Abstract
Grain filling may be limited by the joint source and sink capacity in modern wheat cultivars, indicating a need to research the co-limitation of yield by both photosynthesis and the number and potential size of grains. The extent to which the post-anthesis source may be limiting final grain size can be estimated by partial degraining of spikes, while defoliation and shading treatments can be useful to estimate if any excess photosynthetic capacity exists. In the current study, degraining was applied to a set of 26 elite spring wheat cultivars from the International Maize and Wheat Improvement Center (CIMMYT)’s core germplasm (CIMCOG) panel, while lamina defoliation and shading through stem-and-leaf-sheath covering treatments were applied to a subset of the same cultivars. Responses to source treatments in grain weight, pre-anthesis reserve contribution to grain weight, dry-matter translocation efficiency, and flag-leaf and spike photosynthetic rate were measured and compared to an unmanipulated control treatment. Grain weight responses to degraining among cultivars ranged from no response to increases of 28%, suggesting a range of responses from sink limitation, to probable source and sink co-limitation of grain growth. Grain weight’s response to degraining increased linearly with the years of cultivar release from 1966 to 2009, indicating that the current highest yield potential CIMMYT spring wheats have a co-limitation of grain growth by source and sink. This may have been due to an increase in grain sink strength with years of cultivar release with no commensurate increase in post-anthesis source capacity. The relatively low decreases in grain weight with defoliation compared to decreases in light interception by defoliation indicated that sink limitation was still likely predominating in the cultivars with co-limitation. The stem-and-leaf-sheath covering treatment decreased grain weight by nearly 10%, indicating that stem-and-leafsheath photosynthesis plays a key role in grain growth during grain filling. In addition, pre-anthesis reserve contribution to grain weight was increased by ca. 50% in response to lamina defoliation. Our results showed that increasing the post-anthesis source capacity, through increases in stem-and-leaf-sheath photosynthetic rate during grain filling and pre-anthesis reserve contribution to grain weight, is an important objective in enhancing yield potential in wheat through maintaining a source–sink balance.
Highlights
Increases in yield potential have been mainly related to increases in grain m−2 rather than to greater individual grain weight, implying that grain yield in wheat is sink-limited during grainAgronomy 2020, 10, 1527; doi:10.3390/agronomy10101527 www.mdpi.com/journal/agronomyAgronomy 2020, 10, 1527 filling [1]
Regressions of harvest traits on year of release (Supplementary Figure S1a) among the eight historic CIMMYT cultivars evaluated in this study showed that genetic gains in grain yield under yield potential conditions were associated with increases in final aboveground biomass and thousand grain weight rather than increases in grain m−2
The present results highlight the importance of maintaining source strength during grain filling, especially since our results show that modern CIMMYT spring wheat cultivars may be co-limited by source and sink
Summary
Increases in yield potential have been mainly related to increases in grain m−2 rather than to greater individual grain weight, implying that grain yield in wheat is sink-limited during grainAgronomy 2020, 10, 1527; doi:10.3390/agronomy10101527 www.mdpi.com/journal/agronomyAgronomy 2020, 10, 1527 filling [1]. Increases in yield potential have been mainly related to increases in grain m−2 rather than to greater individual grain weight, implying that grain yield in wheat is sink-limited during grain. Aisawi et al [2] reported that grain yield in spring CIMMYT germplasm was associated with grain weight rather than grain m−2 ; in this case, it appeared that yield potential was still associated with increasing grain sink strength but with potential grain weight rather than grain m−2. Degraining experiments carried out in wheat by Gaju et al [6] indicated sink rather than source limitation during grain filling. Consistent with sink limitation of grain growth, Ahmadi et al [7] showed that post-anthesis defoliation did not affect grain yield or individual grain weight in one high yielding winter bread wheat cultivar. The strength of post-anthesis sink limitation had been reduced by wheat breeding under
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