Effect of shoot removal on remobilization of carbon and nitrogen during regrowth of nitrogen-fixing alfalfa

Coupled hydraulics and carbon economy underlie age-related growth decline and revitalisation of sand-fixing shrubs after crown removal

Crown removal revitalises sand-fixing shrubs that show declining vigour with age in drought-prone environments; however, the underlying mechanisms are poorly understood. Here, we addressed this knowledge gap by comparing the growth performance, xylem hydraulics and plant carbon economy across different plant ages (10, 21 and 33 years) and treatments (control and crown removal) using a representative sand-fixing shrub (Caragana microphylla Lam.) in northern China. We found that growth decline with plant age was accompanied by simultaneous decreases in soil moisture, plant hydraulic efficiency and photosynthetic capacity, suggesting that these interconnected changes in plant water relations and carbon economy were responsible for this decline. Following crown removal, quick resprouting, involving remobilisation of root nonstructural carbohydrate reserves, contributed to the reconstruction of an efficient hydraulic system and improved plant carbon status, but this became less effective in older shrubs. These age-dependent effects of carbon economy and hydraulics on plant growth vigour provide a mechanistic explanation for the age-related decline and revitalisation of sand-fixing shrubs. This understanding is crucial for the development of suitable management strategies for shrub plantations constructed with species having the resprouting ability and contributes to the sustainability of ecological restoration projects in water-limited sandy lands.

Physiological, Hormonal and Metabolic Responses of two Alfalfa Cultivars with Contrasting Responses to Drought

Alfalfa (Medicago sativa L.) is frequently constrained by environmental conditions such as drought. Within this context, it is crucial to identify the physiological and metabolic traits conferring a better performance under stressful conditions. In the current study, two alfalfa cultivars (San Isidro and Zhong Mu) with different physiological strategies were selected and subjected to water limitation conditions. Together with the physiological analyses, we proceeded to characterize the isotopic, hormone, and metabolic profiles of the different plants. According to physiological and isotopic data, Zhong Mu has a water-saver strategy, reducing water lost by closing its stomata but fixing less carbon by photosynthesis, and therefore limiting its growth under water-stressed conditions. In contrast, San Isidro has enhanced root growth to replace the water lost through transpiration due to its more open stomata, thus maintaining its biomass. Zhong Mu nodules were less able to maintain nodule N2 fixing activity (matching plant nitrogen (N) demand). Our data suggest that this cultivar-specific performance is linked to Asn accumulation and its consequent N-feedback nitrogenase inhibition. Additionally, we observed a hormonal reorchestration in both cultivars under drought. Therefore, our results showed an intra-specific response to drought at physiological and metabolic levels in the two alfalfa cultivars studied.

Evolution of Increased Photosynthetic Capacity and Its Underlying Traits in Invasive Jacobaea vulgaris

The evolution of increased competitive ability (EICA) hypothesis and the shifting defense hypothesis (SDH) predict that evolutionary changes occur in a suite of traits related to defense and growth in invasive plant species as result of the absence of specialist herbivores. We tested how this suite of traits changed due to the absence of specialist herbivores in multiple invasive regions that differ in climatic conditions with native and invasive Jacobaea vulgaris in a controlled environment. We hypothesized that invasive J. vulgaris in all invasive regions have i) a higher plant growth and underlying traits, such as photosynthetic capacity, ii) lower regrowth-related traits, such as carbohydrate storage, and iii) an increased plant qualitative defense, such as pyrrolizidine alkaloids (PAs). Our results show that invasive J. vulgaris genotypes have evolved a higher photosynthetic rate and total PA concentration but a lower investment in root carbohydrates, which supports the SDH hypothesis. All the traits changed consistently and significantly in the same direction in all four invasive regions, indicative of a parallel evolution. Climatic and soil variables did differ between ranges but explained only a very small part of the variation in trait values. The latter suggests that climate and soil changes were not the main selective forces on these traits.

Overexpression of GsGSTU13 and SCMRP in Medicago sativa confers increased salt-alkaline tolerance and methionine content

Tau-class glutathione S-transferases (GSTUs) are ubiquitous proteins encoded by a large gene family in plants, which play important roles in combating different environmental stresses. In previous studies, we constructed a Glycine soja transcriptional profile, and identified three GSTUs (GsGSTU13, GsGSTU14 and GsGSTU19) as potential salt-alkaline stress-responsive genes. Two of them, GsGSTU14 and GsGSTU19, have been shown to positively regulate plant salt-alkaline tolerance. In this study, we further demonstrated the positive function of GsGSTU13 in plant salt-alkaline stress responses by overexpressing it in Medicago sativa. Stress tolerance tests suggested that GsGSTU13 transgenic lines showed better growth and physiological indicators than wild alfalfa (cv. Zhaodong) under alkaline stress. Considering the shortage of methionine in alfalfa, we then co-transformed GsGSTU13 into two main alfalfa cultivars in Heilongjiang Province (cv. Zhaodong and cv. Nongjing No. 1) together with SCMRP, a synthesized gene that could improve the methionine content. We found that GsGSTU13/SCMRP transgenic alfalfa displayed not only higher methionine content but also higher tolerance to alkaline and salt stresses, respectively. Taken together, our results demonstrate that GsGSTU13 acts as a positive regulator in plant responses to salt and alkaline stresses, and can be used as a good candidate for generation of salt-alkaline tolerant crops.