Overreduction states of photosystem I (PSI) cause PSI photoinhibition. To examine the degree of PSI photoinhibition, changes in its electron transport activity and core-subunit content were evaluated. In contrast, the involvement of peripheral subunits, such as the light-harvesting complex I (LHCI), is less discussed in PSI photoinhibition. Here, we aimed to elucidate whether the light-harvesting functions of LHCI are altered by PSI photoinhibition in rice leaves. To this end, a new method to estimate the functional antenna size of the PSI-LHCI complex was developed using a far-red light-dependent absorption change in the oxidized reaction center chlorophyll (P700 +) in intact leaves, and the obtained kinetics were analyzed using the double Gompertz model. Subsequently, our original parameter, kfast (Far Red-Photon Flux Density) -1, showed a linear response to leaf LHCI content, and the parameter decreased with a decrease in P700 content by PSI photoinhibition in leaves. Furthermore, we found that the susceptibility of LHCI to PSI photoinhibitory treatment decreased with increasing in growth light intensity. Our results suggest that LHCI is a target of PSI photoinhibition, as well as the core-subunits, and rice plants can lower the risk of LHCI photoinhibition through light acclimation responses.

Tropospheric ozone (O 3) significantly reduces rice yield. Koshihikari, a leading Japanese rice cultivar, has been recognized as an O 3 tolerant cultivar; however, the mechanisms underlying its high O 3 tolerance remain unknown. Therefore, we aimed to elucidate the genetic and physiological mechanisms underlying high O 3 tolerance in Koshihikari. A series of chamber experiments were conducted to examine photosynthesis, growth, yield-related traits, and gene expression profiles under chronic O 3 conditions in Koshihikari and Takanari, with contrasting O 3 tolerance. Koshihikari showed no reduction in unhulled-grain weights due to higher total dry weight under chronic O 3 conditions, whereas Takanari showed significantly lower grain weights. The high O 3 tolerance in Koshihikari was attributed to its highly stable photosynthetic performance and increased biomass allocation to its leaf blades. RNA-seq and gene co-expression network analyses revealed that the genes involved in photosynthesis and carbohydrate metabolism are associated with contrasting O 3 tolerance. OsRbcS3, encoding the RuBisCO small subunit, showed contrasting expression profiles between the two cultivars. In Koshihikari, OsRbcS3 was identified as a hub gene candidate in the gene co-expression network, which was highly correlated with photosynthetic performance. These results suggest that OsRbcS3 plays a key role in the genetic mechanisms underlying the high O 3 tolerance of Koshihikari.

Drought stress is a major limiting factor for crop growth and yield. Water availability in the field can cyclically change between drought and rewatering conditions, depending on precipitation patterns. Concurrently, light intensity under field conditions can fluctuate, inducing dynamic photosynthesis and transpiration during crop growth period. The present study aimed to characterize carbon gain and water use in fluctuating light under drought and rewatering conditions by conducting gas exchange measurements in two major crops, namely rice and soybean. In both crops, drought stress reduced steady-state photosynthesis and/or photosynthetic capacity, and delayed photosynthetic induction even when it had relatively small impact on photosynthetic capacity, suggesting that the drought effects on photosynthesis should be evaluated based on induction, maximum, and steady states. This delayed photosynthetic induction resulted in a substantial loss of carbon gain under fluctuating light conditions, which can be a limiting factor for crop growth and yield in the field. Meanwhile, rewatering after drought conditions completely recovered photosynthetic capacity and induction in both crops, whereas drought experience would be memorized to slow down the stomatal opening. Therefore, the stability of photosynthetic induction can be a promising target to improve drought tolerance during crop breeding in the future.