Development of a hemp (Cannabis sativa L.) simulation model 1. General introduction and the effect of temperature on the pre-emergent development of hemp

In recent times, there has been a revival of interest in hemp (Cannabis sativa L.), principally as a source of fibre in paper and pulp manufacture. Studies assessing the production potential and optimum crop management of hemp could benefit from the use of a simulation model that captures crop growth and development processes in response to management, genotypic, soil and climate factors. Such a model would complement the more traditional agronomic field trial programs by helping to identify the need, extent and nature of such trials, and by extrapolating limited field results across both temporal and spatial dimensions. The hemp model described in the final paper in this series, divides crop phenology into 5 phases, the first of which includes the pre-emergent processes of germination and the subsequent elongation of hypocotyl and radicle. This first paper reports on a study into the response of these pre-emergent processes to temperature. The primary objectives were to establish a simple model for predicting the duration from sowing to emergence and to obtain estimates for the cardinal temperatures of hemp growth and development. Cardinal temperatures are required for the estimation of thermal time, which drives phenological development and canopy expansion in the hemp model. The germination response of the hemp cultivar Kompolti was measured at 13 different temperatures in incubators set between 1 and 55°C. Similarly, the response of radicle and hypocotyl elongation to temperature was measured at 8 different temperatures in incubators set between 10 and 40°C. Development rates for each phase of pre-emergent development were then calculated from time response plots of germinant number, hypocotyl and radicle length. Finally, piecewise linear models were fitted to plots of development rate versus temperature in order to calculate thermal time durations for each phase and cardinal temperatures for hemp growth and development. Estimates of the optimum and maximum temperatures from the elongation study were relatively consistent, with average values of 28.6 and 40.7°C, respectively. Base temperature estimates were less consistent, ranging from 1.4°C for the hypocotyl linear phase, to 6.2°C for the radicle lag phase. This variability made it difficult to identify a common base temperature for use in the hemp model. However, there was some evidence to support previously reported base temperature estimates ranging from 0 to 2.5°C. Assuming a common base temperature of 1°C, the average thermal time requirements for germination and the lag and linear phases of hypocotyl elongation were 24.1°Cd, 44.5°Cd and 1.34°Cd/mm, respectively.