Diaspore Dimorphism, Awn Hygroscopicity and Adaptive Significance in a Winter Annual Bromus tectorum (Poaceae)

Abstract

Bromus tectorum, a winter annual plant, produces dimorphic diaspores: complex diaspores with multi-awns and simple diaspores with one awn. However, there is no information available about the role of awns and the germination characteristics of dimorphic diaspores. Dispersal germination and awns hygroscopicity of the dimorphic diaspores were assessed. The complex diaspore with multi-awns can easily be dispersed long distances from the mother plant by mammals. The simple diaspores with one awn are tightly attached to the mother plant. Caryopses from the two types of diaspores exhibited non-deep physiological dormancy at maturity, which can be released by dry storage and GA₃ treatment. The awns have hygroscopic activity and can move in response to changes in moisture, moving the complex diaspore (the seed) into the soil. The seedling emergence from complex diaspores was significantly higher than those from simple diaspores at all burial depths. Germination of caryopses on the soil surface was poor. The optimal planting depth for both types of diaspores' emergence is 1-2 cm. The distinct characteristics of dimorphic diaspores and the beneficial influence of hygroscopic awns on dispersal, germination, and seedling establishment have significant ecological implications for B. tectorum's successful reproduction in unpredictable cold deserts.

Keywords: after-ripening; dimorphic diaspore; dispersal; hygroscopic awns; seedling emergence; self-burial of seeds.

Figure 1

Spikelet composition of Bromus tectorum. (A) Spikelet consisting of glumes and florets, (B) Simple diaspore and complex diaspore. 1 and 2 are fertile caryopses. 3, 4, 5 and 6 are sterile florets. An awn is attached to the top of the lemma. The awn number of a diaspore is equal to the number of sterile florets plus 1. The adjacent bars are 1 mm. (C) The length of the awn and lemma of florets at different positions in the spikelet. Different lowercase letters indicate significant difference (Tukey HSD, p < 0.05) between the lengths of each awn or lemma across different positions. Error bars = ±1 SE. n = 10.

Figure 2

Microhairs and spines on the lemma and awn of Bromus tectorum microscopically. (A) Diaspores. (B) Lemma. Microhairs on the back and margin of the lemma. (C) Awn. Spines on the awn.

Figure 3

Percentage of diaspores remaining on mother plants of Bromus tectorum at 0, 30, 60, 90 and 105 days after diaspore maturity. Error bars = ±1 SE. n = 3 mother plants.

Figure 4

(A) Germination percentages of fresh dimorphic diaspores of Bromus tectorum at different temperatures. (B) Effect of GA₃ treatment on germination of dimorphic diaspores at 15/5 °C. Different uppercase letters indicate significant difference (t-test, p < 0.05) between germination percentages of the two different diaspore morphs at the same treatment levels. Different lowercase letters indicate significant difference (Tukey HSD, p < 0.05) between germination percentages of the same diaspore morph across different treatments. Error bars = ±1 SE. n = 3 replicates.

Figure 5

Germination percentages of complex and simple diaspores of Bromus tectorum during dry storage. Different lowercase letters indicate significant difference (Tukey HSD, p < 0.05) across all times, regimes, and temperature conditions. Error bars = ±1 SE. n = 3 replicates.

Figure 6

The movement of complex diaspores of Bromus tectorum. The relationship between the average angle of complex diaspore awns and the relative humidity (r.h.) of the surrounding air is presented. (A) Complex diaspores were exposed to 0.9 r.h. and (B) 0.2 r.h. for two hours. (C) Effect of relative humidity on angle between the awns. Different lowercase letters indicate significant difference (p < 0.05) among different relative humidity.

Figure 7

The water uptake curves for dimorphic diaspores of Bromus tectorum. Different uppercase letters indicate significant difference (t-test, p < 0.05) between the two different diaspore morphs at the same times. Different lowercase letters indicate significant difference (Tukey HSD, p < 0.05) among different times of the same diaspore morph. Error bars = ±1 SE. n = 3 replicates, each with 100 replicate diaspores.

Figure 8

Cumulative emergence of seedlings from complex diaspores and simple diaspores of Bromus tectorum planted at varying depths in the soil. Error bars = ±1 SE. n = 3 replicates.

Figure 9

Mean maximum, mean minimum, and mean monthly temperature in Urumqi, Xinjiang, China (2013–2023). Temperature and rainfall data were obtained from the local government.