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).