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Key-words:Nile tilapia (Oreochromis niloticus), biodiversity resistance, biological invasion, species richness, biomass Nile tilapia (Oreochromis niloticus) is one of the most widely cultured species globally and has successfully colonized much of the world. Despite numerous studies of this exotic species, how differences in native communities mitigate the consequences of Nile tilapia invasion is unknown. Theory predicts that communities that are more diverse should be more resistant to exotic species, an effect that is referred to as "biotic resistance", but these effects are spatially dependent and organismspecific. Field surveys and laboratory experiments were conducted to test the theory of "biotic resistance" and ascertain the relationship between native species richness and the invasion of Nile tilapia. In the field, we found that as native species richness increased, the biomass of Nile tilapia was significantly reduced. Consistent with results from the field, our manipulative experiment indicated that the growth of Nile tilapia was negatively related to native species richness. Thus, our study supports the theory of "biotic resistance" and suggests that species biodiversity represents an important defense against the invasion of Nile tilapia. RÉSUMÉLa diversité des espèces protège contre l'invasion du tilapia du Nil (Oreochromis niloticus) Le tilapia du Nil (Oreochromis niloticus) est l'une des espèces les plus largement élevées à l'échelle mondiale et il a colonisé avec succès la plupart des pays. Malgré de nombreuses études de cette espèce exotique, comment les diffé-rences dans les communautés indigènes influencent les conséquences de l'invasion du tilapia du Nil est une donnée inconnue. La théorie prédit que les communautés qui sont plus variées devraient être plus résistantes aux espèces exotiques, un effet qui est dénommé « résistance au stress biotique », mais ces effets sont spatialement dépendants et propres à chaque organisme. Des études de terrain et des expériences de laboratoire ont été menées afin de tester la théorie de la « résistance au stress biotique » et de détermi-ner la relation entre la richesse des espèces indigènes et l'invasion du tilapia du Nil. Sur le terrain, nous avons constaté que, quand la richesse des espèces indigènes augmente, la biomasse du tilapia du Nil est considéra-blement réduite. Conformément aux résultats sur le terrain, notre expérience de manipulation indique que la croissance du tilapia du Nil est négativement associée à la richesse des espèces indigènes. Ainsi, notre étude confirme la théorie de la « résistance au stress biotique » et suggère que la biodiversité des espèces représente un important moyen de défense contre l'invasion du tilapia du Nil. Mots-clés :

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The improvement of grain filling is the key issue for promoting wheat thousand grain weight and grain yield. The levels of polyamines (PAs) significantly affect grain filling in cereals, but the mechanism by which PAs affect grain filling in wheat is unclear. In the present study, six wheat cultivars whose grain filling differed were used, and their grain-filling characteristics and endogenous PA contents were measured. In addition, exogenous PAs were supplied during the wheat grain-filling period. The grain-filling characteristics, hormone levels, starch contents, and gene expression [based on RNA sequencing (RNA-seq)] in the grain were analyzed. The objective of the present study was to investigate the effects of PAs on grain filling in wheat. The results suggested that the direct synthetic pathway from putrescine (Put) to spermidine (Spd) in the grain was a key factor in promoting grain filling and thousand grain weight in wheat. Spd through regulates the grain-filling rate of inferior grain during the early grain-filling period to affecting the grain filling and thousand grain weight of wheat. The promotive effect of Spd on the grain filling of inferior wheat grain was notably related to carbohydrate metabolism in that grain. Spd significantly increased the zeatin (Z) + zeatin riboside (ZR) contents but reduced the ethylene (ETH) evolution rate in the inferior grain. In addition, Spd significantly increased the sucrose synthase (SS) and acid invertase (AI) activities in the inferior grain. These effects of Spd led to increased sucrose content in the inferior grain. These reasons might explain why Spd significantly promoted the filling and weight of inferior wheat grain.

Alleviating the negative effects of low temperature at the booting stage on wheat grain number and yield is important for maintaining high and stable wheat yields.In this study, we selected wheat varieties with different levels of cold resistance, simulated low temperature at the booting stage and exogenously applied trehalose to study its effect on wheat floret development under low-temperature stress and explore the physiological mechanism of that process. The results showed that higher levels of endogenous trehalose benefited wheat resistance to low-temperature stress.Exogenous trehalose inhibited floret degeneration and increased floret fertility in the apical spikelets, thus significantly alleviating the decrease in grain number per spike caused by low temperature. The results of the metabolomics analysis as well as the physiological indexes showed that trehalose significantly promoted the assimilation of nitrogen in florets under low-temperature stress, thus reducing the accumulation of ammonium ions in the plant cells. In addition, exogenous trehalose significantly increased the endogenous spermidine level in florets, increased their glutathione and ascorbic acid contents, promoted the glutathione-ascorbic acid cycle, and thereby reduced the contents of reactive oxygen species in florets. This may be an important physiological mechanism by which trehalose alleviates the effect of low-temperature stress on floret development in wheat.