QTL For Leaf Wax Is a Precursor To Functional Staygreen Related Responses At Grain Setting , Filling and Physiological Maturity In Sorghum Under Drought and Heat Stess.

Staygreen as a drought tolerance phonotype has been severally reported in both C3 and C4 plants. Based on screening for rate of senescence and green leaf area at maturity, four major QTLs for staygreen (stg) in sorghum have been shown to individually exhibit tolerance to post flowering drought-induced senescence.  However, physiological mechanisms for this drought response have not been fully elucidated.   Here we show that severe heat treatment combined with partial terminal drought treatment, elicits physiological responses that clearly partition the four main staygreen loci (Stg 1, 2, 3, 4) into three categories of drought and heat tolerance phenotypes, which may point to a unique phenological, stress-dependent association of genomic regions for staygreen and leaf wax, from flag leaf emergence to physiological maturity of grain in sorghum. Five replications of  BTx642-derived near isogenic lines in the post flowering senescent Tx7000 background were planted in four greenhouse conditions in summer (2012) and spring 2013). Several data points were generated at regular intervals for transpiration, stomatal conductance, fluorescence quantum yield, leaf temperature depression, leaf total cuticular wax, and leaf spectral reflectance, from flag leaf emergence to 35 days after pollination. The isolines that exhibited lower WL from flag leaf formation to five days after pollination had higher level of embryo abortion, poor grain filling onset, rolled leaves but elevated stomatal conductance under heat treatment. The results suggest that Stg 1, 2 3 and 4 strategies interact differently with leaf wax  to repackage the way plant respond to drought, heat or a combination of both stresses.