Timing and height of defoliation affect vegetative growth and floral development in grain sorghum

Abstract. In earlier work, we found that the near complete defoliation of grain sorghum [Sorghum bicolor (L.) Moench] seedlings delayed panicle initiation and anthesis. Several aspects of the required defoliation remain unclear, however, including which parts of the seedling’s foliage need to be removed, the timing of defoliation, and what effects differing defoliation treatments have on the morphology of plants that re-form after defoliation is terminated. To answer these questions, sorghum plants (cv. Boomer) grown under natural (c. 11.5 h) or extended (14 h) photoperiods were defoliated during the vegetative development phase. Treatments removed the fully exposed leaf-blade and/or the partially exposed and still expanding leaves and were varied by commencing and ceasing defoliation at different times, by cutting the plants at different heights, and by leaving some green leaf area on the plant. All defoliation treatments, except the one in which only the fully exposed leaf-blade was removed, resulted in delays in panicle initiation and anthesis. Defoliation treatments terminating on the same date, yet commencing between the second and fifth leaf stages, the latter just prior to panicle initiation in control plants, gave the same delay to panicle initiation. Serial defoliation at 3– 4-day intervals maintained the plants in a vegetative state. Subsequent plant development and growth were associated with the morphology of plants when defoliation was terminated, thus were influenced by the height at which defoliation was performed. Plants defoliated above the first ligule took longer to initiate reproductive development and re-formed bigger plants than did those defoliated above the second ligule. Defoliation did not always reduce the plant biomass at anthesis compared with that of control plants. We interpret these responses as evidence that the signal to initiate reproductive development in sorghum originates in the partially exposed expanding leaves and possibly the leaf primordia, and that removal of those leaves resets the plant’s developmental program to an earlier phase. For farmers of rain-fed crops this is an exciting result, since it now seems likely that post-sowing management, via defoliation, can be developed to control flowering time and adjust the yield potential of crops in line with the amount of in-crop rain.