To understand how light intensity influences plant morphology and photosynthesis in the forage crop alfalfa (Medicago sativa L. cv. Zhongmu 1), we investigated changes in leaf angle orientation, chlorophyll fluorescence, parameters of photosynthesis and expression of genes related to enzymes involved in photosynthesis, the Calvin cycle and carbon metabolism in alfalfa seedlings exposed to five light intensities (100, 200, 300, 400 and 500 μmol m⁻² s⁻¹) under hydroponic conditions. Seedlings grown under low light intensities had significantly increased plant height, leaf hyponasty, specific leaf area, photosynthetic pigments, leaf nitrogen content and maximal PSII quantum yield, but the increased light-capturing capacity generated a carbon resource cost (e.g., decreased carbohydrates and biomass accumulation). Increased light intensity significantly improved leaf orientation toward the sun and upregulated the genes for Calvin cycle enzymes, thereby increasing photosynthetic capacity. Furthermore, high light (400 and 500 μmol m⁻² s⁻¹) significantly enhanced carbohydrate accumulation, accompanied by gene upregulation and increased activity of sucrose and starch-synthesis-related enzymes and those involved in carbon metabolism. Together, these results advance our understanding of morphological and physiological regulation in shade avoidance in alfalfa, which would guide the identification of suitable spatial planting patterns in the agricultural system.

Autotoxicity is a form of intraspecific allelopathy, in which a plant species inhibits the establishment or growth of the same species through the release of toxic chemical compounds into the environment. The phenomenon of autotoxicity in crops is best traced in alfalfa (Medicago sativa). A close relative of alfalfa, M. truncatula, has been developed into an excellent model species for leguminous plants. However, it is not known whether M. truncatula has autotoxicity. In this study, M. truncatula root exudates showed a negative impact on the growth of M. truncatula seedlings, indicating autotoxicity. Detailed analyses with plant extracts from M. truncatula and alfalfa revealed varying degrees of suppression effects in the two species. The extracts negatively affected seed germination potential, germination rate, radicle length, hypocotyl length, synthetic allelopathic effect index, plant height, root growth, fresh weight, dry weight, net photosynthetic rate, transpiration rate, and stomatal conductance in both M. truncatula and alfalfa. The results demonstrated that autotoxicity and allelopathic effects exist in M. truncatula. This opens up a new way to use M. truncatula as a model species to carry out in-depth studies of autotoxicity and allelopathy to elucidate biochemical pathways of allelochemicals and molecular networks controlling biosynthesis of the chemicals.

Over half of the alpine meadows in the Qinghai-Tibet Plateau (QTP) are degraded due to human activities. Soil degradation from overgrazing is the most direct cause of grassland degradation. It is thus important to synthesize the effects of multiple soil degradation indicators on the belowground biomass of plants and soil microorganisms in the degraded QTP. We studied the diversities and structures of soil bacterial and fungal communities using soil bacterial 16S rRNA and the fungal ITS gene under four degradation gradients, D1: lightly degraded, D2: moderately degraded, D3: highly degraded, and a non-degraded control site (CK). The bacterial Shannon diversity in D3 was significantly lower than that in D1 (p < 0.001), and the bacterial richness index in D3 was significantly lower than that in D1 (p < 0.001). There was no difference in soil fungal diversity among the different degradation levels; however, soil fungal richness decreased significantly from CK to D3. The phyla Actinobacteria, Acidobacteria and the genus Mortierella were differed significantly under the four degradation gradients. Plant litter mass and root C/N ratio were important factors associated with bacterial and fungal diversity and richness. These results indicated that alpine meadow degradation can lead to variations in both microbial diversity and the potential functioning of micro-organisms in the QTP.

Pearl millet (Pennisetum americanum (L.) K. Schum) has been proven as a potential remediation plant of the pollution caused by atrazine. Plants used in remediation can release root exudates to communicate with rhizosphere microorganisms and accelerate the removal of pollutants in soil. However, the response of pearl millet root exudates under atrazine stress has remained unclear. In this study, hydroponic experiments were conducted at Northeast Agricultural University, Harbin, China, to investigate the oxidative stress response and the changes in composition of root exudates in pearl millet plants that were exposed to 19.4 mgL^-1 of atrazine, compared to the untreated control. The experiment was established as six treatments with exposure to no atrazine for 2, 4 and 6 days (CK-2, CK-4, CK-6) and 19.4 mgL^-1 atrazine for 2, 4 and 6 days (AT-2, AT-4, AT-6), respectively. The results suggest that the growth of the seedlings changed slightly when exposed to atrazine for 2 days. The content of thiobarbituric acid reactive substances exposed to atrazine for 6 days increased 26% compared with the treatment that was exposed for 2 days. Moreover, the reactive oxygen species in test plant obviously increased when exposed to atrazine for 6 days. In addition, the activity of superoxide dismutase increased from 30.82 ug^-1 to 37.33 ug^-1 fresh weight after 6 days of exposure to atrazine. The results of a nontargeted metabolomic analysis suggest that carbohydrate metabolism, fatty acid metabolism and amino acid metabolism in pearl millet were obviously affected by the oxidative stress caused by atrazine. The contents of sphinganine and methylimidazole acetaldehyde in CK-6 increased by 5.14 times and 2.05 times, respectively, compared with those of CK-2. Furthermore, the contents of (S)-methylmalonic acid semialdehyde and 1-pyrroline-2-carboxylic acid decreased by 0.56 times and 0.5 times, respectively, compared with the AT-6. These results strongly suggest that the changes observed in the composition of root exudates in pearl millet seedlings can be attributed to the oxidative stress caused by atrazine.