Synergistic and Additive Effects of Humic Substances and Sugarcane Filter Cake on Papaya Physiology, Gene Expression, and Yield

Reliance on mineral fertilization in papaya cultivation raises sustainability concerns and drives demand for validated organic alternatives. This study tested whether integrating humic substances (HS) and sugarcane filter cake (FC) would stimulate photosynthetic physiology, upregulate carbon metabolism gene expression, and increase fruit yield in ‘Golden’ papaya while outperforming conventional NPK fertilization. A 12-month field experiment was conducted in a randomized complete block design with a factorial arrangement of four HS doses (0, 90, 180, and 270 mL plant−1) combined with two FC doses (0 and 60 kg plant−1) plus an NPK control, measuring photosynthetic pigments, gas exchange, relative expression of rbcL, ACC oxidase, invertase, relative growth rate, and fruit yield. Combined HS and FC increased chlorophyll a by up to 205%, chlorophyll b by 277%, and carotenoids by 208% relative to unamended controls. Gene expression was strongly induced: rbcL reached 202-fold, invertase 156-fold, and ACC oxidase 84.8-fold above control values. Photosynthetic rate followed a quadratic dose-response peaking near 90 mL plant−1 HS. Fruit yield nearly doubled under the optimal combined treatment (115 t ha−1) compared with unamended controls (62 t ha−1) and NPK fertilization (66 t ha−1). These results confirm that HS and FC act synergistically as dual-purpose amendments, improving soil fertility while biostimulating papaya physiology through coordinated upregulation of photosynthetic capacity and carbon partitioning toward reproductive sinks.