Agriculture Reference
In-Depth Information
Budbreak and Leaf Development
The timing of budbreak in species of temperate regions is usually in response to a
combination of photoperiodic cues and spring warming (Lechowicz 2001). The control
of budbreak in tropical species is less clear, but in species from seasonal climatic
regimes the water balance of the plant itself serves as a cue (Borchert 1994). At the
time of budbreak, embryonic leaves expand by absorbing water, in some cases with
further cell divisions (Dengler and Tsukaya 2001; Barthélémy and Caraglio 2007).
The duration of the period of leaf expansion depends on four factors: (1) the number
of primordial cells, (2) the rate of cell division, (3) the duration of the phase of cell
division, and (4) the size of the individual mature cells (Gregory1956). Newly
emerged leaves often are brightly colored and only become green at full expansion
(Dominy et al. 2002). Full expansion of the leaf typically requires of the order of
10-15 days from budbreak, but this timing varies substantially and is influenced by
both environmental conditions and phylogenetic considerations. It should be noted
that terrestrial monocotyledons with graminoid growth forms, such as sedges
(Hirose et al. 1989) and grasses (Bowes 1997), as well as the gymnosperm
Welwitschia
,
all have a different mode of leaf development in which basal meristems continu-
ously form new leaf tissues. Hence in these plants, the leaf has different age tissues
with the tip oldest and the base youngest (Mooney and Ehleringer 1997).
Because leaves are the primary organs of plant productivity, the logical benchmark
for leaf maturation is attainment of full photosynthetic capacity. Instantaneous rates
of photosynthesis are influenced by environmental conditions such as ambient
temperature, vapor pressure deficit, atmospheric CO
2
level, and soil water potential,
as well as plant condition and stage of development, but ultimately are most depen-
dent on irradiance (Larcher 2001; Lambers et al. 1998). Given the very dynamic
nature of photosynthetic rates, what single value might serve as an index of leaf
maturation and more generally as an index of leaf function? It is reasonable to focus
initially on the response of photosynthetic rate to irradiance, the flow of photons on
which this biochemical process depends. Although the net photosynthetic response
to irradiance varies among and within plant species, the basic shape of the response
curve is consistent (Fig.
2.6
). At very low irradiance, respiratory loss of CO
2
is
greater than photosynthetic gains, but as irradiance increases photosynthesis
predominates and net gains of CO
2
increase to an asymptote. This asymptotic rate
of net photosynthesis under saturating irradiance and otherwise optimal conditions
is referred to as photosynthetic capacity,
A
max
. Photosynthetic capacity is commonly
taken as the cardinal value most useful in assessing foliar function and plant adapta-
tion (Wright et al. 2004).
In many, but not all, species photosynthetic capacity develops steadily after
budbreak, reaching its maximal value when the leaf is fully expanded (Saeki 1959;
Šesták 1981; Hodanova 1981; Castro-Diez et al. 2005; Warren 2006). This pattern
is typical of relatively short-lived leaves, but in species with longer-lived leaves
months can pass until full photosynthetic capacity is attained. For example, in
Abies
veitchii
, leaves appear in June but maximum photosynthetic capacity is reached
