Measures of Light in Studies on Light-Driven Plant Plasticity in Artificial Environments

Abstract

Within-canopy variation in light results in profound canopy profiles in foliage structural, chemical, and physiological traits. Studies on within-canopy variations in key foliage traits are often conducted in artificial environments, including growth chambers with only artificial light, and greenhouses with and without supplemental light. Canopy patterns in these systems are considered to be representative to outdoor conditions, but in experiments with artificial and supplemental lighting, the intensity of artificial light strongly deceases with the distance from the light source, and natural light intensity in greenhouses is less than outdoors due to limited transmittance of enclosure walls. The implications of such changes in radiation conditions on canopy patterns of foliage traits have not yet been analyzed. We developed model-based methods for retrospective estimation of distance vs. light intensity relationships, for separation of the share of artificial and natural light in experiments with combined light and for estimation of average enclosure transmittance, and estimated daily integrated light at the time of sampling (Qint,C), at foliage formation (Qint,G), and during foliage lifetime (Qint,av). The implications of artificial light environments were analyzed for altogether 25 studies providing information on within-canopy gradients of key foliage traits for 70 species × treatment combinations. Across the studies with artificial light, Qint,G for leaves formed at different heights in the canopy varied from 1.8- to 6.4-fold due to changing the distance between light source and growing plants. In experiments with combined lighting, the share of natural light at the top of the plants varied threefold, and the share of natural light strongly increased with increasing depth in the canopy. Foliage nitrogen content was most strongly associated with Qint,G, but photosynthetic capacity with Qint,C, emphasizing the importance of explicit description of light environment during foliage lifetime. The reported and estimated transmittances of enclosures varied between 0.27 and 0.85, and lack of consideration of the reduction of light compared with outdoor conditions resulted in major underestimation of foliage plasticity to light. The study emphasizes that plant trait vs. light relationships in artificial systems are not directly comparable to natural environments unless modifications in lighting conditions in artificial environments are taken into account.

Global leaf trait estimates biased due to plasticity in the shade

“…Q int was averaged for the 50 days after the start of leaf development, or for the actual number of days since the start of leaf development for leaves younger than 50 days or for leaves with an average life span of less than 50 days. This approach is motivated by reports that foliage trait acclimation to integrated light stabilizes about 30-60 days after leaf formation in both woody and herbaceous canopies 21,22 .…”

“…Leaf trait acclimation to integrated light has been reported to stabilize about 30-60 days after leaf formation in both woody and herbaceous canopies 21,22 , but age-dependent modifications are possible for some plant growth forms 23 and such modifications can further alter the correlative relationships within the leaf economics spectrum 24 . In particular, in evergreen woody species, M a and N a plasticity has been reported to decrease with increasing leaf age, with limited changes in A a plasticity 23 .…”

“…Although trait databases on occasion record sampled leaves as originating from either full-sun or shaded conditions, such dichotomous categories of 'sun' and (in particular) 'shade' are highly ambiguous. In reality, leaves experience strong light gradients within a canopy 3 , across gap-understory continua 4,31 and with changes to light availability during their development 22 . The extent of lightdriven trait plasticity necessitates a more detailed reporting of the daily average leaf-incident integrated photosynthetically active quantum flux density during leaf development.…”

Leaf age dependent changes in within-canopy variation in leaf functional traits: a meta-analysis

“…Given the evidence that within-canopy acclimation of leaf traits is driven by total integrated light rather than by relative light or instantaneous light (Chabot et al 1979; Niinemets and Keenan 2012), this analysis is based on integrated quantum flux density ( Q int ) that was estimated for each study in a consistent manner as explained in detail in Niinemets et al (2015). In that study, Q int was defined as a standardized estimate of light availability that corresponds to daily integrated quantum flux density averaged for 50 days after the start of foliage development, or to daily integrated quantum flux density averaged for the actual number of days since the start of foliage development when leaves were either younger than 50 days or when the average leaf life-span for the given species was less than 50 days ( Q int,b50 ; (Niinemets et al 2015).…”

Thermal acclimation of leaf respiration consistent with optimal plant function

“…Evergreen broadleaf trees show higher than predicted thermal 354 sensitivities of both R d and V cmax , but the fitted slope of their ratio versus growth 355 temperature (-1.8% ± 0.4%) is identical with our predicted value (-1.8%). 356Microclimatic acclimation(Niinemets & Keenan 2012), the likelihood that many 357 measured leaves were at least partially shaded(Keenan & Niinemets 2017), and 358 genetic variations involving different plant strategies may all have contributed to the 359 within-site variations in R d and V cmax reflected in the vertical scatter of points inFig. 2.…”