通过灌溉提高土壤含水量（SWC）是一种潜在、有效的缓解高温胁迫的调控措施。在提高土壤含水量缓解高温影响的过程中，田间条件下基于叶绿素荧光的光合特性响应受到了有限的关注。本研究在华北平原开展了2年田间试验（2019-2020年），以郑单958（ZD958）和先玉335（XY335）为材料，在灌浆期设置三个试验处理（正常生长条件（CK）、大田升温（H）和大田升温+水分调控（HW）），研究田间高温对玉米冠层光合的影响及水分调控效应。与H处理相比，HW处理下冠层温度降低1-3℃，净光合速率（P_n）提高20%。此外，HW处理显著提高了两个品种的实际光合速率（Phi2）、线性电子流（LEF）、可变荧光（F_v）和最大光能转换效率（F_v/F_m）。同时，发现两个品种对叶绿素荧光的响应存在差异。HW处理显著提高了ZD958的类囊体质子电导率（gH⁺）和最大荧光（F_m），提高了XY335的叶绿体ATP合酶质子电导率（vH⁺）和最小荧光（F₀）。结构方程分析进一步表明，土壤水分含量与P_n、LEF和F_v/F_m呈显著正相关。提高土壤水分含量可通过延缓叶片衰老，延长光合作用有效时间，改善Phi2、LEF、F_v和F_v/F_m，提高叶片光合能力。综合本研究结果表明，提高SWC以增强灌浆期叶片光合作用，是玉米生产中适应气候变暖的一项重要技术措施。

本研究在2017年使用12个玉米品种进行了两个播期的田间试验，2019年使用10个玉米品种进行了田间试验。2017年早播的玉米产量在6.5 到14.6 t ha^-1之间，晚播的玉米产量在9.3 到12.7 t ha^-1之间，2019年玉米产量在5.9 到7.4 t ha^-1之间，收获时的籽粒含水量分别在29.8-34.9%, 29.4-34.5%和31.9-37.1% 范围内。较大的最大叶面积有利于高产，叶片衰老速度快有利于后期籽粒脱水，根系结构紧凑有利于高产和籽粒快速脱水。较强壮的茎秆提高了玉米的抗倒伏能力，但在收获时却保持了较高的籽粒含水量，这对玉米高产低含水量是一个挑战。高产低籽粒含水量的玉米品种具备灌浆速率快，灌浆时间长，灌浆后期籽粒脱水速率快的特点。灌浆后期较高的日间温度可以通过影响籽粒灌浆和脱水进而提高玉米产量和降低籽粒含水量，说明调整播期可能可以作为达到籽粒机械收获的一个策略。

Received  2 August, 2017    Accepted  31 October, 2017
 © 2018, CAAS. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)
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    Applying mathematic models to evaluate absorbed-N effects on dry matter production at different developmental stages would help determine proper nitrogen management according to crop demands and yield target. Two field trials were carried out for establishing absorbed-N effects on dry matter production (ANEDr) model, using uniform design in 2010–2011 and 2012–2013 winter wheat growing seasons in Hebei Province, China. Another field trial was carried out in 2010–2011 for model validation. Dry matter and N concentration in leaf and non-leaf organs were measured at setting, jointing, anthesis, and maturity. Theory of best linear unbiased prediction (BLUP) was applied to analyse the N effects of leaf and non-leaf organs on dry matter production. Within ANEDr model, four N-affected phases at each stage were concerned, leaf absorbed-N effect before this stage, non-leaf organ absorbed-N effect before this stage, leaf absorbed-N effect at this stage, and non-leaf organ absorbed-N effect at this stage. In addition, developmental processes, genotype characters and temperature were three factors that determine each N effect. It was demonstrated that ANEDr model can precisely quantify absorbed-N effects on dry matter production with high correlation coefficient (r=0.95). Comparing with other models, ANEDr model considered both leaf and non-leaf organs according to developmental processes of winter wheat, showed higher flexibility and simplicity, thus could be applied to different environments, cultivars and crops after parameter adjustment.

The solar radiation intensity and duration are continuously decreasing in the major wheat planting area of China. As a consequence, leaf senescence, photosynthesis, grain filling and thus wheat yield shall be affected by light deficiency. Therefore, two winter wheat (Triticum aestivum L.) cultivars, Tainong 18 (a large-spike cultivar) and Ji’nan 17 (a multiple-spike cultivar), were subjected to shading during anthesis and maturity under field condition in 2010–2011 and 2011–2012. Under the slight shading treatment (S1, 88% of full sunshine), leaf senescence was delayed, net photosynthesis rate (Pn) and canopy apparent photosynthesis rate (CAP) were improved, and thus thousand-kernel weight (TKW) and grain yield were higher as compared with the control. However, mid and severe shading (S2 and S3, 67 and 35% of full sunshine, respectively) led to negative effects on these traits substantially. Moreover, superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities in flag leaf were significantly greater under slight shading than those in other treatments, while the malondialdehyde (MDA) content was less than that under other treatments. In addition, the multiple-spike cultivar is more tolerant to shading than large-spike cultivar. In conclusion, slight shading after anthesis delayed leaf senescence, enhanced photosynthesis and grain filling, and thus resulted in higher grain yield.

The calculation method of potential evapotranspiration (PET) was improved by adopting a more reliable PET estimate based on the Penman-Monteith equation into the standardized precipitation evapotranspiration index (SPEI) in this study (SPEIPM). This improvement increased the applicability of SPEI in North China Plain (NCP). The historic meteorological data during 1962–2011 were used to calculate SPEIPM. The detrended yields of maize from Hebei, Henan, Shandong, Beijing, and Tianjin provinces/cities of NCP were obtained by linear sliding average method. Then regression analysis was made to study the relationships between detrended yields and SPEI values. Different time scales were applied, and thus SPEIPM was mentioned as SPEIPMk-j (k=time scale, 1, 2, 3, 4,…, 24 mon; j=month, 1, 2, 3,..., 12), among which SPEIPM3-8 reflected the water condition from June to August, a period of heavy precipitation and vigorous growth of maize in NCP. SPEIPM3-8 was highly correlated with detrended yield in this region, which can effectively evaluate the effect of drought on maize yield. Additionally, this relationship becomes more significant in recent 20 yr. The regression model based on the SPEI series explained 64.8% of the variability of the annual detrended yield in Beijing, 45.2% in Henan, 58.6% in Shandong, and 54.6% in Hebei. Moreover, when SPEIPM3-8 is in the range of –0.6 to 1.1, –0.9 to 0.8 and –0.8 to 2.3, the detrended yield increases in Shandong, Henan and Beijing. The yield increasing range was during normal water condition in Shandong and Henan, where precipitation was abundant. It indicated that the field management matched well with local water condition and thus allowed stable and high yield. Maize yield increase in these two provinces in the future can be realized by further improving water use efficiency and enhancing the stress resistance as well as yield stability. In Hebei and Beijing, the precipitation is less and thus the normal water condition cannot meet the high yield target. Increasing of water input and improving water use efficiency are both strategies for future yield increase. As global climate change became stronger and yield demands increased, the relationship between drought and maize yield became much closer in NCP too. The research of drought monitoring method and strategies for yield increase should be enhanced in the future, so as to provide strong supports for food security and agricultural sustainable development in China. Received 12