Carbon Assimilation and Photosystem Responses of Two Rice Cultivars Acclimated to Sub- and Supra-Optimal Day/Night Temperature Regimes.

The projected increase in nighttime air temperatures (T_N) of 20% and 40% by 2100 in the US under lower and higher greenhouse gases emissions scenarios, respectively, could reduce rice yields through impairing biochemical and physiological processes. Therefore, an experiment was conducted to quantify the effects of projected sub- and supra-optimal day/night temperatures on carbon assimilation and photosystem response mechanisms in two genetically diverse rice cultivars during reproductive growth. The two rice cultivars, IR6 and Rex, were grown in total of 240 PVC pots containing soil medium of pure fine sand under uniform natural conditions until flowering. At flowering, nine different day/night temperature treatments (TTs) namely 33.4/21.4 (control), 33.4/23.6, 33.4/25.7, 36.4/24.4, 36.4/26.9, 36.4/29.3, 30.4/18.4, 30.4/20.3, 30.4/22.1 ^oC, were imposed to each of 12 plants per cultivar, and maintained until harvesting using nine controlled sunlit growth chambers (SPAR units). The LiCOR 6800 and 6400 photosynthesis systems were used to determine carbon dioxide and light response functions from 1000-1400 hours between 7-15 days after the start of TTs initiation. In addition, net photosynthesis (A_N), gross photosynthesis (A_G), mid-day dark respiration (R_D), maximum quantum yield (F_vF_m), leaf thickness, pigment content, relative injury (RI), and cell membrane thermostability (CMT) were also determined. The maximum rate of Rubisco carboxylation (V_cmax) and maximal rate of electron transport (J_max), derived from the response functions, declined with increasing T_N. A significant decline in A_N, R_D, and A_G was also observed with increasing T_N under control and supra-optimal TTs. Cultivar Rex exhibited significantly higher J_max, R_D, and pigment contents than IR6 across TTs. The understanding of genetic, biochemical and physiological processes between the two diverse cultivars under varied temperature regimes from this study could be used in breeding tools to enhance the tolerance in the US rice cultivars for future climatic condition.