Abstract
This study aimed to explore the suitable light quality condition for ex vitro acclimation of M9 apple plantlets. Light quality treatments were set as followed; monochromatic LEDs (red (R), green (G), blue (B)) and polychromatic LEDs (R:B = 7:3, 8:2 and 9:1; R:G:B = 6:1:3, 7:1:2 and 8:1:1). Plant height of R, R9B1, and R8G1B1 treatments were significantly higher than the other treatments. The number of leaves and SPAD value of B were significantly higher than the other treatments. Root fresh weights of R9B1 and R7G1B2 treatments showed an increase of at least 1.7-times compared to R, G and R8B2. R8G1B1 accumulated higher starch contents than the other treatments. Photosynthetic rate of R9B1 and R8B2 were significantly higher than the other treatments. In terms of stomatal conductance and transpiration rate, treatments with high blue ratio such as B, R7B3 had higher values. Rubisco concentration was high in R and B among monochromatic treatments. In conclusion, red light was effective to increase photosynthetic rate and biomass and blue light increased chlorophyll content and stomatal conductance. Therefore, for R9B1 and R8G1B1, a mixture of high ratio of red light with a little blue light would be proper for the acclimation of in vitro-propagated apple rootstock M9 plantlets to an ex vitro environment.
Highlights
Plant tissue culture is an effective method for mass production of virus-free plantlets [1]
Thirty apple plantlets per treatment were transplanted into the deep flow technique (DFT) system, which was judged to be suitable for acclimation and growth of in vitro-propagated apple plantlets and a newly developed nutrient solution for apple plantlets was supplied to the apple plantlets [28]
Shin et al [42] observed that leaf elongation and expansion of in vitro-cultured Doritaenopsis plants could be promoted by red light, which supports our result that the vigorous leaf growth was observed in R and R9B1 (Figure 4B)
Summary
Plant tissue culture is an effective method for mass production of virus-free plantlets [1]. Typical environmental characteristics of in vitro such as high relative humidity, low light intensity, and artificial supply of sugar and growth regulator through culture medium are major factors that are associated with low survival rates in transplanting virus-free plantlets to ex vitro [2]. Light is a crucial factor affecting photomorphogenesis and photosynthesis of plants, which have a significant influence on the acclimation of the in vitro-propagated plantlets. In previous studies related to acclimation of in vitro-propagated plantlets, the effect of light intensities on photosynthetic capacity and the activity of enzymatic antioxidants have been examined in various crops [6,7,8,9] but the studies showing the effect of light quality are limited. Red light induces the accumulation of biomass and stem elongation [10], while blue light induces changes in Agronomy 2020, 10, 1017; doi:10.3390/agronomy10071017 www.mdpi.com/journal/agronomy
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