The ‘peculiar shade nature’ of cyanic leaves: a perspective in anthocyanin research
Though cyanic leaves have a suite of morphoanatomical- and biochemical-related traits that closely resemble those usually displayed by green leaves growing in low light (true shade leaves), they also display features that are uncommon in shaded green leaves. First, while shade leaves typically saturate photosynthesis at much lower PPFD compared to sunny leaves, photosynthesis in cyanic leaves saturates at very similar or even higher PPFD than do the green counterparts (Fig. 3). This results simply on the ability of their epidermal anthocyanin filter in effectively absorbing photons otherwise available to chlorophylls, especially to Chlb: the actual quantum yield for CO2 assimilation is lower at moderate or greater at high light irradiance in red compared to green leaves (Tattini et al., 2014; 2017, Fig. 3). There is evidence indeed that red leaves have higher photosynthesis during the central hours of the day (Tattini et al., 2017), thus adding further support to the effective photoprotective functions of anthocyanins. Second, stomatal conductance in red leaves is higher or very similar compared to that of green leaves (Liakopoulos et al., 2006; Nikoforou et al., 2011; Tattini et al., 2017). This is unusual for true shade leaves, in which the excess of green compared to blue (and red) photons opposes the opening and the development of stomata (Chen, Xiao, Li, & Ni, 2012; Poorter et al., 2019). However, cyanic leaves sense both higher blue/green and red/green ratio compared to true shade leaves, and this may promote stomata opening (Smith et al., 2017). The matter deserves further investigation aimed at evaluating the relative contribution of blue, green and red signals (Merzlyak et al., 2008) perceived by cyanic leaves, to the downstream molecular events regulating the development and the aperture of stomata (Inoue & Kinoshita, 2017; Hiyama et al., 2017; Kang, Lian, Wang, Huang, & Yang, 2009).
The profound morpho-anatomical adjustments imposed by the epidermal anthocyanin shield makes complex the analysis of the ‘photosynthetic performance’ of cyanic vs acyanic leaves and, hence, of the photoprotective role of anthocyanins. For instance, Tattini et al. (2017) have shown that mesophyll conductance to CO2(gm) is substantially lower in purple than in green basil leaves growing in full sunlight (similar results have been observed in Acer platanoides , Fini unpublished data). This conforms to previous observations that gm is usually lower in shaded than in sun-exposed green leaves (Campany, Tjoelker, von Cammer, & Duursma, 2016; Peguero-Pina et al., 2016). We hypothesize that the anatomical adjustments imposed by the shade-avoidance response, such as the accumulation of chloroplasts toward the periclinal cell walls (Wada, 2016) may force cyanic leaves to ‘unusually high’ stomatal conductance (compared to green leaves) to counter large limitations to CO2 diffusion through the mesophyll (Tattini et al., 2017). As a result, the drawdown from actual (calculated from response curves of AN to changes in chloroplast CO2 concentration, AN/Cccurves) to apparent (calculated from response curves of AN to changes in intercellular CO2concentration, AN/Ci curves) carboxylation efficiency (Vc,max) was indeed markedly higher in red compared to green basil leaves (Tattini et al., 2017). The lower carboxylation efficiency of red compared to the green counterparts reported in previous studies (Carpenter, Keidel, Pihl, & Hughes, 2014; Nikoforou, Nikopoulos, & Manetas, 2011; Ranjan et al., 2014) may merit extensive re-evaluation. In turn, this poses serious methodological issues regarding the effective photoprotective potential of cyanic vs acyanic leaves. We recall that photoprotection, a qualitative parameter in its nature, closely relates to photoinhibition and, hence, suitably quantified by high light-induced depression of photosynthesis, rather than photosynthesis per se . However, in most studies the degree of photoinhibition in cyanic vs acyanic leaves has been estimated through light-induced declines in Fv/Fm(∆Fv/Fm) and/or ΦPSII(∆ΦPSII). Nonetheless, when leaves largely differ for both gs and gm, neither ∆Fv/Fm nor ∆ΦPSII are good proxies of photoinhibition: gs, and particularly gm are major constraints to photosynthesis, especially under high light irradiance. We conclude that quantifying the relative contribution of diffusional limitations to photosynthesis in cyanic vs acyanic leaves long-exposed to excessive light (when photoprotection really makes sense) will significantly improve our understanding on the actual photoprotective role(s) of anthocyanins.