Identification of beta-lactam antibiotics that have negligible effects on plant regeneration is a critical step towards the establishment of a reliable Agrobacterium-mediated transformation protocol for perennial trees. In the present report, we have evaluated the effects of the novel beta-lactam antibiotics meropenem and timentin on plant regeneration of a perennial woody fruit plant, Citrus sinensis, in comparison with the commonly used betalactam cefotaxime. It was observed that, in contrast to cefotaxime, meropenem and timentin had a positive or no detrimental effect on the shoot regeneration from epicotyl explants. Residual effects of the beta-lactams from shoot regeneration medium also affected the subsequent ability of the roots to elongate. The addition of meropenem and/or timentin in the rooting medium mostly improved or did not affect the rooting ability of the adventitious shoots. These data indicated that meropenem and timentin can positively replace cefotaxime in Agrobacterium-mediated transformation of C. sinensis.

Soil and water salinity cause physiological disorders in sensitive plants, such as altered gas exchange in citrus genotypes. However, it is possible to analyse these effects and to identify genotypes tolerant to salt stress. An experiment was carried out in order to evaluate the tolerance of citrus genotypes considering the irrigation with saline water during rootstock formation. The study took place under greenhouse conditions in Pombal county, Paraiba, Brazil. A randomised block design was used, with treatments arranged in a factorial scheme (5 x 8). The two factors were: [i] five salinity levels of irrigation water (0.8, 1.6, 2.4, 3.2 and 4.0 dS m -1 ) and [ii] eight genotypes of citrus rootstocks: 1. 'Santa Cruz Rangupur' lime (LCRSTC); 2. common 'Sunki' mandarin (TSKC) x 'Swingle' citrumelo (CTSW) -028; 3. TSKC x CTSW -033; 4. TSKC x CTSW -041; 5. 'Volkamer' lemon (LVK) x 'Rangpur' lime (LCR) -038; 6. 'Florida Sunki' mandarin (TSKFL); 7. TSKC and 8. 'Florida' rough lemon (LRF). Gas exchange was evaluated at 15, 30 and 60 days after the beginning of the saline water application. High water salinity reduces gas exchange in citrus genotypes, which was more evident at 15 days from the beginning of stress. The genotypes 'Santa Cruz Rangpur' lime, TSKC x CTSW -041, LVK x LCR -038 and 'Florida' rough lemon exhibit a satisfactory physiological behaviour during the first 30 days of exposure to the stress, showing moderate tolerance to salt stress. The genotypes TSKC x CTSW -033 and common 'Sunki' mandarin are the most sensitive to salinity. Key words: Citrus spp. Poncirus hybrids. Salinity. Assimilation rate. Tolerance. ResumoA salinidade do solo e da água ocasionam distúrbios fisiológicos em plantas sensíveis, a exemplo das trocas gasosas em genótipos de citros, podendo-se identificar, a partir dessas variáveis, genótipos tolerantes e a resposta das plantas à salinidade. Assim, objetivou-se avaliar a tolerância de genótipos de citros à salinidade na fase de formação de porta-enxertos. O experimento foi desenvolvido em casa de vegetação, no Município de Pombal, Paraíba, empregando-se o delineamento experimental de blocos casualizados com três repetições e tratamentos arranjados em esquema fatorial, 5x8, relativos à

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