Scaling relationship between stomatal density and stomatal area among 12 Magnoliaceae species: -2/3 self-thinning rule of stomata and implication for stomatal conductance

Xuchen Guo1, Ülo Niinemets2, Peijian Shi1*, Kexin Yu1, Lin Wang1, Meng Lian1, Qinyue Miao1, Weihao Yao1, Jianhui Xue1,3*, Karl J. Niklas4,*
1College of Life Science, Nanjing Forestry University, Nanjing 210037, China
2Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 1, 51006 Tartu, Estonia
3Institute of Botany, Jiangsu Province and Chinese Academy Sciences, Nanjing, 210014, China
4School of Integrative Plant Science, Cornell University, Ithaca, NY 14853, USA

Stomata play a crucial role in regulating water loss and CO₂ uptake, and maintaining leaf thermal balance. Stomatal density (SD) and stomatal area (SA) are key traits that affect the maximum stomata conductance and the rate of change of stomatal openness upon environmental changes. However, the relationship between SD and SA remains controversial. Stomatal guard cells contain chlorophyll, and accordingly, increases in stomatal area per unit leaf area (SDA=SD.SA) might reduce light penetration to leaf interior, and thus, SD and SA could follow the “self-thinning” rule. We examined leaves from 12 Magnoliaceae species, exploring three potential scaling exponents of SD versus SA: (1) -1 < exponent < 0, leading to an increase in SDA with rising SD; (2) exponent = -1, resulting in SDA remaining constant despite changes in SD; (3) exponent < -1, causing SDA to decrease as SD increases. Reduced major axis (RMA) method regression and non-linear regression were applied to fit the scaling relationship of SD versus SA. Correlation analyses were conducted to examine the relationships between SD and SA for each species, between SD and the coefficient of SA variation, and between the SDA and the maximum pore water vapor diffusion conductance (gwmax) for pooled data. SD, SA, coefficient of SA variation, and gwmax varied among species, and species with higher SD tended to possess a smaller SA, but larger coefficient of SA variation, and gwmax. Within species, SD values were negatively correlated with SA in 8 out of 12 species. The scaling exponent of SD vs. SA fitted by RMA (exponent = -0.707) was slightly lower than that fitted by non-linear regression (exponent = -0.669, close to -2/3) in the pooled data. There were positive correlations between SD and the coefficient of SA variation, between the coefficient of SA variation and SDA, and between SDA and gwmax. Stomata adhered to the -2/3 self-thinning rule, increasing both the coefficient of SA variation and SDA as SD increased. This allometric scaling implies that the increase of SD leads to an enhancement in the maximum stomatal conductance despite concomitant reductions in SA.