Rice is the most important source of calories for 50% of the world’s population. In the face of population growth and urbanisation, rice production must increase at rates unseen this century to feed the growing global population [1]. For the past three decades, the annual growth rate for rice production has decreased from 2.73%, in 1980s to 0.88% since 2000 [2]. The message is that overall yields must increase, or significant improvements made to minimize yield-reducing factors such as postharvest loss of grain. An additional challenge to increasing rice production is the effect of climate change, especially global warming, which decreases the yield of rice and increases factors that affect recovery of intact grains after polishing [3], [4].

The mean global air temperature has increased by 0.3 to 0.6°C over the past century [5]. Further, the average temperature for July 2010 was 0.55°C warmer than that from 1951–1980 [6]. Higher minimum temperature reduces the yield of rice and the mechanism is thought to be due to higher loss of carbon through increased respiration [7]. By contrast, higher maximum temperature has been shown to raise yield [4], presumably through improving accumulation of assimilates, lower respiration, higher tillering, leaf-area expansion, stem elongation, faster grain filling, and crop phonological development [3].The impact of high temperature on rice production is not only seen on yield but also on grain quality. High temperature during both the day and the night has been shown to increase chalkiness, which causes grains to break during dehulling and polishing, leading to a decrease in the amount of acceptable or marketable polished rice [8], negating any yield gain in the field. Coupled with urbanisation and shrinking farm size, new varieties will have to have both higher field yield of paddy rice and of the yield of whole grains of rice that remain after polishing (head rice yield, HRY), and a path to the latter is through understanding the factors leading to chalk, to enable breeding programs to target those factors in new climate-ready germplasm.

Many studies of the genetic and environmental effects on chalk indicate that heredity and climate conditions, especially high air temperature during the grain-filling period, are the most important factors affecting chalkiness [9], [10], [11], [12], [13]. However those conclusions were based on just a few varieties grown in controlled temperature conditions [12]. Such conditions often do not take account of other climate variables such as solar radiation, humidity and rainfall. In addition, it has been shown that a discrepancy exists between estimates of the effects of warmer temperatures on crop yields based on field conditions, versus those derived from modeling and experiments conducted under controlled conditions [14]. The objective of the present paper is to progress our understanding of the effect of climatic conditions on rice yields, by stepping beyond paddy yield and out of the glasshouse, to understand how different climate factors affect the quality, and therefore acceptability of rice, using a panel of 39 varieties grown in the field over eight seasons in tropical conditions.

Recent research has shown that the mean global surface air temperature has increased over the past century, and other reports indicate that this will have its greatest impact in the rice-producing countries of Asia [4]. In the present paper, we show, as others have shown, that there has been a significant increase in the minimum and mean temperature, accompanied by a significant decrease in diurnal variation over the past 30 years at Los Baños in the Philippines (Table 1).

Higher temperature during grain-filling has been shown to affect rice quality in a number of studies [12], [15], [16], [17], [18], [19], [20]. High temperature during the early stages of grain-filling leads to an increase in the rate of endosperm cell division with the result that cell walls are thin, fewer granules are initiated, and the deposition of starch, on fewer building blocks within thin-walled cells, leads to higher PGWC and large chalky areas [21], [22]. High temperatures during the night have also been shown to lead to higher chalk values [19]. In some studies using controlled growth conditions, temperature was the only variable measured, leading to a general conclusion that temperature is the main factor affecting chalkiness and HRY. In the present study, using four years of field-grown rice, we show that maximum temperature increased over the whole grain-filling stage in the DS, and decreased over grain-filling in the WS (Table 2), but in contrast to other studies, the average chalkiness and the PGWC over the four years were both significantly and reproducibly higher in the cooler WS than in the hotter DS (Table 2). Moreover, we show that the association between chalkiness and temperature is not significant at most of the stages of grain-filling (Table 3).

HRY was lower in the WS than in the DS over the four years (Table 3). Furthermore, the difference in maximum temperature over grain-filling between the 2005DS, with the lowest chalkiness and 2^nd highest HRY, and 2006WS, which had the highest chalkiness and lowest HRY, was close to three degrees (Table S2), while minimum temperature was similar. Taken together, these data demonstrate (i) that the environmental conditions of the WS lead to fewer and less valuable whole grains, than in the DS, (ii) that temperature alone is unlikely to be the major factor that leads to higher chalk and lower HRY, and (iii) other climate elements, or interactions between them, play a more significant role than temperature on the postharvest yield and quality of rice.

Solar radiation and sunshine were significantly different between the wet and dry seasons over four years (Table 1, Table 2), and the main climate parameters that associated with chalkiness in the field conditions over the four years were VPD, which associated negatively, and RH which associated positively (Table 4). Both of these factors influence transpirational capacity and therefore [23]. Both of these parameters correlated with chalkiness at every stage of grain-filling, and they also correlated throughout grain-filling, and in the opposite directions, with HRY (Table 4). A further demonstration of this is shown in Table S2, which compares climate factors during grain-filling in the 2005DS and 2006WS. Average chalk in the 2006WS was the highest of the dataset, at 22.79%, and in the 2005DS it was the lowest at 9.09% (Table 3). HRY was also about 15 percentage points higher in the 2005DS compared with the 2006WS. In these two seasons, the minimum and mean temperatures were similar throughout grain-filling (Table S2). However solar radiation, sunshine, evaporation and VPD were all substantially lower in the 2006WS, and rainfall and RH were higher, perhaps leading to less capacity for transpirational cooling to take place.

Transpirational cooling has a significant effect on the temperature of the canopy, and it is also linked with the dynamics of sucrose transport from sources in leave and stem to sinks, such as the developing endosperm, with significant implications for yield [24]. Within the grain, the phenotype of chalk appears as incompletely filled starch granules [8]. If capacity for transpirational cooling affects yield, then it affects the allocation of substrate to the developing panicle, and therefore, most likely affects chalkiness as well. Atmospheric VPD follows a diurnal pattern, being lowest at sunrise and increasing to a maximum at around 15:00 [25], where it would create the greatest moisture potential, leading to stomatal opening and transpiration. Dry matter accumulation follows a linear relationship withincreasing VPD under both controlled and uncontrolled climatic conditions at IRRI [26], further supporting the importance of transpirational cooling for the development of grains that are densely packed with well-developed and mature starch granules.

Chalkiness is the not the only parameter affected by climate in the WS; HRY is also affected. Figure 1 shows clearly that chalkiness and HRY are closely related, suggesting that the capacity for transpirational cooling is necessary also to minimize post-harvest losses of grain. Since chalkiness and HRY are the two factors that are used to determine the price of rice on both domestic and international markets, transpirational cooling appears to be an ecessary trait for maximal value of the rice market chain. The data suggest that global warming per se might not be the major factor that leads to losses in yield and quality of rice, but the consequences of global warming, together with the climate of the WS, lead to higher RH and lower VPD, which lead to decreased evaporation and therefore capacity for transpirational cooling.

Table 2 shows that over four years, radiation, sunshine, VPD and evaporation are higher, and rainfall and humidity are lower in the dry seasons compared with the wet seasons, further indicating the antagonistic effect of global warming in a WS climate, whereas global warming could have a synergistic effect in a DS environment, by increasing VPD and consequently transpirational cooling, enabling the canopy to remain cool despite the higher air temperature.

The data in the present paper suggest that new, climate-ready varieties of rice should carry allelic combinations that lead to increased capacity for transpirational cooling, especially those that are grown in the wet season, in order to obtain the maximum number of whole and high quality grains. Variability for this trait has already been demonstrated in rice [24] providing the phenotypes that can be used to discover the best alleles for this trait.

All varieties used in the present study were developed by IRRI and have been widely grown in South and South East Asia over the past 30 years. The relationships between quality traits and weather parameters analyzed based on average data reflect the overall trend about how climate change affects rice grain quality. Results of single variety analysis could guide breeders on parental selection for progeny resistant to climate change. It is very interesting that very few significant associations were detected between HRY and weather parameters in some varieties (Fig. 2). Among these, the low chalk varieties such as IR 60, IR 64, PSB RC 88 and PSB RC 94 gave high HRY in both DS and WS, provided a set of potential parent lines in breeding programs targeting grain quality.