Flowering of Sugarcane Genotypes under Different Artificial Photoperiod Conditions

Flowering is essential in any breeding programme, particularly in genetic introgression programmes in which flowering synchronism is required. In this work, 16 sugarcane genotypes were evaluated using three different flowering-inductive photoperiod treatments (30 s, 45 s, and 1 min of a daily photoperiod decrease from 12 h 55 of light). Each genotype was planted in 43 L pots (equal proportion of soil, sand, and substrate) with three tillers per pot. Plants with 4 to 6 internodes were placed in a photoperiod facility with three controlled chambers, each chamber containing all 16 genotypes (one treatment per chamber). The temperature range (21 °C to 32 °C) and humidity were the same for all of the treatments. The flower induction started in September 2010 and ended in April 2011. The plant elongation and flag leaf and inflorescence emergence were recorded and the pollen viability was evaluated by using the iodine staining method. The photoperiod facility provided suitable conditions for flowering, as the plants in the three treatments successfully flowered. The genotypes in the three treatments behaved differently in relation to the date of panicle emergence, with the treatment of 45 s showing better results. The results here presented contribute to synchronise flowering for desired sugarcane crosses, particularly those from introgression programmes between commercial cultivars and species from the Saccharum complex.

• Artificial Induction
• Leaf Flag
• Photoperiod Facilities
• Physiologic Flowering
• Pollen

1. Introduction

In Brazil, sugarcane crosses have been performed under natural conditions in the Northeast coastal region, which has ideal climate conditions for floral induction. However, sugarcane breeders have found difficulties in synchronising flowering in specific crosses, such as those between commercial cultivars and Saccharum spontaneum to broaden the genetic base of sugarcane and to increase the fibre content and biomass.

The process of inflorescence formation is difficult to define because it depends on the genotype, weather and changes that occur during the growing season. The flowering stimulus occurs during 18 - 25 days ([1] ); in the Southern Hemisphere, the meristem differentiation for inflorescence formation occurs in February, March and April, and flowering occurs in April, May and June ( [2] [3] ).

Among the external factors that influence flowering induction, photoperiod is of high importance. Although sugarcane behaves as a short-day plant, successive long nights are also required to induce flowering ([4] ). The ideal day length for flowering appears to be approximately 12 h 55 min and constitutes the day length adopted for most of the sugarcane cultivars grown worldwide ( [5] [6] ). Even at locations where the inductive photoperiod conditions occur, the emergence of inflorescences may not be uniform, revealing that the temperature is also important for flowering ([7] ). It is believed that the minimum temperature rarely falls below 18˚C and the maximum never exceed 32˚C in areas with abundant flowering ( [6] [8] ). Moreover, temperatures below 21˚C can delay growth and panicle emergence ([1] ). The pollen grain viability is also affected by the temperature, mainly cold temperatures, resulting in pollen grains that are unviable; in fact, night temperatures below 15˚C before or during flowering may cause anther abortion and male sterility ( [5] ).

Another important factor that affects sugarcane flowering is the soil moisture, which can interfere with tassel formation, as a water deficit during the inductive period can delay flowering. Indeed, a lack of water inhibits the translocation of photoassimilates to the apex, and therefore elongation of the inflorescence peduncle and anthers exposure ( [9] ).

Photoperiod management in areas of unfavourable natural conditions of flowering can be conducted by using photoperiod facilities. Controlled temperatures have been adopted by some sugarcane breeding programmes in Australia, South Africa and the USA to synchronise panicle emergence among desirable cultivars. In Brazil, the first fully automated photoperiod facility capable of simulating the ideal conditions of temperature and photoperiod to induce sugarcane flowering was built at the Campinas Agronomic Institute (IAC)―Sugarcane Center, which is an unfavourable site for natural sugarcane flowering ( [10] ).

Due to the instability of sugarcane flowering and pollen viability in São Paulo State, techniques that artificially synchronise flowering and maintain pollen viability have a great impact on sugarcane breeding programmes. Moreover, the inductive photoperiod of the Brazilian sugarcane genotypes is still unknown mainly due to a lack of studies related to flowering induction at photoperiod facilities. Therefore, the present study aimed to study the behaviour of 16 sugarcane genotypes using different inductive flowering photoperiod treatments. The results will contribute to synchronise flowering to planning crosses and also help breeders in genetic introgression programmes between commercial sugarcane cultivars and wild sugarcane species, especially those from Saccharum spontaneum.

2. Methods

2.1. Material

The experiment was conducted at the IAC―Sugarcane Center at Ribeirão Preto, São Paulo State in the period of September 2010 to April 2011. Sixteen sugarcane genotypes (Table 1) with high sucrose and fibre contents were evaluated for flowering induction under different artificial photoperiod conditions.

2.2. Experimental Design

The 16 sugarcane genotypes were planted in pots of 43 L with equal amounts of clay soil, sand and substrate (Plantmax^®), with the recommended fertilisation, maintaining three tillers of each genotype per pot. The pots were placed randomly on three mobile wagons in separate chambers with controlled photoperiods (Figure 1). Induction began when the genotypes had 4 to 6 well-formed internodes to ensure maturity for flowering ( [1] ).

Table 1

Sugarcane genotypes evaluated for flowering inductive photoperiod

Genotypes
IACSP94-2101	IACSP00-8206	CTC12	SP89-1115
IACSP93-2060	IACSP97-2055	CTC8	SP90-1638
IACSP96-7569	IACSP95-5094	CTC6	SP80-1842
IACSP00-8095	CO213	CTC15	RB867515

Figure 1

Photoperiod facility. A―External view with the mobile wagons; B―Photoperiod chambers; C―Internal chamber; D―Light structure in the celling; E―Automation Software; F―Automation painel

2.3. Flowering Induction

The treatments were 30 s (Treatment I), 45 s (Treatment II) and 1 min (Treatment III) of a decrease in the daily photoperiod, starting from 12 h 55 min ( [8] [9] ) and continuing until the end of the experiment. The irradiation was controlled in a photoperiod facility by a combination of incandescent and fluorescent light bulbs programmed to switch on/switch off to simulate sunrise and sunset, respectively, with the option of a daily reduction in the photoperiod.

The opening and closing of the photoperiod chamber gates and the movement of the wagons was controlled automatically according to the programmed period of light and external temperature. The temperature was controlled in the range of 21˚C - 32˚C, with an internal optimum control at 27˚C for the three treatments. To achieve this, reversible air conditioners (hot/cold) were installed in each photoperiod chamber, and devices that detect temperature were placed both inside and outside of the photoperiod chamber. The automation of the photoperiod chamber allowed the identification of the optimal conditions for induction and flowering.

The induction began in September 2010, and daily evaluations were performed until the end of April 2011. During the induction period, all of the morphological changes that indicate flowering, such as elongation and the date of flag leaf emergence were recorded as also the date of panicle emergence. In the period of inflorescence emergence, the relative humidity was maintained above 80 % (to simulate an intense period of rain).

The inflorescences were harvest to test the pollen viability under the artificial flower induction. Mature anthers were mixed with iodine 0.1 N solution to visualise the percentage of blue pollen grains in relation to the total stained pollen grains in a microscope slide ( [11] ). The inflorescences received a score ranging from 1 (female or pollen receptor) to 9 (male or pollen donor) based on the number of blue stained pollen grains ( [11] ) (Table 2).

3. Results

The first sign of flower induction which is characteristic of the Saccharum species, i.e., plant elongation, was observed at 60 days after the beginning of the flower induction treatment, indicating that the genotypes responded to the treatments. Among the tillers of the same genotype, in Treatments I (30 s) and II (45 s), some were etiolated leading to a visual lack of uniformity compared to treatment III (1 min).

Overall, the period of flag leaf emission was 110 to 179 days from the beginning of induction, and the emergence of inflorescences occurred at 137 to 207 days after the beginning of induction (Table 3) for all of the treatments.

Table 2

Pollen viability scores according to the percentage of blue stained pollen (%) using 0.1 N iodine method

Percentage of blue pollens (%)	Score
0	9
1 to 9	8
10 to 19	7
20 to 40	6
41 to 60	4
61 to 80	3
81 to 99	2
100	1

Table 3

Number of days of artificial photoperiod induction until leaf flag and inflorescence emission for 16 sugarcane genotypes at three different treatments

References

1. Clements, H.F. and Awada, M. (1967) Experiments on the Artificial Induction of Flowering in Sugarcane. International Society of Sugar Cane Technologists, 12, 795-812.

2. Humbert, R.P. (1974) El cultivo de la ca&ntildea de azucar. 6th Edition, Editorial Continental, México DF.

3. Srivastava, R.P., Singh, S.E., Singh, P. and Singh, S.B. (2006) Artificial Induction of Flowering in Sugarcane under Sub-Tropical Conditions—A Successful Approach. Sugartech, 8, 184-186.

4. Taiz, L. and Zeiger, E. (2010) Plant Physiology. 6th Edition, Sinauer Associates, Sunderland.

5. Berding, N. (1981) Improved Flowering and Pollen Fertility in Sugarcane under Increased Night Temperature. Crop Science, 21, 863-867. http://dx.doi.org/10.2135/cropsci1981.0011183X002100060016x

6. Berding, N., Pendrigh, R.S. and Dunne, V. (2007) Can Flowering in Sugarcane Be Optimized by Use of Differential Declinations for the Initiation and Developmental Phases? Proceedings of International Society of Sugarcane Technologists, Durban, 699-711.

7. Pereira A.R. ,et al. (1985)Previs&atildeo do florescimento em cana-de-a&ccedilúcar Comunica&ccedil&atildeo da Pesquisa Agropecuária 3, 15-16.

8. Berding N. ,et al. (1995)Improving Flowering through Breeding: Progress and Prospects International Society of Sugar Cane Technologists 17, 162-171.

9. Moore, P.H. and Nuss, K.J. (1987) Flowering and Flower Synchronization. In: Heinz, D.J., Ed., Sugarcane Improvement through Breeding, Elsevier, Amsterdam, 273-311. http://dx.doi.org/10.1016/B978-0-444-42769-4.50012-6

10. Scarpari, M.S., Garcia, J.C., Xavier, M.A., Landell, M.G.A., Mendonca, J.R. and Melloni, M.L.G. (2011) Building and Automation of the First Photoperiod Facility Made in Brazil for Sugarcane Breeding. Proceedings of the 10th Germplasm and Breeding 7th Molecular Biology Workshop, Maceió, 39-39.

11. Machado Jr., G.P. (1987) Melhoramento da cana-de-a&ccedilúcar. In: Paranhos, S.B., Ed., Cana-de-a&ccedilúcar: Cultivo e utiliza&ccedil&atildeo, Funda&ccedil&atildeo Cargill , Campinas, 165-186.

12. Berding, N., Pendrigh, R.S. and Dunne, V. (2010) Pursuing Higher Efficacy for Managed Photoperiodic Initiation of Sugarcane Flowering in the Tropics. Proceedings of Australia Society of Sugar Cane Technologists, Bundaberg, 234-250.

13. Berding, N. and Hurney, A.P. (2005) Flowering and Lodging, Physiological-Based Traits Affecting Cane and Sugar Yield. What Do We Know of Their Control Mechanisms and How Do We Manage Them? Field Crops Research, 92, 261-275. http://dx.doi.org/10.1016/j.fcr.2005.01.015

14. Melloni, M.L.G., Scarpari, M.S., Pinto, L.R., Perecin, D., Xavier, M.A. and Landell, M.G.A. (2014) Selfing Rate Estimation in Sugarcane under Unfavorable Natural Conditions of Crossing by Using Microsatellite Markers. Genetics and Molecular Research, 13, 2278-2289. http://dx.doi.org/10.4238/2014.March.31.8

15. Moore, P.H. (1987) Physiology and Control of Flowering. Proceedings of Copersucar International Sugarcane Breeding Workshop, Piracicaba, 101-127.