Résumé
Considering the mean global temperature increase of 0.7°C from the start of the industrial era and the prediction window of additional increase of 1 to 3.7°C by the end of the century, adaptation through crop improvement and adjustment of cultural practices is essential. One way of achieving this challenge is to better take into account the plant response to temperature into crop models. In the case of rice, mostly grown in tropical and subtropical regions, the increase in temperature goes with systematic yield reduction which makes this challenge even more critical. Two main situations need to be analyzed: the impact of (i) daytime heat on spikelet sterility and (ii) nighttime temperature on respiration. (i) Coping with heat stress at day time involves different options that account for the ability of plants to escape (early anthesis time), avoid (panicle cooling through transpiration) or tolerate (presence of key genes) heat at flowering. First, variability in the time of day of anthesis was correlated with the mean of climatic variables calculated for the 7-day period before flowering, over four distinct field locations and seasons and four contrasted varieties. The best predictive variables (negative correlations) were Tmin and VPD, with high values of both being associated with early times. Second, depending on conditions, panicle temperature varied between 9°C below and 2°C above air temperature at 2 m. A significant positive correlation was obtained between spikelet sterility rate and maximum panicle temperature at flowering, whereas no correlation was obtained with air temperature. By extrapolation, this correlation predicted minimal sterility with a panicle temperature of 30 °C, and 50 % sterility with a panicle temperature of 33-34 °C. Third, a genome-wide association study of the sterility rate of 167 traditional and modern varieties (grown for six consecutive days at 37°C between 8 am and 2 pm at anthesis) detected 91 significant associations grouped into 12 independent regions located on eight chromosomes. The highest heat tolerance was detected for N22, an aus variety from India, and Peh Kuh, a traditional indica variety from Taiwan. (ii) Global temperature increase has been higher at night than at day time and night temperature is predicted to increase by 3°C by 2050. While no escape or avoidance pathway can address the effect of high night temperature on respiration as it occurs at night when the plant cooling system is minimal and mostly at any time during plant cycle so that the plant cannot really escape it, tolerance is seen as a major option. As a preliminary step, the focus was conducted here on the change of respiration with temperature of 2-4 contrasted varieties grown in field and controlled environments. While the increase of night respiration with temperature rising from 21 to 31°C was 2.4-fold without acclimation, it was only between 1.2 and 1.7-fold with acclimation. In the same way, the maintenance respiration, which was estimated by assimilate starvation at 34% of the night respiration, increased by a factor of 1.49 when temperature rose from 21 to 31°C. These figures are lower than the common assumption of the Q10=2 rule that overestimates the effect of increasing night temperature on respiration in acclimated conditions. Even if the cost in carbohydrates of night respiration over crop duration varied from 8 to 20% of the potential shoot dry matter depending on the conditions, the additional burden due to increased night temperature was only 1 to 7%. As a general conclusion, avoidance (for daytime stress only) and tolerance (for both day and night time stresses) appear as the main genetic improvement pathways to cope with the thermal component of climate change. A Q10=1.5 rule seems closer to the reality to account for respiration changes with temperature into crop models where the estimation of canopy temperature is essential to predict spikelet sterility. (Texte intégral)