Role of bundle sheath conductance in sustaining photosynthesis competence in sugarcane plants under nitrogen deficiency

Roles for leakiness and O2 evolution in explaining lower-than-theoretical quantum yields of photosynthesis in the PEP-CK subtype of C4 plants

Theoretically, the PEP-CK C4 subtype has a higher quantum yield of CO2 assimilation (Φ CO 2 ) than NADP-ME or NAD-ME subtypes because ATP required for operating the CO2-concentrating mechanism is believed to mostly come from the mitochondrial electron transport chain (mETC). However, reportedΦ CO 2 is not higher in PEP-CK than in the other subtypes. We hypothesise, more photorespiration, associated with higher leakiness and O2 evolution in bundle-sheath (BS) cells, cancels out energetic advantages in PEP-CK species. Nine species (two to four species per subtype) were evaluated by gas exchange, chlorophyll fluorescence, and two-photon microscopy to estimate the BS conductance (gbs) and leakiness using a biochemical model. Average gbs estimates were 2.9, 4.8, and 5.0 mmol m-2 s-1 bar-1, and leakiness values were 0.129, 0.179, and 0.180, in NADP-ME, NAD-ME, and PEP-CK species, respectively. The BS CO2 level was somewhat higher, O2 level was marginally lower, and thus, photorespiratory loss was slightly lower, in NADP-ME than in NAD-ME and PEP-CK species. Differences in these parameters existed among species within a subtype, and gbs was co-determined by biochemical decarboxylating sites and anatomical characteristics. Our hypothesis and results partially explain variations in observedΦ CO 2 , but suggest that PEP-CK species probably use less ATP from mETC than classically defined PEP-CK mechanisms.

Drought-triggered leaf transcriptional responses disclose key molecular pathways underlying leaf water use efficiency in sugarcane (Saccharum spp.)

Drought is a major constraint to sugarcane (Saccharum spp.) production and improving the water use efficiency (WUE) is a critical trait for the sustainability of this bioenergy crop. The molecular mechanism underlying WUE remains underexplored in sugarcane. Here, we investigated the drought-triggered physiological and transcriptional responses of two sugarcane cultivars contrasting for drought tolerance, 'IACSP97-7065' (sensitive) and 'IACSP94-2094' (tolerant). After 21 days without irrigation (DWI), only 'IACSP94-2094' exhibited superior WUE and instantaneous carboxylation efficiency, with the net CO₂ assimilation being less impacted when compared with 'IACSP97-7065'. RNA-seq of sugarcane leaves at 21 DWI revealed a total of 1,585 differentially expressed genes (DEGs) for both genotypes, among which 'IACSP94-2094' showed 617 (38.9%) exclusive transcripts (212 up- and 405 down-regulated). Functional enrichment analyses of these unique DEGs revealed several relevant biological processes, such as photosynthesis, transcription factors, signal transduction, solute transport, and redox homeostasis. The better drought-responsiveness of 'IACSP94-2094' suggested signaling cascades that foster transcriptional regulation of genes implicated in the Calvin cycle and transport of water and carbon dioxide, which are expected to support the high WUE and carboxylation efficiency observed for this genotype under water deficit. Moreover, the robust antioxidant system of the drought-tolerant genotype might serve as a molecular shield against the drought-associated overproduction of reactive oxygen species. This study provides relevant data that may be used to develop novel strategies for sugarcane breeding programs and to understand the genetic basis of drought tolerance and WUE improvement of sugarcane.

Photosynthetic resource-use efficiency trade-offs triggered by vapour pressure deficit and nitrogen supply in a C4 species

Trade-offs in resource-use efficiency (including water-, nitrogen-, and light-use efficiency, i.e., WUE, NUE, and LUE) are an important acclimation strategy of plants to environmental stresses. C₄ photosynthesis, featured by a CO₂ concentrating mechanism, is believed to be more efficient in using resources compared to C₃ photosynthesis. However, response of photosynthetic resource-use efficiency trade-offs in C₄ plants to vapour pressure deficit (VPD) and N supply has rarely been studied. Here, we studied the photosynthetic acclimation of Cleistogenes squarrosa, a perennial C₄ grass, to controlled growth conditions with high or low VPD and N supply. High VPD increased WUE by 12% and decreased NUE by 16%, the ratio of net photosynthetic rate (A) to electron transport rate (J) (A/J) by 7% and the apparent quantum yield by 6%. High N supply tended to reduce NUE and increased maximum phosphoenol pyruvate carboxylation rate by 71% and slightly increased WUE. Stomatal conductance showed acclimation to VPD according to the Ball-Berry model, while a balanced cost of carboxylation and transpiration capacity was found across VPD and N treatments based on the least-cost model. WUE correlated negatively with NUE and LUE indicating that there was a trade-off between them, which is likely associated with acclimations in stomatal conductance and CO₂ concentrating mechanisms.

High nitrogen inhibits biomass and saponins accumulation in a medicinal plant Panax notoginseng

Nitrogen (N) is an important macronutrient and is comprehensively involved in the synthesis of secondary metabolites. However, the interaction between N supply and crop yield and the accumulation of effective constituents in an N-sensitive medicinal plant Panax notoginseng (Burkill) F. H. Chen is not completely known. Morphological traits, N use and allocation, photosynthetic capacity and saponins accumulation were evaluated in two- and three-year-old P. notoginseng grown under different N regimes. The number and length of fibrous root, total root length and root volume were reduced with the increase of N supply. The accumulation of leaf and stem biomass (above-ground) were enhanced with increasing N supply, and LN-grown plants had the lowest root biomass. Above-ground biomass was closely correlated with N content, and the relationship between root biomass and N content was negatives in P. notoginseng (r = -0.92). N use efficiency-related parameters, NUE (N use efficiency, etc.), N_C (N content in carboxylation system component) and P _n (the net photosynthetic rate) were reduced in HN-grown P. notoginseng. SLN (specific leaf N), Chl (chlorophyll), N_L (N content in light capture component) increased with an increase in N application. Interestingly, root biomass was positively correlated with NUE, yield and P _n. Above-ground biomass was close negatively correlated with photosynthetic N use efficiency (PNUE). Saponins content was positively correlated with NUE and P _n. Additionally, HN improved the root yield of per plant compared with LN, but reduced the accumulation of saponins, and the lowest yield of saponins per unit area (35.71 kg·hm^-2) was recorded in HN-grown plants. HN-grown medicinal plants could inhibit the accumulation of root biomass by reducing N use and photosynthetic capacity, and HN-induced decrease in the accumulation of saponins (C-containing metabolites) might be closely related to the decline in N efficiency and photosynthetic capacity. Overall, N excess reduces the yield of root and C-containing secondary metabolites (active ingredient) in N-sensitive medicinal species such as P. notoginseng.

Nitrogen application alleviates salt stress by enhancing osmotic balance, ROS scavenging, and photosynthesis of rapeseed seedlings (Brassica napus)

Nitrogen application could alleviate salt stress on crops, but the specific physiological mechanism is still unclear. Therefore, in this study, a pot experiment was conducted to explore the effects of different application rates of nitrogen (0, 0.15, 0.30, and 0.45 g·kg^-1) on the growth parameters, osmotic adjustment, reactive oxygen species scavenging, and photosynthesis of rapeseed seedlings planted in the soils with different concentrations of sodium chloride (1.5, 3.5, 5.5, and 7.5 g·kg^-1). The results showed that nitrogen could alleviate the inhibition of salt on rapeseed growth, and improve the antioxidant enzyme activities and the contents of non-enzymatic substances, K⁺, soluble protein (SP), soluble sugar (SS), and proline. Besides, there was a significant correlation between the indexes of active oxygen scavenging system, osmoregulation system, and photosynthesis. Therefore, applying appropriate amount of nitrogen can promote the growth and development of rapeseed seedlings under salt stress, accelerate the scavenging of reactive oxygen species, maintain osmotic balance, and promote photosynthesis. This study will improve our understanding on the mechanism by which nitrogen application alleviates salt stress to crops.

Elevated CO2 concentration increases photosynthetic sensitivity to nitrogen supply of sorghum in a genotype-dependent manner

We hypothesized that elevated [CO₂] only increases sorghum photosynthesis under low nitrogen availability and evaluated whether cultivars BRS373 (grain), BRS511 (saccharine) and BRS655 (forage) differ in their sensitivity to nitrogen and [CO₂]. Plants were grown in growth chambers where air [CO₂] was 400 (a[CO₂]) or 800 (e[CO₂]) μmol CO₂ mol^-1 and supplied with nutrient solution containing 211 (HN) or 48 (LN) ppm N for 45 days. Photosynthetic traits were measured in fully expanded leaves as well as leaf nitrogen and biomass accumulation. e[CO₂] increased the sensitivity of photosynthesis to LN, with all sorghum cultivars having lower maximum Rubisco carboxylation rate, effective quantum efficiency of PSII and stomatal conductance at LN than at HN. As compared to HN, LN caused lower photosynthesis of BRS373 at a[CO₂] and lower maximum PEPC carboxylation rate at e[CO₂]. Actually, the metabolic limitation of photosynthesis by LN (L_m) was high in BRS373 at a[CO₂] and slightly reduced at e[CO₂]. On the other hand, L_m was increased in BRS511 and BRS655 at e[CO₂]. Based on photosynthesis, the grain cultivar BRS373 was the most sensitive to LN. Although the number of leaves and of tillers and the leaf area were lower at LN than at HN for BRS373 and BRS655 after 45 days of growth, shoot biomass was not significantly affected. We found significant variation in photosynthetic responses to LN and e[CO₂] among sorghum cultivars, likely associated with different patterns of nitrogen and carbon partitioning. Such findings must be considered when predicting crop performance in a changing environment.