Repeated Foliar Glycine Betaine Applications in Cool-Season Greenhouse Passion Fruit: Effects of Application Frequency on Growth and Marketable Yield

Glycine betaine (GB) is used as an abiotic-stress biostimulant, but the performance of repeated foliar applications in perennial fruit crops under commercial conditions remains poorly defined. We evaluated four application strategies delivering cumulative GB inputs of 0, 42.5, 59.8, or 99.7 kg·ha−1 to greenhouse-grown ‘Tainung No. 1’ passion fruit (Passiflora edulis × P. edulis f. flavicarpa) during the cool season. Greenhouse temperature, vegetative growth, leaf greenness, visible reproductive development, marketable yield, and fruit quality were assessed. During the treatment period, 59 h below 10 °C were recorded, predominantly at night (83.1%). The 42.5 and 59.8 kg·ha−1 strategies did not consistently improve crop performance relative to the control. The 99.7 kg ha−1 strategy was associated with lower main-vine length during crop development, although final main-vine length at 144 DAT did not differ significantly from the control. The first visible flower bud appeared at a significantly higher node under 99.7 kg·ha−1 than in the control (27.1 vs. 20.8). Its appearance was significantly later under 99.7 than under 42.5 kg·ha−1 (29.9 vs. 19.6 DAT), but did not differ significantly from the control (21.0 DAT). The 99.7 kg ha−1 strategy also reduced marketable fruit number and yield by 14.6% and 13.8%, respectively. Fruit dimensions, individual fruit mass, and most quality traits were not consistently affected by GB, whereas harvest date more consistently influenced fruit quality. Under the recorded fluctuating thermal conditions, the intensive GB strategy was unfavorable, supporting crop-specific validation before repeated use.