After a long enough period of adaptation, plants no longer
appear to be TPU-limited
TPU limitation is characterized by the responses of photosynthesis to
increasing CO2 (McClain & Sharkey 2019). Once the plant
becomes TPU-limited, elevating CO2 results in elevated
PMF and NPQ, while reducing φII andgH+ through the thylakoid membrane. In addition,
the shape of the A /Ci curve is distinct:
with increasing CO2, A remains constant or
marginally decreases due to reduced export of photorespiratory
intermediates (Busch, Sage & Farquhar 2018). After 30 h of acclimation
to elevated CO2, evidence of TPU is gone (Fig. 2). Thus,
acute TPU limitation is probably a brief condition during which the
consumption and production of free phosphate come back into balance, and
TPU limitation is instead diagnosed by the regulatory effects that
result. Once qE is supplanted by photoinhibition
it becomes difficult to assess TPU limitation.
It is generally thought that extended periods of time in high light and
low CO2 will cause damage to the photosynthetic
apparatus, but data reported here show that extended periods of high
CO2 are deleterious while low CO2 are
not as bad. This is interpreted as TPU being a stressful condition that
causes regulatory responses that result in a loss of TPU behavior. The
acclimation shown here prevents plants from experiencing TPU stress.
Debate has recently surfaced about the relevancy of TPU limitation to
global models (Lombardozzi et al. 2018; Rogers et al.2020). TPU limitation is rarely diagnosed as the limiting factor of
steady-state photosynthesis in the wild (Sage & Sharkey 1987). We
believe that this is due to the relatively fast adaptation to TPU
limiting conditions. Within a day of acclimation to very high
CO2, TPU limitation would not be diagnosable from gas
exchange or fluorescence analysis. TPU limitation would only happen
transiently. For this reason, we agree that TPU limitation as an
explicit parameter of photosynthesis need not factor into global models
of photosynthesis. However, it is important as a component of the
regulatory network of photosynthesis.
It is currently unclear as to why TPU capacity did not increase in
response to elevated CO2 (Fig. 1). If maximizing
photosynthesis were the only concern, the plant would produce extra
enzymes for processing end products to relieve TPU limitation instead of
reducing other photosynthetic capacities. Some experiments have been
done previously connecting TPU capacity with low temperature, another
primary cause of TPU limitation (Sharkey & Bernacchi 2012) due mostly
to the high temperature sensitivity of sucrose-phosphate synthase (Stitt
& Grosse 1988). Plants grown in low temperature produced significantly
more sucrose synthesis enzymes (Guy et al. 1992; Holaday et
al. 1992; Hurry, Strand, Furbank & Stitt 2000). We know therefore that
plants which have been TPU limited can produce more
end-product-synthesis enzymes, so it seems like an obvious inefficiency
for plants to lose photosynthetic capabilities. This conundrum may
reflect the interaction between plant growth and photosynthesis. Some
analyses indicated that photosynthetic rate is not the best predictor of
plant growth (Körner 2015). Factors controlling growth rate and
photosynthetic rate may not always work in concert. Growth is more
temperature sensitive than is photosynthesis and so it may be that at
low temperature growth limits photosynthesis while at high temperature
photosynthesis limits growth. In this case, while the plant may look
like it is performing inefficiently, it may simply be growing as fast as
possible and any additional photosynthesis would not be useful. Thus far
it has been difficult to establish explicit causality connecting sink
regulation to TPU limitation (Paul & Foyer 2001) but efforts have been
reported (Fabre et al. 2019; Dingkuhn et al. 2020). Recent
work on SnRK1, the Target of Rapamycin complex, and interactions with
trehalose 6-phosphate signaling may eventually help explain the
interaction between plant growth and photosynthetic rate (Sulpiceet al. 2009; Smeekens, Ma, Hanson & Rolland 2010; Lastdrager,
Hanson & Smeekens 2014; Shi, Wu & Sheen 2018; Brunkard 2020; Peixotoet al. 2021).