YIELD AND PERFORMANCE
Microtubers vs Plantlets for Minituber
Production
Microtubers are commonly harvested aseptically or are
fungicide-treated, dried for a time or suberized in the dark at
20 C in open fiats, then cold-stored in refrigerators (4-5 C) to
meet dormancy requirements. Greening in vitro (16/8 h d/n
under 30 ~mol m-'~y 1 for 10 d) prior to harvest may reduce
shrinkage and improve sprout emergence following storage
(Naik and Sarkar 1997). Microtubers are easier to transport
and handle than plantlets and are less delicate, so require less
aftercare, when planted in a greenhouse or screenhouse
(Wang and Hu 1982; Hoque et al. 1996). If microtubers are compared directly with whole plantlets, for essentially the same number of propagules, microtuber production takes 1-2
months longer than production of plantlets and microtubers
must be harvested, stored, and their dormancy needs met
before they can be planted (Ahloowalia 1994). For these reasons Ranalli (1997) questioned their advantage over plantlets
for minituber production, but this point has been very difficult
to resolve. Microtuber productivity comparisons are confounded by the many variables involved in their production,
storage, pre-emergence treatment, and cultural practices following planting. Of particular import are microtuber size, number of eyes, physiological age, and dormancy-breaking
treatments. Comparisons with other propagules are equally
confounded by a number of factors including differences in
plantlet production, time in culture, genotype (Ahioowalia
1994), size of cutting (whole plantlets vs nodal cuttings), nodal
position (Ali et al. 1995), and after-effects of nutrient and
growth regulator levels.
With respect to yield in nursery beds, microtubers produced similar total fresh weights, but fewer minitubers than
plantlets did; yield on a per stem basis was greater for the
plantlets at two planting densities (Wiersema et al. 1987). When
microtubers of 10- to 28-ram diameter were planted at a density
of 100 m -2 the yields were greatest for microtubers that had
been stored longest (7 months storage, 500 minitubers m-2) and
least for those stored for the shortest time (3 months storage,
200 minitubers m -s) (Molet 1991). Results were similar for
microtubers in the 7- to 9-ram range; larger, physiologically
older microtubers out-produced smaller or younger microtu-
bers (D6sir6 et al. 1995) in high-density nursery beds. Microtu-
bet yields were similar to stem cuttings (Ahloowalia 1994); the
latter outperformed whole plantlets. For example, stem cut-
tings outperformed plantlets in a greenhouse minituber pro-
duction system (three to five times greater yield) but were not
compared with microtubers (Ali et al. 1995).