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

Stem elongation and growth of potato plantlets under three DIF (differ ence in photoperiod and dark period temperatures) levels, -9, 0 and +9 , combined with two PPF (photosynthetic photon flux) levels, 70 (low) and 140 (high) mu mol m(-2) s(-1) provided by white cool fluorescent l amps, under 16 h day(-1) (long) or 8 h day(-1) (short) photoperiods, w ere studied. Four nodal cuttings were cultured for 21 days on 0.6 X 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 X 10(-3) m(3) pol ycarbonate boxes. Each box had two 10 mm holes covered with microporou s 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 d egrees C for all treatments, and CO2 concentration and relative humidi ty were maintained at 400-500 mu mol 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 we re significantly enhanced by long photoperiod and high PPF, but affect ed little by DIF level. Specific leaf area was little affected by phot operiod, but decreased under high PPF and under low DIF. Long photoper iod and high PPF led to an increase in the fresh and dry weights maint aining similar percentage dry matter and to enhanced root growth. Unde r the same amount of integrated PPF, fresh and dry weights of leaf, st em, root and whole plantlet were significantly higher under the long p hotoperiod and low PPF conditions than under the short photoperiod and high PPF conditions. Because of suppressed root growth under short ph otoperiod, shoot to root dry weight ratio increased under short photop eriod, but was not affected by DIF. It is suggested that under photoau totrophic 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, whic h are desirable for transfer to ex vitro conditions.