Simulating seed number in grain sorghum from increases in plant dry weight

GDD, growing degree days; PBS, SORKAM with Eq. [16], from A-540 to A-180 (360 GDD), approximating the first half of the panicle development-appearance of primary branches and spikelets appear on the developing panicle; PBS sub I , interval encompassing panicle branch and spikelet appearance; PBS/ EA, SORKAM containing the empirically derived equations [i.e., representing panicle branch -spikelet appearance interval (A-540 to A - 180) and panicle elongation-anthesis interval ( A - 180 to A +180)] with a switch to select the equations to simulate seed number. The switch compares the mean WATCO values for the two intervals and then directs the model to apply the equation associated with the interval where water stress was more severe (i.e., where the WATCO value was smaller) to simulate seed numbers for the whole 720-GDD interval; RMSE, root mean squared error; SORKAM, grain sorghum plant growth model; WATCO, water stress coefficient; WATCOdw, water stress coefficient for dry weight; WATCO sub Ie , water stress coefficient for leaf area; WATCO sub dw/Ie , the SORKAM with its original equations for estimating seed number but containing a switch that applies the water stress coefficient, WATCO sub dw/Ie , or WATCO sub Ie , during the intervals of panicle branch-spikelet appearance (A-540 to A-180) and panicle elongation-anthesis (A -180 to A + 180) depending on levels of water stress. The switch compares the mean WATCOlc values of the two intervals and then applies WATCO sub Ie to the interval where water stress was more severe and WATCO sub dw to the interval where water stress was less severe. If water stress levels are equal, WATCO sub Ie is applied to both intervals.Simulation of seed number for crop models is important in identifying cultural practices, which enhance yield stability. Field and crop simulation studies examined the relationship between dry weight accumulation and seed number per plant to potentially improve the capability of the grain sorghum [Sorghum bicolor (L.) Moench] model, SORKAM, to simulate seed number. The best estimates of seed numbers were obtained from plant dry weight accumulated during the 360 growing degree day intervals encompassing panicle branch-spikelet formation (PBS sub I ) and panicle elongation through anthesis (EA sub I ). Comparison of observed vs. simulated seed numbers using SORKAIVTs original equations accounted for 57% of the variation in seed number, but it underestimated high seed numbers. Accumulated plant dry weight for the PBS sub I and EA sub I intervals accounted for 49 and 64% of the variation in seed number, respectively. Simulation of seed numbers improved when the more sensitive water stress coefficient (for leaf area, WATCO sub Ie ) was applied to the interval (PBS sub I or EA sub I ) experiencing the highest water stress while the less sensitive water stress coefficient (for dry weight, WATCO sub dw ) was applied to the interval experiencing the lowest water stress. The slope from the regression of observed on simulated seed numbers was 0.80 (r sup 2 = 0.57) for SORKAM with the WATCO sub Ie switch compared with 0.59 (r sup 2 = 0.57) in the original SORKAM model. Hence, the timing and recovery of water stress during the panicle development period was important in estimating seed number of sorghum. Abbreviations: E, SORKAM model containing Eq. [14], from A-480 to A +120 (600 ODD), approximating the time interval used to simulate seed number; EA, SORKAM with Eq. [17], from A-180 to A -180 (360 GDD), approximating the second half of the panicle development-panicle elongation through anthesis; EA sub I , interval encompassing panicle elongation through anthesis;