Projected day/night temperatures specifically limits rubisco activity and electron transport in diverse rice cultivars

Ambient temperature has a profound effect on agricultural production. Generally, sub- and supra-optimal temperatures have a negative effect on growth/yield, provided the deviation for optimal is large enough. On average, mean global temperatures are projected to increase by 0.3 to 0.6 °C per decade over the next century. Nighttime temperatures (T _N ) are projected to increase faster (20%–40% by 2100) than day maximum temperatures (T _D ). Therefore, an experiment was conducted to determine the effects of projected sub- and supra-optimal day/night temperatures on two diverse rice (Oryza sativa) cultivars, IR6 and Rex, during reproductive growth. At flowering, nine different day/night temperature treatments (TTs) were imposed and maintained until harvesting using sunlit growth chambers (SPAR units). Alterations to plant metabolism were evaluated using CO ₂ response functions, also, to survey gas exchange readings under ambient conditions. Net photosynthesis (A _N ) declined under both T _D and T _N effects. The decline in maximum carboxylation rate allowed by Rubisco (V _cmax ) and maximal rate of electron transport allowed by the thylakoid reactions (J _max ), with increasing T _N mainly contributed to the decline in A _N . Dark respiration rates (R _D ) declined with increasing temperatures could be a cultivars acclimation mechanism to prevent excessive carbon losses. Cultivars varied mainly for primary photochemistry, with cultivar Rex exhibited significantly higher R _D , and pigment contents but lower J _max than IR6 across TTs. The information on genetic, biochemical and physiological processes in these two diverse cultivars under varied temperature regimes could be useful for developing breeding tools to enhance thermotolerance in US rice cultivars for future climatic conditions.