4. Discussion
Results from this study showed that shade avoidance cues from early emerging weeds (weeds that emerge with the sugar beet) have a more detrimental impact on sugar beet growth and development than late-emerging weeds. Sugar beet responded to an early shade avoidance cue by adjusting leaf orientation (hyponasty) to optimize light interception (Figure 3 and 4). Even when the weed was removed, the hyponastic response remained. These responses result in a reduced number of leaves and reduced leaf area for light capture and photosynthesis (Figure 3), thereby reducing shoot and root biomass (Figure 6). These results suggest a constitutive response where most, or even all, of the season-long shade avoidance response (SAR) in sugar beet growth is fixed by the time the sugar beet crop has reached the two true-leaf stage (approximately 330 GDD after planting). Removing weeds after that time had a small impact on the number of sugar beet leaves, but did not allow recovery of leaf area or biomass production, indicating that a constitutive shade avoidance response (CoSAR) may play a key role in crop yield loss due to weeds.
Studies have shown that stem and petiole extension and hyponasty are among the most common SARs (Cerrudo et al., 2017; Franklin & Whitelam, 2005; Yang & Li, 2017). Leaf orientation is an important factor influencing light interception in plants. This is because leaf angle relative to the direction of sunlight determines both the photosynthetic performance of the leaves at the top of the canopy as well as the amount of light available to lower leaves (Van Zanten, Pons, Janssen, Voesenek, & Peeters, 2010). At the canopy level, steeper leaf angles reduce mutual shading and thus, maximize light interception (Mullen, Weinig, & Hangarter, 2006). Since reduced R:FR is a cue for impending competition, it is not surprising that hyponasty is an intrinsic part of the shade avoidance syndrome (Van Zanten et al., 2010). Sugar beet has a rosette growth habit in the first season of growth and thus, hyponasty appears to be the strategy for projecting leaves for optimal light interception. We observed shade avoidance-induced hyponasty in the form of increased leaf angles in our study (Figure 3 and 4); however, sugar beet did not increase petiole or leaf length in response to shade avoidance cues as might be expected. In fact, we observed a reduction in petiole length, total leaf length, and petiole proportion of leaves in response to season-long shade avoidance cues in our greenhouse study (Figure 5).
Previous studies have found that reduced R:FR resulted in fewer leaves in A. thaliana (Xie et al., 2017) maize (Page, Liu, Cerrudo, Lee, & Swanton, 2011), and soybean (Green-Tracewicz, Page, & Swanton, 2012). We have previously shown that leaf number is reduced by season-long shade avoidance cues in three different sub-species ofB. vulgaris (Schambow et al., 2019). Here, we show that although the first two leaf pairs were least affected with respect to leaf size and area (Figure 5), subsequent sugar beet leaf development was reduced substantially even if weeds were removed early in plant development (Figure 3). Compared to season-long weed-free treatments in this study, the number of sugar beet leaves were reduced 10% if weeds were present for the first 16 days after emergence, and only reduced 12% if weeds were added 16 days after emergence and remained for the next 51 days (the average time from two true-leaf stage to harvest in this study). Early-emerging weeds have more impact on sugar beet leaf development compared to late-emerging weeds.
The relationship between sugar beet leaf area (Figure 3 E & F) and the duration of weed presence follows a similar trend as number of leaves, with early presence of the shade avoidance cue initiating an irreversible season-long response. Since leaf area is a function of leaf size and number, it appears the reduction in sugar beet leaf area was at least partly due to reduced number of leaves. Results from the greenhouse study showed that, in addition to reduced number of leaves, reflected FR light reduced sugar beet leaf width, petiole length, and total leaf length as well (Figure 5). Differences in sugar beet leaf size were most pronounced after the first six true-leaves had appeared, which corresponds to the transition from leaf canopy-dominated growth to root cambium development (followed by storage root and sugar accumulation dominated phase) (Milford, 2006). Most capacity for storage root size and weight is laid down by the sixth true leaf stage (Milford, 2006).
Although increased petiole elongation is a common response to shade avoidance cues (Weijschedé, Martínková, De Kroon, & Huber, 2006), we observed a reduction in petiole length (Figure 5C), even as a proportion of total leaf length (Figure 5E), from sugar beet plants exposed to weed-reflected light in the greenhouse study. Sugar beet did not increase petiole length in response to shade avoidance cues as we had predicted based on previous work in other plant species. This illustrates the importance of conducting research on crop species with different growth habits, rather than generalizing from more commonly studied species like Arabidopsis , maize, and soybean.
The reduction in sugar beet leaf size and leaf number reduced leaf biomass, and subsequently, reduced root biomass (Figure 6). It is clear from our results that weeds present at sugar beet emergence had a significantly greater impact on sugar beet growth compared to weeds added later in the season. Many previous studies have also shown that early emerging weeds have a substantial effect on sugar beet yield. Early weed removal is often recommended in sugar beets (Jalali & Salehi, 2013). Our results demonstrate that a substantial portion of reduced yield due to weeds may be due to shade avoidance, rather than solely resource depletion as is commonly thought. To protect sugar beet yield potential, ensuring a weed-free environment at emergence may be critical.
It is important to state that crop yield loss due to shade avoidance is typically understood in the context of consistent FR light stimulation, but our understanding of shade avoidance responses at the molecular level is not well-connected to whole-plant responses in agronomically relevant contexts (Guo et al., 2017; Shikata et al., 2014). Alternative splicing occurs in response to far-red light at the seed stage, and changes development into the seedling stage, but the effects on further growth are not well understood after this point (Penfield, Josse, & Halliday, 2010; Shikata et al., 2014). There is evidence that phytochrome interacting factor (PIF) proteins are upregulated specifically in response to weed competition, although these do not occur in all species (Horvath et al., 2019; Horvath et al., 2015; Page, Tollenaar, Lee, Lukens, & Swanton, 2009). The gene which controls leaf area index (LAI) during the shade avoidance response is also involved in growth-phase transition and thus, can affect organ size and whole-plant biomass (Fu et al., 2012; Toriba et al., 2019; Wang, Schwab, Czech, Mica, & Weigel, 2008; Wu et al., 2009; Xie et al., 2017). This latter observation is particularly intriguing in light of differential leaf area development shown in Figure 3, that a CoSAR can play a significant role in crop yield loss opens a potential pathway for improved breeding and bioengineering strategies.
We conclude that sugar beet exhibits a constitutive shade avoidance response when exposed to early-emerging weeds causing season-long impacts that affect growth, development, and yield potential. Thus, this work has significant practical implications for weed management (e.g. timing of weed removal) as well as crop genetic improvement. However, our study system ensured a strong reflected R:FR light ratio at the time of sugar beet emergence, and the weed density or proximity of weeds to the sugar beet plants required to cause a similar response under more typical field conditions remains unknown. This will no doubt vary by weed species [e.g. differences in reflectance, growth habit (erect vs prostrate), etc], crop row spacing, and other agronomic factors. Although it is uncommon for weeds to be extremely dense in the field at the time of sugar beet emergence, there is growing interest in ‘planting green’, where a cash crop is seeded directly into established cover crops, and this work suggests that practice may have unforeseen impacts on crop yield potential.
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Table 1. Treatment descriptions showing the start and end of weed treatments in growing degree days (GDD)