Air and spikelet temperature
Mean daily air temperature in the greenhouse during the experimental period was maintained very close to the target temperature of 30/24 °C day/night in 2003 [29.2 (SD 1.23)/24.4 (0.42) °C] and in 2004 [29.3 (1.21)/24.0 (0.40) °C]. However, in 2003 ambient temperatures in the greenhouse reached 38 °C mid-afternoon at 65 and 66 DAS. Although IR64 flowered peak, there was no spikelet sterility observed in control plants as anthesis occurred during mid-morning, while temperatures were cooler. Azucena, which flowered at 93 DAS, was unaffected by these high ambient temperatures.
Air temperature in the growth cabinets was maintained at 30 (SD 0.13), 35 (0.16), and 38 °C (0.09) during the day in both years. Spikelet temperatures recorded in 2004 were 29.6, 33.7, and 36.2 °C, i.e. 0.4, 1.3, and 1.8 °C below ambient temperatures, similar to differences observed elsewhere in rice (Satake, 1995) and in peanut flowers in the same growth cabinets (Vara Prasad et al., 2001). It was assumed that the difference between spikelet and air temperature was the same in the growth cabinets in 2003 and therefore spikelet (tissue) temperatures are used throughout this paper.
Effect of temperature on flowering pattern
The pattern of flowering at 30 °C was similar on the first, second, or third day of flowering in IR64 and Azucena (Fig. 1) although the total number of spikelets at anthesis over the 3 d period was lower, on average, in Azucena (109) than in IR64 (164). Flowering in IR64 at 30 °C started between 09.30 h and 10.00 h, reached at peak at 1100 h and ended by about 1300 h. Azucena started flowering slightly earlier and ended later, but with peak flowering also at 11.00 h.
The pattern of flowering and hence the total number of spikelets that reached anthesis per day (
Table 2) differed significantly (
P (
P peak, and end of flowering occurring about 30 min earlier in the day, as well as increasing the mean number of spikelets at anthesis per day from 49.7 to 59.3. In Azucena, the timing of flowering was similarly affected, but, in contrast to IR64, the number of spikelets reaching anthesis was significantly reduced, from 44.3 to 28.3 spikelets per day. In IR64, spikelet production at 36.2 °C increased over the 3 d period and hence had a positive effect on the number of potential seed sites; by contrast, spikelet number in Azucena declined over the 3 d period, reducing the potential number of seed sites.
Effect of time of spikelet anthesis relative to a high temperature episode on spikelet fertility
In Experiments 1 and 2, spikelets of Azucena and IR64 that had reached anthesis prior to (Experiment 1 and 2), and after (Experiment 2), exposure to temperatures of 29.6, 33.7, and 36.2 °C for varying durations were marked as well as spikelets that reached anthesis during the treatment. In both experiments and genotypes the spikelet fertility of spikelets that reached anthesis prior to the temperature treatments was affected by the subsequent temperature, and in 2004 by previous temperature, and this is illustrated by IR64 exposed to a 2 h temperature treatment in Experiment 2 (Fig. 2).
In IR64 there were significant effects of temperature (P time of spikelet anthesis (P (P in spikelets reaching anthesis during the 2 h temperature treatment decreased linearily from 0.90 to 0.30 OR (i.e. slope –0.06 logit % fertility °C–1) as temperature increased from 29.6 °C to 36.2 °C. Spikelet fertility in spikelets that reached anthesis 1 h before the temperature treatments were imposed was also affected by subsequent temperature in a similar manner. A comparison of regressions showed these two ‘timings’ had a common slope but different intercepts (P=0.0018). Spikelets that reached anthesis after the temperature treatment, however, were not significantly affected by the previous temperature and this regression was significantly different (P=0.034) from the other two.
Interaction between duration of exposure and temperature on spikelet fertility
In IR64, the logit analysis showed that temperature had a significant (P effect of duration or a temperaturexduration interaction (Fig. 3). By contrast, in Azucena there was a significant effect of temperature (P xduration interaction (P
In IR64, average spikelet fertility declined from 0.88 to 0.27 OR as temperature increased from 29.6 °C to 36.2 °C, but only from 0.66 to 0.56 OR as duration increased from 1 h to 6 h. A comparison of regressions of logit spikelet fertility against duration confirmed that there was no interaction with duration (
P >0.05), the sensitivity to duration being –0.095 (±0.0266) logit % spikelet fertility h
–1 at each temperature. A comparison of regressions of logit % spikelet fertility on temperature (not presented) also revealed no effect of duration (
P >0.35) and a common regression line describing the response to temperature could be fitted, where
y=14.70 (±1.20)–0.42 (±0.036) logit % spikelet fertility °C
–1. Thus, in IR64, spikelet sterility increased by 0.09 OR per °C from 29.6 °C to 36.2 °C and there was no interaction with duration of exposure.
Although duration effects were not significant in IR64, it is apparent that fertility did decline with duration and this may be confounded by the number of spikelets exposed to temperature, and hence with escape from the stress. For example, with a 1 h treatment, spikelets that open towards the end of this period will be exposed to less cumulative temperature than those opening at the start, giving some scope for escape. With a 6 h exposure, by contrast, most spikelets will be exposed for at least 1 h and the chance of escape is much less. The relation between logit % spikelet fertility and the proportion of spikelets opening during the temperature treatments was therefore plotted (Fig. 4). In IR64 at 29.6 °C, 76% of spikelets reached anthesis during the 1 h treatment and there was, as expected, no effect of proportion and hence duration on fertility at this non-stress temperature. At 36.2 °C, where only 31% of spikelets had reached anthesis in the 1 h treatment compared with >90% in the 4 h treatment, there was a small but non-significant (P=0.06) effect of proportion.
Spikelet fertility in Azucena at 29.6 °C averaged 0.64 OR, similar to 2003 (not presented) and lower than in IR64. On average, spikelet fertility decreased from 0.64 to 0.08 OR as temperature increased from 29.6 °C to 36.2 °C. At 29.6 °C and 33.7 °C spikelet fertility in Azucena, like IR64, exhibited little or no response to duration. However, at 36.2 °C, there was a marked effect of duration longer than 2 h and spikelet fertility was reduced to
Fig. 3). A comparison of regressions of logit % spikelet fertility against duration confirmed that the response to duration at 36.2 °C was significantly different (
P °C and 33.7 °C, which had parallel slopes.
Regressions of IR64 and Azucena were also compared at each temperature to test whether Azucena was more sensitive to higher temperatures than IR64. At 29.6 °C and 33.7 °C there was no difference in the sensitivity of IR64 and Azucena, slopes being 0.008 and –0.052 logit % spikelet fertility h–1, respectively. However, at 36.2 °C slopes were significantly (P different. Azucena was therefore more sensitive to high temperature than IR64.
While the response to temperature in IR64 can be described by a quantitative relation between fertility and spikelet temperature, without interaction with duration, that of Azucena cannot be modelled in this way. It was therefore examined whether the response of Azucena in 2003 and 2004 could be described by a cumulative temperature response above a threshold value, a common form of stress response used in crop models (Fig. 5). There was a strong negative relation (r2=0.86) between logit % spikelet fertility and accumulated hourly spikelet temperature above a threshold of 33 °C, suggesting that the interaction between temperature and duration can be modelled in this way.
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