Biomedical Engineering Reference
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available water holding capacity of soil decreases to 50 % or less. In this study,
under hot dry conditions that prevailed late in the growing season, irrigation led to a
2.7-fold increase in seed yield. Moreover, measurements of photosynthesis, leaf
water potential, and seed
13
C all indicated that nonirrigated plants suffered severe
drought stress. The shallow root system and inefficient water use in the interspecific
hybrid PSR23 and related varieties perhaps come from the genetic background of
C. viscosissima
, whose native habitat
δ
is primarily riparian (relatively wet)
areas [
67
].
The seasonal water use in cuphea varies with sowing date where early sowing
allows roots to penetrate deeper into the soil profile. Cuphea's root system is very
shallow with most roots (80 % or more) in the upper 20 cm of the soil, but roots may
penetrate up to 50 cm if planted early [
73
]. Water use efficiency of seed production
(WUE) ranged from 1.2 to 2.0 kg ha
1
mm
1
for cuphea grown under dryland
condition in west central Minnesota [
73
] and tended to be highest for early-sown
plants (late April to early May). In a separate study conducted on the same soil type,
WUE was as high as 2.4 kg ha
1
mm
1
for irrigated cuphea. Water use efficiency in
cuphea appears to be relatively low as compared with other oilseed crops. For
instance, WUE of canola and barley (
Hordeum vulgare
L.) is about 10 kg ha
1
mm
1
[
75
,
76
]. Water use efficiency for sunflower (
Helianthus annuus
L.), soybean, and
wheat fluctuates between 5 and 6 kg ha
1
mm
1
[
75
]. Flax (
Linum usitatissimum
L.), however, has a WUE very similar to cuphea of about 2.0 kg ha
1
mm
1
[
75
]. Based on the relationship of seasonal water use and seed yield of cuphea,
Gesch et al. [
74
] estimated that cuphea requires about 460-490 mm of moisture
during the growing season, depending on soil type and climate, to maximize seed
yield.
Cuphea Agronomics
Cuphea Seeding Requirements
Seeding depth, soil packing, and seeding rate are also factors that influence plant
stand establishment. Roath [
57
] observed that cuphea seedling emergence was
greatly reduced with seeding depths greater than 13 mm. Roath [
57
] reported that
soil packing increased pure live seed emergence by 14 %. This may be due to better
seed/soil contact and greater seed hydration for germination. Soil water within the
first 20 mm of the soil is likely less than at deeper-profile depths. The recommended
seeding depth according to the cuphea grower's guide is 13 mm [
77
].
In a recent study conducted by Berti et al. [
12
], soil packing increased pure live
seed emergence and plant stand when seeds were broadcasted on the soil surface
(Fig.
13.2
). When drilled, pure live seed emergence was highest at 13-mm depth
compared with 25-mm depth. This confirms the need to seed cuphea to less than
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