Effects of narrow plant spacing on root distribution and physiological nitrogen use efficiency in summer maize

The objective of this study was to understand the effects of plant spacing on grain yield and root competition in summer maize(Zea mays L.). Maize cultivar Denghai 661 was planted in rectangular tanks(0.54 m × 0.27 m × 1.00 m) under 27 cm(normal) and 6 cm(narrow) plant spacing and treated with zero and 7.5 g nitrogen(N) per plant. Compared to normal plant spacing, narrow plant spacing generated less root biomass in the 0–20 cm zone under both N rates, slight reductions of dry root weight in the 20–40 cm and 40–70 cm zones at the mid-grain filling stage, and slight variation of dry root weights in the 70–100 cm zone during the whole growth period. Narrow plant spacing decreased root reductive activity in all root zones, especially at the grain-filling stage. Grain yield and above-ground biomass were 5.0% and 8.4% lower in the narrow plant spacing than with normal plant spacing, although narrow plant spacing significantly increased N harvest index and N use efficiency in both grain yield and biomass, and higher N translocation rates from vegetative organs. These results indicate that the reductive activity of maize roots in all soil layers and dry weights of shallow roots were significantly decreased under narrow plant spacing conditions, resulting in lower root biomass and yield reduction at maturity. Therefore, a moderately dense sowing is a basis for high yield in summer maize.

Foliar application of sodium hydrosulfide(NaHS),a hydrogen sulfide(H2S) donor,can protect seedlings against heat stress in wheat(Triticum aestivum L.)

Temperature extremes represent an important limiting factor to plant growth and productivity. Low concentration of hydrogen sulfide （H2S） has been proven to function in physiological responses to various stresses. The present study evaluated the effect of foliar application of wheat seedlings with a H2S donor, sodium hydrosulfide （NariS）, on the response to acute heat stress. The results showed that pretreatment with NariS could promote heat tolerance of wheat seedlings in a dose-depen- dent manner. Again, it was verified that H2S, rather than other sulfur-containing components or sodion derived from NariS solution, should contribute to the positive role in promoting wheat seedlings against heat stress. To further study antioxidant mechanisms of NariS-induced heat tolerance, superoxide dismutase （SOD, EC 1.15.1.1 ）, catalase （CAT, EC 1.11.1.6） and ascorbate peroxidase （APX, EC 1.11.1.11 ） activities, and HzS, hydrogen peroxide （H2O2）, malonaldehyde （MDA）, and soluble sugar contents in wheat seedlings were determined. The results showed that, under heat stress, the activities of SOD, CAT, and APX, H2S, H2O2, MDA, and soluble sugar contents in NaHS-pretreated seedlings and its control all increased. Meanwhile, NaHS-pretreated seedlings showed higher antioxidant enzymes activities and gene expression levels as well as the H2S and soluble sugar levels, and lower H2O2, MDA contents induced by heat stress. While little effect was detected in antioxidant enzymes activities and soluble substances contents in pretreated wheat seedlings compared with its control under normal culture conditions （data not shown）. All of our results suggested that exogenous NariS could alleviate oxidative damage and improve heat tolerance by regulating the antioxidant system in wheat seedlings under heat stress.

Photosynthetic Characteristics and Antioxidative Metabolism of Flag Leaves in Responses to Nitrogen Application During Grain Filling of Field-Grown Wheat

A two-factorial experiment was conducted with two wheat cultivars, SN1391 （large spike and large grain） and GC8901 （multiple spike and medium grain）, and two nitrogen （N） application rates （12 and 24 g N m^-2）, to investigate the responses of photosynthetic characteristics and antioxidative metabolism to nitrogen rates in flag leaves of field-grown wheat during grain filling. The results showed that the content of N and chlorophyll （Chl） in wheat flag leaves decreased after anthesis and the net photosynthetic rate （Pn）, effective quantum yield of PS Ⅱ （Фps n）, efficiency of excitation capture by open PS Ⅱ reaction centers （Fv′/Fm′）, and photochemical quenching coefficient （qp） began to decrease at 14 days after anthesis. However, the maximal efficiency of PS Ⅱ photochemistry （Fv/Fm） decreased slightly until the late period of senescence and the nonphotochemical quenching coefficient （NPQ） increased during flag leaves＇ senescence, As a result, a conflict came into being between absorption and utilization to light energy in flag leaves during senescence, which might accelerate the senescence of flag leaves. Compared with GC8901, the lower plant population of SN1391 during grain filling was helpful to maintain the higher content of photosynthetic pigment, activity of PS II, and Pn in flag leaves during senescence, The delayed decrease in antioxidative enzyme activity and the lower degree of membrane lipid peroxidation in the senescing leaves of SN1391 were beneficial to protect the photosynthetic apparatus, which lead to the prolonged duration of CO2 assimilation. With the increase of N application, the Chl content of SN1391 flag leaves and the efficiency of excitation captured by open PS Ⅱ centers increased. At the same time, the thermal dissipation in SN1391 flag leaves at high N （HN） treatment decreased and Фps Ⅱ improved greatly, which were favorable to the increase of Pn. The SOD, POD, CAT and APX activities in the flag leaves of SN1391 increased markedly at HN treatment, indicating that these enzymes could clean more active oxygen and decrease the degree of membrane lipid peroxidation. In this way, the ability of SN1391 to protect photosynthetic apparatus was improved with the increase of N. In the HN treatment, the decreased activity of PS Ⅱ and increased thermal dissipation resulted in the decline of Pn in flag leaves of GC8901. Meanwhile, the decreased antioxidative enzyme activities and the increased degree of membrane lipid peroxidation had indirect and unfavorable influences on CO2 assimilation. This implied that the conflict between absorption and utilization to energy in senescing leaves was an important reason which induced and accelerated the senescence of wheat leaves in the field. The photosynthetic characteristics and antioxidative metabolism of flag leaves during grain filling were markedly different among wheat cultivars. The effects of nitrogen rate on the photosynthetic and senescent characteristics of flag leaves also varied with wheat cultivars.

The effects of grain texture and phenotypic traits on the thin-layer drying rate in maize(Zea mays L.) inbred lines

To explore the relation of maize grain texture and phenotypic traits with grain thin-layer drying rate,we observed the ultra-structure of maize grain,and tested three traits about the maize grain texture and four phenotypic traits.The vitreous part percentage was different（P〈0.05） among different maize inbred lines.There was a significant relationship between the drying rate with grain texture and phenotypic traits.Main factors that influenced the drying rate were different during different drying stages.New results observed that empirical constants（k and N） in drying equation were different for seed of the 30 inbred lines of maize.The k of simplified diffusion equation and N of page equation were significantly influenced by both grain texture and phenotypic traits.These results could be used as guideline parameters for drying maize seeds having different grain characteristics during different drying stages.