Consequences of incongruency in diurnally varying resources for seedlings of Rumex crispus (Polygonaceae)

The incongruency of diurnally varying resources essential to plants may detrimentally affect plants early in their development as indicated by reduced water use efficiency and carbon gain. Typical diurnal patterns of light and CO₂ availability in a midsized temperate herbaceous or forest gap were simulated in specially designed growth chambers. A sinusoidally varying CO₂ treatment (400 ppm minimum, 800 ppm maximum) approximated the diurnal cycle of CO₂ at the soil surface, while a steady-state CO₂ treatment (600 ppm) with the same average CO₂ concentration provided a control. Crossed with these two CO₂ treatments were two light regimes, one with 3 h of high light (850 μmol·m^-2·s^-1) in the morning (west side of a gap), and the other with 3 h of high light in the afternoon (east side). All treatments received baseline low light (55 μmol·m^-2·s^-2) for 14 h during the day. Rumex crispus was selected as a model species because of its rosette leaves, which grow close to the ground where diurnal CO₂ variation is greatest. The relative timing of diurnal variations in light and CO₂ significantly affected seedling water use efficiency, carbon gain, and morphology. Total biomass, photosynthetic rates, daily integrated carbon, water use efficiency, and leaf area were enhanced by morning exposure to high light. Seedlings that were exposed to peak values of light and CO₂ incongruently, i.e., those plants receiving intense afternoon light with diurnally varying CO₂, were detrimentally affected relative to control plants receiving intense afternoon light with steady-state CO₂. The results of this experiment indicate that the incongruent availability of required resources-such as light and CO₂-can detrimentally affect performance relative to when resources are congruent. These contrasting resource regimes can occur on the east and west side of gaps.