The effect of high temperature and high atmospheric CO2 on carbohydrate changes in bell pepper (Capsicum annuum) pollen in relation to its germination

Pollen viability and germination are known to be sensitive to high temperat ure (HT). However, the mode by which high temperature impairs pollen functi oning is not yet clear. In the present study, we investigated the effect of high temperature on changes occurring in carbohydrate of bell pepper (Caps icum annuum L. cv. Mazurka) pollen in order to find possible relations betw een these changes and pollen germination under heat stress. When pepper pla nts were maintained under a moderate HT regime (32/26 degreesC, day/night) for 8 days before flowers have reached anthesis, pollen count at anthesis w as similar to that found in plants grown under normal temperatures (NT 28/2 2 degreesC). However, the in vitro germination, carried out at 25 degreesC, of pollen from HT plants was greatly reduced. This effect matched the mark ed reduction in the number of seeds per fruit in the HT plants. Maintaining the plants at high air CO2 concentration (800 mu mol mol(-1) air) in both temperature treatments did not affect the in vitro germination of pollen fr om NT plants, but restored germination to near the normal level in pollen f rom HT plants. Under NT conditions, starch, which was negligible in pollen at meiosis (8 days before anthesis, A-8) started to accumulate at A-4 and c ontinued to accumulate until A-2. From that stage until anthesis, starch wa s rapidly degraded. On the other hand, sucrose concentration rose from stag e A-4 until anthesis. Acid invertase (EC 3.2.1.26) activity rose parallel w ith the increase of sucrose. In pollen from HT plants, sucrose and starch c oncentrations were significantly higher at A-1 pollen than in that of NT pl ants. Under high CO2 conditions, the sucrose concentration in the pollen of HT plants was reduced to levels similar to those in NT pollen. In accordan ce with the higher sucrose concentration in HT pollen, the acid invertase a ctivity in these pollen grains was lower than in NT pollen. The results sug gest that the higher concentrations of sucrose and starch in the pollen gra ins of HT plants may result from reduction in their metabolism under heat s tress. Elevated CO2 concentration, presumably by increasing assimilate avai lability to the pollen grain, may alleviate the inhibition of sucrose and s tarch metabolism, thereby increasing their utilization for pollen germinati on under the HT stress. Acid invertase may have a regulatory role in this s ystem.