Photoperiod and plant growth: a review

Long-day (LD) treatments frequently promote an increase in dry weight in plants that otherwise grow in short days (SD). Responses can be substantial. Increases for grass species have averaged around 52%. However, LD treatments (day extension, night-break, etc) frequently vary in their effectiveness. A direct promotional effect of LD treatment on leaf expansion and total photosynthetic area provides one possible mechanism. Of 50 species surveyed, 41 (82%) gave larger or longer leaves in LD, six (12%) gave larger or longer leaves in SD, and three (6%) showed no leaf extension response. Increase in leaf area is frequently accompanied by an increase in specific leaf area (SLA) and, in such cases, it is clear that an increase in leaf area occurs ahead of an increase in dry weight. Increased SLA is frequently associated with reduced net assimilation rate (NAR), but this is generally more than compensated for by greater leaf area. In other cases, increase in leaf area in LD treatment is not accompanied by increased SLA. In such cases, dry weight gain may itself be driving leaf area expansion, possibly mediated by an influence of LD on assimilate partitioning. Increases in leaf size are associated with increases in both cell size and cell number, and a photoperiodic growth stimulus has been shown to be transmissible from mature leaves to developing leaves. Gibberellins may be implicated, but responses to exogenous gibberellins and LD treatment are rarely exactly the same. Parallels are noted between LD treated leaves and low-irradiance, 'shade' leaves, suggesting that, in some cases, this may be because plants respond to the average irradiance over the lit period. Chlorophyll per unit leaf area is occasionally increased by LD treatment, which may increase photosynthesis and constitute a second mechanism that increases dry weight. Furthermore, lighting at a lower irradiance over a longer period may be more efficient than a high irradiance SD treatment due to the hyperbolic relationship between PAR and photosynthesis. Low intensity LD lighting can, at least in part, offset respiration. The LD growth phenomenon has yet to be exploited in commercial crop production, but the potential benefits are clear.