Improvement of light capture in oilseed rape canopies by a potential novel method of canopy management

Study to test the effect of alkaline pH on oilseed rape leaf expansion.

Year of Publication2008

Crop canopies function as collectors of solar energy and exchange carbon dioxide, oxygen and water vapour. Changes in canopy structure affect these processes and directly affect the growth and yield (Milford, 1991). Yields of oilseed rape are often variable and below their potential, especially after vigorous growth early in the season (Scott et al., 1999). The seed yield in oilseed rape is primarily determined by the dry matter accumulation of the crop prior to flowering (Mendham, 1981). An excessive canopy during pod filling would also hinder maximum photosynthesis due to poor light distribution resulting in yield reduction. The aim of canopy management in oilseed rape is to establish a Green Area Index (GAI) of 3.5 at the start of seed filling (Scott et al., 1995). However, Lunn et al., (2003) states that a GAI of 3 is also important just prior to flowering and also showed that a GAI above or below 3 did not lead to any yield benefits (Lunn et al., 2001). A GAI of 3 will intercept about 90-95 % of the incident solar radiation (Lunn et al., 2003). Foliar application of fungicides with plant growth regulator action has become agronomic practice in oilseed rape canopy management (Lunn et al., 2003). With public concern over the use of agrochemicals, new methods to reduce leaf expansion are sought. A possible new method of canopy management may lie in the leaf growth response to alkaline pH. It is hypothesised that as a stress response some plants increase their xylem sap pH (Gollan et al., 1992) which when the xylem sap reaches the leaf induces closure of the stomata via an ABA dependent mechanism (Bacon et al., 1998). Alkaline pH has been shown to reduce leaf elongation in barley when fed with artificial xylem sap of acidic and alkaline pH (Bacon et al., 1998). Another mechanism suggested to be responsible for the reduction in leaf area by high pH is the acid growth theory (Cleland, 1991). With the aim of examining the effect of pH on oilseed rape leaf expansion, experiments using a leaf assay were carried out in controlled environments, and foliar application of different pH buffer solutions has also been studied in a greenhouse environment. In the leaf assay detached shoots were fed artificial xylem sap with pH ranging from 5 to 11. Leaf area was significantly reduced (P=0.023) by 80% over four days for shoots fed pH 8 when compared to control plants. The foliar application studies have shown inconsistent results, possibly due to penetration difficulties or high leaf apoplastic buffering capacity. Further studies are needed to either improve penetration into the leaf or overcome apoplastic buffering capacity.

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