Levels of cytokinins and abscisic acid (ABA) and the expression of senescence-related genes were investigated in two maize (Zea mays L.) cultivars of different senescence type, cv. P3845 (stay-green) and cv. Hokkou 55 (earlier senescent), in a field study. The delay in leaf senescence in P3845 was correlated with increased levels of chlorophyll and nitrogen and a higher photon-saturated photosynthetic rate (P(sat)). Compared with the earlier senescent Hokkou 55, P3845 showed enhanced contents of cytokinins (trans-zeatin riboside, t-ZR; dihydrozeatin riboside, DHZR; isopentenyladenosine, iPA) and reduced levels of ABA in its leaves. In roots, P3845 had increased levels of t-ZR, DHZR, and ABA, but decreased concentrations of iPA. It was concluded that a higher rate of cytokinin transport from roots to leaves contributes to the delay of senescence in P3845. By contrast, the translocation of ABA from roots to shoots may be blocked in the stay-green cultivar, which also results in retarded leaf senescence. P3845 ear leaves contained more malondialdehyde (MDA) and higher catalase (CAT) and superoxide dismutase (SOD) activities than Hokkou 55. Since the accumulation of the mRNAs for Rubisco small subunit (rbcS), phosphoenolpyruvate carboxylase (PEPC), and SOD peaked after Chl content and P(sat) had reached their maxima, it is speculated that when leaf senescence is initiated, Chl contents decrease first, followed by the degradation of the photosynthetic apparatus and of photosynthesis-related enzymes. See1 and See2 encode senescence-related cysteine proteases; their mRNAs were most abundant in yellowing leaves, suggesting that these proteins are involved in the process of senescence rather than its initiation. mRNAs of both genes were more abundant in Hokkou 55 than in P3845, which suggests a regulation of leaf senescence at the transcriptional level.
Overapplication of N and P and insuffi cient supply of K are considered primary reasons for restriction of yield improvement in the North China Plain. Optimized nutrient management practices based on soil testing and yield targets have been developed. Other large scale fi eld experiments have indicated that additional improvement for yield and nutrient use benefi ts is needed. Th e objective of this study was to evaluate the eff ects of the optimized nutrient management system on yield, nutrient uptake, nutrient utilization, and profi t in the North China provinces of Shanxi, Hebei, Shandong, and Henan. Treatments consisted of a check without fertilizer use (CK); a balanced, optimum nutrient application (OPT); the farmers' practice (FP); and a series of nutrient omission treatments (minus N, P, and K, respectively). Th e results indicated that the OPT optimized grain yield, nutrient use effi ciency, and profi tability. Maize (Zea mays L.) yield increased by 12.2% at Shanxi and 18.5% at Hebei, respectively. Inputs of N and P across the wheat (Triticum aestivum L.) and maize system at the four sites was reduced by 13% (266 kg N ha −1 ) and 45% (430 kg P 2 O 5 ha −1 ), while K input was increased by 43% (265 kg K 2 O ha −1 ). Th e OPT improved both measurements of nitrogen use effi ciency (NUE); agronomic nitrogen effi ciency (AE N ) and nitrogen recovery effi ciency (RE N ) in the majority of cases. Although the OPT tested in this study increased yields and nutrient uptake, there remains considerable potential to improve AE N and RE N further for this intensive winter wheat-summer maize rotation system.
Resistance to grape anthracnose [Elsinoë ampelina (de Bary) Shear] was evaluated in 13 known Vitis species and five taxonomically undescribed grapes native to China. One hundred and eight clones of Chinese Vitis species were tested under field conditions between 1990 and 1992. Berry infection did not occur in these species. Leaves displayed strong resistance to anthracnose, although intraspecific variations were observed. There was no relationship between anthracnose resistance and geographical origin of the species. Results from this study indicate that oriental grape species are useful for disease-resistance breeding.
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