Abstract
Photosynthetic carbon assimilation rates are highly dependent on environmental factors such as light availability and on metabolic limitations such as the demand for carbon by sink organs. The relative effects of light and sink demand on photosynthesis in perennial plants such as trees remain poorly characterized. The aim of the present study was therefore to characterize the relationships between light and fruit load on a range of leaf traits including photosynthesis, non-structural carbohydrate content, leaf structure, and nitrogen-related variables in fruiting ('ON') and non-fruiting ('OFF') 'Golden Delicious' apple trees. We show that crop status (at the tree scale) exerts a greater influence over leaf traits than the local light environment or the local fruit load. High rates of photosynthesis were observed in the ON trees. This was correlated with a high leaf nitrogen content. In contrast, little spatial variability in photosynthesis rates was observed in the OFF trees. The lack of variation in photosynthesis rates was associated with high leaf non-structural carbohydrate content at the tree level. Taken together, these results suggest that low carbon demand leads to feedback limitation on photosynthesis resulting in a low level of within-tree variability. These findings provide new insights into carbon and nitrogen allocations within trees, which are heavily dependent on carbon demand.
Highlights
Within-plant microclimate gradients, such as those due to incoming light, air temperature, humidity, and wind, locally affect organ function and structure
Leaves respond to multiple constraints in various ways; these responses were gathered into a framework built over a large range of species, the so-called “leaf economics spectrum” (Wright et al, 2004; Niinements et al, 2015)
This study aims to determine how the spatial variability resulting from local acclimation to intercepted light could be modulated by source-sink relationships
Summary
Within-plant microclimate gradients, such as those due to incoming light, air temperature, humidity, and wind, locally affect organ function and structure. The authors have linked these photosynthetic profiles to available light and traits involved in light harvesting and photosynthetic capacities, such as nitrogen (N) content per r unit area (Hollinger, 1996; Makela et al, 2008; Mc Murtrie and Dewar, 2011; Niinemets et c al., 2004; Thornley, 2004) These relationships have led to “the optimization hypothesis”. S Following this hypothesis, the C and N economy is oriented towards investment of leaf N and u leaf biomass for maximum light interception and C return on protein invested (Mooney, n 1979; Bloom, 1985) This optimization has been considered in models that account for a within-tree N distribution and leaf gas exchanges that depend on incoming radiation at the individual scale (e.g., Le Roux et al, 2001, Prieto et al, 2012; Buckley et al, 2013) and in
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