Effects of alternating temperature on cactus seeds with a positive photoblastic response

Temperature is an important factor governing cactus seed germination. Particularly, alternating regimes can have positive, neutral and negative effects on germination. We studied the effects of constant and alternating temperatures on the germination of positive photoblastic seeds of six cactus species native to Argentina. Seeds of all species failed to germinate in darkness, irrespective of temperature regimes. Seed germination was significantly affected by temperature. For constant temperature, Cereus hankeanus, Echinopsis atacamensis, E. terscheckii and Parodia aureicentra showed a higher proportion of maximum germination than E. ancistrophora and Gymnocalycium saglionis. In E. atacamensis and E. terscheckii, the effect of alternating temperature was neutral, as no differences were registered among the maximum germination values obtained for both temperature regimes. The other species showed reduced germination under alternating temperatures in relation to the maximum values obtained at constant temperatures, indicating a negative effect of temperature alternation. In addition, our results confirm the idea that alternating temperatures do not promote the germination of positive photoblastic cactus seeds in darkness.