Croatian Journal of Forest Engineering (Apr 2011)
Phenological Asynchronization as a Restrictive Factor of Efficient Pollination in Clonal Seed Orchads of Pedunculate Oak (Quercus robur L.)
Abstract
The knowledge of the flowering biology of forest trees, and thus also of Pedunculate Oak, is exceptionally important when planning to mass-produce genetically high-quality seeds at clonal seed orchards (CSO) for the purpose of reforestation and restoration of forest cultures. The production of genetically high-quality seeds at CSOs, which assumes a high degree of genetic diversity, can only be guaranteed if 20 to 60 clones of different genotypes contribute to pollination (transferring of pollen from stamen to stigma). However, for a successful fertilization, which would guarantee the desired quality and quantity of seeds, phenological synchronization of the clone flowering within CSOs is also important. The beginning of pedunculate oak bud burst is under significant genetic control, which opens up the possibility to make the right selection of plus trees, with the goal of achieving a permanent phenological synchronization of flowering and flushing of clones with which the CSOs have been established. However, the effect of poor weather, such as heavy rainfall, too high or too low relative humidity of air, low temperatures, frosts, hail, etc., can sometimes disrupt favorable phenological synchronization of clones, also lowering the quality and quantity of seeds produced. The objectives of this study were (i) to determine the phenological synchronization of clone flowering within CSOs, (ii) to look into the possible effect of weather on the duration of receptivity of female flowers and pollen shedding, (iii) to determine the degree of correspondence between particular phenophases of flushing and flowering.The study has been conducted at the experimental field founded in the spring of 2008 within Rasadnik Brestje (Forest Administration Zagreb, Forest Office Dugo Selo). The experimental field has been founded by planting ramet in accordance with the experimental design of randomized section system with repetitions (sections). Each clone is represented with one ramet in each section. Ramets originate from the clones of pedunculate oak, from which clonal seed orchards were founded in the regions under respective Forest Administrations (FAs) Vinkovci (VK), Našice (NA) and Bjelovar (BJ). Phenological observations of flushing and flowering were performed simultaneously, twice a week, until the leaves were fully developed on all clones, i.e. until all male flowers ceased to shed pollen or until all female flowers ceased to be receptive. The phenology of flushing was monitored in accordance with the methodology which distinguishes 7 phenophases in the flushing of pedunculate oak, while the phenology of flowering was monitored according to the methodology developed on the example of cork oak (Quercus suber L.). In this study, during the phenological monitoring of flowering, special attention was given to phenophases of male flower development, which describe the beginning and the end of pollen shedding, i.e. to phenophases of female flowers, which describe the beginning and the end of their receptivity. Microclimatic conditions during flowering (from April 7 to May 23) in 2010 were measured via automatic meteorological station set up at the experimental field.From a total of 145 clones, 103 were flowering at the experimental field in 2010, 8 of which only with male flowers, 31 only with female flowers, and 63 with both male and female flowers. Protogyny was established for the clones flowering with both male and female flowers, except for the clone VK 43 where protandry was established. The receptivity of female flowers of protogynous clones occurred on average 7 days before the male flowers began shedding pollen, while the VK 43 clone started releasing pollen a day before female flowers started to be receptive.Generally speaking for all the clones represented at the experimental field and flowering in 2010, pollen shedding started on April 15, and ended on May 22. The receptivity of female flowers started on April 13, and ended on May 13. From the moment the first clone started shedding pollen untill shedding stopped, 37 days have elapsed, while the receptive period of female flowers lasted 30 days.The receptive period of female flowers for the clones in the region of FA BJ and VK started on April 14, while this period started on April 13 for the clones in the region of FA NA. The end of the receptive period for BJ clones was recorded on May 13, for NA clones on April 20, and for VK clones on May 11. The receptive period of BJ female flowers lasted for 29 days, of NA for 9 days, and of VK for 28 days. There were two »critical» periods (the first from April 19 to 22, and the other on May 3) during which a sudden termination of receptivity in female flowers was recorded in a great number of clones. During the first period with a more pronounced effect, the termination of receptivity occurred in almost all of the clones within all three CSOs where flowers started to be receptive before April 22. The average duration of the receptive period of female flowers in all of the clones that stopped being receptive in this period amounted to 4.2 days. In the second and less pronounced period, there was a termination of receptivity of female flowers in a half of VK clones, whose receptivity mostly began after April 22. The average duration of the receptive period of female flowers, where clones stopped being receptive after May 3, amounted to 9.2 days. Considering the duration of the receptive period of female flowers, statistically significant difference has been determined among the clones whose receptivity of female flowers ended before April 22 and the clones whose female flowers stopped being receptive after April 22. The above-mentioned critical periods occurred in both cases after a sudden drop in the relative humidity of air, which was recorded on April 19 and 29, and low values of minimum air temperatures, which were recorded on April 19, 28 and 29. Shortly before or during the occurrence of »critical» periods, precipitation was also recorded.The shedding of pollen started on April 19 for the clones in the region of FA BJ and NA, while the clones in the region of FA VK started shedding pollen on April 15. BJ clones stopped shedding pollen on May 20, NA clones on May 3, and VK clones on May 13. Pollen shedding lasted 31 days in BJ clones, 14 days in NA clones, and 28 days in VK clones. Similarly to the receptivity of female flowers, two »critical» periods, when a significant number of clones stopped shedding pollen, can also be noted (the first from April 27 to 29, and the second on May 10). During the first period, most of the NA clones stopped shedding pollen. During the second period, a sudden halt in shedding pollen was recorded in almost all VK clones that started shedding pollen after May 3. The duration of pollen shedding does not differ with respect to »critical« periods when cessation of shedding occurred, and it amounts to 5.6 days for all the clones, on average. The abovementioned »critical« periods, in both cases connected with shedding of pollen, occurred during a period with high temperatures and low relative humidity of air, without precipitation.Phenological synchronization of flowering is the result of bud-burst synchronization in clones within CSOs and weather conditions, at the moment of receptivity of female flowers and shedding of pollen. In that respect, a combination of phenological synchronization of flowering and unfavorable weather conditions, which caused the sudden termination and short receptive period of female flowers, was most drastically reflected through the clones from CSO NA. Only a smaller number of clones from CSO NA could have participated in mutual pollination in 2010, while in CSOs VK and BJ most of the clones had this opportunity thanks to the unsynchronized bud burst, which had in turn enabled a number of clones to avoid unfavorable weather conditions.Based on the comparison between an average number of days that clones needed to enter each of the studied phenophases of flushing and flowering, it has been determined that the onset of the receptive period of female flowers occurred almost at the same time with phenophase of flushing L4 (first leaves are developed still mostly in the bud), the onset of pollen shedding, and the termination of receptive period of female flowers at the same time with phenophase L6 (the leaves are formed but still longitudinally curved), while the flushing phenophase L7 (the leaves are fully formed and smooth) coincided with the termination of receptivity of female flowers.The results showed that the phenological synchronization of flowering in 2010 was significantly affected by unfavorable weather conditions, which shortened the duration of the receptive period of female flowers to a greater degree, while they had no significant effect on the duration of pollen shedding period. Such course of events most severely affected the clones originating from CSO NA, which had started the period of receptivity of female flowers and pollen shedding of male flowers approximately at the same time, in contrast to the clones originating from CSOs Vinkovci and Bjelovar, which started their receptive periods of female flowers and pollen shedding periods of male flowers at different times.
