Seasonal effects on the relationship between photosynthesis and leaf carbohydrates in orange trees

Abstract

<jats:p>To understand the effect of summer and winter on the relationships between leaf carbohydrate and photosynthesis in citrus trees growing in subtropical conditions, ‘Valencia’ orange trees were subjected to external manipulation of their carbohydrate concentration by exposing them to darkness and evaluating the maximal photosynthetic capacity. In addition, the relationships between carbohydrate and photosynthesis in the citrus leaves were studied under natural conditions. Exposing the leaves to dark conditions decreased the carbohydrate concentration and increased photosynthesis in both seasons, which is in accordance with the current model of carbohydrate regulation. Significant negative correlations were found between total non-structural carbohydrates and photosynthesis in both seasons. However, non-reducing sugars were the most important carbohydrate that apparently regulated photosynthesis on a typical summer day, whereas starch was important on a typical winter day. As a novelty, photosynthesis stimulation by carbohydrate consumption was approximately three times higher during the summer, i.e. the growing season. Under subtropical conditions, citrus leaves exhibited relatively high photosynthesis and high carbohydrate levels on the summer day, as well as a high nocturnal consumption of starch and soluble sugars. A positive association was determined between photosynthesis and photoassimilate consumption/exportation, even in leaves showing a high carbohydrate concentration. This paper provides evidence that photosynthesis in citrus leaves is regulated by an increase in sink demand rather than by the absolute carbohydrate concentration in leaves.</jats:p>

Source-sink relationships in two soybean cultivars with indeterminate growth under water deficit

“…As previously reported (Ainsworth et al 2004(Ainsworth et al , 2011Ribeiro et al 2012), the up-regulating in A i was attributed to the increase of sink demand by both bloom and pod formation processes ( Figure 6). However, it is …”

“…The A i increase observed through the phenological stages of transition, i.e., at 45 DAE, may be related to the growing demand of photoassimilates caused by the emergence of blooms and pod formations (Figures 4b,e;6a,b). As stated by other authors, the photosynthesis dynamics seems to be controlled by the source-sink relationship, i.e., any increase in carbohydrates demand by sink tissues tends to increase the photosynthesis in source leaves as well (Ainsworth et al 2004(Ainsworth et al , 2011Ribeiro et al 2012). At the beginning of bloom, the plant growth rates were at high levels and only 20% of total aboveground dry matter (DM) and 66% of final plants height (H) had been achieved (Figures 3a,c,d,f ).…”

“…The dynamic of carbon assimilation in plants is functionally controlled by a source-sink relationship (Ainsworth et al 2004;Ribeiro et al 2012). Sources are the sites where net assimilation of CO 2 occurs while sinks are tissues where photoassimilates are allocated for growth or storage.…”

“…Although the CO 2 assimilation rate is genetically determined, exogenous factors, such as water deficit, exerts substantial control over it (Liu et al 2004a). Endogenous factors can also reduce the CO 2 assimilation rate by a negative feedback effect on photosynthesis due to high leaf carbohydrate content (Mondal et al 1978;Ribeiro et al 2012). In soybean, the pods and seeds are the major sink tissues and their size and number per plant determine grain yield (Declaux et al 2000;Egli and Bruening, 2004).…”

Moderate warm temperature improves shoot growth, affects carbohydrate status and stimulates photosynthesis of sweet orange plants

“…The leaf photoassimilate exportation/consumption (PEC) was calculated according to Ribeiro et al (2012):…”