Senescence is a natural, energy-dependent, physiological, developmental and an ecological process that is controlled by the plant's own genetic program, allowing maximum recovery of nutrients from older organs for the survival of the plant, as such; it is classified as essential component of the growth and development of plants. In some cases, under one or many environmental stresses, senescence is triggered in plants. Despite many studies in the area, less consideration has been given to plant secondary metabolites, especially the role of VTCs on plant senescence. This review seeks to capture the biosynthesis and signal transduction of VTCs, the physiology of VTCs in plant development and how that is linked to some phytohormones to induce senescence. Much progress has been made in the elucidation of metabolic pathways leading to the biosynthesis of VTCs. In addition to the classical cytosolic mevalonic acid (MVA) pathway from acetyl-CoA, the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, originating from glyceraldehyde-3-phosphate (GAP) and pyruvate, leads to the biosynthesis of isoprenoid precursors, isopentenyl diphosphate and dimethyl allyl diphosphate. VTCs synthesis and emission are believed to be tightly regulated by photosynthetic carbon supply into MEP pathway. Thus, under abiotic stresses such as drought, high salinity, high and low temperature, and low CO 2 that directly affect stomatal conductance and ultimately biochemical limitation to photosynthesis, there has been observed induction of VTC synthesis and emissions, reflecting the elicitation of MEP pathway. This reveals the possibility of important function(s) of VTCs in plant defense against stress by mobilizing resources from components of plants and therefore, senescence. Our current understanding of the relationship between environmental responses and senescence mostly comes from the study of senescence response to phytohormones such as abscisic acid, jasmonic acid, ethylene and salicylic acid, which are extensively involved in response to various abiotic and biotic stresses. These stresses affect synthesis and/or signaling pathways of phytohormones to eventually trigger expression of stress-responsive genes, which in turn appears to affect leaf senescence.Comparison of plant response to stresses in relation to patterns of VTCs and phytohormones biosynthesis indicates a considerable crosstalk between these metabolic processes and their signal to plant senescence.

This study investigated the influence of both shaking duration and number of trees per bale on postharvest needle characteristics such as percentage needle loss, needle retention duration and explored the physiological roles of endogenous ethylene and volatile terpene compounds (VTCs). To accomplish these objectives, 25 six-year-old trees were detached and exposed to a range of shaking durations (0 to 60 sec.), and 30 six-year-old detached trees were exposed to baling treatments from 0 to 5 trees. Response variables measured were percent needle loss, needle retention duration, average water use, ethylene and volatile terpene compound evolution. Trees shaken for 60 seconds lost 16% less needle compared to control, which was consistent with the decrease in percent needle loss with increasing shaking duration. Baled trees lost 13% more needles compared to control, but percent needle loss was observed to decrease with increasing number of trees in a bale. These trends corresponded with increasing ethylene and VTC evolutions, where the longer the shaking duration or larger number of trees in a bale, the higher the ethylene and VTC evolutions. One can therefore draw inference that mechanical perturbation as a result of shaking and baling induce biosynthesis and regulation of ethylene and VTC in balsam fir trees in an effort to regulate postharvest needle abscission.