Plastic Orchids – Not Just Your Garden Variety
Being flexible is an important way to cope with changes. Whether it’s taking a different route to work because of construction or making your own cup of coffee at home instead of starting your day with a skinny soy caramel mocha vanilla latte from the establishment of your choice, this sort of fluid nature just makes life easier. These sorts of changes work great for us conveniently mobile organisms, but in the case of sessile organisms like plants, these sorts of behavioral changes aren’t an option. When times are tough, plants rely not only on specialized morphology and adaptation, but also a relatively high degree of phenotypic plasticity to get through rough times.
Phenotypic plasticity is when a genotype can have multiple phenotypes. In the simplest cases, it’s when a single set of genetic information can produce a variety of different effects. So for example, a genotype in an individual plant could code for “make leaves”, and when they’re produced, the leaves can come in two shapes: pinnate (skinny and pointy) or round. A plant stuck in the shade would then make more round leaves to catch what little sunlight makes it through, while a plant in high sunlight might make more pinnate leaves to prevent sun damage and/or let sunlight filter through to leaves lower down on the stem.
While phenotypic plasticity does occur in other organisms, plants as a whole exhibit a great diversity of trait plasticity including phenology (timing of life stages), reproduction, and photosynthesis.
Photosynthetic plasticity is the ability to adjust photosynthetic rate when conditions are unfavorable. This can range from adjusting photosynthetic rate in different parts of the plant to entire shifts from one photosynthetic pathway to the next. One of the most frequently discussed shifts are “facultative” CAM plants, which will adjust their photosynthetic pathway from (typically) C3 to CAM under drought stress.
C3 is a more ancestral form of photosynthesis, with high photosynthetic power and low water retention, whereas CAM is the reverse, maximizing water retention and survival while sacrificing energy efficiency. This incredible degree of plasticity is important for a number of reasons:1) photosynthetic hybrids could represent a transitory step in evolution from one type of photosynthesis to the next, 2) allow for survival in typically harsh climates with seasonally fertile or water abundant events, or 3) allow encroachment into other environments.
Compartmentalized C3-CAM Photosynthetic Plasticity
While typical forms of facultative C3-CAM shifts occur throughout an entire plant, there are also instances where this instead occurs in different parts of the plant. Rodrigues et al. found this to occur in two orchid cultivars (Cattleya walkeriana and Oncidium “Aloha”), where C3 and CAM photosynthesis could be performed simultaneously within the same individual in different parts of the plant. Specifically, there are vast differences between pseudobulb, root, and leaf photosynthetic activity. The former two are frequently discounted, as leaves are typically the site of intense photosynthesis and roots are typically buried underground. However Rodrigues et al. found that pseudobulbs are a vital organ and site of CAM metabolism, as well as observing root photosynthetic capabilities, as orchids in nature tend to be hanging from trees with green roots suspended in air.
Pseudobulbs are a bulbous structure found at the base of orchids, where the leaves and stem spring from. The outer layers of the pseudobulbs are lignified with no air space between cells, however, the subepidermal layers differed greatly. In C. walkeriana, the the subepidermal layer was also completely lignified, whereas in O. “Aloha”, intercellular air spaces existed and that all of their bundle sheaths directly connected to some sort of air space.
Rodrigues et al. found that C. walkeriana ultimately had greater morphological adaptations for drought tolerance, including higher degrees of leaf succulence, impermeable pseudobulbs, and non-plastic roots. On the other hand, O. “Aloha” are the opposite, with thinner non-plastic leaves, aerated pseudobulbs, and photosynthetically plastic roots.
What these differences mean is that C. walkeriana represents the typical idea of C3-CAM plasticity, where the leaves can shift photosynthetic pathways and are the main sites of photosynthesis, whereas pseudobulbs and roots act more or less as storage bins. In O. “Aloha”, photosynthesis is heavily compartmentalized, as pseudobulbs and roots express high CAM activity during drought conditions, and roots facilitate CO2 fixation at night. Their leaves, however, continue to perform C3 photosynthesis and the aeration of the pseudobulbs does mean they aren’t as great at retaining water.
Phenotypic plasticity is a convenient feature to have, considering plants can’t get up and walk over to a sunnier, wetter spot. While plasticity of physical traits is certainly advantageous, physiological plasticity can give certain plants a competitive edge when it comes to survival.
What makes the Rodrigues et al. 2013 study such an interesting paper is that it challenges the common belief that photosynthetic pathways are singularly determined by the major photosynthetic site: the leaves. While this could be true for plants that aren’t fleshy and green all-around like trees and shrubs, it’s important to avoid sweeping generalizations especially in organisms with unique characters like orchids. The Oncidium “Aloha” used in this study belongs to a genus labelled as C3 photosynthesizers because of the classical idea that leaves typically do most or all of the heavy lifting in many plants. As shown by this study, that’s not necessarily always the case and we should continue to question the assumptions most people take for granted. While discovery is an important and exciting part about being a scientist, it isn’t the only way to move forward. Building on old answers and testing readily-accepted beliefs is just as momentous as landing a big discovery and, in many ways, can be just as rewarding.