Atmospheric CO2 concentration effects on rice water use and biomass production

Abstract

Numerous studies have addressed effects of rising atmospheric CO2 concentration on rice biomass production and yield but effects on crop water use are less well understood. Irrigated rice evapotranspiration (ET) is composed of floodwater evaporation and canopy transpiration. Crop coefficient Kc (ET over potential ET, or ETo) is crop specific according to FAO, but may decrease as CO2 concentration rises. A sunlit growth chamber experiment was conducted in the Philippines, exposing 1.44-m2 canopies of IR72 rice to four constant CO2 levels (195, 390, 780 and 1560 ppmv). Crop geometry and management emulated field conditions. In two wet (WS) and two dry (DS) seasons, final aboveground dry weight (agdw) was measured. At 390 ppmv [CO2] (current ambient level), agdw averaged 1744 g m-2, similar to field although solar radiation was only 61% of ambient. Reduction to 195 ppmv [CO2] reduced agdw to 56±5% (SE), increase to 780 ppmv increased agdw to 128±8%, and 1560 ppmv increased agdw to 142±5%. In 2013WS, crop ET was measured by weighing the water extracted daily from the chambers by the air conditioners controlling air humidity. Chamber ETo was calculated according to FAO and empirically corrected via observed pan evaporation in chamber vs. field. For 390 ppmv [CO2], Kc was about 1 during crop establishment but increased to about 3 at flowering. 195 ppmv CO2 reduced Kc, 780 ppmv increased it, but at 1560 ppmv it declined. Whole-season crop water use was 564 mm (195 ppmv), 719 mm (390 ppmv), 928 mm (780 ppmv) and 803 mm (1560 ppmv). With increasing [CO2], crop water use efficiency (WUE) gradually increased from 1.59 g kg-1 (195 ppmv) to 2.88 g kg-1 (1560 ppmv). Transpiration efficiency (TE) measured on flag leaves responded more strongly to [CO2] than WUE. Responses of some morphological traits are also reported. In conclusion, increased CO2 promotes biomass more than water use of irrigated rice, causing increased WUE, but it does not help saving water. Comparability with field conditions is discussed. The results will be used to train crop models.

Rice with reduced stomatal density conserves water and has improved drought tolerance under future climate conditions

“…In predicted future higher CO 2 climates, it has been suggested that plants will be more water-use efficient as enhanced photosynthetic A allows stomata to be less open, meaning less water will be lost (Keenan et al, 2013). However, despite grain yields increasing in experiments where rice is grown at elevated CO 2 , a greater volume of water is used than at current CO 2 levels, indicating that, in the future, rice cultivation may be even more water intensive than it is today (Kumar et al, 2017).…”

“…Rising CO 2 levels are expected to result in a warming of 1-4°C in global atmospheric temperatures by the end of the century, and the frequency of heat spikes will also increase (Meyer et al, 2014). Such dramatic rises in temperature are expected to lead to negative impacts on rice yields even in the presence of increased atmospheric CO 2 (Ainsworth, 2008;Kumar et al, 2017). Rice is particularly sensitive to heat stress, with the majority of growth stages being affected once temperatures exceed 35°C (Redfern et al, 2012).…”

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

“…Elevated [CO 2 ] typically decreases stomatal conductance, resulting in a decrease in ET and increase in canopy temperature (T c ) for rice (Shimono, Nakamura, Hasegawa, & Okada, 2013;Yoshimoto, Oue, & Kobayashi, 2005) and other crops (Leakey et al, 2009 and references therein). However, Kumar, Quick, Barrios, Sta Cruz, and Dingkuhn (2017) recently reported an increase in ET from a rice canopy under elevated [CO 2 ]. These different results suggest that further research is still necessary on how energy balance in a rice canopy changes in response to elevated [CO 2 ] conditions.…”

Assessment of empirical methods for estimation of reference evapotranspiration in the Brazilian Savannah

“…Thus, alternative approaches that support limited data are needed, which has led to a large number of related studies applied to various climatic conditions. Evapotranspiration, regardless of evaluated spatiotemporal scale, is extremely variable, because it is driven by the evaporative demand of the atmosphere (Kumar et al, 2017). This feature justifies the knowledge of its dynamics in a delimited geographical area, as these are areas of federal units, the states.…”

“…In agreement with Allen et al (1998), these characteristics would be similar to the evapotranspiration of an extensive grass surface, with uniform height, which grows actively, totally shadows the soil without any water limitation. This hypothetical condition is known as reference evapotranspiration (ETo), and when multiplied by a specific crop coefficient (kc), satisfactorily estimates the water demand of a plant community, since it normalizes the ET observed in variable climatic conditions (Kumar et al, 2017). In the last decades, several methods were developed to estimate ETo (Penman, 1948;Blaney and Criddle, 1950;Makkink, 1957;Turc, 1961;Priestley and Taylor, 1972;Hargreaves and Samani, 1985;Oudin et al, 2005).…”