Nitrogen allocation for CO2 fixation promotes nitrogen use in the photosynthetic compensation of soybean under heterogeneous light after a pre-shading.

The spatial and temporal distribution of sunlight around plants is constantly changing in natural and farmland environments. Previous studies showed that the photosynthesis of crops responds significantly to heterogeneous light conditions in fields. However, the underlying mechanisms remain unclear. In the present study, soybean plants were treated by heterogeneous light after a pre-shading (SH-HL) to simulate the light condition in relay strip intercropping. Gas exchange and nitrogen (N) of leaves were measured to evaluate the photosynthetic performance, as well as photosynthetic N- and water-use efficiency (PNUE and PWUE). Chlorophylls (Chl) and Rubisco were analyzed as representative photosynthetic N components. Results suggest that SH-HL treated soybean exhibited evident photosynthetic compensation as the net photosynthetic rate (Pn) increased significantly in unshaded leaves, from which the export of photosynthates was enhanced. Under SH-HL, leaf N concentration remained relatively stable in unshaded leaves. While Chl concentration decreased but Rubisco concentration increased in unshaded leaves, indicating preferential allocation of leaf N for CO2 fixation. Results also showed that PNUE increased and PWUE decreased in unshaded leaves under SH-HL. Therefore, we suggest that within-leaf N allocation for CO2 fixation in unshaded leaves rather than within-plant N distribution to unshaded leaves drives the photosynthetic compensation under heterogeneous light after a pre-shading. However, enhanced water loss from unshaded leaves is a cost for efficient N-use under these conditions. Supplementary Information: The online version contains supplementary material available at 10.1007/s12298-023-01392-8.

Comparative study on the different responses of maize photosynthesis to systemic regulation under light heterogeneity.

Photosynthetic performance of a leaf is widely recognized to be systemically regulated by distal parts within the same plant. However, the effects of systemic regulation on different plant materials cannot be generalized. In this work, two cultivars of maize (Zea mays L.), 'Rongyu 1210' (RY) and 'Zhongdan 808' (ZD), were selected for a comparative study on the different responses of photosynthesis to light-dependent systemic regulation. After the growth of plants in heterogeneous light, the net photosynthetic rate of newly developed leaves increased in RY but decreased in ZD. A distinct capacity of CO2 fixation and assimilation between these two cultivars is also suggested. In ZD, the area of vascular bundles declined obviously, suggesting a restriction on carbohydrate export, which is also indicated by an increase in starch content. Resulting excessive accumulation of carbohydrates is proposed to inhibit the carbon assimilation, and eventually the photosynthesis. A decline in the area of bundle sheath cells also suggests a restriction on carbon assimilation. In contrast, these restrictions were unlikely to present in RY. This study reveals that the response of leaf photosynthetic performance to light heterogeneity is largely dependent on the systemic regulation of carbon assimilation, as well as carbohydrate export in maize.