Comparison of the abilities of vegetation indices and photosynthetic parameters to detect heat stress in wheat

Abstract With the changing of the climate, the detection of heat stress as early as possible has become increasingly important for wheat ( Triticum aestivum L.) production. Previous studies have demonstrated that photosynthetic parameters can serve as indicators of the stress conditions, and vegetation indices (VIs) provide the ability to non-destructively monitor photosynthetic parameters. However, it remains unclear whether VIs can be used to detect heat stress in a similar manner as the photosynthetic parameters. In addition, the optimal VIs for indicating heat stress and detecting the stress status are also currently unknown. In the present study, a heat stress experiment was designed with four temperature levels [T1, 17 °C/27 °C (T min /T max ), T2 (25 °C/35 °C), T3 (29 °C/39 °C), and T4 (33 °C/43 °C)] and three treatment durations [three days (D1), six days (D2) and nine days (D3)]. Three photosynthetic parameters [leaf chlorophyll content (LCC), net photosynthesis rate ( P n ), and maximum efficiency of photosystem II ( F v / F m )] and 17 published VIs were selected to compare their sensitivity and assess their feasibility for detecting heat stress. The results showed that F v / F m was the most sensitive photosynthetic parameter to heat stress and had the ability to indicate the start and end of heat stress at the slight level or the early stage. The chlorophyll index-red edge (CI red-edge ), normalized difference red edge index (NDRE) and photochemical reflectance index (PRI) were sensitive to heat stress owing to their close relationships with photosynthetic parameters. Among these three VIs, PRI displayed the highest sensitivity. Nevertheless, the sensitivity of PRI was less than that of F v / F m , and it failed to detect the beginning and end of heat stress lasting for three days. The ability of PRI to detect heat stress became similar to that of F v / F m when the duration of heat stress was increased to seven days. In conclusion, F v / F m is the optimum indicator for detecting early-stage heat stress, in which only the photosynthetic functions change. In contrast, PRI, a non-destructive indicator, works well to indicate relatively late-stage heat stress, in which the chemical and physical characteristics of leaves (e.g., chlorophyll content) are affected.