Cuphea Production and Management - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 295

emergence from soil depths greater than 19 mm. Light has been reported to be
essential for germination in C. viscosissima [ 56 ].
Seed germination can be affected by storage temperatures prior to planting.
Cuphea carthagenensis viability, for example, remained high when seeds were
stored at 25 C, but it decreased rapidly when stored at 5 C. Viability decreased
faster in seeds with seed moisture greater than 100 g kg 1 than seed at 10 g kg 1
moisture. The reduced seed viability is explained by the crystallization of
triacylglycerols (TAGs) [ 58 ]. Most crop seeds are stored at 18 C in germplasm

banks and germinate without problems after cold storage. Cuphea seeds will not
germinate after stored at 18 C. The low temperature exposure does not kill the
seed after storage at 18 C. Death occurs when the seed is hydrated. Seed

imbibition is hindered by crystallized TAGs [ 59 ]. Water contact with crystallized
TAGs is lethal for the seed by altering the balance between hydrophilic and
hydrophobic compounds needed for the transition of the cell to aqueous environ-
ment. The crystals cause massive decompartmentalization and interfere with the
proper arrangement of oleosin (protein covering oil bodies) in the oil bodies
[ 60 ]. Fortunately, poor germination of cuphea seed can be avoided by heating
seeds from cold storage, such as those of C. lanceolata , for 10 min at 45 C before
imbibition [ 59 , 61 ].
Interspecific cuphea germplasm lines developed from crossing C. viscosissima x
C. lanceolata have been shown to exhibit nondormancy [ 3 , 20 ]. An extensive
amount of work has been devoted to studying seed germination and development
of PSR23. Germination, seedling emergence, and vigor of PSR23 cuphea in the
field tend to be low and highly variable [ 8 , 10 - 12 , 25 , 62 ]. To a large extent, this is
due to the heterogeneity of seed maturity at harvest resulting from cuphea's
indeterminate growth. Therefore, any particular seed lot may consist of seed with
a range of weights, moisture content, oil content, and composition, which can
greatly affect germination potential and vigor.
During a 3-year field study in Fargo, ND, Berti and Johnson [ 62 , 63 ] studied seed
development of PSR23 from anthesis to physiological maturity (PM). They found
that an accumulation of 253 growing degree days (using a base of 10 C and
maximum temperature of 30 C) was required from anthesis to reach PM (i.e.,
maximum seed dry matter accumulation), which corresponded to approximately
30 days from anthesis. Seed moisture content at PM, however, varied among years
ranging from 566 g kg 1 the first year to 156 and 52 g kg 1 in the second and third
years, respectively, while maximum seed weight for PSR23 was found to be about
3.3 mg seed 1 in their study. For comparison, Kaliangile [ 64 ] studied seed devel-
opment of C. wrightii and C. lutea and reported that for these two species, PM
occurred at 19 days after anthesis or 219 growing degree days when grown in a
greenhouse under 25/18 C day/night temperatures and a 16 h light cycle. Also,
they reported that the seed moisture content for C. wrightii and C. lutea at PM was
320 and 420 g kg 1 and maximum seed weight was 1.89 and 2.62 mg seed 1 ,
respectively. When harvested at an optimum time for seed yield [ 16 ], seed moisture
of PSR23 cuphea generally ranges from 300 to 450 g kg 1 [ 6 , 17 ] and requires

further drying before storage.