Genotypic variation in source and sink traits affects the response of photosynthesis and growth to elevated atmospheric CO2.

This study aimed to understand the response of photosynthesis and growth to e-CO2 conditions (800 vs. 400 µmol mol-1 ) of rice genotypes differing in source-sink relationships. A proxy trait called local C source-sink ratio was defined as the ratio of flag leaf area to the number of spikelets on the corresponding panicle, and five genotypes differing in this ratio were grown in a controlled greenhouse. Differential CO2 resources were applied either during the 2 weeks following heading (EXP1) or during the whole growth cycle (EXP2). Under e-CO2 , low source-sink ratio cultivars (LSS) had greater gains in photosynthesis, and they accumulated less nonstructural carbohydrate in the flag leaf than high source-sink ratio cultivars (HSS). In EXP2, grain yield and biomass gain was also greater in LSS probably caused by their strong sink. Photosynthetic capacity response to e-CO2 was negatively correlated across genotypes with local C source-sink ratio, a trait highly conserved across environments. HSS were sink-limited under e-CO2 , probably associated with low triose phosphate utilization (TPU) capacity. We suggest that the local C source-sink ratio is a potential target for selecting more CO2 -responsive cultivars, pending validation for a broader genotypic spectrum and for field conditions.

Is triose phosphate utilization involved in the feedback inhibition of photosynthesis in rice under conditions of sink limitation?

This study aimed to understand the physiological basis of rice photosynthetic response to C source-sink imbalances, focusing on the dynamics of the photosynthetic parameter triose phosphate utilization (TPU). Here, rice (Oriza sativa L.) indica cultivar IR64 were grown in controlled environment chambers under current ambient CO2 concentration until heading, and thereafter two CO2 treatments (400 and 800 µmol mol-1) were compared in the presence and absence of a panicle-pruning treatment modifying the C sink. At 2 weeks after heading, photosynthetic parameters derived from CO2 response curves, and non-structural carbohydrate content of flag leaf and internodes were measured three to four times of day. Spikelet number per panicle and flag leaf area on the main culm were recorded. Net C assimilation and TPU decreased progressively after midday in panicle-pruned plants, especially under 800 µmol mol-1 CO2. This TPU reduction was explained by sucrose accumulation in the flag leaf resulting from the sink limitation. Taking together, our findings suggest that TPU is involved in the regulation of photosynthesis in rice under elevated CO2 conditions, and that sink limitation effects should be considered in crop models.

Plasticity of Sorghum Stem Biomass Accumulation in Response to Water Deficit: A Multiscale Analysis from Internode Tissue to Plant Level.

Sorghum is increasingly used as a biomass crop worldwide. Its genetic diversity provides a large range of stem biochemical composition suitable for various end-uses as bioenergy or forage. Its drought tolerance enables it to reasonably sustain biomass production under water limited conditions. However, drought effect on the accumulation of sorghum stem biomass remains poorly understood which limits progress in crop improvement and management. This study aimed at identifying the morphological, biochemical and histological traits underlying biomass accumulation in the sorghum stem and its plasticity in response to water deficit. Two hybrids (G1, G4) different in stem biochemical composition (G4, more lignified, less sweet) were evaluated during 2 years in the field in Southern France, under two water treatments differentiated during stem elongation (irrigated; 1 month dry-down until an average soil water deficit of -8.85 bars). Plant phenology was observed weekly. At the end of the water treatment and at final harvest, plant height, stem and leaf dry-weight and the size, biochemical composition and tissue histology of internodes at 2-4 positions along the stem were measured. Stem biomass accumulation was significantly reduced by drought (in average 42% at the end of the dry-down). This was due to the reduction of the length, but not diameter, of the internodes expanded during water deficit. These internodes had more soluble sugar but lower lignin and cellulose contents. This was associated with a decrease of the areal proportion of lignified cell wall in internode outer zone whereas the areal proportion of this zone was not affected. All internodes for a given genotype and environment followed a common histochemical dynamics. Hemicellulose content and the areal proportion of inner vs. outer internode tissues were set up early during internode growth and were not drought responsive. G4 exhibited a higher drought sensitivity than G1 for plant height only. At final harvest, the stem dry weight was only 18% lower in water deficit (re-watered) compared to well-watered treatment and internodes growing during re-watering were similar to those on the well-watered plants. These results are being valorized to refine the phenotyping of sorghum diversity panels and breeding populations.

Influence of the radial stem composition on the thermal behaviour of miscanthus and sorghum genotypes.

The hypothesis made is that thermal resistance of sorghum and miscanthus stem pieces taken at well-defined positions of the stem is simply related to their biochemical composition. For miscanthus, two different genotypes and two internode levels were selected. For each region, the stem was divided into three radial layers. For sorghum, two different genotypes were selected and the stem was divided into the same three radial layers. The results show that the thermal analysis is only sensitive to very large variations of compositions. But aside of such large composition differences, it is impossible to correlate thermal effects to biochemical composition even on very small size, well-identified pieces of plant materials. The interplay between sugar-based components, lignin and minerals is totally blurring the thermal response. Extreme care must be exercised when willing to explain why a given plant material has a thermal behaviour different of another plant material.

Grain, sugar and biomass accumulation in photoperiod-sensitive sorghums. II. Biochemical processes at internode level and interaction with phenology.

Sugar accumulation in sorghum (Sorghum bicolor (L.) Moench) stems is a complex trait that is particularly plastic in response to photoperiod. This study investigated sucrose accumulation in a sterile (no grain filling) and fertile near-isogenic line of the photoperiod-sensitive cultivar IS2848 in two greenhouse experiments. Variable phenology was induced by applying a short (12-h PP) and a long (13-h PP) photoperiod. Dynamics of plant growth, phenology, sugar accumulation and related enzyme activities in internodes were investigated. Under 13-h PP, plants flowered 28 days later and attained threefold higher sucrose concentration at anthesis compared with those under 12-h PP. Sucrose accumulation in individual internodes was driven by organ physiological age, not by plant phenology. Competition with grain filling was marginal but greater under 12-h PP (i.e. when sucrose accumulation in internodes occurred after flowering). Enzyme activities showed marked developmental patterns but contributed little to explaining differences between treatments and genotypes. The study demonstrates that sucrose storage physiology in sweet sorghum resembles that of sugarcane (Saccharum spp.) but is more complex due to photoperiod effects on phenology. It confirms the field results on 14 sorghum genotypes contrasting for phenology and photoperiod sensitivity presented in a companion paper. Perspectives for developing sorghum ideotype concepts for food and fuel crops are discussed.