The effects of phosphate deficiency on the composition and photosynthetic CO 2 assimilation rates of fully expanded leaves of sunflower, maize and wheat plants are described. The regulation of photosynthesis by stomatal and mesophyll characteristics of leaves of different phosphate status is analysed and related to structure. Phosphate deficient leaves had small concentrations of inorganic phosphate, Pi, in the tissue water. Rate of photosynthesis in leaves and stomatal conductance were smaller in plants grown with inadequate phosphate when measured under any given light intensity or CO 2 partial pressure. Despite the decrease in stomatal conductance (and without evidence of patchy stomatal closure), the relative stomatal limitation of photosynthesis was similar in the plants grown with deficient or abundant phosphate. However, the mesophyll capacity for photosynthesis was greatly limited by phosphate deficiency. Leaves deficient in phosphate had larger numbers of small size cells per unit leaf area than leaves with adequate phosphate. The total soluble protein content of leaves decreased with phosphate deficiency in all three species; however, the leaf chlorophyll content was decreased only in sunflower and maize and not in wheat. These results suggest that stomatal conductance did not restrict the CO 2 diffusion rate, rather the metabolism of the mesophyll was the limiting factor. This is shown by poor carboxylation efficiency and decreased apparent quantum yield for CO 2 assimilation, both of which contributed to the increase in relative mesophyll limitation of photosynthesis in phosphate deficient plants.

Stands of Scirpus olneyi, a native saltmarsh sedge with C 3 photosynthesis, had been exposed to normal ambient and elevated atmospheric CO 2 concentrations (Ca) in their native habitat since 1987. The objective of this investigation was to characterize the acclimation of photosynthesis of Scirpus olneyi stems, the photosynthesizing organs of this species, to long-term elevated C a treatment in relation to the concentrations of Rubisco and non-structural carbohydrates. Measurements were made on intact stems in the field under existing natural conditions and in the laboratory under controlled conditions on stems excised in the field early in the morning. Plants grown at elevated C a had a significantly higher (30-59%) net CO 2 assimilation rate (A) than those grown at ambient C a when measurements were performed on excised stems at the respective growth Ca' However, when measurements were made at normal ambient C I " A was smaller (45-53%) in plants grown at elevated C a than in those grown at ambient Ca' The reductions in A at normal ambient C a , carboxylation efficiency and in situ carboxylase activity were caused by a decreased Rubisco concentration (30-58%) in plants grown at elevated C ll ; these plant'i also contained less soluble protein (39-52%). The Rubisco content was 43 to 58% of soluble protein, and this relationship was not significantly altered by the growth CO 2 concentrations. The Rubisco activation state increased slightly, but the in situ carboxylase activity decreased substantially in plants grown at elevated C ll • When measurements were made on intact stems in the field, the elevated C a treatment caused a greater stimulation of A (100%) and a smaller reduction in carboxylation efficiency (which was not statistically significant) than when measurements were made on excised stems in the laboratory. The possible reasons for this are discussed.Plants grown at elevated C ll contained more non-structural carbohydrates (25-53%) than those grown at ambient Ca' Plants grown at elevated C a appear to have sufficient sink capacity to utilize the additional carbohydrates formed during photosynthesis. Overall, our results are in agreement with the hypothesis that elevated C a leads to an increased carbohydrate concentration and the ensuing acclimation of the photosynthetic apparatus in C 3 plants results in a reduction in the protein complement, especially Rubisco, which reduces the photosynthetic capacity in plants grown at elevated C a , relative to plants grown at normal ambient C ll • Nevertheless, when compared at their respective growth C a , Scirpus olneyi plants grown at elevated C a in their native habitat maintained a substantially higher rate of photosynthesis than those grown at normal ambient C a even after 8 years of growth at elevated Ca'

Sunflower (Helianthus annuus L. cv Asmer) and maize (Zea mays L. cv Eta) plants were grown under controlled environmental conditions with a nutrient solution containing 0, 0.5, or 10 millimolar inorganic phosphate. Phosphate-deficient leaves had lower photosynthetic rates at ambient and saturating CO2 and much smaller carboxylation efficiencies than those of plants grown with ample phosphate. In addition, phosphate-deficient leaves contained smaller quantities of total soluble proteins and nbulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) per unit area, although the relative proportions of these components remained unchanged. The specific activity of Rubisco (estimated in the crude extracts of leaves) was significantly reduced by phosphate deficiency in sunflower but not in maize. Thus, there was a strong dependence of carboxylation efficiency and C02-saturated photosynthetic rate on Rubisco activity only in sunflower. Phosphate deficiency decreased the 3-phosphoglycerate and ribulose-1,5-bisphosphate (RuBP) contents of the leaf in both species. The ratio of 3-phosphoglycerate to RuBP decreased in sunflower but increased in maize with phosphate deficiency. The calculated concentrations of RuBP and RuBP-binding sites in the chloroplast stroma decreased markedly with phosphate deficiency. The ratio of the stromal concentration of RuBP to that of RuBP-binding sites decreased in sunflower but was not affected in maize with phosphate deficiency. We suggest that a decrease in this ratio made the RuBP-binding sites more vulnerable to blockage or inactivation by tight-binding metabolites/inhibitors, causing a decrease in the initial specific activity of Rubisco in the crude extract from phosphate-deficient sunflower leaves. However, the decrease in Rubisco specific activity was much less than the decrease in the RuBP content in the leaf and its concentration in the stroma. A large ratio of RuBP to RuBP-binding sites may have maintained the Rubisco-specific activity in phosphatedeficient maize leaves. We conclude that the effect of phosphate deficiency is more on RuBP regeneration than on Rubisco activity in both sunflower and maize.plants was due to inhibition of nonstomatal processes rather than to stomatal factors despite a decrease in stomatal conductance. The carboxylation efficiency and the apparent quantum yield for CO2 assimilation were two parameters strongly affected by phosphate deficiency, leading to an increase in the relative mesophyll limitation of photosynthetic rate. Similar effects on carboxylation efficiency and quantum yield in phosphate-deficient leaves of spinach, soybean, and sugar beet have been observed in other studies (4,11,19).Under conditions of a nonlimiting supply of RuBP, the amount and specific activity of Rubisco determine carboxylation efficiency (24), defined as the initial slope of the curve relating A to C1, i.e. dA/dCj when A = 0 (10). Phosphatedeficient leaves produce smaller amounts of RuBP and Rubisco (4,19), and the Rubisco has a lower specific activity (

The effects of extreme phosphate (Pi) deficiency during growth on the contents of adenylates and pyridine nucleotides and the in vivo photochemical activity of photosystem II (PSII) were determined in leaves of Helianthus annuus and Zea mays grown under controlled environmental conditions. Phosphate deficiency decreased the amounts of ATP and ADP per unit leaf area and the adenylate energy charge of leaves. The amounts of oxidized pyridine nucleotides per unit leaf area decreased with Pi deficiency, but not those of reduced pyridine nucleotides. This resulted in an increase in the ratio of reduced to oxidized pyridine nucleotides in Pi‐deficient leaves. Analysis of chlorophyll a fluorescence at room temperature showed that Pi deficiency decreased the efficiency of excitation capture by open PSII reaction centres (φe), the in vivo quantum yield of PSII photochemistry (φPSII) and the photochemical quenching co‐efficient (qP), and increased the non‐photochemical quenching co‐efficient (qN) indicating possible photoinhibitory damage to PSII. Supplying Pi to Pi‐deficient sunflower leaves reversed the long‐term effects of Pi‐deficiency on PSII photochemistry. Feeding Pi‐sufficient sunflower leaves with mannose or FCCP rapidly produced effects on chlorophyll a fluorescence similar to long‐term Pi‐deficiency. Our results suggest a direct role of Pi and photophosphorylation on PSII photochemistry in both long‐and short‐term responses of photosynthetic machinery to Pi deficiency. The relationship between φPSII and the apparent quantum yield of CO2 assimilation determined at varying light intensity and 21 kPa O2 and 35 Pa CO2 partial pressures in the ambient air was linear in Pi‐sufficient and Pi‐deficient leaves of sunflower and maize. Calculations show that there was relatively more PSII activity per mole of CO2 assimilated by the Pi‐deficient leaves. This indicates that in these leaves a greater proportion of photosynthetic electrons transported across PSII was used for processes other than CO2 reduction. Therefore, we conclude that in vivo photosynthetic electron transport through PSII did not limit photosynthesis in Pi‐deficient leaves of sunflower and maize and that the decreased CO2 assimilation was a consequence of a smaller ATP content and lower energy charge which restricted production of ribulose, 1‐5, bisphosphate, the acceptor for CO2.

In this study a comparison of the canopy architecture and the growth and distribution of roots was made in 10-year-old trees of Hevea brasiliensis grown in a severely drought-prone area on the west coast of India under rainfed and irrigated conditions. LAI and light interception increased significantly in the irrigated compared to the rainfed trees. Girth and height of the tree were 29 and 19% more while width and height of the canopy were 19 and 20% more in the irrigated than rainfed trees. There were 22% more primary branches which had 26% more diameter in the irrigated trees than rainfed trees. The branches were inserted on the main trunk at an angle of 58.36° in the irrigated and 44.22° in rainfed trees. The above changes led to more light penetration which altered the light distribution inside the rainfed trees during summer and inhibited leaf photosynthesis particularly in the top canopy leaves. In the rainfed trees most of the growth occurred during the short favorable season immediately after the monsoon between June and October and no growth or even shrinking of the trunk was seen during summer. In the irrigated trees a higher growth was seen throughout the year and summer had no adverse effect. Although there was some difference in the root distribution pattern, the total root density per unit soil volume did not vary between the irrigated and rainfed trees.