Will Plant Performance on Soils Prone to Drought or With High Mechanical Impedance to Root Penetration Be Improved Under Elevated Atmospheric CO₂ Concentration

Abstract

Plants growing on dry soils or on soils with high mechanical resistance to root penetration grow more slowly and exhibit lower stomatal conductance than those growing on moist and loose soils. In most situations in nature where edaphic stresses develop rather slowly (compared to stresses imposed in most pot experiments conducted under controlled conditions), photosynthesis is mainly reduced via stomatal effects rather than via changes in mesophyll capacity for photosynthesis. Elevated CO₂ will induce an increase in the internal partial pressure of CO₂, despite stomatal conductance being lowered even further. Photosynthesis will therefore be improved, and leaf turgor will be increased.

It is widely thought that growth on dry or hard soils is not carbon limited because levels of soluble carbohydrates in the leaves and root cells are increased. It is shown in this paper that growth on soil with high mechanical resistance does respond to elevated CO₂. However, this response is smaller than expected from the increase of carbon assimilation rate because: (a) carbon partitioning is altered so that supplementary carbohydrates are preferentially allocated to the roots; (b) leaf growth sensitivity to internal availability of sugars is lower than in plants growing on loose soils. These alterations of 'sink activity' and carbon partitioning are mediated by unknown signalling factor(s) induced in the roots. It is not known whether the root factors acting in droughted plants are of the same nature. In both droughted and impeded plants the interacting effects of these factors and of ambient CO₂ levels are likely to result in improved transpiration efficiency. More experiments are needed in this area, however, especially to ascertain the relative contribution of changes in growth patterns versus changes in the patterns of water use. In conclusion, the importance of identifying the nature of the sink limitations induced by root signals is emphasised. It is a fundamental area of research to be developed not only for assessing growth responses to rising CO₂ under edaphic stress, but likely also for reconciling conflicting responses of field-grown and pot-grown plants.