Pollination characteristics of two sympatrically distributed Tamarix species in south Xinjiang, China

Tamarix laxa Willd and T. androssowii Litw are two early spring flowering plants, which are widely and sympatrically distributed in south Xinjiang, China. A comparative study of flowering characters, floral rewards, pollination characters, and mating mode in T. laxa and T. androssowii was undertaken in the same natural population. We aimed to explore the effect of floral syndrome on pollination characteristics. Population-and individual-level flowering time and floral longevity were significantly different between species (P < 0.01); the population-level flowering time of T. laxa was longer than that of T. androssowii, but individual-level flowering time and floral longevity of T. androssowii was longer than that of T. laxa. The lengths of each floral parameter (petals, stamens, and pistils) and numbers of pollen and ovules were significantly different between species. Petals, stamens, and pistils were fragrant and nectar volume was significantly positively correlated with pollinator visiting frequency. Visiting frequency and pollen deposited on the stigma by the major pollinator were significantly different between species, and were higher in T. androssowii than in T. laxa. Flowers produced fruits after self-pollination and cross-pollination; thus, they exhibited a mixed mating system. There were significant differences between the self-incompatibility and pollen limitation index in both species. The self-incompatibility index was higher in T. androssowii than in T. laxa, but pollen limitation was higher in T. laxa than in T. androssowii. Thus, both species exhibited pollen limitation due to low pollinator activity.