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

In earlier work, we found that the near complete defoliation of grain sorgh um [Sorghum bicolor (L.) Moench] seedlings delayed panicle initiation and a nthesis. Several aspects of the required defoliation remain unclear, howeve r, including which parts of the seedling's foliage need to be removed, the timing of defoliation, and what effects differing defoliation treatments ha ve on the morphology of plants that re-form after defoliation is terminated . To answer these questions, sorghum plants (cv. Boomer) grown under natura l (c. 11.5 h) or extended (14 h) photoperiods were defoliated during the ve getative 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 plan ts 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 anthes is. Defoliation treatments terminating on the same date, yet commencing bet ween the second and fifth leaf stages, the latter just prior to panicle ini tiation in control plants, gave the same delay to panicle initiation. Seria l defoliation at 3-4-day intervals maintained the plants in a vegetative st ate. Subsequent plant development and growth were associated with the morph ology of plants when defoliation was terminated, thus were influenced by th e height at which defoliation was performed. Plants defoliated above the fi rst ligule took longer to initiate reproductive development and re-formed b igger 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 e xposed expanding leaves and possibly the leaf primordia, and that removal o f 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 se ems likely that post-sowing management, via defoliation, can be developed t o control flowering time and adjust the yield potential of crops in line wi th the amount of in-crop rain.