**3.2 Results**

258 Solar Radiation

developed a simple and convenient empirical model to estimate the contribution of solar radiation to panicle temperature using generalized linear models in Experiment 2. The contribution of solar radiation for panicle temperature was estimated using this model.

The study was conducted at the same location in Experiment 1. We used six rice cultivars in this study. The nursery growing and transplanting methods used were similar to those in Experiment 1. Table 6 shows the seeding dates and dates for measuring panicle temperature. The planting density was 22.2 hills m−2. The area of each plot was 12.8 m2. A basal dressing of 4.0 g m−2 of N, 4.9 g m−2 of P2O5, and 4.3 g m−2 of K2O was applied. Top dressing was not used. Culture methods such as irrigation and pesticide used followed the

> Cultivar Seeding dates Measurement dates 'Fujihikari' April 20 July 17, 18

'Koshihikari' May 18, June 20 August 13, 15, September 6

'Milky Queen' May 18, June 20 August 13, 15, September 6 'IR72' May 18 September 6 'Asahi' May 18 September 6 Table 5. Seeding dates and measurement dates for six *O. sativa* cultivars in Experiment 2.

We measured panicle and leaf temperatures every hour between 0600 and 1200 on the flower opening days (Table 5), using infrared thermography (Neo Thermo TVS-700, Nippon Avionics co. LTD., Tokyo, Japan) with a fixed emissivity factor of l. Weather factors (solar radiation, air temperature, relative humidity, and wind speed) were measured every 5 min using a wireless weather station (Wireless Vantage Pro, Davis Instruments, Hayward, CA, USA) located at Shimane University. Measurement details and calculating weather factors

As there are inherent relationships among weather factors, relatively high correlations may exist among air temperature, solar radiation, vapor-pressure deficit, and wind speed. Therefore, to evaluate their individual effects as well as the cultivar effects on panicle temperature, we analyzed our data by means of generalized linear models and multiple regression procedures using SPSS (Version 14J for Windows, SPSS Japan Inc., Tokyo, Japan). In this analysis, air temperature, solar radiation, and vapor-pressure deficit values were used in one hour intervals between 6000 and 1200 based on Japan Standard Time. The relative contribution of each weather component to panicle temperature was determined using their standardized partial regression coefficients, and the overall strength of the

relationships was quantified using the multiple correlation coefficients.

'Xiaomazhan' April 20 July 29

standard local practices for rice production in Shimane Prefecture.

**3.1.1 Measurements of panicle temperature and weather** 

**3.1 Materials and methods** 

are the same in Experiment 1.

**3.1.2 Statistical analysis** 

The weather in 2011 was variable, in contrast with the record hot weather in 2010 (Fig. 9). Air temperature and solar radiation in mid-July to mid-August were high. In 2011, two strong typhoons affected Japan in late July and late August; as a result, air temperature and solar radiation in late July and late August dropped rapidly.

Fig. 8. Daily mean air temperatures and total solar radiation during the observation period.

The weather during the observation hours (0600–1200) differed every on the measuring days (Table 6). On July 17, air temperature was higher than 30°C, and solar radiation was high (13.5 MJ m−2). On the other hand, solar radiation was low (5.0 MJ m−2) on July 18. On September 6, air temperature was below 22°C, but solar radiation was high (12.1 MJ m−2).


Table 6. Mean air temperature, total solar radiation, mean vapor-pressure deficit, and wind speed between 0600 and 1200 on measuring days in Experiment 2.

### **3.2.1 Meteorological variables, panicle and leaf temperatures**

The air temperature on August 13 was similar to that on August 15; on the other hand, the solar radiation was approximately 58% higher than that on August 15 (Table 6). Solar radiation on August 15 increased between 0600 and 0800, but it decreased between 0800 and 0900 (Fig. 9). The level of solar radiation between 0800 and 1200 on August 15 was lower than that on August 13.

Fig. 9. Solar radiation and air temperature between 0600 and 1200 on August 13 and 15.

Panicle and leaf temperatures in both cultivars were as high as the air temperature between 0600 and 0700. As solar radiation increased, both panicle and leaf temperatures increased above the air temperature, and the differences between the panicle and air temperatures and between the leaf and air temperatures increased. In both cultivars and on both days, the panicle temperature was higher than the leaf temperature by approximately 1°C. On August 15, the increases in panicle and leaf temperatures in both cultivars were small between 0800 and 1200, probably because of lower solar radiation. The differences in panicle and leaf temperatures between 'Milky Queen' and 'Koshihikari' were also small.

Fig. 10. Leaf temperature and panicle temperature in two rice cultivars ('Milky Queen' and 'Koshihikari') between 0600 and 1200 on August 13 (left figure) and 15 (right figure).

The air temperature on September 6 was lower than that on August 13 and 15; on the other hand, solar radiation on September 6 was as high as that on August 13 (Table 6). Solar radiation on September 6 increased linearly between 0600 and 1200 just as on August 13. However, the air temperature between 0600 and 1200 was less than 30°C, although air temperature increased linearly at a rate of 1.5°C hr−1; this rate was higher than that on August 13 (1.4°C hr−1).

Fig. 9. Solar radiation and air temperature between 0600 and 1200 on August 13 and 15.

temperatures between 'Milky Queen' and 'Koshihikari' were also small.

Panicle and leaf temperatures in both cultivars were as high as the air temperature between 0600 and 0700. As solar radiation increased, both panicle and leaf temperatures increased above the air temperature, and the differences between the panicle and air temperatures and between the leaf and air temperatures increased. In both cultivars and on both days, the panicle temperature was higher than the leaf temperature by approximately 1°C. On August 15, the increases in panicle and leaf temperatures in both cultivars were small between 0800 and 1200, probably because of lower solar radiation. The differences in panicle and leaf

Fig. 10. Leaf temperature and panicle temperature in two rice cultivars ('Milky Queen' and 'Koshihikari') between 0600 and 1200 on August 13 (left figure) and 15 (right figure).

The air temperature on September 6 was lower than that on August 13 and 15; on the other hand, solar radiation on September 6 was as high as that on August 13 (Table 6). Solar radiation on September 6 increased linearly between 0600 and 1200 just as on August 13. However, the air temperature between 0600 and 1200 was less than 30°C, although air temperature increased linearly at a rate of 1.5°C hr−1; this rate was higher than that on

August 13 (1.4°C hr−1).

Air temperature (August 13) Air temperature (August 15) Solar radiation (August 13) Solar radiation(August 15)

Fig. 11. Solar radiation and air temperature between 0600 and 1200 on September 6.

Panicle and leaf temperatures in the four cultivars were slightly lower than the air temperature at 0600. As solar radiation increased, panicle and leaf temperatures increased rapidly. In all cultivars, the panicle temperature was higher than leaf temperature by approximately 1–2°C. However, panicle and leaf temperatures between 0600 and 1200 were less than 30°C although they increased at the rate of 1.8°C hr−1, which was a higher rate than that on August 13 (1.4°C hr−1). At 1100, panicle and leaf temperatures in 'Asahi' and 'Koshihikari' dropped, but the reason for this decrease was unclear.

Fig. 12. Leaf (left figure) and panicle (right figure) temperatures in four rice cultivars ('Koshihikari', 'Milky Queen', 'IR72'and 'Asahi') between 0600 and 1200 on September 6.

#### **3.2.2 Evaluation of the effects of solar radiation, cultivar, air temperature, vaporpressure deficit, and wind speed on panicle and leaf temperatures**

The multiple correlation coefficient determined by the general linear model for leaf temperature was 0.911 (*p* < 0.001), and three weather parameters (air temperature, solar radiation, and vapor-pressure deficit) significantly influenced leaf temperature (Table 7). A cultivar effect was not observed. The standardized partial regression coefficient for air temperature was 1.580; on the other hand, the standardized partial regression coefficient for solar radiation was small (0.180). The multiple correlation coefficient determined by the general linear model for panicle temperature was 0.891 (*p* < 0.001), and three weather factors (air temperature, solar radiation, and vapor-pressure deficit) also significantly influenced on panicle temperature (Table 7). A cultivar effect was not detected. The standardized partial regression coefficient for air temperature was 1.430; on the other hand, the standardized partial regression coefficient for solar radiation (0.225) was slightly larger than that of the generalized linear model for leaf temperature.



ns (not significant). \*, \*\*, and \*\*\* (significant at *p* < 0.05, *p* < 0.01, and *p* < 0.001) respectively.

Table 7. Results of multiple regression analysis using general linear models for the correlations between four weather factors (air temperature, solar radiation, vapor-pressure deficit, and wind speed) and panicle and leaf temperatures for every hour during 0600 and 1200.

#### **3.3 Discussion**

The direct effects of solar radiation on panicle and leaf temperatures were smaller than those of air temperature and vapor-pressure deficit because the standardized partial regression coefficient for air temperature was much larger than that for solar radiation (Table 7.). Higher solar radiation on August 13 resulted in higher panicle and leaf temperatures (Fig. 10.), but air temperature was also affected by solar radiation; higher air temperature also resulted in higher panicle and leaf temperatures. Yoshimoto et al. (2007) estimated the panicle temperature in New South Wales, Australia and in Jiangsu Province, China, using their heat balance model to simulate panicle temperature and its transpiration. In New South Wales, solar radiation was higher and exceeded 1000 W m−2 at midday with higher air temperatures of nearly 40°C; on the other hand, solar radiation is approximately 800 W m−<sup>2</sup> at midday with modest air temperatures of nearly 35°C in Jiangsu Province, China. However, the estimated panicle temperature was lower in New South Wales than in Jiangsu Province due to low relative humidity of <50% and high wind speeds (2–6 m s−2). Based on these results, we concluded that the effect of solar radiation on panicle temperature is indirect and through increasing air temperature.

Cultivar effects on panicle and leaf temperatures were not observed. However, differences in cultivar may exist in the ability to reduce panicle temperature through panicle transpiration. The change in the reflectance of the surface of the leaves and panicles may mitigate high solar radiation. Purple panicles with low reflectance and white and hairy panicles with high reflectance may respond differently to solar radiation. Research that includes tissue reflectance and conductance for panicle transpiration is needed to estimate panicle temperature accurately. An attempt should be made to conduct another experiment under a wider range of weather factors under field conditions.
