There are many factors that give rise to this, but the primary reason in planted tanks is that a change in the environment that requires the plant to reprogram its enzymes to make best use of what is available in the current environment. For example, if you move a plant from shade to an area of very high light, the plant can channel energy from producing light capturing pigments in its leaves to producing new leaves or protective pigmentation instead. Green plants are deeper green in shade, as they need to produce more chlorophyll to survive, while many red species only grow very red in high light - as they produce protective pigments under high light.
While older leaves have some plasticity that enables adaptation, there are limits to how much old leaves can adapt. New leaves that are grown in the new environment, however, can be fully optimized to make use of current environmental conditions. When there are drastic changes in the environment, the plant may choose to focus its energy on producing new leaves, while abandoning its older leaves that are no longer suited for the new environment.
Plant stress from poor CO2 or nutrient access also forces the plant to channel energy to new growth in an attempt to survive. For example stem plants that are deprived of CO2 often abandon their older leaves prematurely, in order to channel energy to top growth in an attempt to breach the water surface and gain access to air.
Well-fed plants generally adapt to changes in the tank environment better, as they have more stored energy. Macro nutrients such as nitrogen, phosphates and potassium are mobile in the plant. A long term inadequate supply of nutrients can force plants to scavenge these nutrients from older leaves to fund new growth. Therefore plants grown in overly lean conditions can face an accelerated deterioration of older leaves when growing new leaves due to environmental changes.
This is why when buying plants from the store, checking to make sure that the plants are robust and well grown is important. Healthy, robust plants will have an easier time adjusting when transferred to the tank.
In this picture we transferred a Variegated Rotala macrandra from the shop's tank to ours. In the shop, under lower light and inadequate fertilization, the leaves are greener and the internodes are longer. After being planted into our farm tank, the new leaves produced are fully red, reflecting the higher light and fertilization levels. Notice however, that the old leaves that were grown in the shop did not produce this adaptation. They are programmed to best utilize what was available in the shop tank's environment, not the new tank, and even an improvement in their new environment cannot change that fully. This is primary because there is a significant difference in environment between the shop's tank and our own tank.
After the plant has grown in our farm tank for sometime, we cut off the top portion that has been grown adapted to the tank and replant it, discarding the bottom portion (that was grown in the shop). The fully adapted plant preserves its older leaves longer, and even the old growth is more algae resistant as well. Notice that even the older leaves further down the stem are well colored, in good form as they are adapted to this tank.
In a tank, if you have 100% of adapted growth (adapted growth meaning that the portion of the plant was grown under current tank conditions), the tank will be much more resistant to algae as a whole. This is why replanting new growth, discarding older growth is one of the key actions that can make a tank more algae resistant as a whole.
In the case where the environment changes drastically, and the old leaves that were programmed for a different environment cannot function well - the plant will channel all energy into new growth, even drawing energy and nutrients from old growth to feed the development of new leaves. This hastens the deterioration of old leaves. This is why plants bought from the shop often lose their old leaves even as they grow robust new ones.
Most plant species categorized as 'easy plants' have more plasticity and can adapt existing leaves to a wider range of new conditions. Picky species that are classified as difficult to grow are often difficult because they have difficult adapting to new conditions - many such species, once adapted, are actually quite undemanding plants. The prime example of a plant that is classified as difficult in this manner is Utricularia gramminifolia. It is an absolute weed that isn't particularly demanding once it is settled in, however, many aquarists have issues getting it to adapt to the tank environment.
Utricularia gramminifolia is not particularly demanding where growth parameters are concerned. It can grow just fine in harder or softer water, with higher or low nutrient levels, with high or low CO2 levels. However, it requires a stable, well cycled tank to settle in and grow. Its difficult adaptation phase has lead to many myths about its growth requirements when in actual fact it can grow in almost any tank as long as its stable and well cycled.
Leaves do not have the ability to heal physical damage such as puncture holes or tears. Cells in the plant tissue will form a callus at the site of the wound to seal it, but the leaf itself will not regenerate fully. If the leaf is not too damaged, it can still continue functioning after the wound is sealed. If the damage is significant, the plant will channel energy into generating new leaves instead, and the damaged leaf will deteriorate at an accelerated rate.
For mild cases of nutrient deficiency - for example, a lack of magnesium causing pale leaves - adding the missing nutrient can cause the overall plant to become greener. However, old deteriorating leaves in a plant, especially when the leaf starts to display physical deterioration (such as fragmentation or holes or broken leaf edges), will not renew themselves even if conditions turn favourable - the plant will instead channel energy into producing new leaves. Plants renew themselves continuously in this way, by producing new growth that is optimized to make sure of the current environmental conditions, while shedding older leaves.
This Bucephalandra leaf suffered physical damage, but do not show signs of significant deterioration otherwise. The leaf still functions normally, but one could trim it off for aesthetic reasons.
This Anubias leaf has deteriorated too far to be of remaining value. The plant draws nutrients from the deteriorating leaf and channel it to new growth. This accelerates the disintegration of the leaf and makes it a prime candidate for algae. Notice how algae targets only the damaged leaf. Trimming it off would allow space for new growth and prevent it from shading potential growth beneath.
For stem plants, it is common for the plant to channel most of its energy into new top growth which are closer to the light & flow, while leaving older leaves down the stem to deteriorate. Spacing stem plants out and having more light hit the lower levels can lessen this effect somewhat. However, for many species, the top shoots will crowd out and shade the lower portions of the plant naturally. Hardy species such as Rotala rotundifolia variants may not lose their lower leaves quickly but for many other species top growth density gives rise to deterioration of lower leaves.
During trimming cycles, the top portion of healthy leaves should be cut and replanted, and the bottom deteriorating portion uprooted and discarded. This allows the stem plant bush to be rejuvenated each cycle.
Due to shading and over-crowding, the lower leaves of this bunch of Ludwigia senegalensis have deteriorated significantly. The entire plant should be uprooted , the robust top portion cut off and replanted while the bottoms should be discarded.
Some stem plants (such as the Ludwigia senegalensis here) grow better slightly spaced apart, allowing some light and flow to penetrate the bush. Other species can take some over-crowding and can be suitable to form denser bushes. (Lobelia cardinalis, Staurogyne repens, Limnophila vietnam etc). Neat arrangement and correct spacing between plants should not be underestimated.
For rhizome plants such as Anubias and Bucephalandra species, old leaves that deteriorate should be cut off. The bare rhizome itself can sprout new leaves or shoot tips and cutting off older, damaged leaves give room for new growth. Overcrowded clumps should be separated into individual rhizomes if you want to maximize growth as a whole. Too many folks are afraid of trimming old growth off Bucephalandra - reducing over-crowding and giving space for new growth allows the clump to grow healthier and faster.
In this example, we would cut off all the older/damaged leaves circled in orange. This would give space for fresh new growth. Older deteriorating growth is also more vulnerable to algae; if you have a tank filled with old growth, that makes your tank very vulnerable to algae.
For Rosette plants such as Crypts or Sword plants, cut off damaged leaves at the base. Removing deteriorating old leaves deprives algae of a spawning spot.
To preserve health of old leaves, it is important to minimize shocks to the system; dips in CO2 supply are especially stressful. Nutrient availability play an important role; having inadequate nutrients can lead to premature deterioration of older leaves. Having a well rounded water column fertilization regime is important as is maintaining the substrate richness if root feeding is utilized. Inconsistent nutrient dosing or large disturbances to the tank's ecosystem (such as an ammonia spike) can also cause plant stress and hasten deterioration of older leaves. These same factors also cause algae spores to bloom - often resulting in older leaves getting algae. Repeatedly shifting plants that do not fancy being moved, or allowing weaker species to be crowded out by more aggressive growing species can also stress plants.
For slower growing species such as Bucephalandra, adaptation occurs slowly. If you think about it from the angle that only new growth grown under current tank conditions is fully optimized for the current tank, and couple this with how slowly Bucephalandra produce new leaves - the full adaptation of Bucephalandra clumps will take months. For such species, you need very long term stability to grow large patches of them well. This means applying good plant and tank management techniques not just for weeks but multiple months without a significant break in routine in between.
If you change tank parameters or growth conditions every few weeks - stem plants that grow fast can adapt, but your Bucephalandra that produce just one new leaf per week will never catch up. In unstable tanks, there will be always holes or algae in the older leaves of Bucephalandra. The keyword here is consistency.
If you have persistent holes and algae on the older leaves in Bucephalandra, but feel like you have not been tweaking CO2/light/nutrients much at all, chances are either poor maintenance techniques (failure to maintain low organic waste levels, poor horticulture techniques in trimming, replanting etc) have been applied or the system has been setup poorly (poor flow, too much flow, plant placement errors, poor equipment choices etc).
Perhaps one of the species that we are most well known for is Bucephalandra brownie ghost 2011. It is a slightly more picky Bucephalandra species, and so makes for great feedback whether you are doing things right or wrong across a long time horizon. We started with 3 rhizomes a few years ago, and have around 150 of them by now (plus having sold/given away more than a hundred in between). The other species that we are well known for is Hygrophila sp Chai - we also started with a single plant, and have a large patch of it by now as well.The key to both species isn't the ability to get both plants to grow new leaves - Bucephalandra has super low demands, while Hygrophila sp chai will grow if you throw enough light & CO2 at it - it is the ability to preserve the older leaves and not lose them to melt or algae. Maintaining the tank stability required to not lose the old growth takes a far more delicate and experienced hand than throwing a ton of light and CO2 to get new growth - something less experienced hobbyists are quick to realize when they start growing these species.
Many skilled aquarists space out their plants neatly, and also stagger the height so that aggressive growers do not crowd out and shade plants that require more light & flow. Here, the rock hardscape form physical barriers that prevent taller stem plants from over-crowding and shading the shorter, more delicate species. Tank by Jon Ooi