Light filtered through spines

Two measurement surfaces

Radiation falling on a cactus is partly intercepted by spines and their sheaths. The flux reaching chlorenchyma is altered by shading, reflection and angle of incidence. In Nobel's experiment, Cylindropuntia bigelovii appears under the older name Opuntia bigelovii. Plants with armature intact were compared with plants after its removal to determine the contribution of spines to this balance. [Nobel 1983]

Acidity as a trace of nocturnal metabolism

Plants were maintained with a 12-hour day, at 30°C by day and 20°C at night, with roots supplied with water. The nocturnal increase in acidity was determined from the difference between samples taken at dusk and dawn. This measurement records nocturnal acid accumulation associated with CAM metabolism. [Nobel 1983]

Each light level was maintained for seven days before the response was measured. This allowed the daily photon dose falling on the plant to be compared with the acid accumulation achieved. The daily dose describes the integral of light over time, combining intensity with exposure duration. [Nobel 1983]

A shift in the response curve

With spines intact, 90% of the maximum nocturnal acidity increase was reached at around 23 mol photons m⁻² per day. After spine removal, the value was about 16 mol m⁻² per day. Both doses refer to light incident from outside. The difference was linked to shading of the assimilating surface. [Nobel 1983]

Armature thus changed the relationship between ambient light measurement and conditions in the green tissue. Effect size depends on spine arrangement, angle of incidence and stem properties. This experiment describes the response of a particular species and material under the stated temperature and hydration conditions. [Nobel 1983]

Growth and temperature as separate questions

The longer part of the study compared volume growth in a small number of plants under relatively limited light. Plants without spines grew more strongly. The trial included two individuals per treatment, so its significance lies in the agreement between the response direction and measurements of shading and acid accumulation. [Nobel 1983]

The same publication also analysed the effect of armature on stem temperature, but the modelling part concerned a different cactus. Describing the species, measured trait and method separately preserves the scope of each result. Spines simultaneously participate in the radiation balance, surface protection and contact with animals. Their combined role is considered at whole-plant level. [Nobel 1983][Mylo et al. 2022]