William Hoch, formerly of UW-Madison, Horticulture Dept.
Item number: XHT1019
The bright yellow, orange and red colors of autumn leaves are one of nature’s most beautiful spectacles. Although a familiar annual event, much of our understanding of autumn leaf coloration has, until recently, been based primarily on random observation and speculation. Recent research has done much to explain the functions of this phenomenon.
During summer, the majority of a plant’s nutrients are contained within the leaves. These nutrients, in particular nitrogen and phosphorus, are components of proteins, pigments and other compounds necessary for photosynthesis, the process by which plants use the sun’s energy to manufacture sugars from water and carbon dioxide. These nutrients are valuable resources in nature, and the amount of nutrients a plant possesses directly affects the plant’s capacity to grow and reproduce. Thus, it is important for plants to recover as many of these nutrients as possible before the leaves are discarded in autumn.
The shortening days and cooler temperatures of autumn signal plants to begin a process by which a leaf’s photosynthetic components are disassembled and the nutrients contained within them are moved to stems and roots for storage and re-use the following year. The disassembly of leaf components in autumn is highly organized and complex. However, much of this process takes place before any visible color change in leaves, and thus this surge of activity goes largely unnoticed. It is only during the latter stages of this process that the showy colors of autumn appear.
Chlorophyll is the pigment that gives plants their green color and is used by plants to capture the sun’s energy. Throughout autumn, plants are actively breaking down chlorophyll along with the many other photosynthetic components, and as levels of chlorophyll decline, the brightly colored pigments we associate with autumn leaves become visible. There are two types of pigments that give leaves their bright autumn colors: carotenoids and anthocyanins.
Carotenoids are a class of pigments responsible for the bright yellow colors of fall leaves. Carotenoids are present in leaves throughout the growing season, and are visible only when chlorophyll levels become very low in autumn. These pigments protect leaves from a variety of harmful compounds that are normal by-products of photosynthesis. Without carotenoids, leaf photosynthetic components literally destroy themselves.
Anthocyanins are responsible for many of the reds, oranges, pinks, purples and blues in flowers and fruits, but also produce the brilliant red and orange hues of autumn foliage.
Unlike the yellow carotenoids, anthocyanins are made by plants midway through the autumn disassembly process, when a significant amount of chlorophyll is still present within the leaves. At first, the presence of both the red pigments and chlorophyll results in a dark burgundy appearance, but the brightly colored anthocyanins become increasingly visible as chlorophyll levels continue to decline. Red leaves contain similar amounts of carotenoids as yellow leaves, but in red leaves, the yellow carotenoids are masked by the intensely colored anthocyanins.
Red pigments are produced during autumn to help shade leaf photosynthetic systems from bright sunlight. In many plants, the disassembly of leaf components in fall results in susceptibility to damage from bright light, particularly as temperatures decline. Protection from bright light during autumn is important because damage to the photosynthetic components during this time will reduce a plant’s capacity to recover nutrients from leaves. Plants that do not turn red in autumn are generally more resistant to the effects of bright light during this time, and therefore do not need to produce red pigments. The shading function of the red pigments explains why leaves exposed to direct sunlight are the brightest red, while leaves shaded within the canopy of a plant often show little or no accumulation of these pigments.
While many factors can affect the quality and duration of autumn leaf colors, the single most important factor influencing this phenomenon is the weather during the time that leaves are changing color. Cool, sunny weather will result in the brightest foliage colors. Because plants use the red pigments to shade leaves from bright light, sunny autumn days will result in plants producing more of these pigments.
While cool autumn weather is best for the development of brightly colored foliage, hard freezes may irreparably damage leaves before the disassembly process is complete and the brightest colors have emerged. Conversely, very warm autumn temperatures accelerate the processes within leaves and thus shorten the length of time that the colorful leaves remain on plants. Warm temperatures will also reduce the amount of red pigments produced in leaves.
Other factors can also affect leaf color change in autumn. In general, stresses such as drought, nutrient deficiency and even diseases can increase the production of red pigments, and therefore may improve the color display of autumn leaves. Drought is a common stress in many environments, and unless the water deficiency is so severe that leaves are killed before autumn (a very rare occurrence), drought conditions will generally improve the quality of fall leaf colors.
For more information on leaf color change in autumn: Contact your county Extension agent.
William Hoch, zamani wa UW-Madison, Idara ya Kilimo cha Bustani. XHTML 1019: Rangi za manjano, machungwa na nyekundu za majani ya vuli ni moja ya miujiza maridadi zaidi ya asili. Ingawa ni tukio la kila mwaka, mengi ya ufahamu wetu kuhusu rangi ya majani ya vuli, hadi hivi karibuni, umejengwa hasa juu ya uchunguzi wa nasibu na uvumi. Utafiti wa hivi karibuni umefanya mengi kueleza kazi ya jambo hilo. Wakati wa kiangazi, virutubisho vingi vya mmea viko ndani ya majani. Vyakula vinavyoweza kuhifadhi nishati ya jua, hasa nitrojeni na fosforasi, ni sehemu ya protini, rangi na misombo mingine muhimu kwa photosynthesis, mchakato ambao mimea hutumia nishati ya jua kutengeneza sukari kutoka maji na kaboni dioksidi. Vyakula vina uwezo mkubwa wa kukuza na kuzaa, na kiasi cha virutubisho vinavyopatikana katika mimea huathiri uwezo wa mimea kukua na kuzaa. Kwa hiyo, ni muhimu kwa mimea kupata tena kiasi kikubwa cha virutubisho hivyo kabla ya majani kuondolewa wakati wa vuli. Siku za kupungua na joto la joto la majira ya kupukutika kwa majani huonyesha kwamba mimea inaanza mchakato ambao vipengele vya photosynthetic vya jani huondolewa na virutubisho vilivyo ndani yake huhamishwa kwenye shina na mizizi kwa ajili ya kuhifadhi na kutumia tena mwaka uliofuata. Uvunjaji wa vipengele vya majani katika vuli ni wenye kupangwa vizuri na tata. Hata hivyo, sehemu kubwa ya mchakato huu hufanyika kabla ya mabadiliko yoyote ya rangi ya majani, na hivyo ongezeko hili la shughuli huonekana kwa kiasi kikubwa. Ni katika hatua za mwisho tu za mchakato huo ambapo rangi zenye kuvutia za vuli huonekana. Chlorophyll ni rangi ambayo huwapa mimea rangi yao ya kijani na hutumiwa na mimea kukamata nishati ya jua. Katika majira ya kupukutika kwa majani, mimea huvunja klorofili na sehemu nyingine nyingi za photosynthetic, na kama viwango vya klorofili kupungua, rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi. Kuna aina mbili za rangi zinazotoa majani rangi nyangavu za vuli: karotenoidi na anthocyanins. Carotenoids ni aina ya rangi zinazosababisha rangi ya manjano ya majani ya vuli. Carotenoids zipo katika majani wakati wote wa msimu wa kukua, na zinaonekana tu wakati viwango vya chlorophyll inakuwa chini sana katika vuli. Rangi hizo hulinda majani dhidi ya kemikali mbalimbali zenye kudhuru ambazo hutokezwa na fotosintesisi. Bila karotenoidi, vipengele vya majani vinavyofanyiza mwangaza hujiangamiza vyenyewe. Anthocyanins ni wajibu kwa ajili ya wengi wa nyekundu, machungwa, pink, zambarau na bluu katika maua na matunda, lakini pia kuzalisha rangi nyekundu na machungwa ya majani ya vuli. Tofauti na carotenoids ya manjano, anthocyanins hutengenezwa na mimea katikati ya mchakato wa kuondoa majani, wakati kiasi kikubwa cha klorofili bado kinapatikana ndani ya majani. Mwanzoni, uwepo wa rangi nyekundu na klorofili hufanya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi. Majani ya rangi nyekundu yana kiasi sawa cha carotenoids kama majani ya manjano, lakini katika majani ya rangi nyekundu, carotenoids ya manjano hufichwa na anthocyanins yenye rangi kali. Rangi nyekundu hutokezwa wakati wa vuli ili kusaidia kufunika mifumo ya photosynthetic ya majani kutoka kwa mwangaza mkali wa jua. Katika mimea mingi, kuvunjika kwa vipengele vya majani wakati wa vuli husababisha uwezekano wa kuharibiwa na mwangaza mkali, hasa wakati joto linapopungua. Ulinzi kutoka kwa mwanga mkali wakati wa vuli ni muhimu kwa sababu uharibifu wa vipengele vya photosynthetic wakati huu utapunguza uwezo wa mmea wa kupata virutubisho kutoka kwa majani. Mimea ambayo haibadiliki rangi nyekundu wakati wa vuli kwa ujumla ni sugu zaidi kwa athari za mwanga mkali wakati huu, na kwa hivyo haihitaji kutengeneza rangi nyekundu. Kwa sababu ya rangi nyekundu, majani yaliyo chini ya jua huwa na rangi nyekundu zaidi, lakini majani yaliyo chini ya jua huwa na rangi nyekundu kidogo. Ingawa kuna mambo mengi yanayoweza kuathiri ubora na muda wa rangi ya majani ya vuli, jambo moja muhimu zaidi linaloathiri jambo hili ni hali ya hewa wakati majani yanabadilika rangi. Hali ya hewa yenye baridi na yenye jua itasababisha rangi nyangavu zaidi za majani. Kwa sababu mimea hutumia rangi nyekundu kufunika majani kutokana na mwangaza mkali, siku zenye jua za vuli zitafanya mimea itokeze rangi nyingi zaidi. Wakati hali ya hewa ya majira ya kupumzika ni bora kwa ajili ya maendeleo ya majani ya rangi mkali, baridi kali inaweza irreparably kuharibu majani kabla ya mchakato disassembly ni kamili na rangi mkali zaidi wametokea. Kwa upande mwingine, joto kali sana la majira ya kupukutika kwa majani huongeza mwendo wa mambo yanayotokea ndani ya majani na hivyo kupunguza muda ambao majani yenye rangi nyingi hubaki kwenye mimea. Joto kali pia litapunguza kiasi cha rangi nyekundu zinazotokezwa katika majani. Mambo mengine pia yanaweza kuathiri mabadiliko ya rangi ya majani wakati wa vuli. Kwa ujumla, mvua, ukosefu wa virutubisho, na hata magonjwa yanaweza kuongeza uzalishaji wa rangi nyekundu, na hivyo kuboresha maonyesho ya rangi ya majani ya vuli. Ukame ni shida ya kawaida katika mazingira mengi, na isipokuwa upungufu wa maji ni mbaya sana kwamba majani huuawa kabla ya vuli (kiwango cha nadra sana), hali ya ukame kwa ujumla itaboresha ubora wa rangi za majani ya vuli. Kwa habari zaidi juu ya mabadiliko ya rangi ya majani katika vuli: Wasiliana na wakala wako wa Upanuzi wa kaunti.