Summary
This review reports on the processes associated with carbon transfer and metabolism in leaves and growing organs and the role of long-distance transport and vascular links in the regulation of carbon partitioning in plants. Partitioning is clearly influenced by both the supply and demand for photosynthate and is moderated by vascular connections and the storage capacity of the leaves and pathway tissues. However there appears to be little more than circumstantial evidence either that short distance transfer of carbon within either the source or the sink, or that long-distance transport in the phloem, are limiting photosynthesis or growth directly. Although individual biochemical and physiological processes relating to photosynthesis and growth may be well understood, the factors primarily responsible for the control of carbon partitioning in plants have not been clearly identified. There is a need for a greater understanding of organ initiation and development (source and sink formation and potential size), the clear identification of whether growth is sink or source limited (including possible sink-controlled photosynthesis) and a detailed assessment of the role of storage in buffering developmental and environmental changes in sink and source activity. Also more information is needed on the role of hormonal and nutritional factors in regulating source and sink activity (organ interactions not directly associated with carbon transfer).

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1990.tb00524.x

Tansley Review No. 27 The control of carbon partitioning in plants

Phenotypic plasticity is an important element in the response repertoire of plants (25). It may be manifested as changes in organ (e.g. leaf) morphology as well as in patterns of biomass distribution. Morphological plasticity throughout growth is possible because plant development is modular in form (77, 184, 185). Growth results from the reiteration of basic morphological subunits produced by meristems. Because meristems may develop into either reproductive or vegetative structures, patterns of biomass distribution reflect developmental decisions (121, 172, 180). In this essay, we review data suggesting that at least in certain cases, architectural constraints affect the range of morphological plasticity that can be expressed. Because carbon is frequently viewed as a critical currency of allocation (76, 165), we focus on how these constraints influence assimilate (i.e. carbon) production and utilization. We postulate that when these constraints are present, plants consist not only of morphological subunits (185) but, as Adams (4) first suggested, of physiological subunits as well. We call these integrated physiological units (IPUs) and propose that they are made up of identifiable arrays of morphological subunits that together function as relatively autonomous structures with respect to the assimilation, distribution, and utilization of carbon. If such units of physiological integration exist,

Morphogenetic constraints on patterns of carbon distribution in plants

Constraints on patterns of carbon distribution in plants

Does exogenous application of salicylic acid through the rooting medium modulate growth and photosynthetic capacity in two differently adapted spring wheat cultivars under salt stress

Net photosynthesis, dark respiration and the response to photon flux density were measured on cotton leaves grown in a glasshouse. Leaves at four positions on the plant were examined from their unfolding until 70 days later. Photosynthesis and transpiration per unit of leaf area were unaffected by leaf position and, in all leaves, peak photosynthesis of about 110 ng CO2 cm-2 s-1 was attained 13-15 days after leaf unfolding, when the leaf was 75-90% of maximum area. Photosynthesis was maintained at this rate for only 12 days before declining linearly to values 20% of the maximum when leaves were 70 days old. Transpiration followed a similar pattern reaching a maximum of about 13 µg H2O cm-2 s-1 at 2 kPa vapour pressure deficit (VPD) at 13 days. Stomatal and internal conductances changed in parallel as leaves aged, with the consequence that internal CO2 concentration and water use efficiency remainedessentially constant at 220µll-1 and 16.8 ng CO2 (µg H2O kPa VPD-1)-1 respectively. Youngest and oldest leaves saturated at lowest light levels (400-800 pE m-2 s-1) while 16-18- day-old leaves had light saturation at 1100 µE m-2 s-1. The initial slope of the light response curves increased as leaves expanded up to 10 days age then remained constant at 0.25 ng CO2 cm-2 (pE m-2)-1. Dark respiration reached a maximum of 1.5 ng CO2 mg-1 s-1 5 days after leaf unfolding, when leaf dry weight was increasing most rapidly. The relationship between the consistent pattern of gas exchange with age and the pattern of morphological development is discussed, along with internal factors associated with age-dependent photosynthesis.

Effect of Leaf Position, Expansion and Age on Photosynthesis, Transpiration and Water Use Efficiency of Cotton

A carbon budget for cotton plants at the single leaf, node and whole plant level was constructed using data from four glasshouse experiments. Data were collected on leaf expansion and dry weight growth, net photosynthesis, dark respiration of leaves and stems and on responses of photosynthesis to light in tissues of different age. The potential export of carbon by leaves was calculated as daily net photosynthesis less requirements for growth and dark respiration. The carbon budget for the single leaf showed that the leaf's maximum requirement occurred 7-8 days after unfolding, at the same time as it became a net carbon exporter. Dark respiration used most carbon at days 12-15 but even then the amount was only about 10% of the carbon fixation by the leaf during the day. Potential carbon export reached a peak in 22-day-old leaves, approximating 1 mg C cm-2 day -1 on a sunny day. The main finding from the budget was that carbon production and its utilization by bolls is out of phase both at the node and whole plant level which necessitates considerable movement of carbon among nodes and into and out of storage. This finding was confirmed in a study using 14CO2 which, while supporting the general hypothesis that the plant's carbon is fed into a pool available to all organs, indicated that there are preferred links between node positions in vertical alignment.

HM Rawson

Papers

Carbon Production and Utilization in Cotton: Inferences from a Carbon Budget