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).