Canopy conundrums: building on the Biosphere 2 experience to scale measurements of inner and outer canopy photoprotection from the leaf to the landscape

Abstract

Abstract. Recognising that plant leaves are the fundamental productive units of terrestrial vegetation and the complexity of different environments in which they must function, this review considers a few of the ways in which these functions may be measured and potentially scaled to the canopy. Although canopy photosynthetic productivity is clearly the sum of all leaves in the canopy, we focus on the quest for 'economical insights' from measurements that might facilitate integration of leaf photosynthetic activities into canopy performance, to better inform modelling based on the 'insights of economics'. It is focussed on the reversible downregulation of photosynthetic efficiency in response to light environment and stress and summarises various xanthophyll-independent and dependent forms of photoprotection within the inner and outer canopy of woody plants. Two main themes are developed. First, we review experiments showing the retention of leaves that grow old in the shade may involve more than the 'payback times' required to recover the costs of their construction and maintenance. In some cases at least, retention of these leaves may reflect selection for distinctive properties that contribute to canopy photosynthesis through utilisation of sun flecks or provide 'back up' capacity following damage to the outer canopy. Second, we report experiments offering hope that remote sensing of photosynthetic properties in the outer canopy (using chlorophyll fluorescence and spectral reflectance technologies) may overcome problems of access and provide integrated measurements of these properties in the canopy as a whole. Finding appropriate tools to scale photosynthesis from the leaf to the landscape still presents a challenge but this synthesis identifies some measurements and criteria in the laboratory and the field that improve our understanding of inner and outer canopy processes.

From ecophysiology to phenomics: some implications of photoprotection and shade–sun acclimation in situ for dynamics of thylakoids in vitro

“…However, the distinctive, and therefore probably important, features of NPQ DL lie in its rapid induction upon illumination (albeit less than NPQ DLAZ ) which sustains an enhanced NPQ capacity compared with NPQ DpH (figure 2), combined with its fast dark relaxation, which parallels NPQ DpH and is more rapid than NPQ DLAZ (table 2). Pigment composition and photosynthetic parameters were examined in leaves of different ages in a small canopy that developed on a single cutting in full sunlight in the greenhouse [20].…”

“…Designation of different forms of NPQ on the basis of kinetic responses in vivo follows the subscript conventions proposed previously [20,25]. Measurements of NPQ in pre-dawn assays, or from aluminium-foil shaded areas of exposed leaves, in the absence of deepoxidation of lutein epoxide (Lx) or V were considered to represent DpH-dependent processes (or qE) [26] and were designated NPQ DpH .…”

“…Retention of old leaves in deeply shaded canopies presents but one special case of sun-shade acclimation. Although discussion of these phenomena is usually focused on cost-benefit analyses and structural considerations, we have argued that there may be additional distinctive attributes of photosynthesis in these leaves that involve rather more than repayment of their construction costs [20]. For example, the distinctive pigment composition and de-epoxidation/ epoxidation kinetics in two xanthophyll cycles in woody plants of Mediterranean and tropical forests have attracted attention in this context [21][22][23][24].…”

Do Daily and Seasonal Trends in Leaf Solar Induced Fluorescence Reflect Changes in Photosynthesis, Growth or Light Exposure?

“…Leaf-level photosynthetic measurements have traditionally been conducted with active chlorophyll fluorescence-based approaches, such as the pulse amplitude modulation (PAM) method. While the PAM approach is a quick and well established method [3], the saturating light pulse makes it unfeasible for measurements at the large spatial scales required for precision agriculture and plant phenotyping [4]. In recent years, the focus has shifted to the detection of solar induced fluorescence (SIF), which has shown strong potential as a photosynthesis indicator across spatial scales ranging from the leaf and canopy [5][6][7][8] to global scales [9][10][11][12].…”

Decreased Photochemical Efficiency of Photosystem II following Sunlight Exposure of Shade-Grown Leaves of Avocado: Because of, or in Spite of, Two Kinetically Distinct Xanthophyll Cycles?

“…Because we focus on the relationships between xanthophyll pigment composition and photoprotection in this paper, we will use the subscript designation of different components of NPQ introduced independently from several laboratories (Nilkens et al, 2010;Förster et al, 2011;Jahns and Holzwarth, 2012;Nichol et al, 2012).…”