Mechanism responsible for restricted synthesis and accumulation of lignin in wheat stems under low light conditions

Abstract

At present, stem lodging remains a key factor that limits further increases wheat yields, where it is attributed to the reduce mechanical strength of plant stems in the population due to low light during high-yield cultivation. The accumulation of lignin in the stem directly determines its mechanical properties. However, the mechanism associated with the effects of low light restriction on lignin metabolism in wheat stems is poorly understood, and thus there is not a sufficient theoretical basis for developing technical measures to promote stem lodging resistance under high yield cultivation conditions. Therefore, in the present study, three representative wheat cultivars with strong (Xinong511), medium (Xinong979), and weak (Shannong16) stem lodging resistance were selected as experimental materials. Different light environments were simulated within the population by using sparse and close planting treatments, and the effects of low light were assessed on lignin synthesis and accumulation in wheat stems, and stem lodging resistance. Compared with sparse planting, close planting significantly reduced the net photosynthetic rate in the third, fourth, and fifth leaves by 7.55–33.25 %, 0.09–50.48 %, and 5.64–46.49 %, respectively, the allocation of photosynthetic carbon assimilates by various organs decreased, while the root vitality decreased significantly by 9.53–22.78 %, the uptake of nitrogen by various organs decreased, the accumulation of lignin decreased significantly by 5.92–35.87 %, and the stem breaking strength and stem lodging resistance index decreased by 4.59–26.85 % and 21.40–35.59 %, respectively. Correlation analysis and path analysis showed that the light environment affected the activity and gene expression levels of enzymes related to lignin biosynthesis through both direct and indirect pathways (roots), thereby affecting lignin accumulation, and ultimately leading to weakened stem lodging resistance in wheat. The net photosynthetic rate were lower in the middle and lower leaves under low light conditions. In addition, the root vitality was weakened and the expression levels of genes encoding enzymes related to lignin synthesis in stems were significantly down regulated. Thus, the activities of enzymes related to lignin synthesis were significantly reduced, which limited the synthesis and accumulation of lignin in the basal internodes of stems, thereby, leading to wheat stems with decreased mechanical strength and a significantly increased risk of stem lodging. These findings provide important insights into the mechanism associated with stem strength weakening under high-yield wheat cultivation conditions and a theoretical basis for developing technical measures to enhance stem lodging resistance.