The qTSN Positive Effect on Panicle and Flag Leaf Size of Rice is Associated with an Early Down-Regulation of Tillering

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.

The qTSN4 Effect on Flag Leaf Size, Photosynthesis and Panicle Size, Benefits to Plant Grain Production in Rice, Depending on Light Availability

“…In experiment GH-IRRI 2014, the temporary reduction in population during panicle development (increase of spacing among pots) significantly increased FLA and width ( Figures 1A,B ) but did not increase SPN ( Figure 1C ) or PGDW ( Figure 1D ) despite the greater light resources. This surprising result can, however, be explained by the variability observed among plants in this trial, as tiller number was on average higher, at the time when the treatments were established, in plants grown in higher population density compared to those isolated (see Adriani et al, 2016 ). This initial difference might have hidden the expected, positive effect of population reduction on PGDW.…”

“…Across all experiments and treatments, qTSN4 increased FLA by 38% FLW by 17%, SPN by 22%, PGDW by 7%, and it reduced panicle number per plant by 10% (as presented in Adriani et al, 2016 ). The most consistent effect of qTSN4 was on FLW, in terms of orientation of the effect and the number of environments where it was significant.…”

“…Quadrupling population (Field-IRRI 2014) caused a 76% reduction in PGDW, nearly exactly proportionally to the fraction of PAR resources remaining per plant. The SPN only decreased by 17%, however, and most of the reduction of PGDW was caused by reduced panicle number per plant ( Adriani et al, 2016 ).…”

“…The effects of qTSN4 on 1000 grain dry weight was also inconsistent, as a significant reduction was observed in IR64 background in GH-CNRS and field 2014, but a significant increase was observed in IRRI 146 background in GH-CNRS whereas no effect was observed in the field. In fact, the positive effect of the QTL on SPN was compensated by a negative effect on panicle number per plant (as presented in Adriani et al, 2016 ), resulting in the variable (positive or negative) effects on PGDW.…”

“…PGDW (in g), Spikelet Number per panicle (SPN) on the main stem (recoded by P-TRAP software, Al-Tam et al, 2013 ) were measured at final harvest (physiological maturity of grains). Dry weight was obtained after drying samples at 70°C during 48 h. Plant phenology and development were measured but are presented in another publication ( Adriani et al, 2016 ); accordingly these parameters will not be addressed into detail in the present study.…”