Why do some conifer leaves go bronze in winter?

You may have noticed that certain conifer species’ leaves go a different colour over the winter without dying. This phenomenon is most associated with Buxus (box) and Cryptomeria japonica (Japanese cedar) which take on a range of bronze, reddy-brown colours, as shown in the examples below:

Although it may look a bit alarming, these leaves will go green again as the temperatures warm up. The reason for the colour change is that in these species, sun-exposed leaves at low temperatures shift their pigmentation to protect themselves from excess energy.

During the winter, evergreen trees face the challenge that the sun can be relatively strong, but the temperatures low. Many of the biological processes needed to convert light into sugars rely on enzymes, which don’t work efficiently at very low temperatures. Although it’s possible for plants to acclimate to lower temperaturesref, beyond a certain point the rate of photosynthesis starts to outpace the downstream processes, and excess energy and reactive oxygen species (“ROS”) accumulate. This can permanently damage Photosystem II (part of the photosynthetic apparatus in plants), so a mechanism is needed to avoid that.

The solution adopted by Cryptomeria japonica is to transform some of its chloroplasts into chromoplasts.ref

The part of the plant cell which photosynthesises is the chloroplast, which is a self-contained organelle with its own DNA known as a ‘plastid’ (and also, in conifers, paternally inherited). Normally it synthesises the green pigment chlorophyll. But plastids can also be ‘chromoplasts’, instead of synthesising chlorophyll, they synthesise coloured carotenoidsref which aside from being different colours, also act as anti-oxidants, removing dangerous excess oxygen levels from plant tissues. The pigments found in winter Cryptomeria foliage include Rhodoxanthin (found in Taxus baccata arils), Zeaxanthin (found in marigolds) and Antheraxanthin (found in dandelions).ref These are all carotenoids which are known to safely dissipate excess energy during photosynthesis.ref

So during winter when temperatures fall, green chloroplasts are converted into coloured chromoplastsref, and vice versaref when the temperature rises. The effect of this temporary transformation is to reduce photosynthesis and to increase the plant’s anti-oxidant levels, so that excess energy is dissipated safely and the photosynthetic apparatus is not damaged.

Aside from being observed in Cryptomeriaref and Buxusref, there is also anecdotal evidence that the same process happens in Juniperusref and Thuja (see below for one of my Thuja bonsai – the white bits are frost and the foliage is a distinctly reddy-brown colour). Interestingly these are all ‘flattened’ forms of conifer leaves, and colour-changing behaviour is rarely observed in needle-type leaves, which may instead deal with the problem of winter sun by plugging up their stomata to reduce the rate of photosynthesis. One exception is Pinus contorta ‘Chief Joseph’ which has golden needles in the winter and green ones in the summer.

The conversion of chloroplasts to chromoplasts is also responsible for the colouring of fruit and flowers, but it’s *not* what makes leaves coloured in the autumn (that would be ‘gerontoplasts’ref).

It seems that the conifer species which can do this are all members of Cupressaceae. So if you see pine needles going brown, that is probably the needles dying (which they do naturally after a certain period of time) unless it is a rare form. But if your Cryptomeria, Juniperus or Thuja develops a winter ‘blush’, don’t worry, it will probably come back all green when the temperature warms up.

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