Phenylalanine ammonia-lyase (PAL) which is considered to be one of the main lines of cell acclimation against stress in plants, non-structural carbohydrates (NSC) accumulation and chlorophyll fluorescence parameters were quantified in two rice genotypes as a function of two temperature regimes: 22/30 °C (control) and 28/30 °C night/day (high night temperatures -HNT), imposed from heading to milk stage. The rice cultivars chosen were Nagina22 (N22) and BRS Querência (BRS-Quer), which are genotypes tolerant and sensitive to high temperatures, respectively. BRS-Quer genotype highlighted more sensitive responses maintaining higher PAL and peroxidase levels on seventh and twenty-first days after stress imposing. On the other hand, this genotype showed levels of fructose, glucose and sucrose decreasingly across stress period whether compared to N22. Both genotypes showed similarity for most of the chlorophyll fluorescence parameters. However, the photosynthesis induction curve highlighted that HNT caused decreases in some photochemical quenching of fluorescence as well as increases of non-phochemical quenching, affecting more prominently BRS-Quer genotype. N22 maintained unaltered the spikelet sterility and 1000-grain weight across temperature regimes showing a consistent trend with its stem NSC accumulation during stress period. The higher availability of soluble sugars shown by N22 at the end of stress period could be unloaded in spikelet formation and grain fillings contributing in their lower sterility rate and greater 1000-grain weight stability across the environments. These results indicate that selecting genotypes with higher capacity to stem NSC translocation beyond accumulation at HNT could lead to more grain yield stability in future climate scenarios.

Non-structural carbohydrates (NSC) accumulation and photosynthesis traits were studied in two rice (Oryza sativa L.) genotypes maintained under control (22/30 °C -night/day) and at high night temperatures (HNT) (28/30 °C) conditions from heading to milk stage. Rice cultivars were Nagina22 -N22 and BRS Querência -Quer, which are tolerant and sensitive to high temperatures, respectively. The source-sink flow related attributes were tested to understand the nature of NSC accumulation and translocation. Compared to N22, Quer maintained higher stem starch in glucose on seventh day after heading and at milk stage independently of imposed temperatures. However, the levels of starch in glucose were lower for N22 meanwhile their total sugar concentration (TSC) were higher at control and at HNT at milk stage as compared to Quer. N22 maintained unaltered the spikelet sterility and 1000-grain weight across environments showing a consistent trend with its stem NSC translocation. Both genotypes showed similarity in some gas exchange and chlorophyll fluorescence performance suggesting unaffected photosystem II photochemistry, linear electron flux, and CO 2 assimilation. Beyond indicating that source functioning was not the limiting factor for low TSC and starch in glucose levels found in N22 on seventh day after heading stage. Moreover, our data suggest that the higher translocation capacity shown by N22 can be involved in their lower spikelet sterility and 1000-grain weight stability across the environments. These results indicate that selecting genotypes with higher capacity to stem NSC translocation at HNT could lead to more grain yield stability in future climate scenarios.

The study was conducted using physiological approach to identify rice accessions with superior performance when subjected to infra-optimum temperatures during initial development phase (V 2 -V 4 ). Forty-two rice genotypes composed by background essentially indica, japonica and indica/japonica cross with a broad genetic and ecological diversity were used. Plants were grown under initial optimum temperatures gradient (OTG -22/32 °C night/day) until V2 stage; subsequently were subjected to infra-optimum temperature gradient (ITG -13/17 °C night/day) during three days; after all genotypes returned to OTG conditions for seven days to recovery. Principal components analysis (PCA) highlighted that the three principal components account for 75.16% of total variation at the end of evaluated period. There were similar contributions of effective quantum yield (Y(II) -stress) and electron transport rate variables after recovery period (ETR -recovery) for PC1. Interestedly, genotypes highly responsive under initial OTG which showed fast initial biomass accumulation were also highly sensitive to stress when subjected to ITG, with accentuated decreases in their physiological performance. Sel. TB 1211-3 line, CTB 1419, CTB 1444, CTB 1455 and AB 13720 progenies showed greater performance for physiological analyzed variables, being potentially useful for breeding efforts aiming improve cold tolerance in rice at initial phase.

Rice (Oryza sativa L.) can be negatively impacted by supraoptimum temperatures (above 33 °C) during initial reproductive phase (R3-R5); development and adoption of approaches via non-invasive physiological phenotyping can lead to help build new plant types to face the current extreme climatic events such as future forecasts. For this purpose, screening process was designed to progressively decrease the genotypes number via non-invasive phenotyping approaches; beyond to allow the increase of phenotyping dimensionality degree across tiers. In a first-tier (in 2015-2016 growth season), phenotyping procedures involved measurements of dossel temperatures via thermography imaging in a set of 182 accessions of subspecies Indica, Japonica and Indica/Japonica cross from Embrapa’s Rice Breeding, which were cultivated in two sowing dates. About 30% (55) of the initial genotypes number which showed the lower canopy temperatures were selected based on results of multivariate analyses. In a second-tier (2016-2017 crop season), a second field trial was conducted, using polythene shelters structures aiming ensure the heat stress imposing during the critical phases of plant development; during this period, an effective photochemical quantum yield of photosystem II (YII) performance was monitored across set of genotypes. Data obtained are highlighted and discussed allowing suggest appointments about the usability/bottlenecks of thermography as suitable tool for phenotyping in a large scale manner; beside highlight the importance of some physiological responses as part of the basis of rice heat tolerance. Concluding, the LTB 14031 and BRS Pampa genotypes outperformed the set of evaluated genotypes across sowing dates and years relative to their physiological and grain yield components variables; these genotypes are integrating cross-breeding aiming to construct new plants ideotype which can associate higher grain yield performance when grown under non-stressed conditions and capable to maintain great yield stability under hard environments.