Masumi Okada

Rice cultivar responses to elevated CO2 at two free-air CO2 enrichment (FACE) sites in Japan

There is some evidence that rice cultivars respond differently to elevated CO2 concentrations ([CO2]), but [CO2]×cultivar interaction has never been tested under open-field conditions across different sites. Here, we report on trials conducted at free-air CO2 enrichment (FACE) facilities at two sites in Japan, Shizukuishi (2007 and 2008) and Tsukuba (2010). The average growing-season air temperature was more than 5°C warmer at Tsukuba than at Shizukuishi. For four cultivars tested at both sites, the [CO2]×cultivar interaction was significant for brown rice yield, but there was no significant interaction with site-year. Higher-yielding cultivars with a large sink size showed a greater [CO2] response. The Tsukuba FACE experiment, which included eight cultivars, revealed a wider range of yield enhancement (3-36%) than the multi-site experiment. All of the tested yield components contributed to this enhancement, but there was a highly significant [CO2]×cultivar interaction for percentage of ripened spikelets. These results suggest that a large sink is a prerequisite for higher productivity under elevated [CO2], but that improving carbon allocation by increasing grain setting may also be a practical way of increasing the yield response to elevated [CO2].

Genotypic variation in rice yield enhancement by elevated CO2 relates to growth before heading, and not to maturity group

Maturity group (based on the number of days to maturity) is an important growth trait for determining crop productivity, but there has been no attempt to examine the effects of elevated [CO2] on yield enhancement of rice cultivars with different maturity groups. Since early-maturing cultivars generally show higher plant N concentration than late-maturing cultivars, it is hypothesized that [CO2]-induced yield enhancement might be larger for early-maturing cultivars than late-maturing cultivars. To test this hypothesis, the effects of elevated [CO2] on yield components, biomass, N uptake, and leaf photosynthesis of cultivars with different maturity groups were examined for 2 years using a free-air CO2 enrichment (FACE). Elevated [CO2] significantly increased grain yield and the magnitude significantly differed among the cultivars as detected by a significant [CO2]×cultivar interaction. Two cultivars (one with early and one with late maturity) responded more strongly to elevated [CO2] than those with intermediate maturity, resulting mainly from increases in spikelet density. Biomass and N uptake at the heading stage were closely correlated with grain yield and spikelet density over [CO2] and cultivars. Our 2 year field trial rejected the hypothesis that earlier cultivars would respond more to elevated [CO2] than later cultivars, but it is revealed that the magnitude of the growth enhancement before heading is a useful criterion for selecting rice cultivars capable of adapting to elevated [CO2].

Effects of ozone on dry matter partitioning and yield of Japanese cultivars of rice (Oryza sativa L.)

Abstract Rice plants were exposed to ozone in field exposure chambers throughout most of the growing seasons of 3 years from 1987 through 1989. Cultivar ‘Koshi-hikari’ was used for the 1987 and 1989 experiments, and cv. ‘Nippon-bare’ was used for the 1988 experiment. Ozone exposure was performed for 7 h from 09:00 to 16:00 h each day, and ozone concentration in each of the five chambers was maintained at either 0.5, 1.0, 1.5, 2.0, or 2.75 times ambient ozone level. Throughout the growing seasons, plant samples were taken for measuring leaf area and dry weight of plant parts. At harvest, samples were taken to determine grain yield and yield components. The results of the ozone exposures indicated no consistent effect of ozone on leaf area growth, whereas total dry matter decreased with increased ozone concentration. The effect of ozone on total dry matter was particularly evident after heading. Ozone also affected dry matter partitioning. There was increased dry matter distribution to the leaf blades compared with the leaf sheaths, culms, or roots. Rice yield was significantly reduced by ozone. Although the yield was significantly different among the 3 years, there was no significant interaction between the ozone treatment and the year of the ozone exposures. The relationship between ozone concentration and yield loss of rice in this study was comparable to the relationship reported for rice cultivars in California, but was different from that reported for soybeans, which showed much greater ozone-induced yield loss than rice. Among the yield components, 1000 grain weight was significantly reduced by ozone. Harvest index was not affected by the ozone treatment. The above results indicated that rice yield is reduced by ozone in a realistic range of 20–100 nl 1−1. The results for yield components were compared with other reports. The increased dry matter distribution to leaf blade was discussed with regard to its implications for leaf area growth and lodging.

Effect of Low Root Temperature on Hydraulic Conductivity of Rice Plants and the Possible Role of Aquaporins

The role of root temperature T(R) in regulating the water-uptake capability of rice roots and the possible relationship with aquaporins were investigated. The root hydraulic conductivity Lp(r) decreased with decreasing T(R) in a measured temperature range between 10 degrees C and 35 degrees C. A single break point (T(RC) = 15 degrees C) was detected in the Arrhenius plot for steady-state Lp(r). The temperature dependency of Lp(r) represented by activation energy was low (28 kJ mol(-1)) above T(RC), but the value is slightly higher than that for the water viscosity. Addition of an aquaporin inhibitor, HgCl(2), into root medium reduced osmotic exudation by 97% at 25 degrees C, signifying that aquaporins play a major role in regulating water uptake. Below T(RC), Lp(r) declined precipitously with decreasing T(R) (E(a) = 204 kJ mol(-1)). When T(R) is higher than T(RC), the transient time for reaching the steady-state of Lp(r) after the immediate change in T(R) (from 25 degrees C) was estimated as 10 min, while it was prolonged up to 2-3 h when T(R) < T(RC). The Lp(r) was completely recovered to the initial levels when T(R) was returned back to 25 degrees C. Immunoblot analysis using specific antibodies for the major aquaporin members of PIPs and TIPs in rice roots revealed that there were no significant changes in the abundance of aquaporins during 5 h of low temperature treatment. Considering this result and the significant inhibition of water-uptake by the aquaporin inhibitor, we hypothesize that the decrease in Lp(r) when T(R) < T(RC) was regulated by the activity of aquaporins rather than their abundance.

Changes in source-sink relations during development influence photosynthetic acclimation of rice to free air CO2 enrichment (FACE)

Relationships between photosynthetic acclimation and changes in the balance between source-sink supply and demand of carbon (C) and nitrogen (N) were tested using rice (Oryza sativa L. cv. Akitakomachi). Plants were field-grown in northern Japan at ambient CO2 partial pressure [p(CO2)] or free air CO2 enrichment (FACE; p(CO2) ~ 26-32 Pa above ambient) with low, medium or high N supplies. Leaf CO2 assimilation rates (A) and biochemical parameters were measured at 32-36 (eighth leaf) and 76-80 (flag leaf) d after transplanting, representing stages with a contrasting balance between C and N supply and demand in sources and sinks. Acclimation due to FACE was pronounced in flag leaves at each N supply. This was not fully accounted for by reductions in leaf N concentrations, because A/N and Vcmax/N were lower in FACE-grown flag leaves. Acclimation did not occur in the eighth leaf, and A/N and Vcmax/N was not significantly increased in FACE-grown leaves. Soluble protein / sucrose and amino acid / sucrose concentrations decreased under FACE, whereas sucrose phosphate synthase protein levels increased. At flag leaf stage, there was a discrepancy between the demand and supply of N, which was resolved by enhanced leaf N remobilization, associated with the lower Rubisco concentrations under FACE. In contrast to the early growth stage, enhanced growth of rice plants was accompanied by increased plant N uptake in FACE. We conclude that photosynthetic acclimation in flag leaves occurs under FACE because there is a large demand for N for reproductive development, relative to supply of N from root uptake and remobilization from leaves.

The Chilling Injury Induced by High Root Temperature in the Leaves of Rice Seedlings

Root temperature is found to be a very important factor for leaves to alter the response and susceptibility to chilling stress. Severe visible damage was observed in the most active leaves of seedlings of a japonica rice (Oryza sativa cv. Akitakomachi), e.g. the third leaf at the third-leaf stage, after the treatment where only leaves but not roots were chilled (L/H). On the other hand, no visible damage was observed after the treatment where both leaves and roots were chilled simultaneously (L/L). The chilling injury induced by L/H, a novel type of chilling injury, required the light either during or after the chilling in order to develop the visible symptoms such as leaf bleaching and tissue necrosis. Chlorophyll fluorescence parameters measured after various lengths of chilling treatments showed that significant changes were induced before the visible injury. The effective quantum yield and photochemical quenching of PSII dropped dramatically within 24 h in both the presence and absence of a 12 h light period. The maximal quantum yield and non-photochemical quenching of PSII decreased significantly only in the presence of light. On the other hand, L/H chilling did not affect the function of PSI, but caused a significant decrease in the electron availability for PSI. These results suggest that the leaf chilling with high root temperature destroys some component between PSII and PSI without the aid of light, which causes the over-reduction of PSII in the light, and thereby the visible injury is induced only in the light.

Soil and Water Warming Accelerates Phenology and Down-Regulation of Leaf Photosynthesis of Rice Plants Grown Under Free-Air CO2 Enrichment (FACE)

To enable prediction of future rice production in a changing climate, we need to understand the interactive effects of temperature and elevated [CO2] (E[CO2]). We therefore examined if the effect of E[CO2] on the light-saturated leaf photosynthetic rate (Asat) was affected by soil and water temperature (NT, normal; ET, elevated) under open-field conditions at the rice free-air CO2 enrichment (FACE) facility in Shizukuishi, Japan, in 2007 and 2008. Season-long E[CO2] (+200 µmol mol−1) increased Asat by 26%, when averaged over two years, temperature regimes and growth stages. The effect of ET (+2°C) on Asat was not significant at active tillering and heading, but became negative and significant at mid-grain filling; Asat in E[CO2]–ET was higher than in ambient [CO2] (A[CO2])–NT by only 4%. Photosynthetic down-regulation at E[CO2] also became apparent at mid-grain filling; Asat compared at the same [CO2] in the leaf cuvette was significantly lower in plants grown in E[CO2] than in those grown in A[CO2]. The additive effects of E[CO2] and ET decreased Asat by 23% compared with that of A[CO2]–NT plants. Although total crop nitrogen (N) uptake was increased by ET, N allocation to the leaves and to Rubisco was reduced under ET and E[CO2] at mid-grain filling, which resulted in a significant decrease (32%) in the maximum rate of ribulose-1,5-bisphosphate carboxylation on a leaf area basis. Because the change in N allocation was associated with the accelerated phenology in E[CO2]–ET plants, we conclude that soil and water warming accelerates photosynthetic down-regulation at E[CO2].

Paddy Rice Responses to Free-Air CO2 Enrichment

Rice (Oryza sativa L.) is one of the world’s three major crops. It differs from wheat and maize, the world’s top two crops, in the distribution of its production areas: a predominant proportion of the global rice harvest comes from regions at latitudes between 30° N and 30° S, mostly in Asia (FAOSTAT 2004). In contrast, the majority of the wheat and maize crops are produced at higher latitudes. Rice is also quite unique in that the majority (ca. 90 %) of the world’s harvest comes from flooded fields. As such, rice is grown under natural and socioeconomic environments that are different from those for the other major crops. This would further imply that the impacts of global change on rice may differ from those on other crops due to these differences in the growing environment. The importance of rice as the most important food crop in Asia justified the commencement of the Rice FACE project in Japan in 1996. The primary objective of the project was to improve our capability to predict responses of rice plants and paddy ecosystems to increasing atmospheric CO2 concentration ([CO2]). The FACE experiment was conducted for 3 years (1998–2000, Phase 1), and after a 2-year hiatus, for an additional 2 years (2003–2004, Phase 2). This chapter summarizes rice plant responses to elevated [CO2] in the Rice FACE experiment during Phase 1.

Effects of ozone on the light use of rice (Oryza sativa L.) plants

Abstract Effects of ozone on rice growth processes were addressed in terms of light use of plants exposed to ozone in field exposure chambers in experiments in 1987, 1988 and 1989. Incident, reflected and transmitted light fluxes were measured with light sensors set above and below the rice canopy in the exposure chambers. Light-use efficiency (LUE) was calculated from the total dry weight and the accumulated amount of light absorbed by the rice canopies. The results showed increase of light absorption with the increase of leaf area index (LAI) during the vegetative growth, but light absorption was almost constant after heading despite the decrease of LAI due to senescence. While no effects of ozone on light absorption were found, the LUE was decreased by ozone. A quadratic function was fitted to the relationship between mean ozone concentration and LUE during the vegetative growth, and a linear function was fitted to the relationship for the reproductive growth. The effect of ozone on LUE was much greater in the reproductive than in the vegetative stage. Some mechanisms were discussed with regard to the enhancement of the ozone impact on LUE in the reproductive stage.

Diurnal and seasonal variations in stomatal conductance of rice at elevated atmospheric CO2 under fully open-air conditions

Understanding of leaf stomatal responses to the atmospheric CO(2) concentration, [CO(2)], is essential for accurate prediction of plant water use under future climates. However, limited information is available for the diurnal and seasonal changes in stomatal conductance (g(s)) under elevated [CO(2)]. We examined the factors responsible for variations in g(s) under elevated [CO(2)] with three rice cultivars grown in an open-field environment under flooded conditions during two growing seasons (a total of 2140 individual measurements). Conductance of all cultivars was generally higher in the morning and around noon than in the afternoon, and elevated [CO(2)] decreased g(s) by up to 64% over the 2 years (significantly on 26 out of 38 measurement days), with a mean g(s) decrease of 23%. We plotted the g(s) variations against three parameters from the Ball-Berry model and two revised versions of the model, and all parameters explained the g(s) variations well at each [CO(2)] in the morning and around noon (R(2) > 0.68), but could not explain these variations in the afternoon (R(2) < 0.33). The present results provide an important basis for modelling future water use in rice production.