Cellular stages of root formation, root system quality and survival of hybrid Pine cuttings in different temperature environments

Time to first root in cuttings varies under different environmental conditions and understanding these differences is critical for optimizing propagation of commercial forestry species. In Queensland, Australia, the most widely planted forestry taxon is the Slash x Caribbean Pine hybrid (Pinus elliottii var. elliottii x P. caribaea var. hondurensis). We used histology to observe the cellular stages of root initiation and changes in this process between four different controlled temperature environments (15, 25, 30 and 35ºC ± 2ºC) over 16 weeks. We also recorded survival, rooting percentage and root system quality in the different temperature regimes. Temperature environment had no effect on the cellular stages in root formation. Initially callus cells formed in the cortex, then tracheids developed and formed primordia leading to external roots. However, speed of development followed a growth curve with the fastest development occurring at 25ºC and slowest at 15ºC with rooting percentages at week 12 of 80% and 0% respectively. Cutting survival was good in the three cooler temperature regimes (>80%) but reduced to 59% at 35ºC. Root formation appeared to be dependant on the initiation of tracheids because all unrooted cuttings had callus tissue but no tracheids, irrespective of temperature treatment and clone