All indices showed the highest association between years at the grainfilling stage except for PRI and GNDVI between years 2001-2002 and 2002-2003 (Table 5).
RNDVI, GNDVI, and SR have been the most widely used by different authors to study the physiological conditions of plants, and all five SRI (RNDVI, GNDVI, SR, WI and PRI) have been reported by various authors to differentiate genotypes for grain yield under water-stressed conditions in durum wheat (Triticum turgidum L.
Comparing all the indices, those based on NIR (WI, NWI-1, and NWI-2) demonstrated a higher level of association with grain yield compared with the other spectral indices (RNDVI, GNDVI, SR, and PRI) at the heading and grainfilling stages, except for GHIST in year 2001-2002.
In general, values of PRI, RNDVI, GNDVI, and SR decreased from heading to grainfilling (Table 2).
On the other hand, RNDVI, GNDVI, and SR showed a higher correlation between booting and flowering and between booting and grainfilling.
RNDVI, GNDVI, and SR gave significant positive correlations with grain yield at the heading and grainfilling stages in nearly all cases over the 3 yr.
The three NIR-based indices measure the water status at the canopy level, while RNDVI, GNDVI, and SR measure the greenness of the canopy, and PRI is an indicator of radiation use efficiency in plants.
Comparing various indices, the indices based on NIR (WI, NWI-1, and NWI-2) demonstrated consistently higher levels of association and explained a higher proportion of the variability for grain yield compared with the other spectral indices (RNDVI, GNDVI, SR, and PRI).