3.3 Season-long shade avoidance has less effect on the earliest
developing leaves compared to leaves that develop later
In the greenhouse study, season-long weed presence reduced sugar beet
leaf width (Figure 3A), leaf area (Figure 5B), petiole and whole leaf
length (Figure 5C, 5D), which confirmed the field study results.
Differences in sugar beet leaf width, leaf area, or total leaf length
were not apparent on the oldest four leaves (leaf pairs 1 and 2) but
were evident on the third and subsequent leaf pairs. Differences in
petiole length between grass and no-grass treatments were evident on
nearly all leaves (Figure 5C). Petiole proportion (petiole length/whole
leaf length) was greater in the weed-free treatment for all leaf
positions (Figure 5E), but the differences between treatments were more
pronounced in the younger leaves (leaf pairs > 5). For
example, there was 9±2% difference in leaf area between the season-long
weed presence and season-long weed-free treatment leaf pairs ≤4 and
42±11% difference for leaf pairs >4 (Figure 5B).
3.4 Early exposure to shade avoidance cues reduces leaf and
root biomass production
In the greenhouse study, the presence of weeds at sugar beet emergence
until harvest reduced leaf biomass by 35% (15.5 to 10.0 g, P-value =
0.005) and root biomass by 32% (6.5 to 4.4. g, P-value = 0.0002), and
maintained the root to shoot biomass ratio for both treatments (P-value
= 0.57). The leaf biomass reduction is a function of both fewer leaves
developing (19.7 in the weed-free treatment compared to 15.9 in the
weedy treatment, P-value = 0.0002), as well as smaller leaves (Figure
5). More leaves had senesced in the weed-free treatment compared to the
weedy treatment (2.8 and 1.7, respectively, P-value = 0.02).
In the field study, reduction in biomass was entirely attributable to
the early presence of the shade avoidance cues. Presence of weeds at
sugar beet emergence reduced leaf and root biomass by 25 and 32%,
respectively, even if weeds were removed as early as the 2-true leaf
stage (~330 GDD) (Figure 6A, 6C). Longer duration of
weed presence after the 2 true-leaf stage did not reduce biomass
further. In the late-season series, when weeds were added at the sugar
beet two true-leaf stage and remained until harvest, sugar beet leaf
biomass was reduced 18% and root biomass was reduced 23% compared to
the weed-free control (Figure 6B, 6D). The biomass lost from weed
addition in the late-season series followed a linear trend; for every
100 GDD that the sugar beets were kept weed-free, root yield increased
by 3.8% compared to the season-long weedy treatment (Figure 6 D).