Rapid Screening of Thirty-two Commercial Spinach Cultivars for Heat Tolerance under High Temperature

As a cool-season leafy vegetable, spinach ( Spinacia oleracea L.) is highly sensitive to high temperatures, which limits its year-round production in warm climates. Although many commercial cultivars are marketed as “heat tolerant,” their performance under high temperatures in hydroponic systems remains largely unverified. Full-cycle testing of many cultivars is time-consuming and resource intensive, highlighting the need for rapid, early-stage screening methods to identify heat-tolerant cultivars. Therefore, this study evaluated short-term (4-day) responses of 32 commercial spinach cultivars at two constant temperatures (20 and 30 °C) under indoor farming conditions. Changes in canopy area and petiole elongation, along with chlorophyll fluorescence parameters (variable fluorescence/maximum fluorescence and performance index) and soil plant analysis development (SPAD) index, were analyzed to classify cultivar-specific heat tolerance. At 20 °C, ‘Dallas’, ‘Amador’, and ‘Pawnie’ exhibited the greatest canopy expansion (114%–181%), whereas ‘Lizard’, ‘Sioux’, and ‘Space’ remained compact. A high temperature (30 °C) reduced canopy expansion in most cultivars; however, ‘Mandolin’, ‘SV2157’, and ‘SV3580VC’ maintained stable canopy growth, indicating high heat resilience. Compact, yet consistent, performers ‘Lakeside’, ‘Lizard’, and ‘Sioux’ also maintained stable growth under heat, suggesting suitability for production in warm climates. In contrast, ‘Hammerhead’, ‘Reflect’, and ‘Rangitoto’ exhibited poor canopy and petiole development, reflecting low heat tolerance. Chlorophyll fluorescence and SPAD index remained relatively constant, suggesting limited sensitivity to short-term heat exposure.