Leaf modification delays panicle initiation and anthesis in grain sorghum

Water stress at anthesis is the major cause of yield reduction or crop fail ure in grain sorghum [Sorghum bicolor (L.) Moench] in central Queensland. R ainfall is difficult to predict and it is impractical to substantially alte r the timing and amount of water stored in the soil, so we focussed on whet her crop ontogeny could be managed, ultimately giving farmers some capabili ty to align anthesis with in-crop rain. It is widely considered that a sign al, transported from the leaf to the shoot apical meristem, is integral to the onset of panicle initiation and reproductive development. We hypothesis ed that modifying the leaves may interrupt the signal and cause a delay in the onset of reproductive development. Delays in sorghum anthesis associate d with leaf modification treatments applied before panicle initiation were found to be a consequence of delays in panicle initiation. The longest dela ys in panicle initiation were obtained by twice-weekly defoliation above th e second ligule (15 45 days); delays were shorter when plants were defoliat ed above the third ligule (10 41 days) or when only the fully exposed leave s were removed (0 13 days), depending on genotype. Although panicle initiat ion was delayed, leaf initiation continued, so extra leaves were produced. Defoliation of fully irrigated plants, however, generally reduced green lea f area, plant dry weight at anthesis, and grain yield, all by 30-50%. The a pplication of ethephon also delayed anthesis, and changed the pattern but n ot the area of leaf produced, and did not alter grain yield. In rain-fed ag riculture, where grain yields are frequently <50% of irrigated controls, de laying panicle initiation by 2 weeks may provide a better rainfall environm ent during which anthesis and grain-filling will occur. Reductions in green leaf area, although reducing yield potential, may promote a more balanced use of water between vegetative and grain growth. There was sufficient evid ence to indicate that defoliation before panicle initiation could provide s imple post-sowing management to achieve this scenario.