Importance of the green color, absorption gradient, and spectral absorption of chloroplasts for the radiative energy balance of leaves

Atsushi Kume

doi:10.1007/s10265-017-0910-z

Abstract

Terrestrial green plants absorb photosynthetically active radiation (PAR; 400–700 nm) but do not absorb photons evenly across the PAR waveband. The spectral absorbance of photosystems and chloroplasts is lowest for green light, which occurs within the highest irradiance waveband of direct solar radiation. We demonstrate a close relationship between this phenomenon and the safe and efficient utilization of direct solar radiation in simple biophysiological models. The effects of spectral absorptance on the photon and irradiance absorption processes are evaluated using the spectra of direct and diffuse solar radiation. The radiation absorption of a leaf arises as a consequence of the absorption of chloroplasts. The photon absorption of chloroplasts is strongly dependent on the distribution of pigment concentrations and their absorbance spectra. While chloroplast movements in response to light are important mechanisms controlling PAR absorption, they are not effective for green light because chloroplasts have the lowest spectral absorptance in the waveband. With the development of palisade tissue, the incident photons per total palisade cell surface area and the absorbed photons per chloroplast decrease. The spectral absorbance of carotenoids is effective in eliminating shortwave PAR (<520 nm), which contains much of the surplus energy that is not used for photosynthesis and is dissipated as heat. The PAR absorptance of a whole leaf shows no substantial difference based on the spectra of direct or diffuse solar radiation. However, most of the near infrared radiation is unabsorbed and heat stress is greatly reduced. The incident solar radiation is too strong to be utilized for photosynthesis under the current CO2 concentration in the terrestrial environment. Therefore, the photon absorption of a whole leaf is efficiently regulated by photosynthetic pigments with low spectral absorptance in the highest irradiance waveband and through a combination of pigment density distribution and leaf anatomical structures.

Highlights

Green plants absorb incident solar radiation and harness part of that energy in photosynthesis
The initial slopes of the photosynthetic light-response curves in healthy leaves are similar among a wide range of plant species, and the photosynthesis rate is proportional to the incident photon flux density of photosynthetically active radiation (PAR, 400–700 nm)
We assumed that the gray chloroplasts, with an average PAR absorptance of 0.3 and an incident solar radiation of 400 W m−2, have the same spectral direct solar radiation profiles (Fig. 1)

Summary

Introduction

Green plants absorb incident solar radiation and harness part of that energy in photosynthesis. These facts suggest that terrestrial green plants are finetuned to reduce excess energy absorption by photosynthetic pigments rather than to absorb PAR photons efficiently. The mutualistic relationships among leaf anatomical development, chloroplast characteristics, and accessory pigments are considered in terms of the effective use of strong direct solar radiation in the terrestrial environment.

Results

Conclusion