Steady-state and dynamic photosynthetic characteristics of shade-tolerant species Panax notoginseng in response to nitrogen levels

Abstract

Aims Nitrogen (N) is essential for the photosynthesis of plants. Shade plants are mostly exposed to dynamic light under natural growth conditions. However, little is known about the role of N levels in the photosynthetic regulation of shade plants under dynamic light. The objective of present study was to elucidate the mechanisms involved in the effect of N on dynamic photosynthesis in the typically shade-tolerant species Panax notoginseng. Methods The gas exchange parameters and the activity and amount of Calvin cycle enzyme/proteins were examined under dynamic and steady light conditions in P. notoginseng grown under low N (LN, 112.5 kg·hm–2) and high N (HN, 450.0 kg·hm–2), respectively. Important findings N content per unit of leaf area (Narea) was negatively correlated with the induction state at 60s of light (IS60) and positively correlated with the time required to reach 90% of photosynthetic steady state (tP90) and 100% of photosynthetic steady state (tP-steady), suggesting that Narea does not regulate the photosynthetic induction during dynamic light by the total activity of ribulose-1,5-bisphosphate carboxylase (Rubisco). Moreover, a short low light interval only slightly decreased the activity of Rubisco, but significantly reduced the activity of fructose-1,6-bisphosphatase (FBPase) and sedoheptulose-1,7-bisphosphatase (SBPase); when the sunfleck of high intensity appeared suddenly, Rubisco was still highly active, but SBPase and FBPase need to be reactivated to match Rubisco activity, so that photosynthetic induction after low light intervals is largely limited by reactivation of SBPase and FBPase. Furthermore, the content of Rubisco was higher than that of FBPase and SBPase. During the high light period of dynamic light, HN leaves need to activate a higher proportion of FBPase and SBPase and a longer period of time to resume photosynthesis. The results of present study reveal that under dynamic light condition, LN could alleviate the decline of photosynthetic induction rate, while HN exacerbated the decline of photosynthetic induction rate. The limitation in enzymes related to the ribulose-1,5-disphosphate (RuBP) may be one of the reasons why HN exacerbates the decline of photosynthetic induction rate under dynamic light condition.