Calcium requirements of citrus

Abstract

A close relationship was found between the pH of soil suspensions in the SMP buffer solution (pH_smp) and the potential acidity of soils (H + Al) extracted by a neutral calcium acetate solution (r = 0.98), for twenty six soil samples of the State of Sao Paulo, Brazil, This relationship was represented by the equation lnY = 7.76 ‐ 1.053X, which allowed for the calculation of H + Al directly from the values of pH_smp.

With the values of H + Al and the sum of bases, calcium, magnesium and potassium, the cation exchange capacity (CEC), and the base saturation (V) were calculated. Relationships between the base saturation of the soils and the active acidity of soil suspensions were close, both for pH determined in water (r=0.94) and pH determined in 0.01M CaCl₂ solution (r ‐ 0.97). Thus the lime requirement (LR) of soils could be calculated, for given values of pH or base saturation, using the equation LR = CEC (V₂ ‐ V₁)/100, in which V₁ is the base saturation of the soil and V₂ is the expected value upon liming.

The predicted values for lime required to increase the soil pH in water to either 5.5 or 6.0 were comparable to those obtained by the direct use of the SMP buffer method, and were, respectively, two and four times higher than the amounts required to neutralize exchangeable aluminum, considering the criterion LR = Al × 1.5.

The proposed method to determine lime requirement of soils is described in detail and the advantages of its use are discussed.     

Interactions between a crinivirus, an ipomovirus and a potyvirus in coinfected sweetpotato plants

Novel and severe symptoms of chlorosis, rugosity, leaf strapping and dark green islands, designated as sweetpotato severe mosaic disease (SPSMD), were caused by dual infection of Sweet potato mild mottle virus (SPMMV; Ipomovirus ) and Sweet potato chlorotic stunt virus (SPCSV; Crinivirus ) in three East African sweetpotato cultivars (Tanzania, Dimbuka and New Kawogo). The storage root yield was reduced by ∼80%, as compared with healthy plants under screenhouse conditions in Uganda. Plants infected with SPMMV or SPCSV alone showed nonsignificant or 50% yield reduction, respectively. SPCSV reduced resistance to SPMMV in sweetpotato, similar to the situation with resistance to Sweet potato feathery mottle virus (SPFMV; Potyvirus ) that breaks down following infection with SPCSV, followed by development of sweet potato virus disease (SPVD). In single virus infections with SPMMV and SPFMV or their coinfection, cvs Tanzania and Dimbuka were initially systemically infected, displayed symptoms and contained readily detectable virus titres, but new leaves were symptomless with very low virus titres, indicating recovery from disease. In contrast, cv. New Kawogo remained symptomless and contained low SPMMV and SPFMV titres following graft inoculation. These moderate and high levels of resistance to SPMMV and SPFMV, respectively, were lost and cultivars succumbed to a severe disease following coinfection with SPCSV. The synergistic interactions increased titres of SPMMV and SPFMV RNA by ∼1000-fold as quantified by real-time PCR, whereas SPCSV titres were reduced twofold, indicating an antagonistic interaction. Coinfection with SPMMV and SPFMV caused no detectable changes in virus titres or symptom severity.     

拮抗菌对致病疫霉生长及抗性酶活性的影响

为获得更为有效的致病疫霉拮抗菌并探讨拮抗菌的作用机理,本研究利用对峙培养法、滤纸片法测定了10个待测菌株的活体及发酵液的抑菌作用,用分光光度法测定了受显著抑制后再恢复生长的致病疫霉体内多酚氧化酶(PPO)、过氧化物酶(POD)和苯丙氨酸解氨酶(PAL)的活性。结果表明:活体状态下以核桃细菌的抑制作用最强(抑菌率为84.50%);发酵液中以梨黑斑病菌(Alternaria alternate)的抑制作用最强(抑菌率为76.68%),其次是白菜黑斑病菌(A.brassicae)。将梨黑斑病菌和白菜黑斑病菌的发酵液复配后其抑菌率达到了81.31%。受抑制的致病疫霉菌落中紧靠抑菌带边缘的菌丝不能恢复正常生长,较远的菌丝易恢复正常生长;受抑制的致病疫霉体内PPO、POD、PAL活性均明显降低,与对照相比,PPO活性下降了45%~68%,POD下降了27%~58%,PAL下降幅度最大,约80%~90%,这与植物受病菌抑制或侵染后PPO、POD、PAL活性增加相反。