Rate of shoot development (phyllochron) is dependent of carbon availability, shoot type, and rank in peach trees

Crop load or carbohydrate availability, affects the phyllochron, leaf length, and leaf area of proleptic and epicormic shoots on mature, field-grown Prunus persica trees. It is well known that shoot growth rates can be strongly influenced by availability of carbohydrates to support growth. Additionally, carbohydrate availability for vegetative growth is influenced by crop load, since fruits are strong sinks for photosynthates. Thus, while crop load is known to have significant effects on shoot growth rates it is not clear whether this effect is limited to extension growth rates of internodes or whether it also affects shoot development such as the rate at which nodes are added to shoots, i.e., the phyllochron. In this study, we investigated the effect of the presence and absence of the crop on the phyllochron of proleptic and epicormic shoots on mature, field-grown peach trees. Leaf growth measurements were recorded three times per week from the beginning to the end of the growing season and used to calculate the phyllochron on trees from two treatments; one with 100% of the crop left on it (unthinned or fully cropped) and a treatment of trees where the crop was completely removed (non-cropped). The phyllochron fluctuated but generally increased over the season due to a rank effect. The phyllochron was longer on trees with heavy crop loads. Although there were large differences in phyllochrons between treatments in both shoot types, only differences observed in epicormic shoots were statistically significant. Additionally, leaf length and leaf area were also significantly reduced by crop load in both shoot types. These findings indicate that the phyllochron of field-grown peach trees is apparently sensitive to carbohydrate availability in the tree and should be considered when trying to accurately predict the timing of the addition of new phytomers along a shoot.