THE EFFECT OF TEMPERATURE ON FRUIT SET, SEED SET AND SEED GERMINATION IN 'SONIA' x 'HADLEY' HYBRID TEA-ROSE CROSSES
D. P. DE VRIES and LIDWIEN A. M. DUBOIS
Institute for Horticultural Plant Breeding (IVT), P.O. Box 16, 6700 AA Wageningen, the Netherlands
Received 29 January 1986
The effect of temperature on fruit set, seed set and seed germination was studied in 'Sonia' x 'Hadley' Hybrid Tea-rose crosses. 'Sonia' mother bushes were grown at constant temperatures (10, 14, 18, 22, 26°C) in the greenhouses of the phytotron until fruit ripening. Fruit set, fruit weight and number of seeds increased as temperature was higher. Optimum temperatures were found for days to fruit ripening (18°C), seed germination (22°C) and number of seedlings per pollinated flower (22°C). Fruit weight and number of seeds were positively correlated. For crossing and the subsequent growing of seed-bearing plants 22°C was the most favourable temperature. Effects of temperature on pollen tube growth, fertilization and seed germination are discussed.
In crosses between Hybrid Tea-rose cultivars relatively low percentages of available ovules develop into viable seeds. Although we have shown that by pollinating more than once, the number of seeds per fruit may more than double, the seed set was still 30% only (DE VRIES & DUBOIS, 1983). Among the environmental conditions affecting fruit-, and seed set, temperature would be an important factor, although commercial breeders have different opinions about optimal values.
To obtain a first indication of the optimal temperature for breeding, the present study deals with the effect of five constant temperatures, applied from pollination to fruit harvest, on fruit set, seed set and seed germination in 'Sonia' x 'Hadley', a cross, which was also successful in earlier experiments (DUBOIS, & DE VRIES, 1986).
MATERIALS AND METHODS
Mother plants consisted of 2-year-old bushes of the Hybrid Tea-rose 'Sonia' planted in 30 1 buckets and pre-grown in a greenhouse at 20°C. Pollen used in this experiment was harvested from 'Hadley' Hybrid Tea-roses, also grown at 20°C, and dried and stored at 4°C in a dessicator (VISSER et al., 1977).
Experiments were carried out in the greenhouse of the IVT-phytotron, in full day light, at 5 constant temperatures (10, 14, 18, 22 and 26°C) and a relative humidity of 70%. A few days before the petals were to unfold, three buckets were placed at each temperature, where they remained until fruits were harvested. At each temperature 40 flower-bearing shoots were marked for pollination; an experimental unit consisted of one flower. At the end of April flowers were emasculated, when about 3/4 open, and pollinated twice (24 h and 48 h after emasculation) to improve seed set (DE VRIES & DUBOIS, 1983). Fruits (hips) were harvested when orange coloured. The seeds (achenes) of each fruit were sown separately in flats, and stratified for 4 months at 0°C. Seeds were germinated by placing the flats at 22°C. Analysis of variance was carried out for:
Pollination efficiency (PE) is defined as the number of seedlings obtained per pollinated flower.
Temperature significantly affected the number of days to fruit ripening. Fig. 1 shows that the optimum temperature for fruit ripening was 18°C (117 d).
Table 1 shows that fruit set, fruit weight and the number of seeds per fruit, significantly increased as the greenhouse temperature was higher. Seed germination was dramatically affected by the temperature regime under which pollination and subsequent fruit growth had taken place: from 0% at 10°C it rose to an optimum of 70% at 22°C. For PE also, the optimum temperature was 22°C; both at 22 and 26°C, PE was about twice as high as at 18°C.
Fig. 1. The relation between temperature (°C) and the
number of days to fruit ripening in 'Sonia' x 'Hadley'
crosses (bars represent standard deviation).
Table 1. Fruit- and seed characters* of 'Sonia' seed-bearing plants, resulting from crossing with 'Hadley', and grown at 5 constant temperatures.
|10°C||14°C||18°C||22°C||26°C||Fruit set (%)||75a||78a||88b||90b||100c|
|Fruit weight (g)||3.5a||7.6b||9.6c||13.0d||14.3e|
|Number of seeds per fruit||6.4a||10.0b||27.4c||32.6d||37.1e|
|Seed germination (%)||0a||5.6b||42.2c||70.2d||62.7e|
|Pollination efficiency (PE)||0||0.4||10.2||20.6||23.3|
* Means in one row, indicated by the same letter, do not differ significantly (5% level), according to the range test of KEULS.
Both within each temperature, and taking all data together, fruit weight and number of seeds per fruit were very significantly correlated: 10°C (r = 0.85, n = 30), 14°C (r = 0.90, n = 31), 18°C (r = 0.88, n = 35), 22°C (r = 0.89, n = 36), 26°C (r = 0.86, n = 40); for all data: r = 0.92, n = 172. This demonstrates that in Rosa, as in Fragaria (NITSCH, 1950) and Malus (SCHANDER, 1955), the number of seeds greatly determines fruit weight. No significant correlations occurred between the other characters.
DISCUSSION AND CONCLUSIONS
In the present experiment, in which the plants were constantly grown at the same temperatures, temperature effects on fertilization, fruit ripening and subsequent seed germination cannot be separated. As prior to the experiment, the 'Sonia' bushes were grown under the same environmental conditions, the number of ovules per flower available for fertilization is supposed to be the same at each temperature, i.e. approximately 103 (DE VRIES & DUBOIS, 1983). Consequently, the poor seed set below 18°C is likely to be due to slow or arrested pollen tube growth, to poor fertilization and/or to embryo abortion, rather than to under-developed or defective ovules.
In Rosa the temperature effect on the above phenomena has not been studied, but in e.g. Malus and Pyrus growth rate of pollen tubes decreased as temperature was lower (MODLIBOWSKA, 1945). On the other hand, actual fruit set in these crops was even better at low than at high temperatures in the orchard (LU & ROBERTS, 1952). Temperature effects on pollen tube growth and fertilization in Rosa, should be further studied by means of UV-microscopy.
In several fruit crops an effect of temperature on the time of fruit ripening has been observed, but actual optima were rarely demonstrated. In Fragaria fruit ripening was earlier as growing temperature was higher (GRUPPE & KHANZARI, 1975), and in Malus there was a significant negative correlation between the temperature sum from June to September and the picking date (LUTON & HAMER, 1983). In Prunus cerasus, on the other hand, time to fruit ripening was hardly affected by temperature (TUKEY, 1952). The present results demonstrate an optimum for fruit ripening in the Hybrid Tea-rose studied to occur at approximately 18°C.
The germination percentage of seeds of outdoor-grown rose cvs was positively correlated with the temperature prior to fruit harvest, which would indicate greater after-ripening requirements at low than at high growing temperatures (VON ABRAMS & HAND, 1956). It is, however, the experience over the years of the present authors that independent of temperatures during fruit ripening, viable Hybrid Tea-rose seeds are ready for germination after 4 months stratification at 0°C. For the present experiment this may indicate that most seeds at 14 and all at 10°C did not contain a viable embryo, and that fruits were parthenocarpic. Contrary to the small seeds occurring in parthenocarpic fruits after GA treatment (DUBOIS & DE VRIES, 1986), the seeds present were of 'normal' size. For the present cross ('Sonia' x 'Hadley'), temperatures between 18 and 26°C are suitable for pollination and subsequent growing of seed-bearing plants. Measured by the main criterion PE, 22°C or slightly higher appeared to be the most favourable temperature for breeding, thus confirming longstanding experience at IVT with numerous Hybrid Tea cultivars. As long as the separate effects of temperature on gamete functioning, fertilization and embryo growth and development have not been investigated, 22°C may be recommended from pollination to fruit harvest.