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Johnson 1980)

However, a higher temperature with an increase in vapour pressure deficit will increase evaporation, thereby depriving the crucial moisture needed for pollen grain swelling which is inevitable for anther dehiscence. An increase in relative humidity from 45 to 75% also resulted in reduced pollen count on the stigma due to abnormal anther dehiscence (Matsui et al. 1997b). Genotypic differences in anther characteristics between susceptible and tolerant rice genotypes do exist (Table 4.3). Artificial spikelet opening triggered rapid pollen swelling helping in anther dehiscence and subsequent pollen shedding from apical and basal pores (Matsui et al. 1999). The anther basal pore length is considered to have a significant contribution towards pollination under high temperature (Matsui and Kagata 2003) because of its close proximity to the stigmatic surface. The importance of the apical pore under high temperature is not known, but can assist in pollination.

Dehiscence of the anther leading to pollen deposition on the stigma is called as pollination. It has been accepted that the critical number of pollen grains (Matsui and Kagata 2003) and the number of pollen germinating on the stigma (Satake and Yoshida 1978) for a spikelet to be fertile are 20 and 10, respectively. However, 20% of the spikelets having 10 or more germinated pollen grains on the stigma had 0% fertility at 40°C (Matsui et al. 2001). Moreover, Matsui et al. (1997a) recorded 13% spikelet fertility with spikelets having <5 germinating pollen, while spikelets having >20 germinating pollen on the stigma showed good agreement with fertility. Therefore, genotypic variation in pollen germination under high temperatures exists and a fixed number of pollen count leading to the assumption of 50% of them germinating and equating them to fertility can be misleading. Developmental processes beyond pollen germination are sensitive to heat and have been also shown in other crops (Arachis hypogea: Kakani et al. 2002, 2005; Glycine max: Salem et al. 2007).

Table 4.3 Anther characteristics influencing dehiscence in tolerant and susceptible rice genotypes

Tolerant genotypes

Susceptible genotypes

References

Longer anthers Two cell layers (degrading or degraded tapetum and endothecium cells) separate the locule from the lacuna, allowing for easy anther dehiscence Easy and homogeneous anther dehiscence

Anthers dehisce within the spikelet on short filaments, shedding more pollen on the stigma

Comparatively shorter anthers Three cell layers (degrading tapetum, endothecium cells and parenchyma cells) separate the locule from the lacuna, hindering anther dehiscence Abnormal or no anther dehiscence

Anthers do not dehisce or they may dehisce outside the spikelet on loose sagging filaments, with less pollen shed on the stigma

Matsui et al. (2001) Matsui and Omasa (2002)

Matsui et al. (1997b), Yoshida etal. (1981), Satake and Yoshida (1978) Satake and Yoshida (1978)

After pollination, it takes about 30 min for the pollen tube to reach the embryo sac and fertilization will be completed in 1.5-4 h (Cho 1956). Rice pollen is extremely sensitive to temperature and relative humidity (Matsui et al. 1997b) and looses its viability within 10 min of shedding (Song et al. 2001). Critical high temperature for pollen germination ranges from 41 to 45°C (Enomoto et al. 1956). The reduced pollen germination percentage at 39.5°C was attributed to reduced viability prior to shedding (Matsui et al. 1997b). Both pollen germination and pollen tube elongation are inhibited by high temperature, but there exists genotypic differences for these traits (Enomoto et al. 1956; Yamada 1964; Satake and Yoshida 1978). Poor pollination at 37.5°C and pollen germination at 40°C are considered to be the likely reasons for the spikelet sterility (Matsui et al. 2001). A decline of about 7% in rice yields with per 1°C increase in temperature has been reported in India. This reduction is mainly attributed to spikelet sterility at high temperatures (Krishnan et al. 2007).

High temperature of 39°C (±0.5°C) exhibited significant reduction in pollen activity, pollen germination and floret fertility. However, the tolerant cv. Shanyou 63 showed a much slower reduction in the rates compared to the susceptible cv. Teyou559 (Tang et al. 2008). Furthermore, they quantified the growth hormones in the anthers and found a decrease in IAA (Indole Acetic Acid), GA3 (Gibberellic acid), free proline and soluble proteins, but a significant increase in ABA (Absisic acid) content. Nine japonica spp. were screened between 35 and 40°C and a 3°C difference in critical temperature was found causing 50% spikelet sterility between the tolerant genotype (40°C; Akitakomachi) compared to the susceptible genotype (37°C; Hinohikari) (Matsui et al. 2001). Working on three rice genotypes, Yoshida et al. (1981) identified cv. N22 to be highly tolerant when exposed to high temperature at flowering. Subsequent reports have identified the heat tolerant trait during flowering in N22 (Prasad et al. 2006; Jagadish et al. 2008). Rice genotypes were screened for spikelet fertility and yield in different species, ecotypes and cultivars under ambient and high temperature (ambient + 5°C) conditions for the whole crop growth period (Prasad et al. 2006). They concluded that heat tolerance exists in both sub-spp. of O. sativa and it cannot be generalized that indica sub-spp. are more tolerant than japonicas.

Matsui and his associates exposed the plants to high temperature beginning 10:00 h, while Prasad et al. (2006) recorded the beginning of flowering as early as 09:00 h. Moreover, the quantitative effect or acclimation to high temperature occurring on different days of the flowering period is not known. To minimize these problems in studying the response of rice to heat sensitive anthesis stage, a protocol has been documented to identify the spikelets explicitly exposed to high temperature (Jagadish et al. 2007). Considering the sensitivity of rice during anthesis, they exposed the plants to high temperatures (35 and 38°C) for a single day and identified spikelets receiving 1, 2, 4 or 6 h of high temperature by adopting the marking protocol using acrylic paint. One concern with this approach was the manual interruptions with the spikelets ability to cool its microclimate through transpiration resulting in reduced tissue temperature. This was overcome by marking the spikelets after the heat treatment under control conditions giving all spikelets equal opportunity to efficiently utilize their ability to cool under high temperatures. A difference of

0.4, 1.3 and 1.8°C in spikelet tissue temperatures were recorded using copper con-stantan thermocouples at ambient temperatures of 30, 35 and 38°C, respectively in temperature controlled growth cabinets (Jagadish et al. 2007). Moreover, the flowering pattern of both indicas and japonicas is known (Yoshida et al. 1981; Prasad et al. 2006) and hence exposing the plants to 6h (09:00-15:00h) of high temperature included >95% spikelets flowering during the day and each spikelet would be exposed to at least an hour of high temperature sufficient to test the spikelet for tolerance. From this study, it was evident that high temperature (>33.7°C tissue temperature) for < 1 h also affected spikelet fertility. This was reflected in the short duration heat treatments of 1 and 2 h, where spikelets opening 30 min before high temperature exposure would experience next 30 min high temperature apparently enough to reduce spikelet fertility to a certain level (Fig. 4.1), indicating the extreme sensitivity of rice spikelets at anthesis. This marking protocol could also be used to study true cold tolerance during anthesis since the mechanisms leading to sterility at low temperature (Gunawardena et al. 2003) are similar to high temperature (Yoshida etal. 1981).

Identifying heat avoiding genotypes (early morning flowering) could help solve the problem of increasing day temperatures on rice production. However, day/night temperatures and number of warmer nights are predicted to increase in the future (IPCC 2007). Recently, it has been reported that minimum temperature (night) has increased by 1.13°C and the maximum temperature (day) by 0.35°C as evident from 25 years weather data from the International Rice Research Institute, Philippines (Peng et al. 2004). In their study, they showed decreasing yield with increase in minimum temperature. Although the time of occurrence of anthesis and its sensitivity are well known, the degree of heat sensitive processes during panicle initiation

Fig. 4.1 Spikelet fertility in rice cv. IR64 exposed to temperatures of 29.6, 33.7 and 36.2°C for 2h (solid circle and line), and seed-set of spikelets that opened at 29.6°C either 1 h before (open circles and hashed line) or 1 h after (solid square and dotted line) the 2 h treatment. Fitted lines: °y = 15.43 - 0.408x; •y = 14.30 - 0.408x; By = 3.56 - 0.062x. Adapted from Jagadish et al. (2007) and Jagadish (2007)

Fig. 4.1 Spikelet fertility in rice cv. IR64 exposed to temperatures of 29.6, 33.7 and 36.2°C for 2h (solid circle and line), and seed-set of spikelets that opened at 29.6°C either 1 h before (open circles and hashed line) or 1 h after (solid square and dotted line) the 2 h treatment. Fitted lines: °y = 15.43 - 0.408x; •y = 14.30 - 0.408x; By = 3.56 - 0.062x. Adapted from Jagadish et al. (2007) and Jagadish (2007)

and microsporogenesis are unknown. Therefore, identifying heat avoiding genotypes will not solve the problem completely and true heat tolerance identification is crucial to mitigate future climatic changes.

Although genotypic difference in critical heat thresholds in rice is documented (Yoshida et al. 1981), the interaction between high temperature and duration of exposure was studied recently (Jagadish 2007). Present crop models have the geno-typic difference in thresholds incorporated in them but the possibility of an interaction between temperature and duration of exposure is assumed to be non-significant. Response of rice to high temperature has been modeled using daily mean temperature (Horie et al. 1995; Kropff et al. 1995) or including number of days with maximum temperature >34°C (Challinor et al. 2007) and more recently using daily minimum and maximum temperature (Krishnan et al. 2007). Although the above studies included sub-daily temperatures, response to anthesis is extremely sensitive to sub-daily time course of temperature and flowering models with hourly temperature changes are needed, which can be incorporated into crop models for better prediction. Furthermore, quantification of high temperature impact on future crop yields based on predictions is still in its infancy (Challinor et al. 2007) due to less experimental data available.

Following the marking protocol, an interaction between high temperature and duration of exposure in a heat sensitive genotype (Azucena) but not in a moderately tolerant IR64 was identified (Fig. 4.2). The interactive effect could be included into crop models by adopting the cumulative temperature response above a threshold temperature of 33°C (Nakagawa et al. 2002). A similar response was seen in ground nut and has been quantified by cumulative temperature approach (Vara Prasad et al. 1999). Accumulated temperature or thermal time above a threshold can be calculated by TT = (T — 33°C) xt, where T is the day temperature and t, duration of the treatment (Fig. 4.3).

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