Hydrothermal time model of germination: parameters for 36 Mediterranean annual species based on a simplified approach.

摘要

Germination rates and germination fractions of seeds can be predicted well by the hydrothermal time (HTT) model. Its four parameters HTT, minimum soil temperature, minimum soil moisture, and variation of minimum soil moisture, however, must be determined by long-term germination experiments at combinations of several levels of soil temperature and moisture. For some applications of the HTT model it is more important to have approximate estimates for many species rather than exact values for only a few species. We suggest that minimum temperature and variation of minimum moisture can be estimated from literature data and expert knowledge. This allows to derive HTT and minimum moisture from existing data from germination experiments with one level of temperature and moisture. We applied our approach to a germination experiment comparing germination fractions of wild annual species along an aridity gradient in Israel. Using this simplified approach we estimated HTT and minimum moisture of 36 species (Aegilops ovata, Aegilops peregrina, Alyssum strigosum, Atractylis cancellata, Biscutella didyma, Brachypodium distachyon, Bromus fasciculatus, Bupleurum lancifolium, Calendula arvensis, Carrichtera annua, Catapodium rigidum, Chaetosciadium trichospermum, Crepis sancta, Crithopsis delineana, Coronilla scorpioides, Filago contracta, F. desertorum, Geropogon hybridus, Gymnarrhena micrantha, Hedypnois rhagadioloides, Lagoecia cuminoides, Lolium rigidum, Onobrychis squarrosa, Plantago afra, P. cretica, P. ovata, Reboudia pinnata, Reichardia tingitana, Rhagadiolus stellatus, Scabiosa palaestina, Stipa capensis, Torilis tenella, Trisetaria macrochaeta, Urospermum picroides, Valantia hispida and Valerianella vesicaria). Comparison with exact data for three species shows that our method is a simple but effective method for obtaining parameters for the HTT model. HTT and minimum moisture supposedly indicate climate related germination strategies. We tested whether these two parameters varied with the climate at the site where the seeds had been collected. We found no consistent variation with climate across species, suggesting that variation is more strongly controlled by site-specific factors..