UFV-M7: mutant yellow passionfruit genotype with photoperiod insensitivity for flowering

The development and characteristics of UFV-M7, a mutant with photoperiod insensitivity to beginning of flowering, are described. This genotype was fortuitously obtained by regeneration of nodal segments, exposed to gamma radiation. The possibility of incorporating photoperiod insensitivity into cultivars for producing regions is discussed for latitudes where the photoperiod determines the seasonality of harvest.


INTRODUCTION
Yellow passionfruit (Passiflora edulis Sims., Passifloraceae) is a species typical of tropical climates.It belongs to a group of plants requiring long days, demanding in photoperiod and temperature (Nave et al. 2010).The plant needs a minimum of 11 h light day -1 and monthly average temperatures around 20 °C, to ensure floral induction and fruit development (Junqueira et al. 1999, Borges andLima 2009).
In general, the climatic conditions for year-round production of yellow passionfruit are ideal in low-latitude regions near the equator (Cavichioli et al. 2006).Major passionfruit-producing regions are located at latitudes above 15° South.In these regions, the reduction in photoperiod and temperature in the winter adversely affects the passionfruit harvest, which is seasonal.
Mutant plants that can flower when days are short were identified in the yellow passionfruit breeding program of the Federal University of Viçosa (UFV), Minas Gerais, Brazil.Understanding the genetic control of photoperiod insensitivity would facilitate the incorporation of this phenotype in hybrids or cultivars (Bruckner et al. 2005).Genotypes with photoperiod insensitivity to flowering can be used to develop cultivars recommendable for higher-latitude regions, increasing the harvest period.

DEVELOPMENT OF THE MUTANT GENOTYPE
The genotype UFV-M7 was obtained from micropropagated plants from nodal segments explants exposed to gamma radiation.The plant material, derived from superior genotypes identified by the UFV yellow passionfruit breeding program, was initially irradiated with a view to the establishment of varieties resistant to Fusarium oxysporum f.sp.Passiflorae (Flores et al. 2011).The nodal segments in petri dishes containing MS medium were exposed to gamma irradiation (20 Gya) from a 60 Co Gammacell source (0.709 to 0.711 kGy h -1 ) at the center for nuclear energy in agriculture (CENA), in Piracicaba, São Paulo (Flores 2011).After four subcultures, the plants were subjected to two selection cycles in liquid MS medium supplemented with culture filtrate of a F. oxysporum f. sp.Passiflorae isolate, according to the methodology developed by Flores et al. (2012).The selected plants were acclimatized and transferred to an area of historical occurrence of Fusarium in an experimental field of UFV (lat 20° 45' 14'' S, long 42° 52' 54'' W and alt 648 m asl), Viçosa, Minas Gerais, Brazil.The climate is Cwb (tropical highland climate), with dry/cold winters and hot/humid summers.The average annual rainfall is 1,200 mm.Among the surviving plants, it was found that the plant UFV-M7 flowered early and grew flower buds even at photoperiods of less than 11 h light day -1 , demonstrating insensitivity to short photoperiod for flowering of this mutant.

CONFIRMATION OF INSENSITIVITY TO SHORT PHOTOPERIOD FOR FLOWERING
To confirm the insensitivity of UFV-M7 to flower under a photoperiod of less than 11 h light.day - , this genotype was compared with N9 in an experiment.This plant material is unirradiated and normal in terms of photoperiod requirement for flowering (over 11 h light day -1 ).Genotype N9 was derived from a progeny developed by the UFV yellow passionfruit breeding program, based on recurrent selection cycles for fruit yield and quality, with flowering between September and October.

Location
The experiment was carried out in a greenhouse of the Plant Science Department of UFV.The monthly distribution of photoperiod and temperature in the evaluation period is shown in Figure 1.Photoperiod data were obtained by subtracting the hours from sunset to sunrise from 24, by the forecast Center CPTEC/INPE.Temperatures were recorded by a Datalogger Impact IP-747RH, maintained in a greenhouse with hourly data collection.

Propagation of genotypes
The genotypes UFV-M7 and N9, initially present in the field, were propagated by cuttings and grown in a greenhouse.The cuttings were collected with three buds, cut between the middle part and the apex of branches of annual growth.After removing the leaves, the cuttings were planted in dibble tubes (6 x 20 cm) containing commercial substrate, at a depth of 4 cm.They were kept under spray irrigation for 5 min, triggered every 1 hour by a timer, from 6 to 18 o'clock.

Test installation and performance
In May/2011, 60 days after planting the cuttings, seedlings were transplanted to 30 L pots containing a substrate of soil and washed sand (ratio 2:1).Fertilization was applied according to the results of fertility analysis of this substrate.The substrate was maintained near field capacity by trickle irrigation, using one dripper per pot (microjet spray heads) at a flow rate of 14 L/h at 1 kgf cm -2 .Thirty-two plants per genotype were evaluated, with one plant per pot, totaling 64 plants.The seedlings were grown from a single stem (Figure 2A), clipping the lateral branches.When these main stems crossed a wire at a height of 20 cm, they were bent downward, forming a loop.Secondary and tertiary branches were left to grow from this loop, to assess the flowering period (Figure 2B).

Evaluation flowering periods
Two evaluation periods were previously established, considering the variations in photoperiod and temperature throughout the year (Figure 1): Period 1 -May 2011 to August 2011, a cold/dry season with short photoperiod, less than 11 h light day -1 ; Period 2 -September 2011 to December 2012, a warm/humid season with long photoperiod, more than 11 h light day -1 .

The evaluated characteristics
The number of flowering plants in the two pre-established periods was evaluated.The beginning of flowering was

Table 1 .
Morphological traits of two genotypes of yellow passionfruit (Passiflora edulis Sims.), contrasting for beginning of flowering at latitude 20° 45´ 14´´ S, propagated by cuttings and grown in a greenhouse Source: Descriptors established by the National Plant Variety Protection (SNPC) of the Ministry for Agriculture, Livestock and Supply (MAPA)