Growth and flower-bud formation in apple as affected by paclobutrazol, daminozide, and tree orientation in combination with various gibberellins

Application of Paclobutrazol to Mitigate Environmental Stress of Urban Street Trees

Paclobutrazol is a tree growth regulator that is frequently applied by arborists to control tree growth in utility rights of way. Paclobutrazol is also marketed to mitigate tree stresses associated with urban environments. In this study we applied paclobutrazol as a soil drench to honeylocust (Gleditsia triacanthos L var. inermis (L.) Zab.) and Callery pear (Pyrus calleryiana Decne.) trees planted as street trees on two sites in Lansing, Michigan USA. We evaluated physiological and morphological responses for two years after treatment. Application of paclobutrazol increased SPAD chlorophyll index of trees of both species in both years, compared to untreated control trees. Application of paclobutrazol increased leaf water potential of trees on one study site (Downtown) but not the other (Old Town). Paclobutrazol increased gas exchange (net photosynthesis and stomatal conductance) of Callery pear trees on one of four measurement dates (gas exchange was not measured on honeylocust trees). Leaf size of Callery pear trees was reduced following paclobutrazol application whereas leaf size of honeylocust trees was unaffected by paclobutrazol. These results indicate that paclobutrazol can help to reduce stress of trees and improve physiological function under urban conditions. However, paclobutrazol should be viewed as part of a suite of options for arborists and landscapers to manage trees on stressful sites, rather than as a replacement for proper overall care.

Constraints to obtaining consistent annual yields in perennials. II: Environment and fruit load affect induction of flowering

In many commercial fruit crop species, high fruit load inhibits vegetative growth and floral induction. As a result, trees that had a high fruit load will bear few flowers and fruit the following year, along with abundant vegetative growth. We previously discussed how high fruit load interferes with concurrent shoot growth. Here we focus on how high fruit load impacts the process of flowering. Ascertaining the precise time at which specific buds begin the floral transition in each species is challenging. The use of indirect approaches to determine time of floral induction or evocation may lead to questionable conclusions. Annual and perennial plants appear to use conserved proteins for flowering induction and initiation. The accumulation or reduction of transcripts encoding proteins similar to Arabidopsis (annual) FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1), respectively, correlates well with flower induction in several diverse species. The recent use of such markers provides a means to formulate an accurate timeframe for floral induction in different species and holds promise in providing new insight into this important developmental event. A role for hormones in modulating the inhibitory effect of fruit load on floral induction is also discussed.