Action of humic acids in pigeon pea plants under light restriction: morphological and photobiological aspects

Strong light fluctuations, along with changes in light ratio and wavelength, can cause stress in plants, which can be minimized by using humic materials. These materials have biostimulant activity, altering photosynthetic activity. However, it is unclear whether responses to humic materials are light-dependent. Thus, in this study, Cajanus cajan (L.) Millspaugh was used to investigate the dependence of HA effects on photosynthetically active radiation (PAR) levels and to evaluate the impact of shading on HA action in this species. To this end, a humic acid concentration–response trial was conducted (0, 1, 2, 4, and 8 mM C L −1 ), with root morphology being analyzed. Based on the highest concentration obtained, another trial was conducted with HA at different irradiance levels (100%, 50%, and 20%) to determine whether HA's effect depends on light conditions. Root morphology, biomass, biometric variables, and photochemical activity were analyzed. Research has shown that humic acid stimulates root growth. HA increases root density and photosynthetic parameters in plants under light-restricted conditions, reducing the inhibitory effects of light restriction. However, when observing HA treatment at different irradiance levels, its effects on the photosynthetic apparatus and root density decrease as irradiance decreases, suggesting that the magnitude of HA effects may be modulated by light availability; however, it remains unclear whether this reflects a direct light-dependent mechanism or whether plants under low irradiance simply have a reduced metabolic capacity to respond to biostimulants.