During the 1960s and 1970s, plant breeders began collecting landrace strains and
often referred to them as heirloom varieties. The resulting nomenclature of any
strains resulting from genetic crosses may reflect some of the original landrace
origin e.g.‘Grape Kush’, would describe a strain which contains‘Kush’ancestry
collected from Afghanistan/Pakistan. Many of the resulting strains from those
original crosses are still used in present-day genetic crosses, which has resulted in
much confusion amongst cannabis producers as to the correct identification of the
original landrace strains, the number of existing landrace strains, and the nature of
the genetic backgrounds. The use of the term landrace may also be inappropriately
used by growers or commercial breeders to describe strains that may have little
original heritage from the landrace e.g.‘Acapulco Gold’.
Genetic diversity amongst present-day cannabis plants is high compared to
populations of other weedy plant species that are annuals, wind-pollinated and have
gravity-dispersed seeds (Hamrick 1989 ; Lynch et al. 2017 ). The sub-speciessativa
of cannabis contain plants which appear to be less variable and relatively more
homogenous than theindicagroup, which suggests that the regional differences
within theindicagroup are higher. Many producers describe theindicaplants to
have broader leaflets whilesativaplants are observed to have narrow leaflets
(Fig.19.1); however,indicaplants with leaflets which are indistinguishable in
width and size fromsativaplants have been observed. This was also noted by Hillig
( 2005 ) when he described populations of cannabis plants originating from different
parts of the world as part of a cannabis species study. A more detailed description of
the distinguishing features betweenindicaandsativaplant types is provided by
Chandra et al. ( 2013 ).
19.3 Flower Induction in Cannabis
Although there are many strains currently being grown for the production of
marihuana, a common feature they share with their ancestry is the trigger for
induction offlowering as well as the length of theflowering period. Cannabis plants
that originate from latitudes beyond 60° N can be induced toflower by increasing
the night-time duration period compared to strains of equatorial origin, where plant
age is the determining factor in initiatingflowering. Plants that are reported to have
an equatorial or subequatorial ancestry generally produce much higher harvestable
biomass when allowed toflower for up to 10 weeks compared to plants that are
from latitudes greater than 60° N, where biomass is similar between 8 and 10 week
flowering duration (Potter 2009 ). Many marihuana producers induce plants to
flower by placing them in long periods of darkness, followed by a 12 h day/night
photoperiod and alter the length offlowering duration according to whether the
plants are presumed to be ofindicaorsativaancestry. Our observations suggest that
many strains used in production actually have equal optimumflower duration times
19 Assessing Genetic Diversity inCannabis sativa... 397