Effects of latitude and extreme temperatures on the expression of cleistogamous flowers and pollen viability in soybean (Glycine max) under field conditions.

Developing soybean flowers exposed to higher maximum temperatures during the week preceding full anthesis show a strong association with cleistogamy, but without a latitudinal gradient. Higher mean maximum temperature also had a moderate but significant negative effect on pollen viability. Soybean displays a range of reproductive strategies, including the production of both closed, self-pollinated cleistogamous flowers and open, self- and cross-pollinated chasmogamous flowers, as well as variation in the pollen viability among flowers. The incidence of cleistogamy and pollen viability could be related to stressful environmental conditions. This research examines under field conditions the relationship between latitude and the mean-maximum temperatures (data from National Meteorological Service) to which buds are exposed 1 week before sampling, with cleistogamy and pollen viability. The number of cleistogamous flowers (n = 50 flowers per site) and pollen viability (n = 5 flowers of each type per site, staining pollen with lactophenol to estimate the percentage of non-viable pollen) were measured across a broad latitudinal range (21 sites) and considering mean-maximum temperatures during the week before sampling flowers in full anthesis. The regression models were applied to interpret the response variables disentangling the effects of both factors. The proportion of cleistogamous flowers was high across sites (63.1 to 98%). The model with mean-maximum temperature as fixed factor showed a significant association with the occurrence of cleistogamy and with pollen viability. This trend suggests that higher temperatures (i.e., high mean daily temperatures) one week before anthesis are associated with changes in reproductive soybean traits throughout the latitudinal gradient. Given the observed increase in the frequency and intensity of maximum temperatures during the flowering season associated with climate change, our research implies an impact on the pollination and fertilization of soybean with possible implications for crop production.