<i>S</i>-Benzyl-L-cysteine Inhibits Growth and Photosynthesis, and Triggers Oxidative Stress in <i>Ipomoea grandifolia</i>

L-cysteine, a precursor of essential components for plant growth, is synthesized by the cysteine synthase complex, which includes <i>O</i>-acetylserine(thiol) lyase (OAS-TL) and serine acetyltransferase. In this work, we investigated how <i>S</i>-benzyl-L-cysteine (SBC), an OAS-TL inhibitor, affects the growth, photosynthesis, and oxidative stress of <i>Ipomoea grandifolia</i> plants. SBC impaired gas exchange and chlorophyll <i>a</i> fluorescence, indicating damage that compromised photosynthesis and reduced plant growth. Critical parameters such as the electron transport rate (<i>J</i>), triose phosphate utilization (<i>TPU</i>), light-saturation point (LSP), maximum carboxylation rate of Rubisco (V_cmax), and light-saturated net photosynthetic rate (<i>P</i>_Nmax) decreased by 19%, 20%, 22%, 23%, and 24%, respectively. The photochemical quenching coefficient (q_P), quantum yield of photosystem II photochemistry (ϕ_PSII), electron transport rate through PSII (ETR), and stomatal conductance (<i>g</i>_s) decreased by 12%, 19%, 19%, and 34%, respectively. Additionally, SBC decreased the maximum fluorescence yield (F_m), variable fluorescence (F_v), and chlorophyll (SPAD index) by 14%, 15%, and 15%, respectively, indicating possible damage to the photosynthetic apparatus. SBC triggered root oxidative stress by increasing malondialdehyde, reactive oxygen species, and conjugated dienes by 30%, 55%, and 61%, respectively. We hypothesize that dysfunctions in sulfur-containing components of the photosynthetic electron transport chain, such as the cytochrome b₆f complex, ferredoxin, and the iron–sulfur (Fe-S) centers are the cause of these effects, which ultimately reduce the efficiency of electron transport and hinder photosynthesis in <i>I. grandifolia</i> plants. In short, our findings suggest that targeting OAS-TL with inhibitors like SBC could be a promising strategy for the development of novel herbicides.