Elizabeth A. Webb

The effects of phenotypic plasticity on photosynthetic performance in winter rye, winter wheat and Brassica napus

The contributions of phenotypic plasticity to photosynthetic performance in winter (cv Musketeer, cv Norstar) and spring (cv SR4A, cv Katepwa) rye (Secale cereale) and wheat (Triticum aestivum) cultivars grown at either 20°C [non-acclimated (NA)] or 5°C [cold acclimated (CA)] were assessed. The 22-40% increase in light-saturated rates of CO₂ assimilation in CA vs NA winter cereals were accounted for by phenotypic plasticity as indicated by the dwarf phenotype and increased specific leaf weight. However, phenotypic plasticity could not account for (1) the differential temperature sensitivity of CO₂ assimilation and photosynthetic electron transport, (2) the increased efficiency and light-saturated rates of photosynthetic electron transport or (3) the decreased light sensitivity of excitation pressure and non-photochemical quenching between NA and NA winter cultivars. Cold acclimation decreased photosynthetic performance of spring relative to winter cultivars. However, the differences in photosynthetic performances between CA winter and spring cultivars were dependent upon the basis on which photosynthetic performance was expressed. Overexpression of BNCBF17 in Brassica napus generally decreased the low temperature sensitivity (Q₁₀) of CO₂ assimilation and photosynthetic electron transport even though the latter had not been exposed to low temperature. Photosynthetic performance in wild type compared to the BNCBF17-overexpressing transgenic B. napus indicated that CBFs/DREBs regulate not only freezing tolerance but also govern plant architecture, leaf anatomy and photosynthetic performance. The apparent positive and negative effects of cold acclimation on photosynthetic performance are discussed in terms of the apparent costs and benefits of phenotypic plasticity, winter survival and reproductive fitness.

The oxygen isotopic compositions of silica phytoliths and plant water in grasses: Implications for the study of paleoclimate

Abstract Information about climatic conditions during plant growth is preserved by the oxygen-isotope composition of biogenic silica (phytoliths) deposited in grasses. The oxygen-isotope composition of phytolith silica is dependent on soil-water δ 18 O values, relative humidity and evapotranspiration, and temperature during plant growth. Phytolith and plant-water δ 18 O values for C3 ( A. breviligulata ) and C4 ( C. longifolia ) grasses from natural and greenhouse sites in southwestern Ontario were used to compare the isotopic fractionation between biogenic silica and water in various parts of these living plants. For non or weakly transpiring tissues (rhizomes, stems, sheaths) in both grass species, the Δ 18 O silica-plant water remained constant at ∼34‰, and the δ 18 O and δD values of plant water collected from pre-dawn and mid-day samplings showed little variation. These plant waters were only slightly enriched in 18 O and D relative to water provided to the grasses. Isotopic temperatures calculated from the silica and plant-water isotopic data matched measured growing temperatures for the region. By comparison, the upper leaf water was extremely enriched in oxygen-18 and deuterium at maximum rates of transpiration relative to water from non-transpiring tissues, as were the calculated, steady-state values for leaf-water δ 18 O and δD. Silica produced in the transpiring tissues (leaf, inflorescence) has higher δ 18 O values than silica from non-transpiring tissues, but the enrichment is modest compared to upper leaf water under mid-day conditions. Leaf phytoliths have formed from plant water typical of average conditions in the lower leaf, where the extreme 18 O-enrichment is not encountered. C. longifolia was also collected from Alberta and Nebraska, where growing conditions are different from southwestern Ontario. Phytoliths at all three sites have a similar pattern of δ 18 O values within the plants, but the isotopic separation between leaf and stem silica increases from 4 to 8‰ as average relative humidity decreases. The difference between actual growing temperature and that calculated using measured δ 18 O values for stem silica and local meteoric water became progressively larger as relative humidity decreased, likely because of evaporative 18 O-enrichment of soil water. Such effects are most pronounced in arid environments and pertinent in grasslands where much of the active rooting zone can be situated at the shallower depths most affected by the 18 O-enrichment of soil water.

Climatic influences on the oxygen isotopic composition of biogenic silica in prairie grass

Abstract Samples of Calamovilfa longifolia were collected from across the North American prairies to investigate the relationship between the oxygen-isotope composition of biogenic silica (phytoliths) deposited in this grass and relative humidity, temperature, and the oxygen-18 enrichment of soil water relative to local precipitation. The δ 18 O values of silica in nontranspiring tissues were controlled by soil-water composition and temperature, whereas the oxygen-18 content of silica formed in leaf and inflorescence tissues was enriched further by transpiration. Accurate calculation of growing temperature was possible only when the oxygen-isotope compositions of both stem silica and soil water were known. However, the oxygen-isotope values of stem phytoliths can be used to calculate the variation in the isotopic composition of soil water across a North American temperature gradient. As plant organic matter decays and phytoliths are transferred to the soil, the temperature and soil-water signals carried by the oxygen-isotope composition of silica from nontranspiring tissues can be masked by the oxygen-18 enrichment of phytoliths from transpiring tissues. However, the overall oxygen-isotope composition of a soil-phytolith assemblage can be related to temperature using an empirical relationship based on temperature and the difference between soil-phytolith and estimated soil-water oxygen-isotope compositions.

The relationship between phytolith- and plant-water δ18O values in grasses

Abstract Information regarding climatic conditions during plant growth is preserved by the oxygen-isotope composition of biogenic silica (phytoliths) deposited in grasses. The O-isotope compositions of phytoliths and the plant water from which they precipitate are dependent on soil-water δ 18 O values, relative humidity, evapotranspiration rates, and temperature. Plant water and phytoliths from two grass species, Ammophila breviligulata (C 3 ) and Calamovilfa longifolia (C 4 ) at Pinery Provincial Park in southwestern Ontario, Canada, were examined to determine the variability in their δ 18 O values. Stem water was unfractionated from soil-water in oxygen isotopic composition and the δ 18 O values of stem silica provide a good proxy for the soil water available to roots during the growing season. Greater spatial and temporal variation in the δ 18 O values of water in the top 5 cm of the soil, and their enhanced sensitivity to evaporative 18 O enrichment, are reflected in the generally higher δ 18 O values of water in the shallow roots and rhizomes of these grasses. Water within the sheath and lower and upper leaf tissues experiences continual evaporation, becoming progressively enriched in 18 O as it moves towards the tip of the leaf. However, the water from which leaf silica precipitates has not acquired the extreme 18 O enrichment predicted using steady-state models, or measured for midday or average daily leaf water. Possible explanations for this behaviour include preferential deposition of silica at night; the existence of a secluded water fraction within the leaf, which experiences smaller diurnal variations in isotopic composition than leaf water at sites of evaporation; kinetic isotope effects during rapid precipitation of leaf silica; and incomplete exchange between the oxygen in the silicic acid and the leaf water.