynthetic capacity did not always accompany higher ramet production in the sun treatment. Pharus latifolius reproduced abundantly in both treatments, and produced more ramets and developed higher maximum photosynthetic capacity under higher irradiance. Streptochaeta spicata also produced a high percentage of reproductive ramets in both treatments and produced more ramets in the sun, did not show any significant differences in photosynthetic parameters between treatments. Streptochaeta sodiroana did not change maximum photosynthetic capacity in the sun, and had higher photosynthetic efficiency and lower mortality in the shade. Stable carbon isotope composition of leaves indicated that all three species developed higher water-use efficiency under higher irradiance. Photosynthetic flexibility may contribute to the ability of P. latifolius to reproduce in treefall gaps, whereas S. spicata and S. sodiroana may maintain the ability to fix carbon efficiently in low irradiance even when growing or persisting in gaps.

Evolution of C 3 and C 4 Plants Along an Environmental Moisture Gradient: Patterns of Photosynthetic Differentiation in Hawaiian Scaevola and Euphorbia Species

The endemic Hawaiian species of Scaevola and Euphorbia grow in a wide variety of native habitats and exhibit a wide range of variation in photosynthetic responses. Light-saturated photosynthetic capacities range from 12.0 to 24.7 μmol CO2 m−-2 s−-1 in the Scaevola species and from 18.2 to 51.4 μmol CO2 m−-2 s−-1 in the Euphorbia species. Within each genus, differences in light-saturated photosynthetic capacity are paralleled by differences in mesophyll and leaf conductances to CO2. Within each habitat, the C4 Euphorbia species exhibits a significantly higher photosynthetic capacity and a significantly higher mesophyll conductance than the corresponding C3 Scaevola species. These differences are greatest in the dry scrub habitat and least in the wet forest habitat. One photosynthetic characteristic that exhibits little variation among the species within each genus, yet that exhibits a consistently large difference between the species within each habitat, is photosynthetic water-use efficiency. The C4 Euphorbia species possess water-use efficiencies that are 2–3½ times as high as those of the C3 Scaevola species, regardless of whether these species are native to very dry or very wet habitats. At present, the ecological significance of this large inherent difference in photosynthetic water-use efficiency is unknown. Indeed, it appears that neither photosynthetic pathway has imposed any major inherent constraints on the ability of the Scaevola and Euphorbia species to diversify into a wide variety of habitats.