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Observations on Physiological Response to Light in Nepenthes

The physiological response of Nepenthes pitcher and leaf morphology due to environmental factors illustrates the interplay between light, metabolism and nutrient uptake.  Unlike most other carnivorous genera, the trapping structure of Nepenthes is well segmented between photosynthetic (leaf) and nutrient trapping (pitcher) structures.  Leaves provide a large surface area for photosynthesis whereas pitcher structure is optimized for prey capture. Pitchers are less suited for photosynthesis due to geometry and other factors.  The devoted biomass allocated for pitchers is costly from a plant energetics perspective.  Resources that need to be devoted to pitcher development and maintenance include water transport, digestive enzyme secretion, nectar glands, crystalline waxy zones, etc... However, the net benefit Nepenthes derive from developing pitchers is clear with 50%-70% of nitrogen content in studied plants attributed to come from prey capture.  (Schulze et al. 1997, Moran et al. 2001).  As nitrogen is absorbed from prey, the photosynthetic rate of the entire plant is enhanced.  (Capó-Baucà et al., 2020).  

Pitcher to leaf ratio in Nepenthes is often directly influenced by light levels when other factors are kept constant.  Changes in growth and appearance generally require PPFD change for at least 1 month .  The plants in photos 1-3 had been subjected to a change in PPFD for 3 months with a 12 hour photoperiod.

  • At low lights levels, leaf area increases and pitchers are generally smaller in size (see photo 1).  Generally, pitchers won't develop much pigment and leaf color remains deep green. Often times plants will be reluctant to pitcher under such conditions.
  • At moderate levels increase, pitcher size increases and pigmentation increases substantially in the pitcher and some color change is noticeable in the leaf (see photo 2). 
  • At high light levels, leaf size reduces while pitcher size increases.  Leaves produce increased amounts of pigments with underlying leaf tones skew more towards yellow.  Leaf thickness and rigidity generally increase significantly. Additionally, red (or in some species like bicalcarata black) spotting may occur throughout the leaf.  This is generally the manifestation of endogenous fungal expression that is expressed when Nepenthes are stressed by various environmental factors. (pers. comm, Borneo Exotics). This affect can be suppressed with the use of fungicides). 

    The characteristic effects become more pronounced the longer plants are subject to intense lighting.  If plants do not capture prey or receive fertilizer in the soil, then the condition increases further and plants may start to decline over time.  However, with prey capture, the leaf and pitcher size may begin to increase although still remain somewhat dwarfed (see photos 4 and 5 below).  It should be noted that fertilization to the root zone will lead to a relative decrease in biomass devoted to pitcher production if they even pitcher at all (Pavlovič, A., et al, 2010).  This makes sense from an energetics perspective.  If nutrients are in good supply then the amount of reward for pitcher production may be diminishing.

    A number of factors could explain the increasing pitcher to leaf ratio as greenhouse light levels increase.  The amount of carbon, nitrogen and other nutrients devoted to pitcher development may reduce the availability of resources for leaf development.  Another possibility could be that the overall low photosynthetic capacity of Nepenthes (Pavlovic A et al., 2007) gets capped at a certain point and therefore the increased PPFD has a net balance with the reduced leaf surface area.  A third possibility could be that higher light levels create warmer, drier environments with more rapid transpiration thus leading to a reduction in leaf size to minimize surface area subject to losses.

    Many Nepenthes in situ grow under fully exposed conditions.  However, they generally don't express overexposure to such a degree (i.e. Mamut Copper Mine, Sabah, Malaysia) as greenhouse grown plants under approximately the same light levels.  The ideal conditions could play a role in this.  Also, greater amounts of nutrients may be available than in horticultural counterparts thanks to the high level of prey availability and larger root area in situ.  A controlled greenhouse study testing increased light levels while supplementing with various levels of fertilizer and prey could be very illuminating.

    From an evolutionary perspective, the cost of devoting more biomass to pitchers when stimulated by high light levels must lead to increased benefits in nutrient gain and subsequent metabolic increase.  Some populations may not present this behavior because they have evolved to respond to environmental cues such seasonal droughts or other factors that would lead to a poor energetic investment in increased pitcher biomass.  These are simply observations into this a fascinating subject area which deserves further examination.

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