Effects of High-Temperature Stress on Physiological Characteristics and Propagation Potential of Pholidota chinensis Seedlings

Pholidota chinensis is an orchid with both medicinal and ornamental value. Due to overharvesting and limited natural reproduction, its wild populations have declined sharply. Although aseptic sowing enables large-scale propagation, climate-driven extreme heat poses a threat to seedling production. However, the physiological responses and propagation potential of P. chinensis seedlings under heat stress remain poorly understood. This study examined six-month-old seedlings under four temperatures (34℃, 36℃, 38℃, 40℃) for up to 96 h. Photosynthetic performance was assessed via UV-Vis spectroscopy and chlorophyll fluorescence (Fv/Fm), and propagation potential via stem segment regeneration. Unexpectedly, high-temperature stress induced a non-monotonic and temperature-specificpattern of pigment alteration: 34℃ reduced total chlorophyll to ~40% of initial value, whereas 36℃, 38℃, and 40℃ maintained 70%–95%, revealing that pigment content did not decline monotonically with increasing temperature. Pigment degradation became increasingly synchronized at higher temperatures, with chlorophyll b and carotenoid absorbance strongly correlated with chlorophyll a peaks (r > 0.85–0.90, P < 0.01). In contrast, Fv/Fm declined in a strictly temperature-dependent manner, from 0.74 to 0.50 (34℃), 0.45 (36℃), 0.41 (38℃), and 0.25 (40℃) at 96 h. Notably, 36℃ showed signs of partial PSII recovery, while the 36–38℃ range represented a critical transition zone, with 38℃ showing sustained suppression without clear recovery. Fv/Fm correlated strongly with proliferation coefficient (r > 0.85, P < 0.01), with proliferation severely compromised below Fv/Fm = 0.40 and completely lost at 0.25. These findings demonstrate that pigment content and photosynthetic functionality respond to heat stress through distinct mechanisms in P. chinensis seedlings. The strong Fv/Fm–regenerative capacity correlation supports chlorophyll fluorescence as a rapid, non-destructive indicator for early stress diagnosis and heat-tolerant material screening.