Plastic response to early shade avoidance cues has season-long effect on Beta vulgaris growth and development
Abstract: Early emerging weeds are known to negatively affect crop growth but the mechanisms by which weeds reduce crop yield are not fully understood. In a 4-yr study, we evaluated the duration of weed-reflected light on sugar beet (Beta vulgaris L.) growth and development. The study included an early-season weed removal series and a late-season weed addition series of treatments arranged in a randomized complete block, and the study design ensured minimal direct resource competition. If weeds were present from emergence until the two true-leaf sugar beet stage, sugar beet leaf area was reduced 22%, leaf biomass reduced 25%, and root biomass reduced 32% compared to sugar beet grown season-long without surrounding weeds. Leaf area, leaf biomass, and root biomass were similar whether weeds were removed at the two true-leaf stage (approximately 330 GDD after planting) or allowed to remain until sugar beet harvest (approximately 1240 GDD after planting). Adding weeds at the two true-leaf stage and leaving them until harvest (~1240 GDD) reduced sugar beet leaf and root biomass by 18 and 23%, respectively. It appears sugar beet responded to weed presence by adjusting carbon allocation and leaf orientation to optimize light interception.
Keywords: environmental plasticity; light quality; shade avoidance; weed competition; weed removal timing; yield potential
1. Introduction
Plants are exposed to heterogenous environments comprising different levels of stress and competition for resources. As sessile organisms, plants often exhibit morphological plasticity in response to adverse environmental conditions. Many of these responses involve developmental trade-offs that affect plant growth and development and their ability to respond to future environmental cues (Page, Tollenaar, Lee, Lukens, & Swanton, 2010; Weinig & Delph, 2001).
Mechanisms described as optimality, balanced growth, or functional equilibrium suggest that plants allocate more resources to the organ(s) which is(are) acquiring the resource that is currently the most limiting (Gedroc, McConnaughay, & Coleman, 1996; Poorter et al., 2012; Shipley & Meziane, 2002). Plants will allocate more resources to root growth if the limiting factor is below-ground (e.g. nutrients, water) and more resources to shoot growth if the limiting factor is light (Fazlioglu, Al‐Namazi, & Bonser, 2016; Freschet, Violle, Bourget, Scherer‐Lorenzen, & Fort, 2018). The ecological advantage of this functional response is clear; by allocating more resources to the plant organ acquiring the most limiting resource, plants are able to acquire the needed resources for growth and reproduction. However, the timing of these responses is critical. For example, plants subjected to moderate levels of moisture stress show little to no changes in size and root mass. However, once plants are subjected to severe moisture stress biomass is reduced by > 50%, and allocation to roots increases strongly at the expense of shoot growth (Padilla, Miranda, Jorquera, & Pugnaire, 2009). Plants might not respond very strongly to relatively short periods of moisture stress because of the unpredictability of precipitation. Increasing allocation to roots too quickly might result in suboptimal growth after the restoration of the water supply (Poorter et al., 2012). This is also true for other environmental responses, such as stem elongation responses light cues of neighbor proximity (Smith, 1992; Smith & Whitelam, 1997; Weinig & Delph, 2001).
One particularly important environmental cue with economic consequences, i.e. early allocation vs. later growth tradeoffs, is shade. Light reflected by green vegetation has a reduced ratio of red (R) to far-red (FR) light. Low R:FR thus provides a reliable cue of neighboring plants that may impose future competition for light and specifically competition for light wavelengths for photosynthesis. Exposure to light with a low R:FR ratio results in a series of developmental changes that serve to circumvent being shaded by other plants. These changes are collectively referred to as shade avoidance syndrome (McLaren & Smith, 1978; Smith & Whitelam, 1997; Weinig & Delph, 2001; Whitelam & Smith, 1991).
The developmental changes set off by shade-avoidance response (SAR) include differential allocation to roots, seeds, and leaves (Page et al., 2010; Schambow, Adjesiwor, Lorent, & Kniss, 2019). When these effects are combined, crops exhibiting the shade avoidance syndrome in response to weeds constitute a potentially significant source of yield loss in agriculture (Page et al., 2012). We have previously shown that leaf number is reduced by season-long shade avoidance cues in three different sub-species of B. vulgaris . The reduction in number of leaves resulted in reduced leaf area and biomass (Schambow et al., 2019). In the current study, we experimentally manipulated the duration of the shade avoidance cue to assess the mechanism by which B. vulgaris growth is reduced. Specifically, we wanted to test whether shade avoidance cues during the early and late stages of B. vulgaris growth have the same effects on morphology and growth. It has long been known that early emerging weeds (weeds that emerge with the crop) have more detrimental impact on crop yield than late-emerging weeds (Dew, 1972; O’Donovan, Remy, O’Sullivan, Dew, & Sharma, 1985; Swanton et al., 1999). In corn, 5% of potential yield is lost within 183 growing degree days (corresponding to 3 to 5 leaf stage) after planting due to weeds (Page et al., 2012).
It is logical to ascribe this phenomenon to the reduced crop growth from early-season resource depletion from weed competition. However, root interaction and growth resource (e.g. water, nutrients, and light) competition is very minimal early in the growing season. Thus, growth reduction from early-season weed presence could be due to other competitive effects of weeds rather than resource depletion. Rajcan, Chandler, and Swanton (2004) found that corn seedlings detected changes in light quality caused by the presence of grass (which was used to simulate low-growing weeds) and responded by adjusting carbon allocation and leaf orientation to optimize the light interception. 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). We hypothesize that early-season reflected FR light from neighboring weeds would induce hyponasty and increase allocation to petiole extension growth, thereby reducing root biomass of sugar beet. To test this hypothesis, we used a study design that prevents any direct resource competition, making it possible to assess the sole effect of reflected light on sugar beet growth. The understanding of shade avoidance responses in sugar beet and the role shade avoidance cues plays in yield loss during weed competition could provide novel insights into the critical period for weed control in this crop.
2. Materials and methods