Cold acclimation ability and photosynthesis among species of the tropical Coffea genus

Three Coffea species (C arabica cv. Icatu, C. canephora cv. Apoata and C. dewevrei) were tested in order to identify and study the mechanisms of tolerance to low, non-freezing temperatures. Several photosynthesis-related parameters were monitored during a 20-day period of gradual temperature decrease, from 25/20degreesC (day/night) down to 15/10degreesC, during chilling treatments (15/4degreesC), and upon rewarming (25/20degreesC). Differences were found among species, both during low temperature exposure and during rewarming. In general, Coffea species showed cold-induced photoinhibition of photosynthesis, which was attributable to biochemical (in vivo ribulose-1,5-bisphosphate carboxylase/oxygenase activity and carbohydrate synthesis) and biophysical (antennae functioning, photosystem II efficiency and linear electron transport) inactivation, rather than to stomatal constraints. The moderately low temperature of 15/10degreesC was enough to cause a negative impact on net photosynthesis (A), mostly due to low (initial) rubisco activity in all species. However, C. arabica cv. Icatu showed a higher tolerance to chilling and recovered quickly and completely upon rewarming, as assessed from the impacts on the photosynthetic machinery (e.g. A(max), F-o, F-v/F-m, F-v'/F-m', q(p), phi(e), rubisco activity) and on carbohydrate metabolism. Such lesser effects are likely to be related to the strong increases and higher contents of zeaxanthin, lutein and beta-carotene that presumably increased the ability to dissipate excitation energy and contributed to protect the photosynthetic apparatus. During cold exposure, a significant reduction of the alpha/beta carotene ratio, which is considered an acclimation feature, was observed solely in C. arabica cv. Icatu. However, C canephora cv. Apoata and, especially, C dewevrei showed to be highly cold-sensitive. In these latter species, the photoinhibitory impairments to photosynthesis were stronger, probably due to the lower contents of protecting pigments during chilling conditions that lead to a higher vulnerability to excess excitation energy. Moreover, the mesophyll impairments (e.g. A(max), F-v/F-m, phi(e)) became significant even at moderately low temperatures of 15/10degreesC, and a lower ability to recover after chilling exposure was observed. The limitation of in vivo rubisco activity and A(max) may have been due to substrate limitation, but disturbances in sugar metabolism could also play an important role in the expression of chilling sensitivity in C. canephora cv. Apoata and C. dewevrei.