Environmental change attracts particular attention by biologists concerned with the performance of biological systems under stress. To investigate these, dose–response relationships should be clarified. It was previously assumed that the fundamental nature of biological dose–responses follows a linear model, either with no threshold or with a threshold below which no effects are expected to occur in biological endpoints. However, substantial literature, including widespread documentation in plants, has revealed that hormesis commonly occurs. Hormesis is highly generalized and can be utilized as a quantitative measure of biological plasticity. Conditioning induced by adaptive responses also occurs in the framework of hormesis and is of particular importance to environmental change biology with regards to evolutionary adaptations. This paper presents additional evidence for hormetic dose responses induced by temperature in plants. The current understanding on hormesis provides a perspective for next generation environmental change research. Hormesis should have a central role in environmental change biology of vegetation.

As photoinhibition primarily reduces the photosynthetic light use efficiency at low light, sunfleck-induced photoinhibition might result in a fatal loss of carbon gain in the shade leaves within a canopy with barely positive carbon balance. We hypothesized that shade leaves at the lower canopy might retain a certain amount of leaf nitrogen (N_L) to maintain energy consumption via electron transport, which contributes to circumventing photoinhibition during sunflecks to keep efficient utilization of low light during the rest period of daytime. We investigated excess energy production, a potential measure of susceptibility to photoinhibition, as a function of N_L distribution within a Japanese oak canopy. Optimal N_L distribution, which maximizes canopy carbon gain, may lead to a higher risk of photoinhibition in shade leaves during sunflecks. Conversely, uniform N_L distribution would cause a higher risk of photoinhibition in sun leaves under the direct sunlight. Actual N_L distribution equalized the risk of photoinhibition throughout the canopy indicated by the constant excess energy production at the highest light intensities that the leaves received. Such a homeostatic adjustment as a whole canopy concerning photoinhibition would be a key factor to explain why actual N_L distribution does not maximize canopy carbon gain.

Accumulated evidence show that reactive species play a dual role in plants as well, with likely biphasic responses. This suggests that photoprotective mechanisms may also show similar patterns because they are highly related to reactive species. The nonphotochemical quenching (NPQ) is an index of heat dissipation of excitation energy in the antenna system. We present here preliminary evidence from some published studies showing significant biphasic response of NPQ to increasing doses of stress, with U-shaped or inverted U-shaped dose–response relationships, typical of hormesis. This evidence provides a remarkable perspective for designing novel studies where the fate of light energy will be seen through the lens of hormesis.

A clear-cutting of canopy trees during winter often causes severe foliar damage during the following spring in forest floor seedlings of Abies sachalinensis, a typical shade-tolerant evergreen coniferous species. The maximum photochemical efficiency of photosystem II after an overnight dark adaptation showed a temporary decrease immediately before budbreak in 1-year-old shoots of A. sachalinensis seedlings grown under full sunlight in a nursery, suggesting “springtime photoinhibition” related to the phenology of evergreen coniferous species. In the field, a greater rate of canopy tree cutting during winter was associated with more severe photoinhibition in the following spring, immediately before budbreak, which subsequently resulted in a reduction in carbon gain in 1-year-old shoots, and consequently suppressed the growth of current-year shoots. Although photoinhibition under low temperature is a well-known factor to determine the survival rate of tree seedlings during winter in cool regions, the present study additionally proposes that the temporary increase in the susceptibility to photoinhibition in springtime i.e. “springtime photoinhibition” would be a constraint for the regeneration of coniferous seedlings especially when the canopy trees are removed during winter.

The density and architecture of leaf veins determine the network and efficiency of water transport within laminae and resultant leaf gas exchange and vary widely among plant species. Leaf hydraulic conductance ( K _leaf ) can be regulated by vein architecture in conjunction with the water channel protein aquaporin. However, our understanding of how leaf veins and aquaporins affect leaf hydraulics and stomatal conductance ( g _s ) remains poor. By inducing blockage of the major veins and inhibition of aquaporin activity using HgCl ₂ , we examined the effects of major veins and aquaporins on K _leaf and g _s in species with different venation types. A vine species, with thick first-order veins and low vein density, displayed a rapidly declined g _s with high leaf water potential in response to vein blockage and a greatly reduced K _leaf and g _s in response to aquaporin inhibition, suggesting that leaf aquaporins are involved in isohydric/anisohydric stomatal behaviour. Across species, the decline in K _leaf and g _s due to aquaporin inhibition increased linearly with decreasing major vein density, possibly indicating that a trade-off function between vein architecture (apoplastic pathway) and aquaporin activity (cell-to-cell pathway) affects leaf hydraulics.

We investigated seasonal patterns of water relations in current-year leaves of three evergreen broad-leaved trees (Ilex pedunculosa Miq., Ligustrum japonicum Thunb., and Eurya japonica Thunb.) with delayed greening in a warm-temperate forest in Japan. We used the pressure-volume method to: (1) assess the extent to which seasonal variation in leaf water relations is attributable to leaf development processes in delayed greening leaves versus seasonal variation in environmental variables; and (2) investigate variation in leaf water relations during the transition from the sapling to the adult tree stage. Leaf mass per unit leaf area was generally lowest just after completion of leaf expansion in May (late spring), and increased gradually throughout the year. Osmotic potential at full turgor (Ψ_o^ft) and leaf water potential at the turgor loss point (Ψ_w^tlp) were highest in May, and lowest in midwinter in all species. In response to decreasing air temperature, Ψ_o^ft dropped at the rate of 0.037 MPa °C⁻¹. Dry-mass-based water content of leaves and the symplastic water fraction of total leaf water content gradually decreased throughout the year in all species. These results indicate that reductions in the symplastic water fraction during leaf development contributed to the passive concentration of solutes in cells and the resulting drop in winter Ψ_o^ft. The ratio of solutes to water volume increased in winter in current-year leaves of L. japonicum and E. japonica, indicating that osmotic adjustment (active accumulation of solutes) also contributed to the drop in winter in Ψ_o^ft. Bulk modulus of elasticity in cell walls fluctuated seasonally, but no general trend was found across species. Over the growing season, Ψ_o^ft and Ψ_w^tlp were lower in adult trees than in saplings especially in the case of I. pedunculosa, suggesting that adult-tree leaves are more drought and cold tolerant than sapling leaves. The ontogenetic increase in the stress resistance of I. pedunculosa may be related to its characteristic life form because I. pedunculosa grows taller than the other species studied.