Assessing leaf senescence in tall fescue (Festuca arundinacea Schreb.) under salinity stress using leaf spectrum

Salinity is one of the major abiotic factors that adversely affect turfgrass quality. One of the symptoms of salinity injury is accelerated leaf senescence. Detecting such injury may guide to a better maintenance of turfgrass. The objective of this study was to assess leaf senescence in tall fescue under salinity stress using reflectance leaf spectrum. Two tall fescue cultivars, 'Tar Heel II' and 'Wolfpack' were watered to field capacity at the 4-leaf stage with a Hoagland solution that contained 16 g L-1 of NaCl and CaCl2 (1:1 w/w). Compared to control, production of Malondialdehyde (MDA) in the 4th leaf of salinity treated plants increased starting at 1 week after treatment (WAT). Chlorophyll a (Chl(a)) content in the 4th leaf of salt treatment was lower than the untreated control starting at two WAT. No difference of Chl(b) content in the 4th leaf was detected until 3 WAT. Salt treatments resulted in larger wetting angle on the adaxial side of the 4th and 5th leaves compared to the control, which may be contributed to epicuticular wax accumulation as confirmed by scanning electron microscopy (SEM) of the leaf surface. The senescent leaves also showed higher levels of degradation of grana and thylakoids in the chloroplasts than the control. As a result, senescent leaves had lower quantum yield of photosystem II compared to the control. Three hyperspectral reflectance indices were tested in the study, mSR(750/705) [(R-750-R-445)/(R-750+R-445)], mND(750/705) [(R-750-R-705)/(R-750 + R-705-2R(445))], and SI710/760 [P-710/R-760] where R is the relative reflectance at a given wave length. All were shown to be very sensitive to detect senescing due to salinity stress, which may be used in turfgrass management.