Increasing canopy photosynthesis in rice can be achieved without a large increase in water use—A model based on free‐air CO₂ enrichment

Abstract

Achieving higher canopy photosynthesis rates is one of the keys to increasing future crop production; however, this typically requires additional water inputs because of increased water loss through the stomata. Lowland rice canopies presently consume a large amount of water, and any further increase in water usage may significantly impact local water resources. This situation is further complicated by changing the environmental conditions such as rising atmospheric CO concentration ([CO ]). Here, we modeled and compared evapotranspiration of fully developed rice canopies of a high-yielding rice cultivar (Oryza sativa L. cv. Takanari) with a common cultivar (cv. Koshihikari) under ambient and elevated [CO ] (A-CO and E-CO , respectively) via leaf ecophysiological parameters derived from a free-air CO enrichment (FACE) experiment. Takanari had 4%-5% higher evapotranspiration than Koshihikari under both A-CO and E-CO , and E-CO decreased evapotranspiration of both varieties by 4%-6%. Therefore, if Takanari was cultivated under future [CO ] conditions, the cost for water could be maintained at the same level as for cultivating Koshihikari at current [CO ] with an increase in canopy photosynthesis by 36%. Sensitivity analyses determined that stomatal conductance was a significant physiological factor responsible for the greater canopy photosynthesis in Takanari over Koshihikari. Takanari had 30%-40% higher stomatal conductance than Koshihikari; however, the presence of high aerodynamic resistance in the natural field and lower canopy temperature of Takanari than Koshihikari resulted in the small difference in evapotranspiration. Despite the small difference in evapotranspiration between varieties, the model simulations showed that Takanari clearly decreased canopy and air temperatures within the planetary boundary layer compared to Koshihikari. Our results indicate that lowland rice varieties characterized by high-stomatal conductance can play a key role in enhancing productivity and moderating heat-induced damage to grain quality in the coming decades, without significantly increasing crop water use.

Quantifying the Feedback Between Rice Architecture, Physiology, and Microclimate Under Current and Future CO2 Conditions

“…Especially the high photosynthetic capacity of Takanari results in a damped response to atmospheric warming when CO 2 levels rise, as the high stomatal conductance and transpiration associated with this higher photosynthesis of Takanari resulted in a larger evaporative cooling effect compared to Koshihikari. The difference in canopy temperature between the two varieties (~0.8 ° C) is greater than that reported in our recent model study (~0.5 ° C; Figure 7‐c in Ikawa et al, ), which assumed the same canopy architecture between the two varieties. This emphasizes the importance of considering differences in architectural effects between varieties in future modeling studies.…”

“…Resulting from a reduced radiative in‐canopy interception, daytime in‐canopy temperatures of Koshihikari were lower compared to Takanari (Figure a). However, lower (~0.5 ° C) leaf temperatures were found in Takanari, which likely resulted from the higher stomatal conductance of Takanari and its associated higher evaporative cooling rates known for this variety (Ikawa et al, ). Paradoxically, this did not result in a more humid canopy (Figure b).…”

“…temperatures were found in Takanari, which likely resulted from the higher stomatal conductance of Takanari and its associated higher evaporative cooling rates known for this variety (Ikawa et al, 2018). Paradoxically, this did not result in a more humid canopy (Figure 4b).…”

“…Takanari has been identified as a potential future candidate to increase rice grain yield in Japan. Compared to Koshihikari, Takanari maintains higher levels of (leaf) photosynthesis (e.g., Chen et al, ) and exhibits higher stomatal conductance (30–40%) (Chen et al, ; Hasegawa et al, ; Ikawa et al, ), resulting in an increase in yields levels of up to 21% (Hasegawa et al, ). Furthermore, Takanari is able to achieve higher productivity in elevated [CO 2 ] conditions, due to its high sink capacity and photosynthetic activity, and showed greater yield enhancement under elevated CO 2 compared to Koshihikari (Hasegawa et al, ; Hasegawa et al, ; Nakano et al, ; Zhang et al, ).…”

“…The high stomatal conductance, thus transpirational cooling of Takanari (e.g., Ikawa et al, 2018), becomes evident in the diurnal course of canopy temperature (Figure 6b). Throughout the day, Takanari maintains a lower canopy temperature, while the canopy temperature of Koshihikari showed a strong increase (Figure 6b).…”