Spine influences on PAR interception, stem temperature, and nocturnal acid accumulation by cacti

Abstract

Abstract. The influence of spines on nocturnal acid accumulation was examined for two Crassulacean acid metabolism plants, the barrel cactus Ferocactus acanthodes (Lemaire) Britton & Rose var. lecontei (Engelm.) Lindsay and the cylindropuntia Opuntia bigelovii Engelm. var. bigelovii. The ambient photo‐synthetically active radiation (PAR) required for 90% saturation of nocturnal acid accumulation by O. bigelovii was 23 mol m−2 d−1 for stems that were 32% shaded by spines and 16 mol m−2 d−1 when the spines were removed. For F. acanthodes, 90% saturation occurred for an ambient PAR of 45 mol m−2 d−1 where the stem was 78% shaded by spines and 21 mol m−2 d−1 when the spines were removed. For comparison, the ambient PAR averaged 21 mol m−2 d−1 at various times of the year and stem positions for O. bigelovii fully exposed to solar irradiation. Although spines can reflect some PAR toward the stem surface, their presence decreases the PAR incident on the stem surface, thereby decreasing the nocturnal acid accumulation. For O. bigelovii, periodically removing the spines led to a 60% greater increase in stem volume over 21/2 years. The effect of variation in spine‐shading on stem surface temperature was determined for F. acanthodes using computer simulations and an energy budget model. A substantial reduction in daytime stem temperatures and a smaller increase in night‐time temperatures as the spine‐shading increased had very little influence on nocturnal acid accumulation for both winter and summer days. Thus, the main effect of spines on the metabolism of cacti is apparently to reduce the PAR incident on the stem surface and thereby to reduce productivity.