A model co-ordinating the elongation of all leaves of a sorghum cultivar was applied to both Mediterranean and Sahelian conditions

Sorghum leaf development was analysed at plant level by analysing the time-course of elongation and identifying the beginning and end of the elongation phases of each leaf blade. This was done with destructive and non-destructive measurements in 14 experiments carried out during several growing periods in Southern France and Sahelian Africa. Elongation of each blade was characterized by the succession of a nearly exponential phase and a linear phase. For a given blade and provided that time was expressed in thermal units, initiation, beginning and end of the linear phase, and time-courses of elongation rate were strikingly similar in all experiments, except in environments with a maximum air temperature close to 40 degrees C and a maximum vapour pressure deficit close to 6 kPa. The relative elongation rate during the exponential phase declined with leaf number from 0.08 to 0.02 degrees Cd(-1), while the duration of this phase increased from 140 to 320 degrees Cd. By contrast, the absolute elongation rate during the linear phase was nearly constant from leaf 8 onwards. This phase was shorter than the exponential phase regardless of leaf position, but accounted for the largest part of blade length. A strict pattern of leaf development was observed at the whole plant level, whereby dates of elongation events and leaf and ligule appearance, represented on a thermal time scale, were linearly related to phytomer number. This pattern exhibited a simultaneous elongation cessation of the last-formed leaves and a mismatch between real and apparent (from leaf to ligule appearance) elongation duration.