Photosynthetic costs and benefits of adaxial/abaxial leaf reddening in understory plants

Background/Question/Methods

Understory plants in temperate and tropical forests often exhibit red/purple coloration of their lower (abaxial) leaf surfaces. The functional significance of this pigment pattern in leaves (if any) is not fully understood. Here we test the hypothesis that abaxial anthocyanin pigments protect leaves from photoinhibition through light attenuation during periodic bursts of high sunlight in the forest understory, without interfering with light capture and photosynthesis during shade conditions.  Experiments were conducted using Colocasia esculenta ‘Mojito’, a cultivar exhibiting abaxial (lower) and adaxial (upper) anthocyanin variegation within individual leaves.  Tissues with the following mesophyll pigment patterns were compared: green upper, green lower (GG), green upper, red lower (GR), red upper, green lower (RG), and red upper, red lower (RR).  Light response of photosynthesis under red/blue LED versus ambient sunlight was used to quantify optical effects of anthocyanin on attenuation of green light under a range of light conditions for each colored tissue combination. Photoinhibition of photosynthesis (quantified via chlorophyll fluorescence) was also measured before, during, and after shade-grown plants were exposed to 3.5 hours of full sunlight (PPFDs >1500 mol μm^-2 s^-1) to compare relative high light stress in a simulated canopy gap.

Results/Conclusions

Consistent with our hypothesis, highest photosynthesis under low PPFDs were observed in GG tissues and GR tissues (which did not significantly differ), and lowest rates in RG and RR.  During exposure to full sunlight, photoinhibition was greatest in GG, lowest in RR and RG, and intermediate in GR.  Photoinhibitory effects were most dramatic after two hours of high light exposure.  Anthocyanic tissues also exhibited symptoms of shade adaptation relative to acyanic tissues in the order (from least to greatest shade acclimation): GG, GR, RG, RR.  We conclude that abaxial anthocyanin pigmentation in understory plants represents a “best of both worlds” strategy, whereby the photoprotective benifits of anthocaynins are utilized in high-intensity sun patches, without significantly interfering with light absorption and carbon gain during low-light conditions.