OJIP-Derived Indices of Photochemical Characteristics in Sunflower and Maize: A Crop Type- and Cultivar-Level Assessment

All oxygenic photosynthetic organisms rely on photosystems (PS) for light-dependent reactions. While C4 crops typically demonstrate greater photosynthetic capacity compared to C3 crops, differences in PS functions between these two types remain poorly resolved. Here, we used fast chlorophyll a fluorescence induction (OJIP transient) to derive indices of PS functions in five cultivars each of C3 sunflower and C4 maize, comparing performance at both species and cultivar levels. OJIP transients provide robust phenotypic indices of PSII performance (photosynthetic performance index, PIabs) and relative PSI electron transfer efficiency (estimated as the probability of electron transfer from QB to PSI acceptors, ΔVIP/ΔVJP). Despite lower relative chlorophyll reading (SPAD), sunflowers exhibited higher PIabs, averaging 2.20 versus 1.16 in maize. PIabs also varied significantly among cultivars within each species (1.00–1.38 in maize; 1.85–2.64 in sunflower), yet this intraspecific variation was considerably smaller than the twofold difference between crop types. The ΔVIP/ΔVJP ratio was 25.65% higher in sunflower than in maize, and although cultivar-level variation was significant, the species contrast remained dominant. SPAD differed significantly among maize cultivars (54.8–65.4) but not among sunflower cultivars, with maize consistently higher (62.3 vs. 42.3). Principal component analyses further showed that SPAD was positively related to PIabs and negatively related to ΔVIP/ΔVJP in both crops. Together, our findings reveal pronounced differences in photochemical characteristics between sunflower and maize, with differences between crop types larger than cultivar-level variation.