Leaf age and seasonal effects on light, water, and nitrogen use efficiency in a California shrub

Oecologia 9 Springer-Verlag 1983 Oecologia (Berlin) (1983) 56:348-355 Leaf Age and Seasonal Effects on Light, Water, and Nitrogen Use Efficiency in a California Shrub C. Field* and H.A. Mooney Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA Summary. Photosynthetic capacity, leaf nitrogen content, and stomatal conductance decreased with increasing leaf age in the chaparral shrub, Lepechinia calycina, growing in its natural habitat. Efficiency of resource use for three resources that potentially limit photosynthesis did not, however, decrease with increasing leaf age. Light-use efficiency, given by the quantum yield of photosynthesis at low light intensities, was unaffected by leaf aging but decreased slightly through the winter and spring growing season. Water-use efficiency, the ratio of photosynthesis to transpiration at light saturation and with a constant water vapor concentration gradient, was not affected by leaf aging or seasonal change. Nitrogen-use efficiency, the ratio of photosynthesis at light saturation to leaf nitrogen content did not change with leaf age but was lower in the leaves with the highest specific weights. This ensemble of leaf-age effects is consistent with the hypothesis that aging represents resource redistribution and not uncontrolled deterioration. Introduction Photosynthetic capacity is strongly influenced by leaf age. Every leaf has a life history in which it passes from net resource importer to net resource exporter and sometimes back. The long-term carbon gain of any plant is as sensitive to variation in leaf life history, as it is to variation in maximum photosynthetic capacity (Mooney and Gulmon 1982). While the physiology of leaf aging has been intensively studied, its adaptive consequences are not well understood. Leopold (1961) postulated a number of advantages of leaf senescence, but these address the issue of whether leaves should be maintained indefinitely. Identifying the range of selective forces that have shaped leaf aging at ages well below the age of abscission requires not only a description of the aging process, but also an examination of its consequences. Leaf aging is generally characterized by predictable changes in photosynthetic capacity, which increases rapidly until near the age of full leaf expansion and eventually begins a long decline (Woolhouse 1967). The decline in photosynthetic capacity is correlated with decreases in stomatal * Present address: Dept. of Biology, University of Utah, Salt Lake City, Utah 84112, USA Offprint requests to: C. Field 0029-8549/83/0056/0348/$01.60 conductance (Davis and McCree 1978), leaf-nitrogen concentration (Osman and Milthorpe 1971), and the activity of several photosynthetic enzymes, including ribulose-l,5bisphosphate carboxylase (Friedrich and Huffaker 1980). Physiological aspects of the leaf aging schedule have been shown to be under precise control of the cell nucleus, acting in response to environmental, hormonal, and innate signals (Thomas and Stoddart 1980). Most studies of the physiology of leaf aging have been neutral with respect to identifying advancing age as deterioration. Parallels between the phenomena of aging in leaves and aging in whole organisms suggest that leaf aging is deterioration. However, the arguments proposed to account for the evolution of senescence in organisms (Medawar 1952; Hamilton 1966; Kirkwood and Holliday 1979)have little relevance to aging in parts of an organism. In a rectapopulation such as the leaves on a single plant (White 1979), decreases in photosynthetic capacity could be manifestations of some inevitable deterioration or they could be the result of reallocating the resources that limit photosynthesis. The present study was undertaken to resolve leaf aging into components due to deterioration and components due to the redistribution of light, water, and nitrogen, three resources required for plant growth. The operational definition of deterioration is a decline, after the age of full leaf expansion, in intrinsic resource-use efficiency, potential photosynthetic carbon gain per unit of invested resource. Differences in the functional roles of light, water, and nitrogen in carbon gain require that resource-use efficiency be defined somewhat differently for each resource. The interception of light and the evaporation of water occur as fluxes that can be considered in the same time frame as photosynthetic CO 2 fluxes. Because nitrogen is not consumed or lost during photosynthesis, steady-state concentrations of leaf nitrogen are a useful index of investment. To center the analysis on intrinsic, rather than realized, resource-use efficiency, we will consider only photosynthesis under controlled levels of light, temperature, humidity, and CO2. Leaf aging in nature logically concerns two separate phenomena, the characteristics of leaves as they age (chronological aging) and the properties of leaves of different ages, at a given time (age-structure dynamics). In complex natural environments, neither chronological aging nor the agestructure dynamics provides a complete picture of the ecologically relevant aspects of leaf aging. The age-structure dynamics are useful because they allow comparisons among 349 leaves of different ages under circumstances where all leaves are in similar environments and subject to common resource constraints. Chronological aging is useful for documenting the life histories of single leaves and for examining the interface between leaf aging and acclimation to seasonal change. This paper describes both forms of leaf aging in a California shrub, Lepechinia calycina (Benth.) Epl. in its natural environment, with emphasis on light-, water-, nitrogen-use efficiency. The approach to evaluating efficiencies will be to first present seasonal and age-related changes in carbon gain and resource investment and to later integrate these components into an analysis of intrinsic resource-use efficiency. Materials and Methods Lepechinia calycina is a facultatively drought-deciduous shrub of the California chaparral. Leaf initiation typically begins with the arrival of winter rains in November or December, and leaf loss is completed during the summer drought, usually by early July. The canopy is relatively open, with leaves distributed throughout the canopy volume. In size, phenology, and commonly associated species, L. calycina is more similar to coastal sage species than it is to chaparral species. L. calycina occurs mainly on the margins of chaparral or in disturbed sites within the chaparral. In both types of sites, coastal sage species often precede chaparral species in a successional sequence (Mooney 1977). The study plants were naturally growing mature shrubs 1 to 1.5 m in height in a chaparral margin on Stanford University's Jasper Ridge Biological Preserve (San Mateo County, California, Lat. 37 ~ 24' N, Long. 122 ~ 13' W). The study site is on a gentle north-facing slope with oak woodland upslope and chaparral downslope. Soil at the site is a gravelly greenstone of the Franciscan series (Page and Tabor 1967). Field measurements were made between August of 1979 and April of 1981. The phenological status of 10 shrubs was assessed every two weeks. To document leaf ages, colored plastic bands were slipped over leaf pairs soon after initiation and locked in place by later leaf expansion (Bazzaz and Harper 1977). Gas exchange characteristics were measured for single, attached leaves using the portable, steady state, humidityand temperature-controlled gas exchange system described by Field et al. (1982). For all measurements, the gas exchange system was used in null-balance mode, with transpiration providing the cuvette humidity, and with photosynthesis balanced by the addition of 1% CO2 in N 2. During measurements, leaves were illuminated with a 300 W coolbeam lamp (General Electric Co., Cleveland, Ohio), and intensity was adjusted with neutral density screens. The gas exchange experiments were designed to allow us to document, and discriminate between, effects due to leaf age and effects due to seasonal change, and to allow us to characterize the interactions between effects of aging and effects of seasonal change. Our basic approach was to examine the gas exchange responses of a number of leaves of known ages at 4 points in the growing season and to use linear regression and multiple linear regression techniques to identify patterns related to either leaf age or sample date. The 4 sample dates, each separated by approximately 40 days, were in late January (early winter), early March (late winter), mid April (early spring), and late May (late spring) of 1980 with some replications in 1981. During each sampling period, we measured photosynthetic capacity at light saturation and light response curves of photosynthesis for at least 5 leaves of known age spanning the range from the youngest leaves larger than 4 cm 2 to the oldest leaves without large necrotic areas. The range of leaf ages at any sample period was limited by the range present on the study plants. The total sample size for measurements at light saturation was 44 leaves ranging in age from 15 to 123 days. The sample size for light response curves was 27 leaves ranging in age from 14 to 110 days. For all these measurements, ambient CO2 concentration was held at 340_+ 20 gmol mol - 1, and the vapor concentration gradient was held at 15 _+2 mmol m o l - 1. Leaf temperatures were controlled at 20 _+0.3 ~ C for the winter measurements and 25_+0.3~ C for the spring measurements. The responses of photosynthesis and stomatal conductance to changes in the vapor concentration gradient were measured for 5 leaves ranging in age from 48 to 104 days. All gas exchange parameters were calculated as described by Field et al. (1982). After the determination of gas exchange characteristics, the individual leaves used for those measurements were harvested and dried at 55 ~ C. These leaves were then used to compute leaf specific weight and were analyzed for total organic nitrogen with an automated micro-Kjeldahl assay (Isaac and Johnson 1976) (Technicon; Tarrytown, New York). Results Phenology In dry sites L. calycina was completely deciduous during the summer drought but in moister sites retained a few small ( < 2 c m 2) green leaves during this period. With the arrival of winter rains, the thick, pubescent summer leaves expanded to 4 to 6 cm 2. Leaves initiated in late autumn began expanding at about the same time as the summer leaves. These leaves expanded slowly, requiring nearly 60 days to reach a final size of 6 to 12 c m 2. As the light intensity increased and temperatures became more favorable for growth, leaf expansion required only 30 to 40 days and final leaf size increased. Leaves maturing in early March ranged in size from 20 to 35 cm 2, with the largest leaves on shaded plants. By mid-April, the end of the rainy season and the beginning of flowering, leaves reached full expansion after about 40 days and attained maximum sizes of 12 to 18 c m 2. In wet sites, the larger leaves were produced later into the growing season. Leaf senescence occurred with two distinct phases. By mid-March, the leaves initiated in November or December were beginning to yellow and abscise. This gradual senescence and abscission of the oldest leaves occurred until early to mid-June, by which time most stems had lost 3 or 4 pairs of leaves. After that, the plants entered a second senescence schedule, marked by a rapid wave of senescence from the oldest to the youngest remaining leaves. The few leaves that remained green over the summer were at the terminals of the non-reproductive stems. During the growing season, the oldest leaves with any green surface area were typically near 110 days old. 350 at,o o ~ \ , ~ "4 T I I ~~ 9 * I L ~\ I ' I season had higher specific weights at all ages (panel to panel in Fig. 1), and leaves of all age classes increased in specific weight throughout their lifespans (vertical lines through Fig. 1). The total variation among leaf specific weights at any sampling period was small relative to the seasonal variation. 1 I~ t Leaf specific \1oo1weight \ / ,gin-2, ~ , Leaf Nitrogen .. r-anywmTer r "r , 0 40 80 Leafage , , ,t t2O 160 200 (days) Fig. 1. Seasonal and leaf-age variation in leaf specific weight. Plots for different seasons are offset such that any vertical line through all plots follows a cohort of leaves initiated on one date. The absence of very young leaves in the spring samples is a consequence of the cessation of leaf initiation _A .3- Photosynthetic Capacity =Car) t "T ] C3 E ::L .2 .~_ (~ (D t 0 ee~ The seasonal increase in leaf specific weight was paralleled by a decrease in leaf organic nitrogen content expressed as a proportion of leaf weight. Mean organic nitrogen content fell from 39.4 mg g- ~ in early winter to 14.3 mg g - ~ in late spring. That this seasonal decrease represented primarily a dilution effect due to increasing leaf specific weight is suggested by the absence of significant seasonal changes in nitrogen per unit of leaf area (Table 1). Over all four sampling periods, the area- and weight-based measures of leaf nitrogen decreased with increasing leaf age (Table 1). .I t z I dan I Feb I I Apr Mar May Date Fig. 2. Seasonal variation in net photosynthesis per unit of leaf weight at light saturation for all leaves at or beyond the age of full leaf expansion. Bars represent 1 standard error Leaf Specific Weight Leaf specific weight (leaf dry weight per unit of area) increased dramatically through the growing season (Fig. 1). Over leaves of all ages, average specific weight increased from 46 g m - 2 in early winter to 113 g m - 2 in late spring. The seasonal change in leaf specific weight was a consequence of two phenomena. Leaves initiated later in the Over leaves of all ages, net photosynthesis per unit of leaf weight at light saturation decreased as the season advanced. Mean values ranged from 0.24 gmol CO2 g-1 s-1 in the early winter to a late spring low of 0.09 ~tmol CO2 g - 1 s- 1 Excluding leaves below the age of full expansion or photosynthetic maturity exaggerates the seasonal decline (Fig. 2). Again combining data for leaves of all ages, photosynthetic capacity per unit of leaf area, however, did not change significantly as the season progressed (Table 2), implying that the seasonal decrease in photosynthetic capacity per unit of leaf weight was a dilution effect due to increasing leaf specific weight. A multiple linear regression reinforces this interpretation, revealing that leaf specific weight had a highly significant negative effect on photosynthetic capacity per unit of leaf weight while sample date had no significant effect (Table 2). For each of the seasonal samples, photosynthetic capacity per unit of leaf area was highest in leaves near the age of full expansion and lower in older and younger leaves (Fig. 3). While the relationships between photosynthetic capacity and leaf age at the late winter and early spring sampling periods express this general trend, the data sets for earlier and later sampling periods were somewhat truncated by the absence of very old or very young leaves at those dates. As a cohort of leaves aged (chronological aging), the changes in photosynthetic capacity per unit area (Fig. 4) paralleled the pattern observed for leaves of several ages at any single sampling period (age-structure dynamics). The Table 1. Summary of linear regressions relating leaf nitrogen content to leaf age, specific weight, and sample date Dependent variable Independent variable y-intercept Slope df r P< N/Wt N/Wt N/Ar N/Wt N/Ar Date LSW Date Age Age 49.5 44.7 2.14 43.6 2.82 - 0.231 -- 0.226 -- 0.003 - 0.260 -0.015 32 32 32 32 32 - 0.917 - 0.639 -- 0.144 - 0.707 -0.590 0.00t 0.001 ns 0.001 0.001 Abbreviations." N/Wt=leaf nitrogen per unit weight (rag g-1), N/Ar=leaf nitrogen per unit area (g m-Z), LSW=leaf specific weight (g m-2), Date = sampling date (days past 10 December), Age = leaf age (days) 351 Table 2. Summary of linear and multiple linear regressions relating photosynthetic capacity to leaf age and seasonal variables Dependent variable Independent variable (s) P/Wt P/Ar P/At P/Wt ~ y-intercept Date Date LSW Date LSW N/Wt N/Wt LSW N/Wt LSW Age N/Ar N/At N/Ar LSW P/Wt P/Wt a, b P/Wt a, b P/Ar P/Ar b P/At a, b df Slope 0.292 11.31 11.71 0.335 -0.001 0.001 --0.0039 -0.0003 -0.002 0.0046 0.0036 - 0.001 0.001 -0.002 - 0.0009 0.366 3.65 4.24 - 0.024 0.048 0.186 0.355 10.72 5.37 6.11 r 32 32 32 31 31 32 24 24 23 23 23 32 25 24 24 P< -0.728 0.021 -0.05 Multiple 0.001 ns ns ns 0.001 0.001 0.001 0.001 ns 0.001 0.01 ns 0.001 0.001 ns 0.678 0.119 0.617 r P< 0.876 0.01 0.942 0.0t 0.962 0.01 0.673 0.01 a Multiple linear regression b Regression based on leaves older than the age of full expansion Abbreviations: P/Wt = photosynthetic capacity per unit weight (gmol COz g- ~ s z), P/Ar =photosynthetic capacity per unit area (gmol CO 2 m -2 s-l) r Io , ~ 16 i i I 60 I 80 t~ Od Late spnng 'E ~ c.) -6 E ::L Iz .~_ 8 I x~:t ief', " ,""~" , r Lotewinter,, Q c/) 0 N;, photosy~hesis~ ;; ~ / ~ ' . "] Earlywinter ] r Q. 4 z 0 40 80 Leaf age 120 160 200 (days) Fig. 3. Seasonal and leaf-age variation in net photosynthesis per unit of leaf area at light saturation. Plots for different seasons are offset such that any vertical line through all plots follows a cohort of leaves initiated on one date seasonal increase in leaf specific weight exaggerated the decline in weight-based photosynthetic capacity associated with chronological aging past the age of full leaf expansion. 0 0 I 20 I 40 I I00 I 120 Leaf age (days) Fig. 4. Variation in photosynthetic capacity per unit of leaf area with leaf age for leaves initiated 9 January 1980 The quantum yield was not significantly affected by leaf age or photosynthetic capacity at light saturation but decreased slightly as the season progressed, from 0.049 in early winter to 0.041 in late spring (Table 3). The Quantum Yield of Photosynthesis and Light- Use Efficiency Stomatal Conductance and Water-Use Efficiency At high light intensities, light-use efficiency is sensitive to photosynthetic capacity and the light intensity required to saturate photosynthesis. A n intrinsic index of efficiency, comparable among leaves with a wide range of photosynthetic capacities, is the slope of the light response curve of photosynthesis at low light intensities. This slope, the quantum yield of photosynthesis, averaged 0.044 real CO2/ mol photosynthetically active radiation over all leaf ages and seasons, on the basis of incident p h o t o n irradiance. Under cuvette conditions, where leaf temperature, ambient CO2, boundary layer conductance, and the water vapor concentration gradient (A w) are the same for all leaves, both water-use efficiency and the calculated CO2 concentration in the intercellular spaces (CO2 internal) are linearly related to the ratio of photosynthesis to stomatal conductance. Water-use efficiency increases and CO2 internal decreases as the ratio of photosynthesis to conductance increases. 352 Table 3. Summary of linear regressions relating quantum yield to leaf age, sample date, and photosynthetic capacity Dependent variable Independent variable y-intercept Slope df r P< QY QY QY Age P/Ar Date 0.048 0.040 0.051 - 0.0001 0.0004 - 0.00006 21 21 23 - 0.278 0.127 - 0.445 ns ns 0.05 Abbreviations." QY = quantum yield of photosynthesis (mol COz/mol photosynthetically active radiation) Table 4. Summary of linear regressions relating conductance and water use to leaf age and seasonal variables Dependent variable Independent variable y-intercept Slope df r P< cond cond WUE" WUE Date P/Ar Age Date Age 0.212 0.0455 0.0016 0.0013 0.718 - 0.0002 0.0131 -0.000 -0.000 0.0005 32 32 27 35 42 - 0.135 0.641 -0.147 0.199 0.252 ns 0.001 ns ns ns ci/e. a Regression based on leaves older than the age of full expansion Abbreviations: cond = stomatal conductance at light saturation (mol m-2 s 1), WUE =water-use efficiency (mol COz/mol HaO), ci/c,= ratio of CO2 internal to CO/ambient at light saturation I I I I I I I i A 4i .2 o "\. 8 83 o g &o ~ o 8 ~~" ~..~. 8 z t~ I E o (/) [ I L I I I I 2 4 6 8 IO 12 14 Net photosynthesis (~mol (302 rn-2 s-I ) o 0 i , i~ I 5 I0 15 20 Aw (mmol mol"l) Fig. 5. The relationship between stomatal conductance to water vapor and net photosynthesis taken from 5 light response curves for leaves in the early spring sampling period. The data were gathere~ at photon irradiances ranging from 0.002 to 1.6 mmol m -2 s -1. Leaf ages were 29 days (dots), 46 days (crosses), 63 days (squares), 78 days (triangles), and 93 days (circles). (y= 0.024+0.012x, r=0.934, P<0.001) Fig. 6. The response of stomatal conductance to the water vapor concentration gradient (A w) for 4 leaves at light saturation. Stomatal conductance is normalized to a value of 1 at a A w of 16mmolmo1-1. Leaf ages were 48 days (triangles), 61 days (circles), 65 days (dots) and 104 days (crosses). (y=0.44+ lO.13/x, r =0.837, P<0.001) Over all leaf ages and sampling dates, stomatal conductance to water v a p o r and photosynthetic capacity per unit area at light saturation were highly correlated (Table 4). A consequence of this correlation is that at light saturation, neither water-use efficiency n o r the ratio o f CO2 internal to CO2 ambient was significantly affected by leaf age or sampling date (Table 4). The mean water-use efficiency was 0.00148 mol CO2/mol H 2 0 or 0.00362 g CO2/g H 2 0 . This value corresponds to a T : P ratio of 276 g H z O / g CO2. The m e a n ratio o f CO2 internal to CO2 ambient was 0.741 and the mean C O / i n t e r n a l was 254 gmol m o l - 1. The strong correlation between stomatal conductance and photosynthetic capacity over m a n y leaves at light saturation was paralleled by similar relationships for single leaves at a range of light intensities (Fig. 5). Leaves o f different ages differed in net photosynthesis at any light intensity but, at any photosynthetic rate, were very similar in stomatal conductance, and hence, water-use efficiency and CO2 internal. In leaves of all ages, stomatal conductance decreased in response to increases in the water v a p o r concentration gradient (A w) (Fig. 6). Normalizing conductance to water 353 Discussion it,/) 0.3 J I Seasonal Trends 0 0 o 0 F 0.2 (D "3 {D t-. (/) o0 0.1 0 ~6 t'- ~ r~ Z O I I0 I 20 I 30 I 40 Leaf nitrogen (rng g-I) Fig. 7. The relationship between photosynthetic capacity per unit of leaf weight and leaf nitrogen content for leaves at or beyond the age of full expansion. (y = 0.008 + 0.0067x, r = 0.89, P < 0.001) vapor to a value of 1 at a A w of 16 mmol tool -~ reveals that although actual conductances were different for leaves ranging from 48 to 104 days in age, the shapes of the curves were very similar, indicating the absence of age-specific deterioration in the response of stomatal conductance to A w. Photosynthetic Capacity and Leaf Nitrogen: Nitrogen- Use Efficiency Combining data for all leaves at light saturation, a linear regression of photosynthetic capacity per unit weight on leaf organic nitrogen concentration is highly significant and has a positive slope (Table 2). If leaves below the age of full expansion or photosynthetic maturity are eliminated from the calculation, the explained variance (r 2) due to the regression is 80% (Fig. 7). Over a greater than 3 fold range of photosynthetic capacities, leaf nitrogen and photosynthetic capacity were linearly related. The slightly positive y intercept in Fig. 7 means that, on average, the ratio of photosynthetic capacity to nitrogen was constant or increased slightly as leaf nitrogen content decreased. Some of this effect is probably due to the fact that increases in leaf specific weight caused photosynthetic capacity and leaf nitrogen to decrease together. Including only leaves older than the age of full expansion, a multiple linear regression of photosynthetic capacity per unit weight on leaf nitrogen and leaf specific weight reveals highly significant effects of both factors, acting in opposite directions (Table 2). The explained variance in this multiple regression is 89%. Adding leaf age as an independent variable in this multiple regression increases the explained variance to 93%, but makes the effect due to leaf nitrogen no longer significant (Table 2). The relationship between area-based measures of photosynthetic capacity and leaf nitrogen is not as strong as the relationship between the weight-based parameters. For leaves beyond the age of full expansion, the linear regression of photosynthetic capacity per unit area on nitrogen per unit area is significant, but the explained variance is only 38%. Variation in leaf specific weight accounts for 7% of the unexplained variance in this regression (Table 2). The phenology of L. calycina is characteristic of the drought-deciduous or semi-drought-deciduous shrubs of the California chaparral and coastal sage (Mooney and Kummerow 1981). The timing of leaf production and abscission in this vegetation type appears to be influenced by both water availability and photoperiod (Nilsen and Muller 1981). In L. calycina, seasonal trends in many leaf characteristics were influenced by the progressive increase in leaf specific weight. Seasonal decreases in photosynthetic capacity per unit weight and nitrogen per unit weight appear to have been largely dilution effects due to increases in leaf specific weight. We found mixed evidence for an inhibitory effect of high leaf specific weight on photosynthetic capacity like that reported by Gulmon and Chu (1981). Photosynthetic capacity per unit area was not significantly affected by leaf specific weight, but adding leaf specific weight as a second independent variable in the regression of weightbased photosynthetic capacity on leaf nitrogen did increase the explained variance (Table 2), indicating that increased leaf specific weight decreased photosynthetic capacity slightly more than it decreased leaf nitrogen. The small seasonal decline in the quantum yield of photosynthesis suggests an inhibitory effect of high leaf specific weight, but other possible mechanisms, including photoinhibition, and changes in absorptance cannot be eliminated. Overall, the seasonal increase in leaf specific weight had little effect on area-based measures of carbon gain or water loss. The adaptive consequences of the seasonal increase in leaf specific weight remain to be elucidated. Leaf Aging Patterns In controlled environmental situations, chronological aging and age specific characteristics at a single time (age-structure dynamics) differ only to the extent that leaves with different initiation dates experience different microenvironments and possess intrinsic physiological differences. While Richards (1934) provides an early caution about assuming that the two phenomena are the same, recent evidence indicates that under growth chamber conditions, the two forms of leaf aging have similar effects on photosynthetic capacity (Constable and Rawson 1980) with apparent differences caused by canopy shading (V/tclavik 1975) or differences in potential photosynthetic capacity in leaves at different nodes (Woodward 1976). In L. calycina, differences between the age-structure dynamics and chronological aging were dominated by the seasonal increase in leaf specific weight. All major area-based parameters including photosynthesis, conductance, and leaf nitrogen, displayed chronological aging trends qualitatively similar to the age-structure dynamics. Weight-based parameters varied with leaf specific weight, creating a pattern in which leaves at any sampling date tended to be more similar than leaves of the same age, compare d across sampling dates. The age-structure dynamics in L. calycina were very similar to patterns obtained for many species in controlled environments. The decrease in photosynthetic capacity with leaf age was slow relative to Perilla frutescens (Woolhouse 1967), wheat (Osman and Milthorpe 1971), soybean 354 (Woodward 1976), and strawberry (Jurik et al. 1979) but comparable to results for plants with greater leaf longevities, including cassava (Aslam et al. 1977), creosotebush (Syvertsen and Cunningham 1977), and coffee (Yamaguchi and Friend 1979). In general, the rate of decline of photosynthetic capacity with increasing leaf age is slower in species with longer leaf durations. The strong correlation between stomatal conductance and photosynthetic capacity is similar to that observed by Aslam et al. (1977) in cassava, Davis and McCree (1978) in bush beans, and Constable and Rawson (1980) in cotton. This correlation, hypothesized by Wong et al. (1979) to reflect the sensitivity of stomatal guard cells to some metabolite of mesophyll-cell photosynthesis, was not eroded by either leaf aging or seasonal trends. The maintenance of the correlation between photosynthetic capacity and stomatal conductance accounted for the constancy of water-use efficiency and the ratio of CO2 internal to CO2 ambient. The absence of a leaf-age effect on the ratio of CO2 internal to CO 2 ambient implies that leaf-age effects on photosynthetic capacity were not caused by changes in stomatal conductance. This result is consistent with the conclusion of several other studies demonstrating that non-stomatal factors are primarily responsible for decreasing photosynthetic capacity with increasing leaf age (Woolhouse 1967; Osman and Milthorpe 1971; Vficlavik 1975; Davis and McCree 1978; Constable and Rawson 1980). The functional significance of leaf abscission prior to the end of the growing season is not clear. The slow decline in photosynthetic capacity did not continue until photosynthetic activity disappeared, as has been reported in many growth chamber studies (Woolhouse 1967; Ludlow and Wilson 1971; Woodward 1976; Davis and McCree 1978). Leaves entered a phase of rapid senescence while photosynthetic capacity was still 40% to 70% of the maximum observed. During this rapid phase, the leaves quickly yellowed and developed large necrotic areas. One hypothesis deserving serious attention is that leaves enter a rapid and ultimate phase of senescence when the normal aging processes result in significant declines in resource-use efficiency. Resource-Use Efficiency We have used several measures of intrinsic resource-use efficiency as probes to search for deterioration during leaf aging. The measures of intrinsic efficiency are indices of physiological integrity that do not necessarily reflect the ratios of photosynthetic carbon gain to resource investment realized in nature. For example, the quantum yield of photosynthesis at low light intensities is a reliable integrator of inputs from the light reactions of photosynthesis (Bj6rkman 1966) and a widely used index of leaf damage at the biochemical level (Bj6rkman 1975; Gauhl 1976; Powles and Osmond 1979), but it is not specifically relevant to the ratio of photosynthesis to light intensity, at saturating light intensities. Similarly, the relationship between photosynthetic capacity at light saturation and leaf nitrogen content appears to reflect the concentration and activity of the primary carboxylating enzyme in C 3 plants (Friedrich and Huffaker 1980), but that relationship may be qualitatively different at lower light intensities (Gulmon and Chu 1981). Overall, seasonal and leaf-age effects on intrinsic resource-use efficiency were small, much smaller than seasonal or leaf-age effects on photosynthetic carbon gain. In the following summary of intrinsic resource-use efficiency, we have included leaves of all ages except when results for leaves below the age of full expansion confound the interpretation for leaves at or beyond the age of photosynthetic maturity. The quantum yield of photosynthesis, an index of intrinsic light-use efficiency, was not significantly affected by leaf age but did decrease slightly as the season progressed. The ratio of photosynthesis to transpiration at light saturation, intrinsic water-use efficiency, was not significantly affected by either leaf age or season. Further, the linear relationship between photosynthesis and stomatal conductance to water vapor for leaves of many ages over a wide range of light intensities (Fig. 5) translates into a ratio of photosynthesis to stomatal conductance (or transpiration with the constant A w) that decreased only slightly as light intensity decreased. The absence of age-specific changes in the shape of the response of conductance to A w (Fig. 6) reinforces the conclusion suggested by the light response data, that the ability of the leaf to adjust stomatal conductance in response to environmental factors was not degraded by leaf aging. The absolute sensitivity of conductance to light and A w did decrease as leaves aged, but these decreases were proportional to the decreases in photosynthetic capacity. For leaves at or beyond the age of full expansion, photosynthetic capacity was highly correlated with leaf organic nitrogen content, using either weight-based or area-based measures, even though leaf ages varied from 37 to 110 days and leaf specific weight varied from less than 40 to more than 120 g m - 2. The regression of photosynthetic capacity on leaf nitrogen has a y-intercept slightly greater than zero (Fig. 7), meaning that the ratio of photosynthetic capacity to leaf nitrogen was constant or even slightly increased with decreasing photosynthetic capacity or leaf nitrogen. Thus, nitrogen-use efficiency was essentially constant over a three fold range of photosynthetic capacities. While this suggests, it does not confirm that nitrogenuse efficiency was constant over the season and for all leaves past the age of photosynthetic maturity. Small effects on nitrogen-use efficiency due to leaf specific weight and leaf age are suggested by the 13% difference between the explained variance in the regression of photosynthetic capacity on leaf nitrogen alone and the multiple regression of photosynthetic capacity on leaf nitrogen, leaf specific weight, and leaf age. For the 9 oldest leaves (older than 80 days), the measured photosynthetic capacities averaged 3.7% below those predicted on the basis of nitrogen content (Fig. 7) while for the 8 youngest leaves (younger than 50 days), the measured capacities averaged 7.5% below those predicted on the basis of nitrogen content. Leaf aging, if anything, slightly increased nitrogen-use efficiency. For the 7 leaves with the highest specific weights (greater than 100 g m-Z), the measured photosynthetic capacities averaged 19.2% below those predicted on the basis of nitrogen content while for the 7 leaves with the lowest specific weights (less than 50 g m 2), the measured photosynthetic capacities averaged 1.2% above those predicted on the basis of nitrogen content. High leaf specific weights, which occurred only late in the season, were associated with decreased nitrogen-used efficiency. In sum, we found no effects of leaf age on intrinsic light-, water-, or nitrogen-use efficiency. Photosynthetic capacity did decrease as leaves aged, but the decreases were paralleled by decreases in leaf nitrogen content and de- 355 creases in stomatal conductance. We found no evidence that either functional aspects of photosynthesis or the integration of photosynthesis and conductance was degraded by leaf aging. All the observations reported here are consistent with the hypothesis that as leaves age, nitrogen is removed, and leaf function responds to that removal in ways that do n o t alter intrinsic resource-use efficiency. In contrast to leaf aging, seasonal advance was assod a t e d with small decreases in the q u a n t u m yield of photosynthesis and probably with small decreases in intrinsic nitrogen-use efficiency. While leaf aging does not appear to represent deterioration, it does involve large changes in leaf characteristics. Other papers in this series (Field 1981, 1983) examine the adaptive significance of those changes. Acknowledgements. Thanks to N. Chiariello, W.E. Williams, and S.L. Gulmon for helpful discussions and to B. Lilley, C. Chu, A. Turner, and R. Mita for technical assistance. The research was supported by NSF grant DEB 78 02067 to HAM and an NSF predoctoral fellowship to CF. References Aslam M, Lowe SB, Hunt LA (1977) Effect of leaf age on photosynthesis and transpiration of cassava (Manihot esculenta). Can J Bot 55:2288 2295 Bazzaz FA, Harper JL (1977) Demographic analysis of the growth of Linum usitatissimum. New Phytol 78:193-208 Bj6rkman O (1966) Photosynthetic inhibition by oxygen in higher plants. Carnegie Inst Wash Year Book 65:446-454 Bj6rkman O (1975) Thermal stability of the photosynthetic apparatus in intact leaves. Carnegie Inst Wash Year Book 74:748-751 Constable GA, Rawson HM (1980) Effect of leaf position, expansion and age on photosynthesis, transpiration and water use efficiency of cotton. Aus J Plant Physiol 7:89-100 Davis SD, McCree KJ (1978) Photosynthetic rate and diffusion conductance as a function of age in leaves of bean plants. Crop Sci 18:280-282 Field C (1981) Leaf age effects on the carbon gain of individual leaves in relation to microsite. In: NS Margaris, HA Mooney (eds), Components of productivity of mediterranean regions: basic and applied aspects. Dr. W Junk, The Hague pp 41-50 Field C (1983) Allocating leaf nitrogen for the maximization of carbon gain: Leaf age as a control on the allocation program. Oeeologia (Berlin) 56 : 341-347 Field C, Berry JA, Mooney HA (1982) A portable system for measuring carbon dioxide and water vapour exchange of leaves. Plant, Cell and Env 5:179-186 Friedrich JW, Huffaker RC (1980) Photosynthesis, leaf resistances, and ribulose-l,5-bisphosphate carboxylase degradation in senescing barley leaves. Plant Physiol 65:1103-1107 Gauhl E (1976) Photosynthetic response to varying light intensity in ecotypes of Solanum dulcamara L. from shaded and exposed habitats. Oecologia (Berl) 22:275-286 Guhnon SL, Chu CC (1981) The effects of light and nitrogen on photosynthesis, leaf characteristics, and dry matter allocation in the chaparral shrub, Diplaeus aurantiacus. Oecologia (Berl) 49 : 207-212 Hamilton WD (1966) The moulding of senescence by natural selection. J Theor Biol 12:12-45 Isaac RA, Johnson WC (1976) Determination of total nitrogen in plant tissue, using a block digestor. J Assoc Off Analyt Chem 59 : 98-100 Jurik TW, Chabot JF, Chabot BF (1979) Ontogeny of photosynthetic performance in Fragaria virginiana under changing light regimes. Plant Physiol 63 : 542-547 Kirkwood TBL, Holliday R (1979) The evolution of aging and longevity. Proc R Soc Lond B 205:531-546 Leopold AC (1961) Senescence in plant development. Science 134:1727-1732 Ludlow MM, Wilson GL (1971) Photosynthesis of tropical pasture plants III leaf age. Aus J Biol Sci 24:1065-1075 Medawar PB (1952) An unsolved problem in biology. HK Lewis, London Mooney HA (1977) Southern coastal scrub. In: MG Barbour, J Major (eds) Terrestrial vegetation of California. Wiley-Interscience, New York p 471~490 Mooney HA, Kummerow J (1981) Phenological development of plants in mediterranean-climate regions. In: F dl Castri, DW Goodall, RL Specht (eds) Mediterranean-type shrublands. Elsevier Scientific Pub Co Amsterdam p 303-307 Mooney HA, Gulmon SL (1982) Constraints on leaf structure and function in reference to herbivory. BioScience 32:198-206 Nilsen ET, Muller WH (1981) Phenology of the drought deciduous shrub Lotus seoparius: climatic controls and adaptive significance. Ecol Monog 51 : 302322 Osman AM, Milthorpe FL (1971) Photosynthesis of wheat leaves in relation to age, illuminance, and nutrient supply II. results. Photosynthetica 5: 61-70 Page BM, Tabor LL (1967) Chaotic structure and decollement in Cenozoic rocks near Stanford University, California. Geol Soc Am Bull 78:1-12 Powles SB, Osmond CB (1979) Photoinhibition of attached leaves of C 3 plants illuminated in the absence of both carbon dioxide and of photorespiration. Plant Physiol 64:982-988 Richards FJ (1934) On the use of simultaneous observations on successive leaves for study of physiological change in relation to leaf age. Ann Bot 48:497-504 Syvertsen JP, Cunningham GL (1977) Rate of leaf production and senescence and effect of leaf age on net gas exchange in creosotebush. Photosynthetica 11:161-166 Thomas H, Stoddart JL (1980) Leaf senescence. Ann Rev Plant Physiol 31 : 83-111 Vficlavik J (1975) Comparison of the changes in net photosynthetic CO2 uptake and water vapour efflux during leaf ontogenesis with the differences between the leaves according to their descending insertion level. Biol Planta 17:411-415 White J (1979) The plant as a metapopulation. Ann Rev Ecol Syst 10:109-145 Wong SC, Cowan IR, Farquhar GD (1979) Stomatal conductance correlates with photosynthetic capacity. Nature 282:424-426 Woodward RG (1976) Photosynthesis and expansion of leaves of soybean grown in two environments. Photosynthetica 10: 274-279 Woolhouse HW (1967) Leaf age and mesophyll resistance as factors in the rate of photosynthesis. Hilger J 11:7-12 Yamaguchi T, Friend DJC (1979) Effect of leaf age and irradiance on photosynthesis of Coffea arabica. Photosynthetica 13 : 271-278 Received July 20, 1982