Effects of spine-shading on aspects of photosynthesis for three cactus species

Abstract

We evaluated the effect of spine removal on three cactus species varying in spine-shading of stems. Turbinicarpus schmiedickeanus showed the greater spine shading (c. 61 %), followed by Mammillaria zephyranthoides (43 %), and Echinocactus platyacanthus (17 %). We evaluated photosynthetic activity [titratable acidity, electron transport rate, potential quantum yield of Photosystem II, effective quantum yield of Photosystem II, and non-photochemical quenching], as well as pigment content (chlorophylls a and b, total chlorophyll, chlorophyll a/b, carotenoids, and total chlorophyll/carotenoids ratio). For T. schmiedickeanus, the species having the higher spine shading, spine removal increased the CAM activity (acidity level) and the effective quantum yield of Photosystem II and diminished both electron transport rate and non-photochemical quenching at 15:00 hours. For M. zephyranthoides, the species having intermediate spine shading, spine removal also increased the acidity level but diminished effective quantum yield of Photosystem II at midday, as well as chlorophyll a, total chlorophyll, and carotenoids, although there was higher non-photochemical quenching (heat quenching) as an adaptation to avoid photosynthetic damage. Spine removal did not affect acidity level or pigment content from E. platyacanthus, the species having the lower spine shading; however, at midday this species without spines showed a decrease of effective quantum yield of Photosystem II, as well as an increase of non-photochemical quenching and electron transport rate. We suggest that a main function of cactus spines is reduction of sun exposure to avoid damage to the photosynthetic apparatus.