Photochemical Reflectance Index and Solar-Induced Fluorescence for Assessing Cotton Photosynthesis Under Water-Deficit Stress.

Rapid and non-destructive assessment of water status is essential to enhance crop performance. This study aimed to evaluate photochemical reflectance index (PRI) and solar-induced fluorescence (SIF) for assessing plant photosynthetic performance and monitoring water status in cotton. A two-year experiment was conducted with three irrigation regimes under a field shed to measure the following indices and parameters at leaf level: hyperspectral indices PRI, structural independent pigment index (SIPI) and water index (WI), solar-induced fluorescence retrieved from reflectance by using Fraunhofer line-depth method, gas exchange and chlorophyll fluorescence. The results showed that PRI decreased > 30% in moderate drought (MD) and >50% in severe drought (SD), compared with control. PRI was found to be positively correlated with net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), actual quantum yield of photosystem II (Φ_PSII), but a negative correlation with non-photochemical quenching (NPQ), amongst which Pn had the highest correlation coefficient (r=0.762). Solar-induced fluorescence at 761 nm (F) had significant correlations with Pn, Φ_PSII and NPQ, but not with Fv/Fm. The relationship between PRI and Pn was stronger at flowering stage (R²=0.86) than for the whole stages (R²=0.54), indicating that PRI could be more effective for assessing net photosynthetic rate of cotton at early water stress. PRI was better correlated with relative water content (RWC) than SIPI and WI. This study shows that PRI is more sensitive to photosynthetic parameters than traditional spectral indices and could be a good indicator to evaluate cotton water status especially during the early water stress. SIF was less sensitive to water stress than PRI at the seasonal scale in cotton.