Transpiration efficiency (TE) contributes to crop performance under water‐limited conditions, but is difficult to measure. Herein, we assess the relationships between TE and surrogate traits and how these interact with water regimes, using isogenic materials: five transgenic events of groundnut and their wild‐type (WT) parent JL 24, among which large variation in TE was previously reported. These five events came from the insertion of transcription factor DREB1A from Arabidopsis thaliana, driven by stress responsive promoter rd29. The events were in T3 generation and had been selected from a preliminary trial for having a large range of variation in the time needed to deplete soil moisture upon exposure to soil drying. Two experiments were conducted, in each case with plants exposed to well‐watered (WW) and water‐stressed (WS) conditions. Significant correlations were found between TE and soil plant analysis development chlorophyll meter readings (SCMR), TE and specific leaf area (SLA), and SLA and SCMR in both experiments. Nevertheless, these significant relationships were confined to the drought stress (DS) treatment. No correlation between TE and Δ13C (carbon isotope discrimination) was found in the present study, regardless of the water regime, and in none of the two experiments. A significant negative correlation was established between TE and the fraction of transpirable soil water (FTSW) threshold values where transpiration declined upon soil drying, in both experiments. The mean vapour pressure deficit in the two different seasons (1.47 kPa and 0.73 kPa) did not affect the ranking of genotypes for TE. It is concluded that surrogates for TE, when used, need careful consideration of the drought stress status of plants at the time of measurement, and that differences in TE might be closely related to how plants respond to soil drying, with high TE genotypes maintaining gas exchange until the soil is dryer than low TE genotypes.

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This study aimed to examine the effect of nitrogen (N) application rate and time on yield, grain filling, starch metabolizing enzymes, and hormones of maize based on a long-term field experiment initiated in 2012. The total N fertilizer dose [(0 (N0), 100 (N1), 200 (N2), and 300 (N3) kg N ha–1] was split into two (T1, one-third at sowing and two-thirds at the six-leaf stage) or three (T2, one-third each at sowing, six-leaf, and eleven-leaf stage) times application. The results showed that the highest yield was obtained under N3T2, N2T1, and N3T2 in 2018, 2019, and 2020, which was 222.49, 185.31, and 194.00% than that of N0 in each year, respectively. N2 and N3 significantly increased the yield through enhancing ears ha–1, grains per plant, and 100-grain weight; however, N2 and N3 did not show a significant difference in yield and above-yield components. In addition, N application time did not significantly change yield under the same N rate. N0 limited the activities of starch metabolizing enzymes, resulting in insufficient accumulation of sucrose and starch. The contents of indole-3-acetic acid, cytokinin, abscisic acid, and gibberellin were decreased under N0 during grain filling. The average grain-filling rate and maximum grain-filling rate (Gmax) and grain weight increment achieving Gmax increased under N2 and N3, and the grain-filling parameters were positively correlated with 100-grain weight. In conclusion, 200 kg N ha–1 with one-third application at sowing and two-thirds application at the six-leaf stage is a suitable N supply way to improve starch metabolizing enzymes, regulate hormone content, and enhance grain-filling rates, and thus increasing the maize yield in the semiarid Loess Plateau of China.

Ammonia oxidizing archaea (AOA) and bacteria (AOB) mediate a crucial step in nitrogen (N) metabolism. The effect of N fertilizer rates on AOA and AOB communities is less studied in the wheat-fallow system from semi-arid areas. Based on a 17-year wheat field experiment, we explored the effect of five N fertilizer rates (0, 52.5, 105, 157.5, and 210 kg ha−1 yr−1) on the AOA and AOB community composition. This study showed that the grain yield of wheat reached the maximum at 105 kg N ha−1 (49% higher than control), and no further significant increase was observed at higher N rates. With the increase of N, AOA abundance decreased in a regular trend from 4.88 × 107 to 1.05 × 107 copies g−1 dry soil, while AOB abundance increased from 3.63 × 107 up to a maximum of 8.24 × 107 copies g−1 dry soil with the N105 treatment (105 kg N ha−1 yr−1). Application rates of N fertilizer had a more significant impact on the AOB diversity than on AOA diversity, and the highest AOB diversity was found under the N105 treatment in this weak alkaline soil. The predominant phyla of AOA and AOB were Thaumarchaeota and Proteobacteria, respectively, and higher N treatment (N210) resulted in a significant decrease in the relative abundance of genus Nitrosospira. In addition, AOA and AOB communities were significantly associated with grain yield of wheat, soil potential nitrification activity (PNA), and some soil physicochemical parameters such as pH, NH4-N, and NO3-N. Among them, soil moisture was the most influential edaphic factor for structuring the AOA community and NH4-N for the AOB community. Overall, 105 kg N ha−1 yr−1 was optimum for the AOB community and wheat yield in the semi-arid area.

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