Plant Quality Indices: Review, Development, and Conceptual Framework

Evaluating plant quality indices is a crucial task in horticulture, agriculture, and forestry, given that these indices quantify key plant production attributes, including biomass allocation, morphological stability, and structural integrity.Despite advancements in the development and testing of plant quality indices, there is an over-reliance on conventional indices, which limits the ability to capture fully the complexity and diversity of modern experiments in plant physiology.Among the most widely used metrics are the Dickson quality index (DQI), the compactness index, and the shoot-to-root ratio (S/R), all of which have long served as standard indicators for assessing seedling vigor and overall plant quality.Although these indices are valuable owing to their simplicity and broad applicability, they often fail to incorporate physiological performance capabilities or capture the full spectrum of morphological variations.In this study, we first provide a concise review of these conventional indices, i.e., their derivation, strengths, and limitations.Building on this synthesis, we then propose a series of complementary plant quality indices formulated to integrate additional morphological and physiological parameters.Specifically, we introduce the balanced morpho-structural index (BMSI), shoot-root efficiency index (SREI), compact biomass index (CBI), top-heavy index (THI), root investment ratio (RIR), morphological complexity index (MCI), leaf efficiency index (LEI), simple growth quality index (SGQI), and structural stability index (SSI), which are derived by integrating important quantitative parameters, in this case, morphological traits (e.g., plant size, leaf number, and branch number) and biomass.In addition, we present the integrated morphophysiological index (IMI) and two variants: a square-root variant applied to the morphological component (IMI morpho ) and a square-root variant applied to the full index (IMI total ).All three integrate quantitative morphological and biomass parameters with physiological indicators, specifically the maximum quantum yield of photosystem II (F v /F m ) and the normalized difference vegetation index (NDVI).Together, these indices constitute a new suite of plant quality indices that complements, rather than replaces, existing ones.By integrating both morphological diversity and physiological soundness, this suite addresses the structural limitations inherent in widely used plant quality indices and provides a more comprehensive assessment of plant quality as a proof-of-concept framework that lays the groundwork for broader application across plant groups and cultivation conditions, pending empirical validation.