Abstract:
Background: Panicle architectural traits in rice (branching, rachis length, spikelet number) are established between
panicle initiation and heading stages. They vary among genotypes and are prone to Genotype x Environment
interactions. Together with panicle number, panicle architecture determines sink-based yield potential. Numerous
studies analyzed genetic and environmental variation of plant morphology, but the plasticity of panicle structure
is less well understood. This study addressed the response of rice panicle size and structure to limited light availability
at plant level for near-isogenic lines (NILs) with IR64 or IRRI146 backgrounds, carrying the QTL qTSN4 (syn. SPIKE) for
large panicles. Full light and shading in the greenhouse and two population densities in the field were implemented.
The image analysis tool P-TRAP was used to analyze the architecture of detached panicles.
Results: The qTSN4 increased total branch length, branching frequency and spikelet number per panicle in IRRI146
background in the field and greenhouse, and in IR64 background in the greenhouse, but not for IR64 in the field.
In the field, however, qTSN4 reduced panicle number, neutralizing any potential yield gains from panicle size.
Shading during panicle development reduced spikelet and branch number but qTSN4 mitigated partly this effect.
Spikelet number over total branch length (spikelet density) was a stable allometry across genotypes and treatments
with variation in spikelet number mainly due to the frequency of secondary branches. Spikelet number on the main
tiller was correlated with stem growth rate during panicle development, indicating that effects on panicle size seemed
related to resources available per tiller.
Conclusions: The qTSN4 effects on panicle spikelet number appear as indirect and induced by upstream effects on
pre-floral assimilate resources at tiller level, as they were (1) prone to G x E interactions, (2) non-specific with respect to
panicle architectural traits, and (3) associated with pre-floral stem growth rate.
