Journal Article

Physiological responses of three deciduous conifers (<i>Metasequoia glyptostroboides</i>, <i>Taxodium distichum</i> and <i>Larix laricina</i>) to continuous light: adaptive implications for the early Tertiary polar summer

M. Alejandra Equiza, Michael E. Day and Richard Jagels

in Tree Physiology

Volume 26, issue 3, pages 353-364
Published in print March 2006 | ISSN: 0829-318X
Published online March 2006 | e-ISSN: 1758-4469 | DOI:
Physiological responses of three deciduous conifers (Metasequoia glyptostroboides, Taxodium distichum and Larix laricina) to continuous light: adaptive implications for the early Tertiary polar summer

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Polar regions were covered with extensive forests during the Cretaceous and early Tertiary, and supported trees comparable in size and productivity to those of present-day temperate forests. With a winter of total or near darkness and a summer of continuous, low-angle illumination, these temperate, high-latitude forests were characterized by a light regime without a contemporary counterpart. Although maximum irradiances were much lower than at mid-latitudes, the 24-h photoperiod provided similar integrated light flux. Taxodium, Larix and Metasequoia, three genera of deciduous conifers that occurred in paleoarctic wet forests, have extant, closely related descendents. However, the contemporary relative abundance of these genera differs greatly from that in the paleoarctic. To provide insight into attributes that favor competitive success in a continuous-light environment, we subjected saplings of these genera to a natural photoperiod or a 24-h photoperiod and measured gas exchange, chlorophyll fluorescence, non-structural carbohydrate concentrations, biomass production and carbon allocation.

Exposure to continuous light significantly decreased photosynthetic capacity and quantum efficiency of photosystem II in Taxodium and Larix, but had minimal influence in Metasequoia. In midsummer, foliar starch concentration substantially increased in both Taxodium and Larix saplings grown in continuous light, which may have contributed to end-product down-regulation of photosynthetic capacity. In contrast, Metasequoia allocated photosynthate to continuous production of new foliar biomass. This difference in carbon allocation may have provided Metasequoia with a two fold advantage in the paleoarctic by minimizing depression of photosynthetic capacity and increasing photosynthetic surface.

Keywords: allocation; carbon balance; leaf area; light-inhibition; paleoecology; photosynthetic end products

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Subjects: Plant Sciences and Forestry

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