Introduction
For a comprehensive understanding of the light incident on plants, it's imperative to characterise its intensity (photon or energy irradiance), duration, quality (spectral composition), and direction (relative location of source and degree of scattering). Plants rely on the absorption of light energy to synthesise organic compounds and sustain metabolic activities throughout their life cycle. Additionally, light acts as a critical environmental cue, guiding plant responses such as photomorphogenesis, phototropism, and circadian rhythms. Optimising light conditions can significantly enhance crop productivity, quality, and resilience for any grower1,2,3.
Definition and Explanation
Light intensity for plant growth is typically quantified as photosynthetic photon flux density (PPFD), measured in µmol photons m-2 s-1. This metric is usually combined with photosynthetically active radiation (PAR), which represents the sum of photons within the 400 to 700nm wavelength range. PAR and PPFD are fundamental concepts used to measure light energy relevant to photosynthesis in plants. While PAR refers to the spectrum of light utilised by plants for photosynthesis, PPFD specifically measures the quantity of photons within this range falling on a given surface area per time unit. Though often used interchangeably, they address distinct aspects of light's impact on plant growth. Understanding the relationship between PAR and PPFD is crucial for providing an optimal growth environment for plants4.
Importance in Plant Physiology
PAR serves as the primary energy source for photosynthetic reactions, converting solar energy into bioenergy-carrying biomass. Despite its importance, PAR is often not directly measured in meteorological stations worldwide, requiring estimation from global solar radiation. PPFD, measured with quantum sensors, provides a more direct assessment of light energy relevant to photosynthesis. However, not all sensors accurately measure PPFD under various light sources, necessitating proper sensor selection and calibration.
The relationship between light intensity (PPFD) and photosynthetic rate follows a curve with three phases: light-limited, light-saturated, and light-inhibited. In the light-limited phase, photosynthetic rate increases with PPFD until other limiting factors emerge (Fig 1). In the light-saturated phase, photosynthetic rate reaches a maximum as the light-dependent reactions operate at full capacity. In the light-inhibited phase, excess light energy causes damage to the photosynthetic apparatus, leading to reduced efficiency and photo-inhibition. Plants employ various mechanisms to cope with high light intensity and minimise photo-inhibition, including leaf orientation adjustments, chloroplast and thylakoid modifications, enzyme regulation, and antioxidant activation5,6,7,8,9,10.
Factors Influencing PAR and PPFD
Sunlight angle: The angle of the sun relative to the plants affects the intensity and duration of sunlight that reaches the leaves. The higher the sun angle, the more PAR and PPFD are available for plants.
Cloud cover: Clouds can reflect, scatter, or absorb sunlight, reducing the amount of PAR and PPFD that reaches the ground. The effect of clouds depends on their type, thickness, and altitude. Some clouds can also enhance PAR and PPFD by creating a diffuse light that penetrates deeper into the plant canopy.
Shading: Shading by trees, buildings, or other structures can block or reduce the direct sunlight that reaches the plants. Shading can also create a microclimate that affects the temperature, humidity, and wind speed around the plants. Shading can be beneficial or detrimental for plant growth depending on the plant species, the intensity and duration of the shade, and the availability of artificial lighting.
Artificial lighting: Artificial lighting can supplement or replace natural sunlight for plant growth. Artificial lighting can be controlled in terms of intensity, duration, spectrum, and direction. Different types of artificial lights have different effects on PAR and PPFD. For example, LED lights can produce high PPFD with low heat and energy consumption, while fluorescent lights can produce a broad spectrum of PAR with moderate heat and energy consumption4,11.
Measurement and Monitoring
Different methods and instruments can be used to measure PAR and PPFD, such as quantum sensors, spectroradiometers, or light meters.
Quantum sensor: These are devices that measure the number of photons in a given area and time interval. They are called quantum sensors because they count individual quanta of light. Quantum sensors can be used to measure PPFD in various environments, such as outdoors, in a greenhouse, or indoor settings. Quantum sensors can be either cosine-corrected or hemispherical, depending on the field of view and the angle of incidence of the light. Cosine-corrected sensors are more accurate for measuring PPFD from a single light source, while hemispherical sensors are more suitable for measuring PPFD from multiple or diffuse light sources.
Spectroradiometer: These are instruments that measure the spectral distribution of light, or how much light there is at each wavelength. They can be used to measure PAR by integrating the light intensity over the PAR range. A spectroradiometer can provide more detailed information about the quality and composition of light than quantum sensors, but they are also more expensive and complex to use.
Light meter: These are devices that measure the intensity of light in a given area, usually expressed in lux or foot-candles. Light meters are not specific to the PAR range, and they do not account for the spectral quality or the photosynthetic efficiency of the light. Therefore, light meters are not very accurate for measuring PAR or PPFD, and they can give misleading results when comparing different light sources or environments4,12,13,14.
When it comes to measuring PAR and PPFD in a greenhouse, the most suitable device is a quantum sensor. Quantum sensors are specifically designed to capture the type and amount of light that plants utilize for photosynthesis, making them accurate, reliable, and easy to use for continuous monitoring. Although spectroradiometers can also measure PAR and PPFD, they are more expensive and complex to operate compared to quantum sensors. Additionally, spectroradiometers may require calibration for the specific light sources used in a greenhouse. On the other hand, light meters, based on human perception of brightness, are not suitable for PAR and PPFD measurements as they do not align with plant responses to light. Moreover, light meters may be influenced by the color and reflectance of greenhouse materials. Overall, while quantum sensors offer the best option for measuring PAR and PPFD in a greenhouse, it's important to ensure that the chosen sensor is properly calibrated and free from errors related to spectral, directional, calibration, and stability factors.
Conclusion
In conclusion, understanding the significance of PAR and PPFD is essential for optimising agricultural practices and ensuring sustainable crop production. By characterising light intensity, duration, quality, and direction, growers can create optimal environments for their crops, maximising productivity and quality while minimising resource inputs.
