Occurrence of repeated drought events : can repetitive stress situations and recovery from drought be traced with leaf reflectance ?

Within the last years a lot of effort has been made to improve irrigation efficiency and early drought stress detection by using various remote sensing techniques. In the present study two different species of wheat (Triticum aestivum and Triticum durum), cultivated in a growth chamber, were used to investigate the effects of drought occurring at different phenological stages. Plant physiological traits and spectral leaf reflectance were used to assess the potential of remote sensing techniques. Drought stress was applied either at flowering and/or at grain filling. Subsequently, a treatment following recovery after drought stress at flowering was set up. The effects of drought were traced by following the changes in plant physiological traits (i.e. photosynthetic rate, leaf conductance, relative and actual leaf water content) as well as in leaf reflectance. Drought resulted in a significant reduction of plant physiological traits and water relations, independently of the time of its occurrence. Rewatering plants after the stress period at flowering resulted in a recovery of plant physiological traits. Single leaf reflectance of plants subjected to drought increased over the entire range of the spectrum. However, five spectral regions with relatively high differences were observed: 520–530 nm, 570–590 nm, 690–710 nm, 1410–1470 nm and 1880–1940 nm. Additionally, three spectral indices were tested towards their applicability for tracing drought stress and subsequent recovery, yielding a reasonable relationship with measured leaf water content, photosynthetic rate and leaf nitrogen content.