The use of mitotic index in seedling assessments

Abstract

The daytime pattern of mitotic index (MI) (percent of apical cells undergoing mitosis) in the shoot apex of Piceaglauca (Moench) Voss (white spruce) containerized seedlings was examined and compared for five cultural treatments. From sowing in March until mid-July, all seedlings were grown under an extended, 23-h photoperiod in a common nursery culture. In mid-July, an array of photoperiod treatments was created, ranging from ambient photoperiod and temperature to different levels of short day length and ambient or controlled, constant temperature. Consistency of MI comparisons among the treatments at different specimen collection times was emphasized rather than treatment effects on MI. Specimens were collected four times a day on two dates: when most seedlings in all treatments were initiating bud scales and when most seedlings were initiating leaf primordia. Patterns of MI were different on each of these dates. It is shown that conclusions about treatment effects on MI can be influenced by the sampling protocol and analytical approach. End of the growing season studies of white spruce and P. glauca × Piceasitchensis (Bong.) Carr. (white × Sitka spruce hybrid) seedlings grown in a greenhouse culture showed that MI below 1% was well correlated with low (<25%) foliage damage, reasonably correlated with stem tissue damage, and not correlated with bud damage resulting from controlled freezer tests to −18 °C. It is concluded that the MI technique could be useful in lifting-date determination, but different MI thresholds must be established for southern, northern, or coastal seed sources. Monitoring MI was not a good alternative to using days to bud break (testing under forcing conditions) to determine bud dormancy status. However, mitotic reactivation of the apical meristem in seedlings overwintering in a nursery bed occurred earlier in the spring than visible signs of growth reactivation (bud swelling and bud break). Studies of growth resumption of western red cedar (Thujaplicata Donn) seedlings in winter revealed that this species would be considered quiescent if tested under a long photoperiod, while under a short photoperiod growth resumption was much slower in early than in mid- and late winter.