shows the gel-phase domains as intramembranous particle-free regions that increase in size and number
as senescence progresses [60].
At the nucleus, senescence causes a progressive condensation of the chromatin that is detectable
through the fluorescence decrease of propidium iodide–stained nuclei. Condensation begins at early
stages and proceeds until it becomes irreversible after the endonucleolytic fragmentation that is typical of
late apoptotic processes [61].
In contrast, there are other structures, such as mitochondria, that remain intact until later phases,
when some swelling or distortion of cristae becomes apparent. The plasmalemma integrity is also main-
tained until the final stages. Cells become progressively more vacuolate with age, and changes in the per-
meability of the tonoplast membrane, surrounding the vacuole, could allow the transfer of cytoplasmic
material into the vacuole, favoring its degradation. Autophagic processes in which organelles become en-
gulfed in vacuole-like structures have been observed [56]. In some cases, tonoplast rupture may cause the
lysis of cells at very late phases [58,62]. With differences depending on cell type, these changes proceed
sequentially until the whole cell is dismantled. For example, a study of the senescence of mesophyll cells
of rice coleoptiles [63] has shown that all cells follow precisely the same temporal sequence of events,
consisting of (1) degradation of chloroplast DNA, (2) condensation of the nucleus and decrease in the size
of the chloroplast with degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase and chloroplast
inner membranes, (3) disorganization of the nucleus, and (4) complete loss of cellular components and
distortion of the cell wall.
B. Physiological Changes
- Leaf Conductance and CO 2 Assimilation
Senescence produces closure of stomata leading to a decline in transpiration. It has been suggested that
the stomata aperture may control the rate of leaf senescence [64]. Because stomata are the main sites of
entrance for CO 2 , it might be speculated that insufficient CO 2 supply could be the cause of the decreased
photosynthetic assimilation observed during senescence. However, experimental measurement of CO 2
concentration in the substomatal cavity [65] suggested that CO 2 does not limit photosynthetic assimila-
tion. Hence, stomatal closure may be more a consequence than a cause of lowered photosynthetic activ-
ity according to the optimal variation hypothesis, which proposes that stomatal conductance adapts to the
photosynthetic capacity of the leaf [66]. It is remarkable that stomatal guard cells remain functional until
very late stages of senescence, far beyond other leaf cells. This may be a result of their lack of symplas-
tic connection with the surrounding cells [67].
- Respiration
Changes in respiratory rate of senescing fruits have been known for a long time. In detached apple fruits,
the respiration rate decreases gradually until a sudden burst, termed climacteric, followed by a further de-
cline in respiratory activity is observed. Fruits are divided into two categories, climacteric and noncli-
macteric, depending on whether or not their respiration shows a sudden peak. In detached leaves and cut
flowers, a climacteric-like rise in respiration has also been observed during senescence in some species
but not in others [68]. The common metabolic feature in climacteric fruits is their ability to produce and
respond to ethylene. It appears that the rise in respiration is a consequence of ethylene action and not of
senescence as such. The main reason for this conclusion is that inhibition of both the biosynthesis and ac-
tion of ethylene eliminates the rise in respiration without preventing eventual senescence. Besides, ethy-
lene treatment enhances respiration of nonripening tomato mutants but does not promote the typical
changes associated with ripening. It appears that ethylene enhances plant respiration by activating a pre-
existent enzymatic potential [68].
Respiratory pathways of senescent plants include glycolysis, the pentose pathway, tricarboxylic acid
(TCA) cycle, and the electron transport pathway, in which some changes have been described [68], but
also an alternative oxidase pathway that is enhanced during senescence [69]. In aging potato tuber slices
the alternative oxidase has been characterized as an integral membrane protein synthetized de novo [70]. It
has been suggested that the alternative pathway is activated when the cytochrome pathway is saturated or
limited, allowing the TCA cycle to function using up excess carbohydrates [71]. In addition, a plant un-
coupling mitochondrial protein (PUMP) has been shown to be induced by low-temperature stress and ag-
ing in potato tubers [72]. PUMP is homologous to the mammalian uncoupling protein of brown adipose tis-
186 PEÑARRUBIA AND MORENO