Leaf lifetime photosynthetic rate and leaf demography in whole plants of Ipomoea pes-caprae growing with a low supply of calcium, a 'non-mobile' nutrient

Abstract

The adaptive significance of leaf longevity has been established in relation to restrictive nutrients that can be retranslocated within the plant. However, the effect of deficiencies in 'non-mobile' nutrients on leaf lifespan and photosynthetic carbon gain is uncertain. Calcium is frequently given as an example of an essential nutrient with low phloem mobility that may alter the leaf senescence process. This study has been designed to estimate leaf lifespan, leaf production (L(p)) and leaf death (L(d)) rates, the age structure of leaves, and the decline in maximum photosynthetic rate (A(max)) with age in plants of Ipomoea pes-caprae growing with a full supply of nutrients and with a low Ca supply. The Ca deficiency produced reductions in L(p) and leaf lifespan compared with control plants. In spite of the differences in the demographic parameters between treatments in control and low-Ca plants, the percentage of leaves of a given leaf age class is maintained in such a way that the number of leaves per plant continues to increase. No relationship was found between Ca supply and A(max). However, the decline in A(max) with leaf senescence was rather sudden in control plants compared with plants growing with a low Ca supply. The importance of simultaneously using the total leaf demographic census and the assimilation rate along with leaf lifespan data in order to understand the performance of whole plants under constrained conditions is discussed.

Fig. 1.

(A) Cumulative number of new (circles) and dead (triangles) leaves and (B) survival probability as a function of time in leaves of Ipomoea pes-caprae cultivated for 3 weeks (white) and 10 months (grey) after transplantation of apical segments. Plants were growing with a full nutrient supply and had previously been pruned. The initial number of leaves was four to seven. In (A), the number of leaves that appeared and shed per plant was counted within a 7-d interval for a period of 70 d; in (B), the leaves that appeared within a 7-d interval were grouped in a cohort, and 11 cohorts per plant were included. Survival probability as a function of time was estimated for a period of 84–126 d. Means for 28 plants per period are shown. Bars represent SEM.

Fig. 2.

Age structure of leaf population in plants of Ipomoea pes-caprae cultivated for 3 weeks (A) or 10 months (B) after transplanting the apical segment and kept thereafter with all nutrients (C, control) or under low Ca supply (D). In the diagrams for the various treatments, the leaves of each age group are represented by a horizontal bar whose length is the percentage of the total leaf population. Age structure was estimated in plants growing for 70–98 d after initiating the treatments. Means for 10–21 plants are shown. Bars represent SEM. Different letters indicate significant differences between leaf age classes at P <0.05.

Fig. 3.

(A) Cumulative number of new (circles and squares) and dead (triangles) leaves and (B) survival probability as a function of time in leaves of Ipomoea pes-caprae cultivated with full nutrient supply (control plants; black) or under a low-Ca nutrient solution (white). Plants were cultivated for 18 months under full nutrient supply before beginning the treatments. Plants were pruned 20 d after initiating the treatments, and measurements taken thereafter. The initial number of leaves was 4–10. In (A), the number of leaves that appeared and shed per plant was counted within a 7-d interval during a period of 49 d; in (B), the leaves that appeared within a 7-d interval were grouped in a cohort, and eight cohorts per plant are included. Survival probability as a function of time was estimated for a period of 119 d. Means for 10 plants are shown. Bars represent SEM.

Fig. 4.

(A) Maximum assimilation rate (A_max) and (B) WUE as a function of leaf age in plants of Ipomoea pes-caprae cultivated with full nutrient supply (control plants; black circles) or under a low-Ca nutrient solution (white squares). White and black arrows indicate leaf half-life in control and low-Ca plants, respectively. Plants were cultivated for 18 months with full nutrient supply before beginning the treatments. Different letters indicate significant differences between treatments at P <0.05; n=12–35 per each leaf age from 8 to 10 different plants. Bars represent SEM.

Fig. 5.

(A) Number of leaves and (B) maximum CO₂ assimilation per leaf age in a whole plant of Ipomoea pes-caprae cultivated with a full nutrient supply (control; black circles and columns) or under a low-Ca nutrient solution (white squares and columns) n=10. Bars represent SEM. The inset is a plot of the differences among treatments between total leaf area and maximum CO₂ assimilation per plant.