ABSTRACT The aim of this work was to evaluate the effect of thermal time methods and to identify the phyllochron and photoperiod response in seedlings of two forest species: Corymbia citriodora and Eucalyptus urophylla. A field experiment was installed under a completely randomized design, in a 2 × 11 factorial scheme, with two forest species and eleven sowing dates (E). Phyllochron is influenced by thermal time methods. The best thermal time method is the one that considers the base and optimum temperature and compares them with daily extreme temperatures. In addition, the influence of photoperiod and air temperature on phyllochron values was also verified. The lowest value occurred in E1 (Corymbia citriodora: 24.6 and Eucalyptus urophylla: 13.2 °C day leaf-1) and the highest in E10 (Corymbia citriodora: 46.4 °C day leaf-1) and E7 (Eucalyptus urophylla: 29.2 °C day leaf-1), suggesting that the phyllochron is influenced by sowing dates.

Resumo: O objetivo deste trabalho foi estimar as temperaturas cardinais e o filocrono para o desenvolvimento foliar das espécies de eucalipto Corymbia citriodora e Eucalyptus urophylla, durante a fase de muda. Um experimento de campo foi realizado em delineamento inteiramente casualizado, em arranjo fatorial 2x11, com as duas espécies de eucalipto, 11 épocas de semeadura e cinco repetições, em Itajubá, MG. As temperaturas cardinais - basal inferior, ótima e basal superior - para o desenvolvimento foliar foram, respectivamente, 8,7, 17,1 e 41,3°C, para C. citriodora, e 11,5, 17,1 e 40,5°C, para E. urophylla. O filocrono de C. citriodora foi de 62,95°C dia por folha, e o de E. urophylla, de 46,03°C dia por folha, o que é indício de que E. urophylla necessita de menor acúmulo energético do que C. citriodora para a emissão de folhas na haste principal.

Abstract: The objective of this work was to estimate cardinal temperatures and phyllochron for the leaf development of the eucalyptus species Corymbia citriodora and Eucalyptus urophylla, in the seedling phase. A field experiment was carried out in a completely randomized design, in a 2x11 factorial arrangement, with the two eucalyptus species, 11 sowing dates, and five replicates, in the municipality of Itajubá, in the state of Minas Gerais, Brazil. The cardinal temperatures - base, optimum, and maximum - for leaf development were, respectively, 8.7, 17.1, and 41.3°C, for C. citriodora, and 11.5, 17.1, and 40.5°C for E. urophylla. The phyllochron of C. citriodora was 62.95°C day per leaf, and that of E. urophylla was 46.03°C day per leaf, which indicates that E. urophylla requires less energy accumulation than C. citriodora for leaf emission in the main stem.

RESUMO Quando a disponibilidade de água no solo é reduzida, as plantas respondem diminuindo a taxa de transpiração, o crescimento e o desenvolvimento, na tentativa de aclimatação à deficiência hídrica. Com o objetivo de quantificar essas respostas em mudas de Corymbia citriodora (Hook.) K.D. Hill & L.A.S. Johnson, foi utilizada a metodologia da fração de água transpirável no solo (FATS), em um experimento conduzido em casa de vegetação, sob o delineamento inteiramente casualizado, sendo dois níveis de suplementação hídrica, duas épocas de aplicação da deficiência hídrica e nove repetições. A FATS e os parâmetros de transpiração, crescimento e desenvolvimento foram medidos diariamente durante a aplicação da deficiência hídrica. A FATS crítica, em que a transpiração, o crescimento e desenvolvimento começam a ser reduzidos, foi distinta nas épocas de aplicação da deficiência hídrica devido à diferença das condições meteorológicas. Os valores elevados de FATS indicaram que a espécie possuiu boa aclimatação à deficiência hídrica no solo, quando comparada com as espécies anuais e outras perenes.

ABSTRACT When water availability in the soil is reduced, plants respond decreasing transpiration rate, growth and development in an attempt to acclimate to water deficit. To quantify these responses in seedlings of Corymbia citriodora (Hook.) K.D. Hill & L.A.S. Johnson, we used the methodology of the Fraction of Transpirable Soil Water (FTSW), in an experiment conducted in a greenhouse, in a completely randomized design with two levels of water supplementation, two application times of water deficiency and nine replicates. The FTSW and the transpiration, growth and development parameters were measured daily during the application of water deficiency. The FTSW threshold, in which transpiration, growth and development begin to be reduced, was variable for different application times of water stress due to the difference in weather conditions. The high values of FTSW indicate that the species has good acclimation to soil water deficit compared to annuals and other perennials species.