Growth of cucumber seedlings under emulated sunlight with artificially reproduced fluctuations in photosynthetic photon flux density

Cucumber seedling growth under constant and fluctuating photosynthetic photon flux density (PPFD) light (CPL and FPL, respectively) was examined to discuss whether plants can acclimate to CPL versus FPL. The FPL was created in a growth chamber by artificially reproducing fluctuations in ground-level sunlight PPFD using phosphor-converted white LEDs. The daily mean PPFD at the canopy surface and photoperiod were the same for the CPL and FPL. The plants were grown for two weeks, with the former and latter half periods under either CPL or FPL, respectively; there were two treatments for the first week and four with the 2 x 2 combinations for the last week. According to the growth analysis for the first week, the net assimilation rate (NAR) tended to be lower under FPL than under CPL, while the leaf area ratio and specific leaf area (SLA) were greater under FPL than under CPL. The lower NAR may be due to a delayed response of photosynthesis to a PPFD increase, i.e., photosynthetic induction, and the non-linearity of the PPFD-response curve of photosynthetic rate. The greater SLA was consistent with previous reports, although the mechanism underlying the response was unclear. For the last week, there was no significant difference in those parameters among treatments, possibly because of a change in canopy structure accompanying self and mutual leaf shading. The relative growth rate and shoot dry mass were also not significantly different among treatments. Thus, although cucumber seedlings initially showed physiological and morphological responses to CPL versus FPL distinctively, the responses did not improve growth under continued respective light conditions and may not necessarily be acclimative at the whole-plant growth level.