Agriculture Reference
In-Depth Information
that an increase in yield is, indeed source-limited or at least that sink capacity is
stimulated by the increased source activity (higher net photosynthetic rate under
eCO
2
). These studies suggested that sink capacity is not necessarily a constraint to
increase yield production by means of
improving photosynthesis (Kimball
et al.
2002
; Ainsworth et al.
2004
).
Apart from the question of how source and sink, or in other words supply and
demand, determine and influence each other, the duality of photosynthesis and
photorespiration makes the issue more complex. Up to one third of C fixed by the
carboxylation activity of Rubisco is again lost by photorespiration. While some
authors propose that photorespiration is of vital importance for plant functioning
(Kozaki and Takeba
1996
), others see these functions as at least partially redundant
and suggest that a reduction of the C lost by this process would improve the
efficiency of photosynthesis and biomass production (Long et al.
2006
; Peterhansel
and Maurino
2011
). However, knowledge about the different roles of photorespi-
ration in plant metabolism and NUE is still limited.
Another important process involved in NUE is the reallocation of nutrients.
Re-use of nutrients from senescing leaves reduces nutrient loss and thereby
increases NUE. Once more N is tightly coupled to C gain and its efficient allocation
from one leaf to another contributes to optimal C fixation (Field
1983
). In this
process older leaves with declining photosynthetic N efficiency are exploited as a
source for N, which is reallocated to young leaves to promote their growth. In this
way N is used efficiently for photosynthesis at a whole plant level (Westoby
et al.
2002
; Escudero and Mediavilla
2003
) and also NUE is increased, as loss is
reduced. Resorption of nutrients from senescing leaves has also been studied for P
(Lajtha
1987
; Chapin and Moilanen
1991
; Killingbeck
1996
). It is generally
assumed that the costs of this process are very low for the plant (Givnish
2002
),
which further supports reallocation of nutrients as a key process for the improve-
ment of NUE. However, nutrients, which are efficiently recycled within the plant
and thereby are not lost during senescence, will also not end up in the decomposi-
tion cycle in the soil. Whether this has negative feedback consequences for the plant
and NUE is not fully understood and much will depend on the particular system.
However, there are speculations about a general trade-off between efficient nutrient
re-sorption in plants and the decomposability of litter (Aerts
1997
).
The translocation of nutrients to the harvestable yield organ follows the same
principles as the allocation to other plant organs. For obvious reasons it is, however,
the most crucial allocation process for yield production and therefore regarded as a
special case that is worthy of additional attention. Plant breeding has resulted in a
wide diversity of crops in which virtually any part of a plant might serve as a yield
organ: roots, stems, leaves, seeds, fruits. However, the six most important crops in
terms of worldwide food and feed production are all grain crops (corn, rice, wheat,
soybean, barley and sorghum) with seeds being the plant organ of interest and grain
filling as a crucial step for yield production (Borr
´
s et al.
2004
; Foulkes et al.
2011
).
The reallocation of N from senescing leaves to the developing seeds is of particular
importance in determining the quality of the crop and thus increasing the efficiency
of reallocating N from leaves to grains is a potential target for improving NUE
