Reflectance indices as nondestructive indicators of the physiological status of Ceratonia siliqua seedlings under varying moisture and temperature regimes

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Abstract
We investigated the use of spectral reflectance techniques to monitor the physiological responses of Ceratonia siliqua L. seedlings exposed to different levels of water availability under normal (25 : 18°C, day : night) and elevated (32 : 21°C, day : night) temperatures. Three spectral reflectance indices (photochemical reflectance index, PRI; water index, WI; red edge position, REP) were measured along with water status, chlorophyll fluorescence and chlorophyll concentration variables in the leaves of well watered, moderately stressed, severely stressed and rehydrated plants under each temperature regime. The PSII effective photochemical efficiency (f2) and the intrinsic efficiency of open PSII centres (F'v/F'm) correlated significantly with PRI, and these three variables loaded heavily onto the same principal component of a three-factor principal component analysis solution. Water concentration (WC) and the succulence index (SI) were more strongly correlated with WI than either water potential (?PD) or relative water content (RWC). Accordingly, WI, WC and SI were combined in the second principal component, and ?PD and RWC in the third. Our results provide clear evidence for interaction between water availability and temperature in the WI and the PRI response segments of the reflectance curves. Elevated temperature inhibited the recovery of WI spectral segments more than that of the PRI segments in SS plants. REP showed a strongly positive linear relationship with leaf total chlorophyll concentration across all water and temperature treatment combinations. PRI, WI and REP are therefore reliable markers that can be used to monitor f2, WC and total chlorophyll concentration, respectively, in C. siliqua seedlings under drought and temperature stress.

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