Screening for Pre- and Post-Flowering Drought Tolerance in Sorghum.

Drought is the most prevalent abiotic stress in crop production.  While sorghum [Sorghum bicolor (L.) Moench] is known as a drought tolerant crop, improvement of this trait is necessary to maintain high levels of production.  The world collection of sorghum contains more than 45,000 accessions, the majority of which are  photoperiod sensitive and have not been adequately screened for either pre- or post-flowering drought tolerance.  Approximately 1500 germplasm lines were evaluated for both pre- and post flowering drought tolerance in the winter nursery where no precipitation was recorded during the growing season.  All lines were grown in 5 m rows using an augmented experimental design. For the pre-flowering test, no additional water was applied during the growing season.  For the post-flowering test, water was applied twice during the first 30 days of growth only.   When stressed before flowering, three responses were noted.  Tolerant types flowered earlier when stressed than they did when not stressed.  Moderately tolerant types flowered at the same time when stressed as when not stressed and susceptible types were delayed in flowering when stressed.  Seed number differences on heads in the middle of the plot and those at the end of the plot were also used to differentiate pre-flowering tolerance.  Post flowering tolerance was evaluated based on chlorophyll differences at flowering and 40 days post flowering, by green leaf retention at 40 days post flowering and final seed size.  Thirty eight lines were observed to have high levels of pre-flowering tolerance and twenty lines had high levels of post-flowering tolerance.  These new sources of tolerance will broaden the genetic basis of drought tolerance in sorghum.