Introduction
In plants, carbohydrates, which are classified as structural
carbohydrates (SCs, including lignin and cellulose) and nonstructural
carbohydrates (NSCs, including soluble sugar, sucrose, fructose, and
starch), are of great importance to energy sources and physiological
metabolism in plant life history (Ögren
2010, Dietze et al. 2014). SCs are
generally used for constructing plant tissue, and NSCs mainly offer
carbon (C) and energy for plant growth, respiration, and production
(Würth et al. 2005,
Dietze et al. 2014).
NSCs are referred to as reservoir pools, and have protective functions,
including osmotically active compounds, chemical chaperones, and
reactive oxygen species (ROS) scavengers
(Newell et al. 2002,
Ivanov et al. 2019). Generally, NSCs only
account for approximately 10% of plant biomass, but their
concentrations in leaves are higher than those in roots and stems under
natural conditions, indicating the vital role of plant leaves in
regulating the C balance between uptake and consumption
(Martínez-Vilalta et al. 2016). The
concentrations of NSCs can mirror the capacity of plant adaption to the
various environmental conditions (Hoch et
al. 2003, Richardson et al. 2015). Under
exposure to various global environmental changes, such as warming,
CO2 enrichment,
ozone
destruction, drought, and N deposition, plant survival, resistance
ability, growth rate, and productivity are primarily determined by
carbohydrate dynamics (Dietze et al.
2014, Martínez-Vilalta et al. 2016).
It is widely believed that N and P are two essential nutrients for plant
photosynthetic C assimilation, and they also limit the net primary
productivity (NPP) in terrestrial ecosystems
(Vitousek and Howarth 1991,
Herbert and Fownes 1995). Traditionally,
N availability constrains plant productivity by limiting leaf initiation
and expansion (Vos and Biemond 1992),
while P availability mainly determines leaf biochemical processes such
as energy exchange and nucleic acid synthesis in plant cells
(Warren 2011). In the herbaceous plant
yellow bluestem (Bothriochloa ischaemum ), the soluble sugar
concentrations was reduced while the starch and total NSC concentrations
were increased by N addition (Xiao et al.
2017). In another study, both above- and below-ground NSC (sugars and
starch) concentrations in yellow bluestems were significantly increased
by N addition (Ai et al. 2017).
Nonetheless, both N and P addition decreased the concentrations of leaf
soluble sugars and starch in two species of grass and forbs in an Inner
Mongolian semi-arid grassland community
(Wang et al. 2017b).
Al-Hamdani and Sirna (2008) also reported
that the starch and total NSC accumulation in Salvinia minimawere significantly lower under N or P addition. However, previous
studies on different plant species (which mainly include herbaceous
species) have found different results regarding the plant’s responses to
external N and/or P addition.
In tropical forests, P is an important limiting factor for plant growth
and productivity (Vitousek et al. 2010),
as soil P availability generally declines with bedrock weathering and
soil age (Walker and Syers 1976).
Therefore, soil P availability in tropical forests may drive leaf NSC
dynamics, which could reflect carbohydrate dynamics (C assimilation by
photosynthesis and consumption by respiration). Although it is widely
recognized that P addition could greatly increase leaf P concentrations
in the tropics (Mayor et al. 2014,
Schreeg et al. 2014,
Wright et al. 2018), very few studies
have investigated how increased leaf P concentrations affect the NSC
dynamics in tropical forests.
A recent study reported that leaves tended to optimally allocate
different functional P fractions (structural P, metabolic P, nucleic
acid P, and residual P) to simultaneously accomplish a series of
physical processes (photosynthesis) in P-limiting tropical forests
(Mo et al. 2019). Given that leaf C
assimilation is closely related to N and P supply
(Kroth 2015), the response of NSC to
long-term N and P addition is relatively fundamental for understanding
the mechanisms and relationships of leaf C assimilation and P allocation
in tropical forests.
Fertilization experiments involving addition of external N and P on
reforestation from degraded sites have been conducted at many locations
globally (Tanner et al. 1992,
Ceccon et al. 2003,
Mayor et al. 2014,
Schreeg et al. 2014,
Li et al. 2015). These experiments could
be efficient ways to evaluate the effects of P limitation on key
biological process (Ågren et al. 2012,
Tessa et al. 2018). In tropical China, N
deposition is projected to continually increase in the future
(Liu et al. 2011). The increased
atmospheric N input may provide higher active N for plant growth in
forest ecosystems (Lu et al. 2008), which
also aggravates the imbalance of soil N:P ratios in tropical forests
(Du et al. 2016).
According to our previous results,
the plant growth in this studied secondary tropical forest has been
proven to be primarily limited by soil P availability
(Mo et al. 2015,
Mo et al. 2019). Here, we employed a
long-term manipulative field experiment to test the response of leaf
NSCs to N and/or P addition in a secondary tropical forest in southern
China. We tried to answer the following questions: how do leaf NSC
(soluble sugars and starch) concentrations respond to continuous N
deposition and P limitation? How does N and P addition regulate leaf NSC
concentrations in this tropical forest? We
hypothesized that: 1)
the
concentrations of leaf NSCs (soluble
sugars and starch) would be primarily regulated by P availability rather
than N availability due to the long-term P deficiency in this tropical
forest, and 2) the NSC concentrations would be reduced and transformed
into leaf biomass along with the increase in leaf structural P fraction
under P addition (Mo et al. 2019).