Seed mass variation potentially masks a single critical water content in recalcitrant seeds

Abstract

For recalcitrant seeds, mortality curves of germination versus water content typically imply a wide range of desiccation sensitivities within a seed population. However, seed to seed differences in water content, during desiccation, may confound our interpretation of these mortality plots. Here, we illustrate this problem for two batches of Vitellaria paradoxa (Sapotaceae) seeds collected in 1996 and 2002. Whole seeds were desiccated to various target water contents (TWCs) using silica gel. During desiccation, smaller seeds in the population dried most rapidly. Consequently, there was a significant linear relationship between whole-seed water content and seed mass during the drying process. In addition, following desiccation to low TWCs, only the largest seeds in the population retained viability. Taken together, this suggests that the larger seeds survived, not as a consequence of great relative desiccation tolerance, but as a result of taking longer to desiccate. Subsequently, the critical water content (CWC) for viability loss was calculated, based on the assumptions that in the seed population whole-seed water content during desiccation was normally distributed and the smallest, and hence driest, seeds were killed first. Using this approach, the driest seeds in the population that were killed, at each TWC, were always below a single CWC (c. 20% and 26% in 1996 and 2002, respectively). In subsequent experiments the effect of seed size variation on the response to desiccation was confirmed by conducting desiccation screens on seeds sorted into two discrete size classes, i.e. the seed-lot heterogeneity in mass was reduced. Using this approach, the mortality curves had a steeper slope. Furthermore, data for 24 tropical tree species from the Database of Tropical Tree Seed Research (DABATTS) revealed that seed lots with less variability in mass had steeper mortality curves. Thus, taken together, the data suggest that, at least for whole seeds, the wide range of desiccation sensitivities typically inferred is an artefact of seed to seed variation in mass, and hence water contents, during drying.

Armazenamento de sementes de Crambe abyssinica Hochst. ex R.E.Fr. em diferentes embalagens e ambientes

“…Contudo, ao final do período de armazenamento, observou-se que as sementes em ambos os ambientes e embalagens apresentaram redução do teor de água, mantendo-se igual ao original. Possivelmente, esses resultados ocorreram graças às propriedades higroscópicas das sementes, que culminam com a manutenção do teor de água em equilíbrio com a umidade do ambiente de armazenamento, pois sementes pequenas, como as de crambe, secam mais rapidamente na população; consequentemente, há uma relação linear significativa entre o teor de água da semente inteira e a massa da semente (Daws et al, 2004).…”

Potencial hídrico do substrato e teor de água das sementes na germinação do crambe

“…Durante a permanência das caixas na câmara foi realizado, periodicamente, o monitoramento da massa úmida das sementes em hidratação, por meio de pesagens, adotando-se como referência o teor de água inicial das sementes até serem atingidos os teores de água desejados de 7,0% (teor de água inicial); 12,8%, 18,4% e 21,9%, correspondentes às sementes que permaneceram durante 6, 12 e 24 horas de hidratação, respectivamente. O teor de água das sementes foi determinado com quatro repetições pelo método da estufa a 105° ± 3°C, por 24 horas, conforme Brasil (2009) e os resultados foram expressos com base na massa das sementes úmidas.…”

“…Os resultados foram expressos em porcentagem média com base no número de plântulas normais (Brasil, 2009); comprimento de hipocótilo -no momento da última contagem, mediuse o comprimento do hipocótilo das plântulas com régua graduada em mm e os resultados foram expressos em cm. plântula -1 ; comprimento da raiz primária -determinado na última contagem do teste de germinação, nas plântulas normais de cada repetição, com o auxílio de régua graduada em mm e os resultados foram expressos em cm.…”

Desiccation tolerance of Tapirira obtusa seeds collected from different environments

“…Among the conditions influencing seed development are hydric availability, time and intensity of light exposure, and availability of nutrients (Fenner, 1991). Daws et al (2004) have found that tolerance to desiccation is also influenced by environmental conditions.…”

“…The environmental condition in which the seed develops influences several characteristics, including its vigor and its tolerance to desiccation (Daws et al, 2004;Delph et al, 1997;Weiner et al, 1997). Among the conditions influencing seed development are hydric availability, time and intensity of light exposure, and availability of nutrients (Fenner, 1991).…”