Seasonal dynamics of shoots growth in <i>Forsythia ovata</i> Nakai plants: Rhythmicity of apical and radial growth

Abstract

The aim of the study was to establish the type (linear or non-linear) and shape (presence of extremes and trends) of the time dependences of apical and radial growth rates for shoots of two types (branching and formation) in woody plants. The object of the study was an introduced in the Middle Urals ornamental early-flowering shrub species Forsythia ovata Nakai. Seasonal dynamics of morphometric parameters of two types of shoots, which differ in intensity of growth and origin (developing from the apical or dormant buds of the skeletal branch respectively), was examined. Also, there was made an assessment of their relationship with air temperature and precipitation values. Apical and radial growth are accompanied by oscillations of increments. The duration of individual phases of oscillations is about 7 or more days, which makes it possible to attribute this phenomenon to infradian growth rhythms. The weak relationship between the characteristics of the observed oscillations and weather conditions indicates the predominantly endogenous nature of the observed morphogenetic processes. The growth of formation shoots begins 4 weeks later than branching shoots. This delay is apparently related to the time necessary for activation of dormant bud growth. Seasonal dynamics curves of rates and accelerations of the apical and radial shoot’s growth have the shape of previously unobserved oscillations with phases fading in amplitude and changing in duration. The growth ends 1–2 months before the environmental conditions become unfavorable. The oscillations of the growth rate of shoots are apparently associated with the action of two or more differently directed factors, which determine the increase in volume of different segments of the shoot. These factors include the different contribution of cell division and elongation to the change in the geometric dimensions of the apical meristem domains. The main differences in the seasonal dynamics of the apical and radial increments for both types of shoots are: the duration of growth (6–8 and 4 weeks, respectively), the number of extremes, the amplitude and the duration of individual phases of oscillations. Higher amplitudes of the growth rate oscillations in shoots formation compared with branching shoots, cause bigger values of the formation shoots total increments. The dynamic curves of the apical growth rates have two maxima, while those of the radial growth have only one. Each type of growth, despite the delay in the onset of the formation shoots, has the same total duration. The shapes of curves of seasonal changes in speeds and accelerations of shoot growth values are also similar for different types of shoots. These similarities indicate that shoots have the same “programs” of growth regulation, ultimately determined by the genotype. The rhythmic growth of both types of shoots during the season suggests the existence of the relationship in time between the rates of division and growth of cells located in functionally different groups of cells in the apical meristem zone.