 Credit: John Bryant.  Flowers of the hazel tree. The large showy catkins are the male flowers. The female flowers are much smaller; one has opened beside the left-hand group of catkins. Credit: John Bryant. John writes … The heading of this blog post takes us back to the last words of my previous outing on these pages in which I wrote about the role of cold weather in regulating aspects of plant growth and development. Seeds of most plants growing in cool temperate regions are dormant – unable to germinate – when they are shed from the parent plant. In many species, dormancy is broken by an exposure to cold conditions, as I discussed in more detail in May. As also mentioned in that previous post, this is equally true of leaf and flower buds in biennial and perennial plants: in technical terms, the buds have to undergo a period of vernalisation (you will probably already know the word vernal which refers to things that happen in Spring such as the vernal equinox). Recent work by Prof Caroline Dean and Prof Martin Howard at the John Innes Centre in Norwich has started to unravel the mechanisms involved in vernalisation of flower buds. In autumn, the flower buds are dormant because the flowering process is held in check by the activity of a repressor gene. The activity of the repressor gene is sensitive to cold and so, during the winter, the gene is slowly switched off and eventually the genes that regulate flowering are able to work. OK then, plants have avoided leaf bud-burst or flowering at an inappropriate time but as Spring arrives, what is it that actually stimulates a tree to come into leaf or induces flowering in a biennial or perennial plant? Spring is characterised by several changes in a plant’s environment but the two most important are the increasing daytime temperature and the steady, day-by-day increase in daylength. In respect of temperature, it is clear that it is the major trigger for Spring-flowering plants. It is often said that Spring comes much earlier than it used to (even though, astronomically, the date of the equinox remains unchanged!). That observation was one of the catalysts for my writing these two blog posts and it has now been borne out by the data. In a recent research project carried out at Cambridge, on the effects of climate change, it was shown that in a range of 406 Spring-flowering trees, shrubs and non-woody plants, flowering now occurs a month earlier than it did in the mid-1980s (1). This ties in with my memories of Spring in Cambridge (I hope you’ll excuse a bit of reminiscing): when I was a student, the banks of the Cam were decorated with crocus flowers at the end of the Lent term, before we went home for Easter; when we came back for the summer term, it was daffodils that dominated the same banks. Now, the crocuses flower in the middle of the Lent term and the daffodils are in bloom at the end of that term.  The mathematical bridge at Queens’ College, Cambridge. This picture was taken in high summer but in spring, the displays of flowers (crocuses and daffodils) on the bank beyond the bridge have been good indicators of the earlier flowering dates referred to in the text. Credit: John Bryant. I will return to the induction of flowering later but now want to think about trees and shrubs coming into leaf. The situation is nicely illustrated by the old saying about oak (Quercus robur & Quercus petrea) and ash (Fraxinus excelsior): ‘Ash before Oak, we’re in for a soak; Oak before Ash, we’re in for a splash’. In colder, wetter Springs, oak budburst was delayed in comparison to ash and, in thinking about the weather, a cold wet Spring was believed to presage a wet summer (a ‘soak’). The folklore illustrates that budburst in oak is temperature-dependent. But what about ash? Its coming into leaf occurs at more or less the same time each year because the main trigger is increasing day-length. Thus, plants (or at least ash trees) have a light detection mechanism that can in some way measure the length of the light period. I need to add that because of climate change, these days, oak is nearly always before ash in respect of leaf production.
Going back to the nineteenth century, Darwin’s experiments on the effects of unilateral illumination clearly showed that plants bent towards the light because of differences in growth rate between the illuminated and non-illuminated side (2). This phenomenon is known as phototropism and shows that light can affect plant growth in a way not directly connected with photosynthesis. This added to previously established knowledge that plants grown in the dark or in very deep shade grew tall and spindly (‘etiolated’) and made little or no chlorophyll. Transfer of etiolated plants into the light slowed down the vertical growth rate and also led to the synthesis of chlorophyll, again showing that light can affect plant growth and development.  The main building at the Beltsville Agricultural Research Centre, Beltsville, Maryland, USA (I have visited the Centre and can reliably state that it, and the work done therein, are as impressive as it looks!). Credit: USDA/ARS.  Stick and ribbon representation of the structure of phytochrome. Credit: Yang, X., et al (2009) (3). These phenomena, and many others, lead us to think that plants must possess light receptors which are able to transduce the perception – and even quantification – of light into effects on growth. Further, these days we would say the effects on growth indicate effects on the expression of genes that control growth. The role of chlorophyll as a photo-reactive molecule, active in photosynthesis, was well known but the effects I am describing cannot be ascribed to chlorophyll since they can occur in its absence. The first of these non-chlorophyll photo-reactive molecules was discovered at the famous Beltsville Agricultural Research Centre in Beltsville, Maryland, USA where Sterling Hendricks and Harry Borthwick showed that red light was particularly effective in promoting several light-dependent developmental processes and that this promotion was reversed by far-red light. They proposed that plants contained a photo-reversible light-detecting molecule that was responsible for transduction of the perception of light into effects on growth and development. Cynics named this as yet unknown light receptor a ‘pigment of our imagination’ but Hendricks and Borthwick were proved right in 1959, when a pigment which had the predicted properties was identified by Warren Butler and Harold Siegelman. Butler called the pigment phytochrome which simply means plant colour or plant pigment. Over subsequent decades it has become clear that plants possess several subtly different variants of phytochrome, each with a specific role and there is no doubt that these are major regulators of the effects of light on plant growth and development. However, as research progressed, it became apparent that not all the effects of light could be attributed to photoreception in the red/far-red region of the spectrum. There must be others, particularly sensitive to light at the blue end of the spectrum (as Charles Darwin had suggested in the 1880s!). At the time of detailed analysis of the effects of blue light, the receptors were unknown – and hence given the name cryptochrome – hidden colour/pigment. Three cryptochrome proteins were eventually identified in the early 1980s. And there’s more! The overall situation is summarised in the diagram which is taken from a paper by Inyup Paik and Enamul Huq (4). It is clear that plants are able to respond to variations in light quality and intensity right across the spectrum. They cannot move away from their light environment but have evolved mechanisms with which to respond to it.  Light receptors in plants. That brings me back to flowering. While there is no doubt that many spring-flowering plants are responsive mainly to ambient temperature (as described earlier) and are thus neutral with regard to day-length, there are many plants which have specific day-length requirements. These are typified by summer-flowering plants such as sunflower (Helianthus annus) and snapdragon (Antirrhinum) which need n days in which daylight is longer than 12 hours (n differs between different species of long-day plants). Similarly, plants that flower in late summer or autumn, such as Chrysanthemum require n days in which there are fewer than x hours of daylight. Note that x may be above 12 hours but the key requirement is that days are shortening.  Flower of Sunflower (Helianthus annuus) accompanied by a White-tailed Bumblebee and a Red Admiral butterfly. Credit: John Bryant. So, the next time you are outside and thinking about your light environment (will it be sunny or cloudy?), just stop to ponder about the marvellous light response mechanisms that are happening in the plants all around you. John Bryant Topsham, Devon July 2024  PS: For those who want to read more about plant function and development, this book has been highly recommended! (1) Ulf Büntgen et al (2022), Plants in the UK flower a month earlier under recent warming
(2) As published in his 1880 book The Power of Movement in Plants. (3) Yang, X., et al (2009). (4) Inyup Paik and Enamul Huq (2019), Plant photoreceptors: Multi-functional sensory proteins and their signalling networks.
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