Interactions between irradiance levels and cactus X virus infection on the crassulacean acid metabolism in Nopalea cochenillifera and Acanthocereus tetragonus plants

Nopalea cochenillifera healthy (Nop-H) and cactus X virus-infected (Nop-I), and Acanthocereus tetragonus healthy (Acan-H) and cactus X virus-infected (Acan-I) plants were grown for two years at high (HL) or low light (LL) levels, i.e., 1900 or 480 μmol m−2 s−1. LL markedly decreased the CAM activity in Nop-H. Whereas, Acan-H maintained high CAM activity rates at HL and LL. When compared to Nop-H, virus infection caused a reduction in the dawn-dusk values of titratable acidity (ÎH+) at HL, but at LL differences were not detected between Nop-H and Nop-I. The latter situation occurred in spite of the higher accumulation of glucan, net diurnal carbon gain and the establishment of dawn-dusk changes in sugars in Nop-I. In Acan-I, the reduction in the CAM activity caused by the virus infection was more conspicuous than in Nop-I at either HL or LL. For all plants, LL brought about an increase in the nitrogen, chlorophyll and water contents, and at each irradiance the values in Nop-I and Acan-I were always lower than in Nop-H and Acan-H, respectively. At LL, there was also an increase in the α-amino-N content in Nop-H and Acan-H ascribed to an enhancement in the pyruvate kinase activity. In Nop-I and Acan-I the increased pyruvate kinase activity above values of healthy plants was not translated into a higher content of a-amino-N regardless of the irradiance. We concluded that Nop-H can be categorized as shade-intolerant, whereas Acan-H can be classified as a shade-tolerant species, and that shade does not have a beneficial effect on virus-infected plants of both CAM species. In all virus-infected plants the concentration of virus particles per gram dry mass was 1.3-fold higher at LL than at HL, and for each species the magnitude of CAM in virus-infected plants was similar at both irradiances.