Stem elongation and growth of Solanum tuberosum L. in vitro in response to photosynthetic photon flux, photoperiod and difference in photoperiod and dark period temperatures

Abstract

Stem elongation and growth of potato plantlets under three DIF (difference in photoperiod and dark period temperatures) levels, −9, 0 and +9, combined with two PPF (photosynthetic photon flux) levels, 70 (low) and 140 (high) μmol m −2 s −1 provided by white cool fluorescent lamps, under 16 h day −1 (long) or 8 h day −1 (short) photoperiods, were studied. Four nodal cuttings were cultured for 21 days on 0.6 × 10 −4 m 3 MS (Murashige and Skoog, 1962, Physiol. Plant., 15: 473–497) agar (8 kg m −3) medium with no added sugar in 3.7 × 10 −3 m 3 polycarbonate boxes. Each box had two 10 mm holes covered with microporous filter to facilitate air exchange (3.6 air exchanges per hour). The average daily temperature in the culture room was set the same at 23 °C for all treatments, and CO 2 concentration and relative humidity were maintained at 400–500 μmol mol −1 and 50–70%, respectively. Stem length was significantly suppressed under 0 or −9 DIF, high PPF and long photoperiod. Stem diameter, leaf area and number of leaves were significantly enhanced by long photoperiod and high PPF, but affected little by DIF level. Specific leaf area was little affected by photoperiod, but decreased under high PPF and under low DIF. Long photoperiod and high PPF led to an increase in the fresh and dry weights maintaining similar percentage dry matter and to enhanced root growth. Under the same amount of integrated PPF, fresh and dry weights of leaf, stem, root and whole plantlet were significantly higher under the long photoperiod and low PPF conditions than under the short photoperiod and high PPF conditions. Because of suppressed root growth under short photoperiod, shoot to root dry weight ratio increased under short photoperiod, but was not affected by DIF. It is suggested that under photoautotrophic conditions a combination of high PPF level, long photoperiod, and zero or negative DIF produces potato plantlets in vitro of short and thick stem with similar number and increased area of leaves, which are desirable for transfer to ex vitro conditions.