Phytochemical Composition and Overall Taste Modulation in Lettuce: Combination of Cultivar and Biofertiliser

This study assessed the impact of biofertilisers on primary and secondary (specialised) metabolites in six lettuce cultivars ('Kiribati', 'Murai', 'Aquino', 'Gaugin', 'Aleppo', and 'Carmesi') grown in anthropogenic soil during an autumn greenhouse experiment. Four treatments were tested: control (no fertilisation), effective microorganisms (EM), <i>Trichoderma</i>, and their combination. Red cultivars showed higher total antioxidant capacity (TAC) and total phenolic content (TPC), with red Lollo 'Carmesi' having the highest TAC, TPC, carotenoids, total soluble solids, sweetness index, and sugars. Red Oak 'Murai' exhibited the highest chlorophyll b and total chlorophyll, while green cultivars 'Kiribati' and 'Aquino' excelled in chlorophyll a and overall taste, respectively. Biofertilisers did not affect TAC or most chlorophyll types but increased TPC (EM by 18.6% and combined treatment by 19.6%) and chlorophyll a (EM by 28.6% and <i>Trichoderma</i> by 23.8%). Combined fertilisers improved taste with reduced glucose and fructose content and sweetness index, though sucrose remained unchanged compared to the control. Major organic acids (malic, citric, and tartaric) were most abundant in 'Murai' and 'Kiribati', unaffected by treatments. Phenolics content peaked in 'Murai' and 'Carmesi', characterised by chicoric and chlorogenic acid, caffeoylquinic acid glucoside, and flavonoids (quercetin derivatives, kaempferol); biofertilisers mainly influenced gallic acid, while kaempferol was affected by all biofertilisers and increased in the range of 12.5-25%. The key sesquiterpene lactones identified were lactucin, lactucopicrin-15-oxalate, and 11β,13-dihydrolactucin. The content of sesquiterpene lactones analysed in this study increased significantly, especially with EM treatment (14.7-185.7%) and combined fertilisers (12.5-128.6%), highlighting the lactone-rich cultivars 'Carmesi' and 'Gaugin'. Red cultivars 'Carmesi' and 'Murai' exhibited the most favourable phytochemical profiles, suitable for cultivation and processing of quality-based products. In contrast, the green cultivar 'Aquino' received the highest sensory scores, delivering the most appealing overall taste despite its lower metabolite content. EM treatment and combined fertilisers are recommended for increasing chlorophyll a, myo-inositol, TSS, propionic acid, TPC, kaempferol, and major lactones under greenhouse autumn conditions.