Photochemical reflectance index and solar-induced fluorescence for assessing cotton photosynthesis under water-deficit stress

Abstract

Rapid and non-destructive assessment of water status is essential to enhance crop performance. This study aimed to evaluate photosynthetic performance and to monitor water status in cotton under field conditions. A two-year experiment was conducted with three irrigation regimes to measure the following parameters: photochemical reflectance index (PRI), structural independent pigment index (SIPI), water index (WI), solar-induced fluorescence (SIF; retrieved from reflectance by using Fraunhofer line-depth method), gas exchange, and chlorophyll fluorescence (CF). 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, transpiration rate, actual quantum yield of photosystem II photochemistry (ΦPSII), but a negatively correlated with nonphotochemical quenching (NPQ). Solar-induced fluorescence around 761 nm (SIF761) had significant correlations with PN, ΦPSII, and NPQ, but not with maximal quantum yield of PS II photochemistry (Fv/Fm). The relationship between PRI and PN was stronger at the beginning of water stress (R2 = 0.86) than for the all stress stages (R2 = 0.54), indicating that PRI could be more effective for assessing PN of cotton at early water stress. PRI was better correlated with relative water content and photosynthetic parameters than SIPI and WI and so it could also be a good indicator to evaluate cotton water status.