As our planet grapples with increasing drought and rising nitrogen levels in the atmosphere, a research study led by researchers at Wuhan Botanical Garden, Chinese Academy of Sciences, and Xinjiang Normal University reveals an important contrast in how C₃ and C₄ grassland plants adapt to changing environments. Published in New Phytologist, the research shows that while both plant types respond similarly to drought, they employ markedly different strategies in nitrogen-use and carbon-allocation.

The study, based on 2,747 root-to-shoot ratio (RSR) observations across the globe, systematically explored these complex interactions. The findings highlight a fundamental decoupling in plant adaptation mechanisms:

The research found both C₃ and C₄ grassland plants exhibit a similar, "convergent" response to drought. As aridity increases, their root-to-shoot ratio rises, indicating a greater investment in belowground biomass. This strategy enhances water acquisition under drier conditions, suggesting that increased root investment is a widespread and fundamental drought adaptation strategy shared by both photosynthetic types, despite their differences in water-use efficiency.

In contrast to their convergent drought responses, C₃ and C₄ plants showed distinctly different pathways for responding to increasing soil nitrogen under arid conditions. For C₃ plants, higher soil nitrogen directly increases root-to-shoot ratio. This means they sustain belowground investment even when nitrogen availability increases. C₄ plants, however, do not directly promote root allocation with increased soil nitrogen. Instead, higher nitrogen boosts their net primary productivity, which indirectly lead to a reduction in their root-to-shoot ratio (Figure 1). This reflects a stronger tendency toward aboveground growth. In other words, when more nitrogen becomes available, C₃ plants tend to "invest in roots", whereas C₄ plants tend to "invest in leaves and shoots".

The study further clarified that while aridity does not fundamentally change the common tendency of C₃ and C₄ plants to increase belowground investment under dry conditions, it significantly influences the consequences of nitrogen availability. These distinct nitrogen use strategies ultimately lead to different carbon-allocation outcomes between C₃ and C₄ plants.

These findings suggest that, under future climate scenarios featuring intensifying aridity and increasing nitrogen deposition, carbon-allocation patterns and carbon-storage potential may diverge further among different types of grasslands. This underscores the critical need for vegetation models to represent C₃ and C₄ plants with greater ecological specificity.

Assistant Professor HU Man at Wuhan Botanical Garden is the first author. Prof. DANG Haishan at Wuhan Botanical Garden and Prof. GAO Jie at Xinjiang Normal University are the corresponding authors. This work was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Hubei Province, and other funding sources.

Figure 1. Pathways through which environmental factors influence root-to-shoot ratio in C₃ and C₄grassland plants, and the differences between the two groups (Image by WBG)