Definition of photosynthetically active radiation (PAR) and its development progress

Abstract: Photosynthesis is one of the most important physiological activities of plant individuals. Green plants use light to drive photosynthesis in the wavelength range of 400 to 700 nm. Photosynthetically active radiation (PAR) can be used to measure the photosynthetic potential in plant individuals or ecosystems. Therefore, the PAR is one of the bridges to link physiology and ecology. However, there is the varying efficiency of different plants using light in this range, particularly with the different plant types and growth stages. As a result, it is a high demand to accurately measure the portion of light radiation for better photosynthesis. Much effort has been made in the fields of ecology, agronomy, meteorology, and remote sensing over the years, including the different definitions and experiments. A series of sensors have been developed, where the PAR sensors have been generally accepted to evaluate photosynthesis. These sensors can be used to measure some parameters, such as the photosynthetic photon flux or its density in the 400 to 700 nm wavelength range. There is a flat and straight line in the spectral response function of these sensors. In this review, the formation of the PAR definition was introduced for the evolution of the PAR sensor. Various technologies were used to correct the original spectral response function of photodetectors, including filtering, and transmittance correction. The spectral response function of the sensor was close to the ideal PAR one. The current optical quantum sensor still shared a wide range of applications for the comparison benchmark with a small error. But it was lacking to perfectly measure the photosynthetic effective radiation. Earlier studies cannot fully quantify the spectral response of photosynthesis under either photometric or radiant units nearly five decades ago. The definition of PAR also failed to completely unify into the spectral response function of PAR measurement sensors in recent years, due to the diversity of applications. Alternatively, the MCCREE curve was considered the standard for the spectral response of photosynthesis. Nevertheless, some challenges remained in the new experimental design and differences between individual photosynthetic and whole-plant growth responses. The MCCREE curve was also replicated with LED lighting and optical filters. However, challenges occurred with the wavelength selection and the higher light intensity levels. A comprehensive spectral response of photosynthesis analysis was required for the different green plants with better controllability over the wavelength, full width at half maximums (FWHM), and higher light intensity. In particular, the wavelength range absorbed by plants was wider than 400 to 700 nm for artificial light illumination and plant growth. There were significant effects of different spectral energy distributions (wavelength and energy ratio) and photoperiods on photosynthesis. It is a high demand to distinguish the effects of these factors on photosynthesis, in terms of plant morphology. As a result, the definition of PAR is still on the way for the development of measuring instruments. Ideally, the definition of PAR should be proposed from the perspective of plant photosynthesis. The spectral response function curve of the sensor should be consistent with the capacity curve of plant photosynthesis. The future trend of the PAR sensor is to program the spectral response function. PAR estimation can be realized at the regional and global scales using remote sensing. Large-scale PAR products can be developed to significantly promote the global carbon cycle and remote sensing.