Abstract

Media summary

Key words

Oryza sativa, TGMS, Fertility behaviour

Introduction

The green revolution in many rice producing countries enabled global rice production to meet the demand of the world’s increasing population. However, by 2025, the world must increase rice production to 880 million tonnes from the present 560 million tonnes. Recent progress in plant breeding research indicated that a significant shift in the yield frontiers could be possible through hybrid rice. China has managed to sustain its output growth since mid 1970’s by switching from modern semi dwarf varieties to hybrid. However, outside China, no tropical country could exploit the hybrid rice technology successfully to the extent expected. This may be partially due to the inherent limitations associated with cytoplamic genic male sterility (CMS) system used in the development of rice hybrids. In this context, environment sensitive genic male sterility (EGMS) is considered as potential alternate to overcome the problems associated with three-line breeding and to surpass the yield plateau. Under tropical conditions, where day length differences are marginal, temperature sensitive genic male sterile (TGMS) system is considered more useful than the photoperiod sensitive genic male sterile (PGMS) system. However, successful exploitation of this novel male sterility system relies on the knowledge on fertility behaviour of TGMS lines. Hence, the present study was undertaken with the specific objective of characterizing a set of promising TGMS lines for their fertility behaviour so as to use them in two-line heterosis breeding.

Materials and methods

A set of 17 putative TGMS lines identified at Tamil Nadu Agricultural University (TNAU), Coimbatore, India and IRRI-bred lines obtained from the Directorate of Rice Research, Hyderabad, were evaluated for pollen and spikelet fertility during summer 1999 at Coimbatore (High temperature condition - 36/22°C) and Hybrid Rice Evaluation Centre, Gudalur, India (Low temperature condition - 30/13°C) simultaneously to identify the promising TGMS lines. Among 17 lines evaluated, seven lines viz., TS6, TS15, TS16, TS18, TS29, TS46 and TS47 were completely sterile with 100 per cent pollen sterility at Coimbatore and 50 to 90 per cent pollen fertility at Gudalur. All these lines except TS15 (due to non availability of sufficient seed) were further characterized for fertility behaviour (Table 1)

Table 1 Source and origin of TGMS lines taken for evaluation

The TGMS lines were raised at fortnightly interval from July 1999 to June 2000. At the time of heading, pollen and spikelet fertility were observed on ten random plants. From the 24 sets of data on pollen fertility, the fertile and sterile phases of each TGMS line, the duration of each phase and the fertility transition phase were identified. The flowering period, in which the lines were completely sterile (100% pollen sterility) was taken as sterile phase. The period, in which the plants recorded more than 50 per cent pollen fertility was considered as fertile phase. The period of partial sterility was considered as the phase of fertility transition. The influence of weather factors viz., maximum temperature, minimum temperature, mean temperature, relative humidity, sunshine hours and photo period (of each day from 26 to one day before heading, average of each factor at different stages of panicle development and the overall mean of each factor throughout the panicle development) on pollen sterility was assessed by simple correlation analysis. The critical stages of panicle development sensitive to temperature were determined from the stages exhibiting significant correlation with pollen sterility. The lowest mean temperature among the temperatures inducing sterility was considered as the critical sterility temperature (CST) and the highest mean temperature inducing fertility was recorded as critical fertility temperature (CFT).

Results and Discussion

Fertile and sterile phases

The preliminary step in exploitation of two-line system of hybrid rice breeding on a large scale is the identification of TGMS lines with stable fertility transformation behaviour. The lines with complete pollen sterility under high temperature condition and more than 30 per cent self seed set under low temperature condition are considered as promising TGMS lines for commercial exploitation (Lu et al., 1994). The multiplication of TGMS lines and seed production of two-line hybrids is not difficult as that of three-line hybrids, which require a maintainer line to multiply sterile lines whereas TGMS lines are fertile during certain temperature regime, in which it can be multiplied by mere selfing. However, the sterile, fertile and the fertility transition phases of TGMS lines need to be determined in different ecological areas, so that the proper seasons and locations for sterile line multiplication and hybrid seed production can be recommended. In this study, the fertility among the plants of a line varied during the fertility transition phase, whereas complete sterility and relatively same degree of fertility were observed among the plants during their sterile and fertile phases, respectively. All lines except TS29 had one distinct sterile phase of more than 50 consecutive days during summer months, whereas TS29 had two sterile phases. This is a favourable feature for successful utilization of these lines. The sterile period was the longest in TS18 (156 days) followed by TS16 (141 days) and TS29 (122 days). The maximum/minimum temperature during the panicle development stages inducing sterility was around 30/20°C. In TS16 and TS18, sterility was observed in plants headed when temperature was less than 30/20°C. It might be due to the occurrence of maximum temperature of more than 30°C at later stages of panicle development or due to the fact that the critical temperature inducing sterility was lower in these lines than that of other lines.

All lines reverted into fertile in two seasons. The first fertile phase was short (13-17 days) with 52 to 72 per cent pollen fertility and 51 to 70 per cent spikelet fertility. Hence, the multiplication of TGMS lines with high purity standards will be difficult during this season because the late tillers may convert into sterile and favour outcrossing, which may affect the purity of TGMS lines. Lu et al. (1998) suggested that the sterile and fertile phases should be atleast for 30 consecutive days for successful utilization of TGMS lines. The second fertile phase was longer in duration for more than 30 days in all the lines except TS29, in which it was only for 16 days. The maximum pollen and spikelet fertility recorded during this period was 63 to 85 per cent and 58 to 70 per cent, respectively.

Relative influence of weather factors on fertility

Correlation analysis between pollen sterility and weather parameters revealed that maximum temperature and mean temperature were the primary factors influencing fertility alteration. The influence of relative humidity was in the negative direction. TS18 was found to be influenced by sunshine hours also. In TS46 and TS47, maximum, mean and minimum temperatures were identified as the primary factors influencing pollen sterility and the subordinate factor was photoperiod in TS46 and sunshine hours in TS47. Influence of sunshine hours and relative humidity on fertility alteration have been observed by Liu et al. (1997) in the TGMS line, 5460S. However, Gao et al. (1996) reported that fertility alteration of TGMS lines was mainly controlled by daily mean temperature and not by photoperiod. Since many factors associated with temperature directly or indirectly influenced the fertility of TGMS lines, the influence of any single factor should be determined by evaluating the lines in phytotron by keeping other factors under control.

Critical stages of panicle development for fertility alteration

There is certain amount of risk in exploiting rice heterosis by means of TGMS, if temperature fluctuation occurs at critical stages of panicle development. Therefore, knowledge on critical thermo-sensitive stages for fertility alteration is useful to determine the most suitable time of sowing the TGMS lines for seed multiplication and hybrid seed production. The appropriate sowing dates of TGMS lines should be determined in such a way that the critical stages of panicle development would be exposed to the required temperature. The stages of panicle development sensitive to environmental factors varied among the lines (Table 2). In this study, the later stages were found more sensitive than the early stages. All panicle developmental stages from differentiation of primary bract primordium (S1) to pollen ripening (S8) were sensitive to temperature in TS46 and TS47. These lines were sterile relatively for a shorter period than the other lines. This might be due to the effect of temperature at all stages of panicle development inducing fertility alteration. If low temperature prevails even for a short period during any stage of panicle development, it will affect the sterility.

Table 2 Critical panicle developmental stages and temperature for fertility alteration

Critical sterility and fertility temperatures

In the present study, the mean temperature inducing fertility alteration during the sensitive stages of panicle development was 24 to 26°C. The critical temperature was found to vary in different TGMS lines as the TGMS genes of these lines are from various sources or transferred into different genetic backgrounds (Table 2). Wu (1997) has reported similar case of varying CST and CFT in the lines developed from single TGMS source (Annong S-1) on different genetic backgrounds. The critical temperature inducing sterility must be relatively low i.e., 23°C in temperate zone and 24°C in subtropics (Yuan, 1998). The lines with more critical temperature (>26°C) are not favourable for commercial exploitation, since even a short fall in temperature during summer months may cause fertility reversion in TGMS lines, which may lead to self seed set. All these lines satisfied the requirements for commercial exploitation. Multiplication of these TGMS lines will also be an easy task as the critical fertility temperature of these lines was 24 to 25°C and this temperature will prevail during the months of November, December and January in most parts of India. But low critical temperature poses problem in sterile line multiplication. The line may be stable for sterility but the seed yield will be low, as it requires very low temperature for transition from sterile to fertile, which can be multiplied only in high altitude areas.

Though the lines evaluated in this study had desirable critical temperature, the difference between CST and CFT was very narrow. It is considered as undesirable, since there is a possibility of fertility reversion due to unusual occurrence of low temperature during hybrid seed production, which may lead to mixture of self seeds with hybrid seeds. It would have been advantageous, if the CFT was still lower and the difference between CST and CFT was wider. However, Wu and Yin (1992) opined that low temperature, which would transform TGMS lines into fertile did not occur frequently during high temperature seasons in tropical countries. Even when it occurred, it lasted for only a few days, thus the purity of hybrid seed would not be affected. In the present study also, the same trend was observed.

It is assumed that the conditions influencing fertility alteration in EGMS lines vary among different lines due to different sources of male sterile genes and the genetic background (Wu et al., 1991; Zhang et al., 1991). In this study, TS29 behaved differently in fertility alteration compared to other lines. Since it is a new TGMS line, allelic test needs to be conducted to ascertain the source of TGMS gene. As suggested by Yuan (1990), this type of variation in response is advantageous because from a single source of TGMS gene, lines with various critical temperatures suitable to various locations can be evolved. The results of this study showed that all TGMS lines have clearly defined fertility expressing temperature regimes, which are frequently available in tropical rice growing countries. They were completely sterile under high temperature condition and exhibited acceptable level of pollen and spikelet fertility under low temperature condition. They can be better exploited as TGMS donor for developing new TGMS lines.

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