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The Role of Betacyanins in Plant Salt Tolerance
<p>Soil salinity is a major threat to future food stability. Almost 20% of irrigated land is currently too saline to grow traditional crops. Moreover, rising sea levels, scarcity of fresh water, and more intense and prolonged periods of drought are exacerbating the problem. Saline soils severely reduce yields of most crop plants. By contrast, halophytes, which naturally thrive on saline substrates, have a variety of mechanisms to tolerate both the osmotic and cytotoxic components of salt stress. There has been concerted scientific effort worldwide to understand these mechanisms, and to introduce genes that may increase salinity tolerance in crop plants. Many halophytes in the Caryophyllales are pigmented red owing to a tyrosine-derived alkaloid called betacyanin. Recent studies using Disphyma australe, a succulent halophyte common on coastal dunes and rocky outcrops throughout New Zealand, have indicated a role for betacyanins in salinity tolerance. This thesis focuses on how the mechanism through which betacyanins might affect salt tolerance mechanisms in D. australe and whether the putative benefits of betacyanins on salt tolerance might be transferred to naturally non-betacyanic plants. Effects of betacyanin on Na+ distribution in salt-stressed leaves of red and green morphs of D. australe were studied using fluorescence microscopy, cryo-scanning electron microscopy with energy dispersive X-ray analysis, and atomic absorption spectrometry (AAS). In betacyanic leaves Na+ accumulated in the epidermis, while in green leaves Na+ was distributed more evenly across the epidermis and mesophyll. Both leaf types had similar numbers of salt glands, but salt secretion rates were higher in red than in green leaves. Betacyanic leaves under salt stress were able to maintain relatively high K+/ Na+ ratios, essential for many metabolic processes, while the leaves of green plants were not. Leaf sections stained with fluorescein diacetate and propidium iodide showed that mesophyll viability decreased significantly in green leaves under salt stress, while there was almost no decrease in mesophyll viability in the presence of betacyanins. Thus, betacyanic leaves might protect the photosynthetically active mesophyll from cytotoxic effects of Na+ by accumulating Na+ in the epidermis instead of the mesophyll. This in turn leads to more efficient salt secretion and higher K+/ Na+ ratios in the mesophyll, resulting in increased mesophyll viability under salt stress. Effects of high apoplastic sodium concentrations on ion flux kinetics in mesophyll tissue was studied using the non-invasive microelectrode ion flux estimation technique. Mesophyll cells of both betacyanic and green leaves showed a highly unusual K+ flux response; most crop plants leak K+ out of cells upon salt stress, but D. australe and the native Australian Disphyma crassifolium both showed K+ influx upon salt stress. Actively taking up K+ from the apoplast to maintain a high cytosolic K+/ Na+ ratio during salt stress might be an entirely new mechanism to combat the cytotoxic stress component of salinity stress in these halophytes. The salt induced K+ uptake was dependent on the presence of Cl- and Cl- was also taken up into mesophyll cells upon salt stress. Taking up both cations and anions at the same time could avoid membrane depolarisation. Voltage-gated channels, which are involved in the salt induced K+ efflux in glycophytes, would not be activated and this could be a new mechanism to avoid a K+ leak during salt stress. To test whether the beneficial effect of betacyanin production on salt tolerance could be transferred to naturally non-betacyanic plants, transgenic betacyanin-over-expression (BtOE) mutants of Nicotiana tabacum were generated by our colleagues at Plant & Food Research Ltd. Betacyanins in leaf discs of N. tabacum were associated with decreased chlorophyll degradation upon high light and high salt stress. Additionally, the decline in maximum quantum efficiency of PSII after high light and salt treatment was significantly greater in green than in betacyanic leaves. Placing a polycarbonate filter with a similar absorption spectrum to betacyanin over green N. tabacum leaf discs had a similar effect to the presence of betacyanin. Thus, betacyanins probably have a photoprotective effect in N. tabacum, which is essential as both high light and salinity can impair photosynthesis. To assess if the salt tolerance enhancing effect of betacyanin production observed in the leaf discs also occurs in whole N. tabacum plants, the ability to recover from exposure to saturating light was assessed. Betacyanic plants were able to fully recover quicker after exposure to saturation light than green leaves. This research shows that the presence of betacyanins during salt stress correlates with an altered Na+ distribution in leaf tissues and a higher salt secretion rate, which contributed to higher mesophyll viability. Moreover, a completely new ion flux response to salt stress was observed in D. australe and D. crassifolium. The observed salt induced K+ uptake into the mesophyll cells during salt stress might be an entirely new mechanism, to maintain a high K+/ Na+ ratio in the cytosol and avoid the cytotoxic effects of Na+ in photosynthetically active tissue. The beneficial effects of betacyanins could also be transferred to non-betacyanic species, by introducing betacyanin production. These results strongly suggest that betacyanins play a role in salt tolerance in halophytes and might be a valuable resource in increasing the salt tolerance of naturally non-betacyanic crop plants.</p>
<p>Soil salinity is a major threat to future food stability. Almost 20% of irrigated land is currently too saline to grow traditional crops. Moreover, rising sea levels, scarcity of fresh water, and more intense and prolonged periods of drought are exacerbating the problem. Saline soils severely reduce yields of most crop plants. By contrast, halophytes, which naturally thrive on saline substrates, have a variety of mechanisms to tolerate both the osmotic and cytotoxic components of salt stress. There has been concerted scientific effort worldwide to understand these mechanisms, and to introduce genes that may increase salinity tolerance in crop plants. Many halophytes in the Caryophyllales are pigmented red owing to a tyrosine-derived alkaloid called betacyanin. Recent studies using Disphyma australe, a succulent halophyte common on coastal dunes and rocky outcrops throughout New Zealand, have indicated a role for betacyanins in salinity tolerance. This thesis focuses on how the mechanism through which betacyanins might affect salt tolerance mechanisms in D. australe and whether the putative benefits of betacyanins on salt tolerance might be transferred to naturally non-betacyanic plants. Effects of betacyanin on Na+ distribution in salt-stressed leaves of red and green morphs of D. australe were studied using fluorescence microscopy, cryo-scanning electron microscopy with energy dispersive X-ray analysis, and atomic absorption spectrometry (AAS). In betacyanic leaves Na+ accumulated in the epidermis, while in green leaves Na+ was distributed more evenly across the epidermis and mesophyll. Both leaf types had similar numbers of salt glands, but salt secretion rates were higher in red than in green leaves. Betacyanic leaves under salt stress were able to maintain relatively high K+/ Na+ ratios, essential for many metabolic processes, while the leaves of green plants were not. Leaf sections stained with fluorescein diacetate and propidium iodide showed that mesophyll viability decreased significantly in green leaves under salt stress, while there was almost no decrease in mesophyll viability in the presence of betacyanins. Thus, betacyanic leaves might protect the photosynthetically active mesophyll from cytotoxic effects of Na+ by accumulating Na+ in the epidermis instead of the mesophyll. This in turn leads to more efficient salt secretion and higher K+/ Na+ ratios in the mesophyll, resulting in increased mesophyll viability under salt stress. Effects of high apoplastic sodium concentrations on ion flux kinetics in mesophyll tissue was studied using the non-invasive microelectrode ion flux estimation technique. Mesophyll cells of both betacyanic and green leaves showed a highly unusual K+ flux response; most crop plants leak K+ out of cells upon salt stress, but D. australe and the native Australian Disphyma crassifolium both showed K+ influx upon salt stress. Actively taking up K+ from the apoplast to maintain a high cytosolic K+/ Na+ ratio during salt stress might be an entirely new mechanism to combat the cytotoxic stress component of salinity stress in these halophytes. The salt induced K+ uptake was dependent on the presence of Cl- and Cl- was also taken up into mesophyll cells upon salt stress. Taking up both cations and anions at the same time could avoid membrane depolarisation. Voltage-gated channels, which are involved in the salt induced K+ efflux in glycophytes, would not be activated and this could be a new mechanism to avoid a K+ leak during salt stress. To test whether the beneficial effect of betacyanin production on salt tolerance could be transferred to naturally non-betacyanic plants, transgenic betacyanin-over-expression (BtOE) mutants of Nicotiana tabacum were generated by our colleagues at Plant & Food Research Ltd. Betacyanins in leaf discs of N. tabacum were associated with decreased chlorophyll degradation upon high light and high salt stress. Additionally, the decline in maximum quantum efficiency of PSII after high light and salt treatment was significantly greater in green than in betacyanic leaves. Placing a polycarbonate filter with a similar absorption spectrum to betacyanin over green N. tabacum leaf discs had a similar effect to the presence of betacyanin. Thus, betacyanins probably have a photoprotective effect in N. tabacum, which is essential as both high light and salinity can impair photosynthesis. To assess if the salt tolerance enhancing effect of betacyanin production observed in the leaf discs also occurs in whole N. tabacum plants, the ability to recover from exposure to saturating light was assessed. Betacyanic plants were able to fully recover quicker after exposure to saturation light than green leaves. This research shows that the presence of betacyanins during salt stress correlates with an altered Na+ distribution in leaf tissues and a higher salt secretion rate, which contributed to higher mesophyll viability. Moreover, a completely new ion flux response to salt stress was observed in D. australe and D. crassifolium. The observed salt induced K+ uptake into the mesophyll cells during salt stress might be an entirely new mechanism, to maintain a high K+/ Na+ ratio in the cytosol and avoid the cytotoxic effects of Na+ in photosynthetically active tissue. The beneficial effects of betacyanins could also be transferred to non-betacyanic species, by introducing betacyanin production. These results strongly suggest that betacyanins play a role in salt tolerance in halophytes and might be a valuable resource in increasing the salt tolerance of naturally non-betacyanic crop plants.</p>
Abstract This article presents the results of research on the biochemical composition of green and yellowing leaves of 64-year-old bipedal ginkgo (Ginkgo biloba L.) seed and pollen trees growing in the Botanical Garden of the Academy of Sciences of Uzbekistan, introduced to Uzbekistan in the last century. Biochemical analyzes recorded the presence of 6 types of vitamins and 44 macro- and micronutrients in the leaves of the ginkgo tree. Vitamin C levels were found to be lower in the seed tree than in the pollen tree. The amount was 35.8 mg/% in the green leaves of the seed tree and 34.4 mg/% in the yellowed leaves. Ginkgo leaves contain important macro-and micronutrients such as Ca, Mg, K, Al, Fe, Cu, Mn, Zn, Mo, Co, I, Se, which are necessary for the vital activity of the human body and normal metabolism. The green leaves of the two-leafed ginkgo pollen tree contained 27577.288 mg/l of calcium, 11562.299 mg/l of potassium, the leaves of the seed tree 13912.903 mg/l of calcium and 7491.462 mg/l of potassium. At the same time, the green leaves of ginkgo contain 3073.807 mg/l – 7977.459 mg/l magnesium, 4353.72-5003.88 mg/l phosphorus, 501.073-515.343 mg/l sodium, 779.750 mg/l– the presence of silicon in the amount of 844.039 mg/l and iron in the amount of 373.023 mg/l – 655.148 mg/l was determined.
Field trials were carried out to test the effect of nitrogen fertilization on the growth, stalks and juice yield, sugar content and ethanol production of sweet sorghum. The experiments were carried out on winter 2013 in the Domonstration Farm of the Faculty of Agriculture, University of Khartoum, Shambat and Experimental Farm of the Soba Research Station for Reclamation of Saline and Sodic soils. The treatments comprised the addition of nitrogen fertilizer (urea) at three levels (0, 43 and 86 Kg N ha-1) and two sweet sorghum genotypes (Baladi and RSSS.11), arranged in randomized complete block design replicated 4 times. Results indicated that there were significant (P ≥ 0.05) differences between genotypes in stem diameter at Soba location, stalk height and leaves area at Shambat location, number of green leaves per plant, stalk juice and ethanol yield in both locations. With the genotype RSSS.11 generally superior over genotype Baladi. Nitrogen fertilization increased significantly (P ≥ 0.05) number of green leaves per plant and leaves area at Shambat location and plant height and juice and ethanol yield in both locations. With the Soba location recorded the highest values over Shambat location and 86 kg N ha-1 recorded the highest ethanol yield (255.6l h-1).
<p>Foliar betalainic plants are commonly found in dry and exposed environments such as deserts and sandbanks. This marginal habitat has led many researchers to hypothesise that foliar betalains provide tolerance to abiotic stressors such as strong light, drought, salinity and low temperatures. Among these abiotic stressors, soil salinity is a major problem for agriculture affecting approximately 20% of the irrigated lands worldwide. Betacyanins may provide functional significance to plants under salt stress although this has not been unequivocally demonstrated. The purpose of this thesis is to add knowledge of the various roles of foliar betacyanins in plants under salt stress. For that, a series of experiments were performed on Disphyma australe, which is a betacyanic halophyte with two distinct colour morphs in vegetative shoots. In chapter two, I aimed to find the effect of salinity stress on betacyanin pigmentation in D. australe and it was hypothesised that betacyanic morphs are physiologically more tolerant to salinity stress than acyanic morphs. Within a coastal population of red and green morphs of D. australe, betacyanin pigmentation in red morphs was a direct result of high salt and high light exposure. Betacyanic morphs were physiologically more tolerant to salt stress as they showed greater maximum CO₂ assimilation rates, water use efficiencies, photochemical quantum yields and photochemical quenching than acyanic morphs. Contrary to this, the green morphs, although possessing the ability to synthesise betalains in flower petals, did not produce betalains in vegetative shoots in response to salt stress. Moreover, green morphs, in terms of leaf photosynthesis, performed poorly under salinity stress. In chapter three I further investigated the physiological benefit of betacyanin accumulation in D. australe. I postulated that betacyanin in the leaves of D. australe can protect the salt stressed chloroplasts from harmful excessive light by absorbing significant amount of radiation. To test this, a novel experimental approach was used; the key biosynthetic step for betacyanin synthesis was identified, which was deficient in vegetative shoots of the green morphs. By supplying the product of this enzymatic reaction, L-DOPA, betacyanin synthesis could be induced in the leaves of green morphs. This model system was used to compare the photoprotective responses of red vs. green leaves. The L-DOPA induced betacyanic leaves showed similar responses (such as smaller reductions and faster recoveries of PSII and less H₂O₂ production than in the green leaves) to naturally betacyanic leaves when exposed to high light and salinity. The differences in photoinhibition between red and green leaves were attributed to the light absorbing properties of betacyanins. L-DOPA treated and naturally red leaves showed lower photoinactivation than green leaves when exposed to white or green light, although not when exposed to monochromatic (red) light. In chapter four, I used a similar experimental model to that in the third chapter and showed that other than photoprotection, betacyanins in leaves may be involved in salt tolerance by enhancing toxic ion (such as Na⁺) sequestration in betacyanic epidermal cells, storing Na⁺ away from sensitive mesophyll tissue. The Na⁺ localization between red and green leaves was compared after salinity treatment by using a sodium binding stain (SBFI-AM) and Cryo-SEM analysis. L-DOPA treated and natural red leaves sequestered Na⁺ ions to the epidermal cell layer. In contrast, green leaves retained Na⁺ in the mesophyll tissue, which suggested that red leaves were better equipped to tolerate salt-specific effects. Therefore, betacyanic plants were more tolerant to applied salinity stress and showed relatively higher growth rates than green morphs. The findings of this thesis provide a significant contribution to our understanding of the role of betacyanins in plants under salinity stress. My data suggest that the multi-faceted properties of betacyanins (such as their photoprotective function, and their involvement in sequestration of toxic ions) clearly provide a benefit to plants under salinity stress.</p>
<p>Foliar betalainic plants are commonly found in dry and exposed environments such as deserts and sandbanks. This marginal habitat has led many researchers to hypothesise that foliar betalains provide tolerance to abiotic stressors such as strong light, drought, salinity and low temperatures. Among these abiotic stressors, soil salinity is a major problem for agriculture affecting approximately 20% of the irrigated lands worldwide. Betacyanins may provide functional significance to plants under salt stress although this has not been unequivocally demonstrated. The purpose of this thesis is to add knowledge of the various roles of foliar betacyanins in plants under salt stress. For that, a series of experiments were performed on Disphyma australe, which is a betacyanic halophyte with two distinct colour morphs in vegetative shoots. In chapter two, I aimed to find the effect of salinity stress on betacyanin pigmentation in D. australe and it was hypothesised that betacyanic morphs are physiologically more tolerant to salinity stress than acyanic morphs. Within a coastal population of red and green morphs of D. australe, betacyanin pigmentation in red morphs was a direct result of high salt and high light exposure. Betacyanic morphs were physiologically more tolerant to salt stress as they showed greater maximum CO₂ assimilation rates, water use efficiencies, photochemical quantum yields and photochemical quenching than acyanic morphs. Contrary to this, the green morphs, although possessing the ability to synthesise betalains in flower petals, did not produce betalains in vegetative shoots in response to salt stress. Moreover, green morphs, in terms of leaf photosynthesis, performed poorly under salinity stress. In chapter three I further investigated the physiological benefit of betacyanin accumulation in D. australe. I postulated that betacyanin in the leaves of D. australe can protect the salt stressed chloroplasts from harmful excessive light by absorbing significant amount of radiation. To test this, a novel experimental approach was used; the key biosynthetic step for betacyanin synthesis was identified, which was deficient in vegetative shoots of the green morphs. By supplying the product of this enzymatic reaction, L-DOPA, betacyanin synthesis could be induced in the leaves of green morphs. This model system was used to compare the photoprotective responses of red vs. green leaves. The L-DOPA induced betacyanic leaves showed similar responses (such as smaller reductions and faster recoveries of PSII and less H₂O₂ production than in the green leaves) to naturally betacyanic leaves when exposed to high light and salinity. The differences in photoinhibition between red and green leaves were attributed to the light absorbing properties of betacyanins. L-DOPA treated and naturally red leaves showed lower photoinactivation than green leaves when exposed to white or green light, although not when exposed to monochromatic (red) light. In chapter four, I used a similar experimental model to that in the third chapter and showed that other than photoprotection, betacyanins in leaves may be involved in salt tolerance by enhancing toxic ion (such as Na⁺) sequestration in betacyanic epidermal cells, storing Na⁺ away from sensitive mesophyll tissue. The Na⁺ localization between red and green leaves was compared after salinity treatment by using a sodium binding stain (SBFI-AM) and Cryo-SEM analysis. L-DOPA treated and natural red leaves sequestered Na⁺ ions to the epidermal cell layer. In contrast, green leaves retained Na⁺ in the mesophyll tissue, which suggested that red leaves were better equipped to tolerate salt-specific effects. Therefore, betacyanic plants were more tolerant to applied salinity stress and showed relatively higher growth rates than green morphs. The findings of this thesis provide a significant contribution to our understanding of the role of betacyanins in plants under salinity stress. My data suggest that the multi-faceted properties of betacyanins (such as their photoprotective function, and their involvement in sequestration of toxic ions) clearly provide a benefit to plants under salinity stress.</p>
<p>Anthocyanin pigments are synthesised in the leaves of many plants, however the adaptive significance of these pigments is not entirely understood. It has been postulated that their red colours may function as visual signals through coevolution between herbivorous insects and their host tree species, though the hypothesis lacks solid empirical evidence. I investigated the leaf signalling hypothesis using Pseudowintera colorata, focusing on five areas: 1) I exploited the natural polymorphism in leaf colour of P. colorata to test the predictions that (i) bright leaf colour is a reliable signal of a plant’s defensive commitment; (ii) insects in the field avoid trees that are brightly coloured; and (iii) the trees with the brightest leaves will have higher fitness. Relative to green leaves, redder foliage contained higher concentrations of polygodial, a sesquiterpene dialdehyde known to have strong antifeedant properties, and incurred less insect feeding damage. Redder trees hosted fewer Ctenopseustis spp. leafroller larvae than neighbouring matched green trees. Contrary to the predictions of the leaf signalling hypothesis, there was no difference in any of the measured fitness parameters between red and green trees, indicating that the leaf colour polymorphism in P. colorata is stable. 2) Many insects are sensitive to volatile organic compounds (VOCs), however the role of VOCs in plant-herbivore signalling has not been investigated. I analysed VOCs released from undamaged, herbivore- and mechanically-damaged red and green leaves of P. colorata, and the olfactory preferences of brownheaded leafroller (C. obliquana) larvae. While the VOC profiles of browsed and unbrowsed leaves were statistically distinguishable, the VOC profiles released from intact, herbivore-, and mechanically-damaged P. colorata leaves did not reliably identify leaf colour. Moreover, naïve and experienced C. obliquana larvae displayed no preference for the volatiles from mechanically-damaged red or green leaves. Therefore, I concluded that VOC compounds are not likely to play a large role in mediating insect herbivore-plant interactions in P. colorata. 3) Studies of leaf signalling rarely consider the influence of the light-absorbing properties of non-green pigments upon photosynthesis. I compared the photosynthetic and photoinhibitory responses of red and green leaves from matched, neighbouring pairs of P. colorata of contrasting colour. Redder P. colorata leaves in the field had a lower maximum photosynthetic assimilation rate than matched green leaves from neighbouring trees. However, I was unable to detect any measurable advantage in terms of photoprotection in the red P. colorata leaves as indicated by chlorophyll fluorescence profiles. My results indicate that the presence of anthocyanin pigments within non-senescing leaves may impose a slight photosynthetic cost to the plant. 4) I used literature searches, field surveys and laboratory bioassays to identify which invertebrate herbivores are most likely to participate in leaf-signalling interactions with P. colorata. Feeding preference bioassays showed that brownheaded leafrollers (C. obliquana and C. herana) and Auckland tree weta (Hemideina thoracica) preferentially consumed leaf material from green than red P. colorata leaves. Results from these bioassays, combined with my field surveys suggest that Ctenopseustis spp. leafroller larvae are the most likely coevolution partners for P. colorata. 5) There is a well-established link between nitrogen deficiency and leaf reddening. Additionally, leaf nutrients can influence foraging behaviour and performance of insect herbivores. I measured N and C contents of leaves from neighbouring matched pairs of red and green P. colorata. There were no significant differences in the amounts of, or ratio between, N and C between matched red and green leaves. This result indicates that differences in colour and herbivory among P. colorata leaves are not attributable to differences in leaf nutrients. Taken together, my results suggest that foliar anthocyanins in P. colorata do function as visual signals, however their effect on herbivory is small. Additionally, interindividual variation in non-senescing leaf colour in P. colorata may be stable due to a trade off between signalling and photosynthesis. Discussions of leaf signalling need to follow the examples of other fields studying the interactions between plants and insects and move from overly simple models to those that incorporate more of the complexity that is observed in the natural world.</p>
<p>Anthocyanin pigments are synthesised in the leaves of many plants, however the adaptive significance of these pigments is not entirely understood. It has been postulated that their red colours may function as visual signals through coevolution between herbivorous insects and their host tree species, though the hypothesis lacks solid empirical evidence. I investigated the leaf signalling hypothesis using Pseudowintera colorata, focusing on five areas: 1) I exploited the natural polymorphism in leaf colour of P. colorata to test the predictions that (i) bright leaf colour is a reliable signal of a plant’s defensive commitment; (ii) insects in the field avoid trees that are brightly coloured; and (iii) the trees with the brightest leaves will have higher fitness. Relative to green leaves, redder foliage contained higher concentrations of polygodial, a sesquiterpene dialdehyde known to have strong antifeedant properties, and incurred less insect feeding damage. Redder trees hosted fewer Ctenopseustis spp. leafroller larvae than neighbouring matched green trees. Contrary to the predictions of the leaf signalling hypothesis, there was no difference in any of the measured fitness parameters between red and green trees, indicating that the leaf colour polymorphism in P. colorata is stable. 2) Many insects are sensitive to volatile organic compounds (VOCs), however the role of VOCs in plant-herbivore signalling has not been investigated. I analysed VOCs released from undamaged, herbivore- and mechanically-damaged red and green leaves of P. colorata, and the olfactory preferences of brownheaded leafroller (C. obliquana) larvae. While the VOC profiles of browsed and unbrowsed leaves were statistically distinguishable, the VOC profiles released from intact, herbivore-, and mechanically-damaged P. colorata leaves did not reliably identify leaf colour. Moreover, naïve and experienced C. obliquana larvae displayed no preference for the volatiles from mechanically-damaged red or green leaves. Therefore, I concluded that VOC compounds are not likely to play a large role in mediating insect herbivore-plant interactions in P. colorata. 3) Studies of leaf signalling rarely consider the influence of the light-absorbing properties of non-green pigments upon photosynthesis. I compared the photosynthetic and photoinhibitory responses of red and green leaves from matched, neighbouring pairs of P. colorata of contrasting colour. Redder P. colorata leaves in the field had a lower maximum photosynthetic assimilation rate than matched green leaves from neighbouring trees. However, I was unable to detect any measurable advantage in terms of photoprotection in the red P. colorata leaves as indicated by chlorophyll fluorescence profiles. My results indicate that the presence of anthocyanin pigments within non-senescing leaves may impose a slight photosynthetic cost to the plant. 4) I used literature searches, field surveys and laboratory bioassays to identify which invertebrate herbivores are most likely to participate in leaf-signalling interactions with P. colorata. Feeding preference bioassays showed that brownheaded leafrollers (C. obliquana and C. herana) and Auckland tree weta (Hemideina thoracica) preferentially consumed leaf material from green than red P. colorata leaves. Results from these bioassays, combined with my field surveys suggest that Ctenopseustis spp. leafroller larvae are the most likely coevolution partners for P. colorata. 5) There is a well-established link between nitrogen deficiency and leaf reddening. Additionally, leaf nutrients can influence foraging behaviour and performance of insect herbivores. I measured N and C contents of leaves from neighbouring matched pairs of red and green P. colorata. There were no significant differences in the amounts of, or ratio between, N and C between matched red and green leaves. This result indicates that differences in colour and herbivory among P. colorata leaves are not attributable to differences in leaf nutrients. Taken together, my results suggest that foliar anthocyanins in P. colorata do function as visual signals, however their effect on herbivory is small. Additionally, interindividual variation in non-senescing leaf colour in P. colorata may be stable due to a trade off between signalling and photosynthesis. Discussions of leaf signalling need to follow the examples of other fields studying the interactions between plants and insects and move from overly simple models to those that incorporate more of the complexity that is observed in the natural world.</p>
Abstract Purpose Sulfur (S) deposition as a global change issue causes worldwide soil acidification, nutrient mobilization and marked changes in plant nutrition. Here, we investigated how S deposition would affect leaf nutrient resorption and how this effect varies with yearly fluctuations in precipitation. Methods In a semiarid meadow exposed to S addition, we measured nitrogen (N), phosphorus (P) and S concentrations in green and senescent leaves of a grass and a sedge and calculated nutrient resorption efficiencies (NuRE) across two years with contrasting precipitation (13% higher and 27% lower than long-term mean annual precipitation). Results Concentrations of N, P, and S in green and senescent leaves generally increased with S addition across the two years, with the exception of N and P concentrations in green leaves of the grass that showed no response or even decreased with S addition. The coupling relationships between N and P concentrations showed interannual variations and tightened by nutrient resorption, as evidenced by stronger N and P correlations in senescent leaves than in green leaves in the wet year. Leaf NuRE convergently decreased with S addition across the two years congruent with soil acidification and increased soil N, P and S availability, while NuRE was higher in the wet year due to lower soil nutrient availability herein. Conclusions This study provides new evidence on the role of nutrient resorption in tightening stoichiometric N:P relationships, and a three-dimensional feedback framework that plant nutrient resorption was favored by higher precipitation to sharpen its tradeoff with soil nutrient availability.
