Pulvinus-based diaheliotropism in cotton: movement–performance relationships under shaded conditions

Abstract

Diaheliotropic leaf movement in upland cotton (Gossypium hirsutum) enhances photosynthetic efficiency under favorable conditions by orienting leaves toward sunlight, yet its potential disadvantages under low PPFD remain unclear. This study tested whether the growth- and efficiency-related returns of diaheliotropic movement decline when photosynthetic carbon gain is limited under low PPFD. An early-maturing genotype was grown for eight days under two PPFD levels (50 and 150 µmol m⁻² s⁻¹). Leaf movement was restricted (P−) by applying water-based acrylic paint to the pulvinus, while unrestricted plants (P+) served as controls. Morphological traits (leaf area, SLA, stomatal features) and physiological parameters (transpiration, water use efficiency, net assimilation rate, relative growth rate) were assessed. Leaf movement was quantified using a non-invasive photographic pixel-counting method. Under moderately shaded PPFD, unrestricted plants showed numerically higher growth-related values and positive correlations between total leaf movement and growth parameters, suggesting a favorable movement–performance relationship. In contrast, under deeply shaded PPFD, restricted plants showed non-significant numerical increases in several growth-related traits, while SLA was the only trait that differed significantly, indicating that the evidence for a movement-related cost under L50 should be interpreted cautiously. Furthermore, the restriction method did not produce detectable stress responses in the measured harvest-time indicators, and the area-based approach provided practical estimates of whole-plant leaf movement. These results suggest a context-dependent shift in the movement–performance relationship in the tested genotype, but the short treatment duration and non-significant growth responses indicate that longer-term, replicated, multi-genotype studies are needed before agronomic implications can be drawn for shaded cropping systems.