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PSRI (Plant Senescence Reflectance Index)

Definition

The Plant Senescence Reflectance Index (PSRI) is a vegetation index specifically designed to detect plant senescence and stress by quantifying the relative abundance of carotenoid pigments compared to chlorophyll. It exploits the spectral shifts in reflectance that occur as leaves age and chlorophyll degrades, revealing the underlying carotenoids which absorb less in the blue and red regions.

Application

PSRI is primarily used to monitor leaf aging, assess plant stress (particularly nutrient deficiency), predict crop maturity and optimal harvest time, and study seasonal vegetation dynamics in both agricultural and natural ecosystems. It is especially valuable in viticulture, fruit orchards, and for monitoring forest health and autumn phenology.

FAQ

What is the formula for PSRI?

PSRI is typically calculated as: PSRI = (Red - Blue) / RedEdge. This formulation uses the difference between red and blue reflectance, normalized by the red edge reflectance, to capture the carotenoid-to-chlorophyll ratio. Some variations use NIR instead of RedEdge, but the RedEdge version is generally more sensitive to senescence.

What does a high PSRI value indicate?

A high PSRI value indicates an increase in carotenoid pigments relative to chlorophyll, which is a hallmark of leaf senescence, nutrient stress (especially nitrogen deficiency), drought, or other environmental stresses. It signals reduced photosynthetic capacity and often precedes visible yellowing or browning of leaves.

What types of sensors are needed to calculate PSRI?

PSRI requires sensors with at least three spectral bands:

1) Blue band (around 450-500 nm);

2) Red band (around 650-680 nm);

3) Red edge band (around 700-730 nm) or NIR band. Multispectral agricultural sensors (e.g., Sentinel-2, drones with RedEdge cameras) are typically used.

What are the main limitations of PSRI?

Limitations include:

1) Sensitivity to soil background and atmospheric effects, especially in sparse vegetation;

2) Requirement for a red edge band for optimal performance, limiting use with some sensors;

3) Can be confounded by other factors that affect pigment ratios, such as species differences or leaf surface properties;

4) May not differentiate between senescence and certain nutrient deficiencies without additional data.