Effect of soil drying on growth, biomass allocation and leaf gas exchange of two annual grass species

Influence of short-term water stress on plant growth and leaf gas exchange was studied simultaneously in a growth chamber experiment using two annual grass species differing in photosynthetic pathway type, plant architecture and phenology: Triticum aestivum L. cv. Katya-A-1 (C-3, a drought resistant wheat cultivar of erect growth) and Tragus racemosus (L.) All. (C-4, a prostrate weed of warm semiarid areas). At the leaf level, gas exchange rates declined with decreasing soil water potential for both species in such a way that instantaneous photosynthetic water use efficiency (PWUE, mmol CO2 assimilated per mol H2O transpired) increased. At adequate water supply, the C-4 grass showed much lower stomatal conductance and higher PWUE than the C-3 species, but this difference disappeared at severe water stress when leaf gas exchange rates were similarly reduced for both species. However, by using soil water more sparingly, the C-4 species was able to assimilate under non-stressful conditions for a longer time than the C-3 wheat did. At the whole-plant level, decreasing water availability substantially reduced the relative growth rate (RGR) of T. aestivum, while biomass partitioning changed in favour of root growth, so that the plant could exploit the limiting water resource more efficiently. The change in partitioning preceded the overall reduction of RGR and it was associated with increased biomass allocation to roots and less to leaves, as well as with a decrease in specific leaf area. Water saving by T. racemosus sufficiently postponed water stress effects on plant growth occurring only as a moderate reduction in leaf area enlargement. For unstressed vegetative plants, relative growth rate of the C-4 T. racemosus was only slightly higher than that of the C-3 T. aestivum, though it was achieved at a much lower water cost. The lack of difference in RGR was probably due to growth conditions being relatively suboptimal for the C-4 plant and also to a relatively large investment in stem tissues by the C-4 T. racemosus. Only 10% of the plant biomass was allocated to roots in the C-4 species while this was more than 30% for the C-3 wheat cultivar. These results emphasize the importance of water saving and high WUE of C-4 plants in maintaining growth under moderate water stress in comparison with C-3 species.