Abstract
The qTSN4 was identified as rice QTL (Quantitative Traits Locus) increasing total spikelet number per panicle and flag leaf area but potentially reducing panicle number depending on the environment. So far, this trade-off was mainly observed at grain maturity and not specifically studied in details, limiting the apprehension of the agronomic interest of qTSN4. This study aimed to understand the effect of qTSN4 and of the environment on panicle sizing, its trade-off with panicle number, and finally plant grain production. It compared two high yielding genotypes to their Near Isogenic Lines (NIL) carrying either QTL qTSN4 or qTSN12, two distinct QTLs contributing to the enlarged panicle size, thereafter designated as qTSN. Traits describing C sink (organ appearance rate, size, biomass) and source (leaf area, photosynthesis, sugar availability) were dynamically characterized along plant and/or panicle development within two trials (greenhouse, field), each comparing two treatments contrasting for plant access to light (with or without shading, high or low planting densities). The positive effect of qTSN on panicle size and flag leaf area of the main tiller was confirmed. More precisely, it could be shown that qTSN increased leaf area and internode cross-section, and in some cases of the photosynthetic rate and starch reserves, of the top 3–4 phytomers of the main tiller. This was accompanied by an earlier tillering cessation, that coincided with the initiation of these phytomers, and an enhanced panicle size on the main tiller. Plant leaf area at flowering was not affected by qTSN but fertile tiller number was reduced to an extent that depended on the environment. Accordingly, plant grain production was enhanced by qTSN only under shading in the greenhouse experiment, where panicle number was not affected and photosynthesis and starch storage in internodes was enhanced. The effect of qTSN on rice phenotype was thus expressed before panicle initiation (PI). Whether early tillering reduction or organ oversizing at meristem level is affected first cannot be entirely unraveled. Further studies are needed to better understand any signal involved in this early regulation and the qTSN × Environment interactions underlying its agronomic interest.
Highlights
Grain yield elaboration in cereals depends on the establishment of panicle number per square meter and panicle size, grain filling rate, and individual grain size (Chen et al, 2008; Gaju et al, 2014)
In IRRI146 background, the reduction rate ranged from 16 to 25.9% (GH Center for Scientific Research (CNRS)) and up to 29.5% in the field (Table 3A). This reduction was associated with a smaller rate of tiller abortion until MAT for the Near Isogenic Lines (NIL) compared to the parents, resulting in a progressive convergence of tiller number of parents and NILs at MAT (Figures 1A,D, 2A,D; Figure S1B; Tables 2, 3)
Fertile tiller number in IRRI146 background in the field kept smaller at MAT in the NIL compared to the parent (22.3 and 19% of reduction under low density (LD) and high density (HD), respectively) as well as for the IR64 parent in HD treatment (21.3 and 24.4% less tillers in NIL and NIL1, respectively, compared to the parent)
Summary
Grain yield elaboration in cereals depends on the establishment of panicle number per square meter and panicle size, grain filling rate, and individual grain size (Chen et al, 2008; Gaju et al, 2014) All these traits are known to compensate per unit area i.e., among plants (Zhang and Yamagishi, 2010) and within the plant where they compete for the same pool of C and N resources (Okawa et al, 2003; Hashida et al, 2013). Amongst these traits in rice, panicle number, and panicle size are those characterized with the highest plasticity under favorable growing conditions with the highest impact on yield elaboration These two traits are determined along plant cycle, based on tillering and green leaf area dynamics, internode reserve remobilization, and reproductive sink size and number. Many studies reported that panicle development, and the determination of spikelet number per panicle, is closely correlated to early plant vigor and underlying traits such as leaf appearance rate (Dong et al, 2004; Streck et al, 2009; Itoh and Shimizu, 2012; Rebolledo et al, 2012), tillering (Lafarge et al, 2002, 2010; Borràs-Gelonch et al, 2012), culm (Fujita and Yoshida, 1984; Wu et al, 2011), and peduncle (Liu et al, 2008) size, plant height (Wang et al, 2014; Chen et al, 2015), or even neck internode diameter (Zhang and Yamagishi, 2010)
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