Summary: By means of meta‐analyses we determined how 70 traits related to plant anatomy, morphology, chemistry, physiology, growth and reproduction are affected by daily light integral (DLI; mol photons m⁻² d⁻¹). A large database including 500 experiments with 760 plant species enabled us to determine generalized dose–response curves. Many traits increase with DLI in a saturating fashion. Some showed a more than 10‐fold increase over the DLI range of 1–50 mol m⁻² d⁻¹, such as the number of seeds produced per plant and the actual rate of photosynthesis. Strong decreases with DLI (up to three‐fold) were observed for leaf area ratio and leaf payback time. Plasticity differences among species groups were generally small compared with the overall responses to DLI. However, for a number of traits, including photosynthetic capacity and realized growth, we found woody and shade‐tolerant species to have lower plasticity. We further conclude that the direction and degree of trait changes adheres with responses to plant density and to vertical light gradients within plant canopies. This synthesis provides a strong quantitative basis for understanding plant acclimation to light, from molecular to whole plant responses, but also identifies the variables that currently form weak spots in our knowledge, such as respiration and reproductive characteristics.

Plant population density is an important variable in agronomy and forestry and offers an experimental way to better understand plant–plant competition. We made a meta‐analysis of responses of even‐aged mono‐specific stands to population density by quantifying for 3 stand and 33 individual plant variables in 334 experiments how much both plant biomass and phenotypic traits change with a doubling in density. Increasing density increases standing crop per area, but decreases the mean size of its individuals, mostly through reduced tillering and branching. Among the phenotypic traits, stem diameter is negatively affected, but plant height remains remarkably similar, partly due to an increased stem length‐to‐mass ratio and partly by increased allocation to stems. The reduction in biomass is caused by a lower photosynthetic rate, mainly due to shading of part of the foliage. Total seed mass per plant is also strongly reduced, marginally by lower mass per seed, but mainly because of lower seed numbers. Plants generally have fewer shoot‐born roots, but their overall rooting depth seems hardly affected. The phenotypic plasticity responses to high densities correlate strongly with those to low light, and less with those to low nutrients, suggesting that at high density, shading affects plants more than nutrient depletion.

Summary: Generalised dose–response curves are essential to understand how plants acclimate to atmospheric CO₂. We carried out a meta‐analysis of 630 experiments in which C₃ plants were experimentally grown at different [CO₂] under relatively benign conditions, and derived dose–response curves for 85 phenotypic traits. These curves were characterised by form, plasticity, consistency and reliability. Considered over a range of 200–1200 µmol mol⁻¹ CO₂, some traits more than doubled (e.g. area‐based photosynthesis; intrinsic water‐use efficiency), whereas others more than halved (area‐based transpiration). At current atmospheric [CO₂], 64% of the total stimulation in biomass over the 200–1200 µmol mol⁻¹ range has already been realised. We also mapped the trait responses of plants to [CO₂] against those we have quantified before for light intensity. For most traits, CO₂ and light responses were of similar direction. However, some traits (such as reproductive effort) only responded to light, others (such as plant height) only to [CO₂], and some traits (such as area‐based transpiration) responded in opposite directions. This synthesis provides a comprehensive picture of plant responses to [CO₂] at different integration levels and offers the quantitative dose–response curves that can be used to improve global change simulation models.

The mechanism of response of plants to vertical light intensity gradients in leaf canopies was investigated. Since shaded leaves transpire less than leaves in high light, it was hypothesized that cytokinins (CKs) carried by mass transport in the transpiration stream would be distributed over the leaf area of partially shaded plants parallel to the gradient in light intensity. It was also hypothesized that this causes the distribution of leaf growth, leaf N and photosynthetic capacity, and possibly chloroplast acclimation as observed in plants growing in leaf canopies. In a field experiment, the distribution of Ca, N and CKs in a bean leaf canopy of a dense and an open stand supported the concept of a role for CKs in the response of N allocation to the light gradient when a decreasing sensitivity for CKs with increasing leaf age is assumed. Both shading of one leaf of the pair of primary bean leaves and independent reduction of its transpiration rate in a growth cabinet experiment caused lower dry mass, N and Ca per unit leaf area in comparison to the opposite not treated leaf. Shading caused a parallel reduction in CK concentration, which supports the hypothesis, but independent reduction of transpiration rate failed to do the same. Application of benzylaminopurine (BA) counteracted the reduction caused by shade of leaf N, photosynthetic capacity and leaf area growth. The experiments show an important role for the transpiration stream in the response of plants to light gradients. Evidence is presented here that CKs carried in the transpiration stream may be important mediators for the acclimation of plants to leaf canopy density.

High midday temperatures can depress net photosynthesis. We investigated possible mechanisms underlying this phenomenon in leaves of Eperua grandiflora (Aubl.) Benth. saplings. This tropical tree establishes in small gaps in the rainforest canopy where direct sunlight can raise midday temperatures markedly. We simulated this microclimate in a growth chamber by varying air temperature between 28 and 38 °C at constant vapor pressure. A decrease in stomatal conductance in response to an increase in leaf-to-air vapor pressure difference (ΔW) caused by an increase in leaf temperature (T_leaf) was the principal reason for the decrease in net photosynthesis between 28 and 33 °C. Net photosynthesis decreased further between 33 and 38 °C. Direct effects on mesophyll functioning and indirect effects through ΔW were of similar magnitude in this temperature range. Mitochondrial respiration during photosynthesis was insensitive to T_leaf over the investigated temperature range; it thus did not contribute to midday depression of net photosynthesis. Internal conductance for CO₂ diffusion in the leaf, estimated by combined gas exchange and chlorophyll fluorescence measurements, decreased slightly with increasing T_leaf. However, the decrease in photosynthetic rate with increasing T_leaf was larger and thus the difference in CO₂ partial pressure between the substomatal cavity and chloroplast was smaller, leading to the conclusion that this factor was not causally involved in midday depression. Carboxylation capacity inferred from the CO₂ response of photosynthesis increased between 28 and 33 °C, but remained unchanged between 33 and 38 °C. Increased oxygenation of ribulose-1,5-bisphosphate relative to its carboxylation and the concomitant increase in photorespiration with increasing T_leaf were thus not compensated by an increase in carboxylation capacity over the higher temperature range. This was the principal reason for the negative effect of high midday temperatures on mesophyll functioning.

Plants growing in dense canopies are exposed to vertical light gradients and show photosynthetic acclimation at the whole-plant level, resulting in efficient photosynthetic carbon gain. We studied the role of cytokinins transported through the transpiration stream as one of probably multiple signals for photosynthetic acclimation to light gradients using both tobacco (Nicotiana tabacum) and Arabidopsis (Arabidopsis thaliana). We show that substantial variation in leaf transpiration parallels the light gradient in tobacco canopies and experimental reduction of the transpiration rate of a leaf, independent of light, is sufficient to reduce photosynthetic capacity in both species, as well as transcript levels of the small subunit of Rubisco (rbcS) gene in Arabidopsis. Mass spectrometric analysis of xylem sap collected from intact, transpiring tobacco plants revealed that shaded leaves import less cytokinin than leaves exposed to high light. In Arabidopsis, reduced transpiration rate of a leaf in the light is associated with lower cytokinin concentrations, including the bioactive trans-zeatin and trans-zeatin riboside, as well as reduced expression of the cytokinin-responsive genes ARR7 and ARR16. External application of cytokinin to shaded leaves rescued multiple shade effects, including rbcS transcript levels in both species, as did locally induced cytokinin overproduction in transgenic tobacco plants. From these data, we conclude that light gradients over the foliage of a plant result in reduced cytokinin activity in shaded leaves as a consequence of reduced import through the xylem and that cytokinin is involved in the regulation of whole-plant photosynthetic acclimation to light gradients in canopies.