Wax and Staygreen May Cosegregate to Rescue Seed Set Failure in Sorghum.

Based on initial field phenotyping, QTL analysis and phenological observations of inbreds from TX642/TX7000 inbreds (RILs), we show that transgressive segregation for total compositional leaf wax combined with staygreen might lend a greater pre-and post-flowering drought and heat tolerance, than the non-staygreen and low wax variants. We phenotyped 100 RILS, and classified them into staygreen or non-staygreen based on visual highest staygreen or lowest staygreen scores, respectively. From each category, plants  consistently within the upper 10^th percentile and the lower 10^th percentile of wax load at anthesis from all field locations were selected and further screened in the greenhouse for wax load, staygreen and percent seed set and yield under normal greenhouse, enhanced drought and elevated heat conditions. Normal watering were done to 40 days after germination (DAG) and continued normally at 1 kilogram (kg) every 2 days under heat treatment and control, but reduced to ½kg every 4 days under drought till physiological maturity of seeds. Temperatures were held at average 31⁰C and 17⁰C day and night respectively for drought and control treatments, and 42⁰C/27⁰C day/night respectively for the elevated heat treatment. Four staygreen isolines (Stg1, Stg2, Stg3, Stg4) were concurrently evaluated for wax and reproductive response against the TX7000 reference (non-staygreen recurrent parent) under similar conditions as the inbreds.  Greenhouse leaf sampling was done at 40 DAG, at 50% flowering, and at 5DAP (days after pollination). Under elevated heat and drought, staygreen-high wax inbreds showed up to 20 per cent higher seed set than staygreen-low wax inbreds, non-staygreen high wax and non-staygreen-low wax inbreds. Individually, staygreen isolines (1, 2 and 4) also showed higher seed set than the pre-anthesis high wax TX7000 suggesting that staygreen genomic loci, if independently inherited might offer advantageous genetic interaction with wax that would significantly reduce seed set abortion and rescue yield.

Key words: wax, staygreen, stress