Abstract
The branch construction of trees is based on phytomers, repetitive subunits defined as node + leaf + axillary meristem + internode. The rate at which phytomers are added to a shoot is termed the phyllochron, which is determined by genetics, endogenous regulation and environmental conditions. The phyllochron is fundamental to understanding the growth of plants. Most phyllochron studies on woody species are of young plants under controlled conditions without consideration for different types of shoots that are present in mature trees. In this 2-year field study, we investigated seasonal patterns of phyllochron development on both proleptic and epicormic shoots of mature peach trees (Prunus persica) exposed to two irrigation treatments. One treatment was not irrigated until significant stress was detected via water potential by pressure bombing. In the second treatment trees were normally irrigated with ~5.84 cm of water each week to match evapotranspirational loss. Midday stem water potential readings were regularly collected to assess the level of water stress experienced by the trees. Measurements of individual leaves and their corresponding internodes were taken along tagged proleptic and epicormic shoots three times per week from the beginning to the end of two growing seasons. Leaf measurements were used to calculate the phyllochron. The phyllochron increased as the season progressed. This increase could not be explained by traditionally accepted effects of temperature or light. The more vigorous epicormic shoots added leaves significantly faster than proleptic shoots on trees in both water deficit and normally irrigated treatments. Additionally, epicormic shoots produced leaves with significantly greater leaf length and leaf area. Midday stem water potentials were more negative in trees in the water deficit treatments only after proleptic shoots stopped growing. The phyllochron did increase while leaf length and leaf area decreased on epicormic shoots of deficit irrigated trees in the 2010 growing season. The phyllochron of both shoot types gradually increased over the season, which was primarily due to an endogenous rank or shoot ageing effect. Differences between shoot types indicate that the phyllochron is variable among shoots on the same tree and is associated with shoot vigour. Water deficit increased the phyllochron and over all shoot growth rate.
Highlights
Growth and development of plants involve interdependent processes resulting from the interaction of genotype and environment (Callaham 1962; Lambers et al 2008)
Leaves were produced along a shoot on average 23 % (2010) and 36 % (2011) faster on epicormic compared to proleptic shoots
The phyllochron of epicormic shoots was less than for proleptic shoots and this was associated with greater mean individual leaf area and leaf length for epicormics shoots
Summary
Growth and development of plants involve interdependent processes resulting from the interaction of genotype and environment (Callaham 1962; Lambers et al 2008). Development and growth are characterized by the repeated formation, expansion and subsequent senescence of basic units, called phytomers (Gray 1879). The time elapsed between the addition of new phytomers can be represented and measured by the appearance of leaves at new nodes (Pagès et al 1996). The number of leaves that emerge per unit of time is termed leaf appearance rate (LAR). The inverse of LAR is termed the phyllochron, the time elapsed between the appearances of successive leaves on a stem (Wilhelm and McMaster 1995) and is most commonly measured in hours, growing degree days (GDD) or growing degree hours (GDH). Shoot development can be conceived as the addition of successive phytomers, added to the shoots during each phyllochron
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