Senescence characteristics of cropsGreenness of crop species; photosynthesis; duration of photosynthesis; leaf area index; leaf area duration; green area duration; relationship of green leaf area duration to yield; crop management and breeding to increase green area duration; limited scope to increase leaf area index; canopy establishment and closure; canopy senescence. The energy band of light absorbed by photosynthetic pigments is squarely within the range of wavelengths to which human vision is receptive (Surridge et al. 2003). This means that throughout most of the history of plant domestication the eye was able to measure non-destructively a plant's physiological status with a sophistication that has become possible with electronic instrumentation only very recently. It is clear that such visual assessments have been important influences on the direction taken by progressive improvements in crop plants from the earliest period of human agricultural activity, and they retain empirical significance to this day. In general, the most useful modern crop species and varieties are greener, establish green tissues faster, retain them longer and are visibly more responsive to agricultural inputs, such as fertilizer, pesticides and irrigation, than their forebears. Greenness is related to crop performance and productivity as an index of photosynthesis. The instantaneous rate of photosynthesis is not consistently predictive of the useful output of most crops. This is seen in modern "improved" cultivars of wheat in comparison with the old "inefficient" varieties they replaced, where improvements in yield have been achieved with no increase - even perhaps a slight decrease - in photosynthetic performance (Evans and Dunstone 1970, Austin et al 1986). On the other hand, it has long been known that the duration of photosynthesis is directly related to yield in many crops under a range of agronomies and environments (Heath and Gregory 1938; Watson 1952). For example, Evans et al (1975) collected data for a number of Triticum aestivum varieties and cultivation conditions and produced a striking correlation between leaf area duration and yield. Variation in assimilation rate will have contributed to the scatter, but the overriding significance of foliar longevity is clear. Relationship between leaf area duration and yield in wheat. Data on grain yield and D were collected by Evans et al. (1975). The regression line shows that yield goes up by more than a tonne for every 100 days of green area duration. Senescence is the major factor limiting D. Leaf area duration (D) is derived from leaf area index (L) thus: L = LA/P where LA is the total leaf area above ground area P. D between times t1 and t2 is given by D = t1∫t2Ldt In graphical terms D is the area beneath the curve of L against time (Watson 1947, 1952, Hunt 1982). The original form of D did not discriminate between photosynthetic area and other above-ground tissue. Green area duration (G) is often employed as an alternative measure of assimilatory opportunity. Wolfe et al. (1988) measured grain yield as a function of G, expressed as green leaf area duration per plant, for a Zea mays crop over two years under contrasting fertiliser and irrigation treatments. Wolfe et al. figure will go here The linear regression fit is highly significant and from the slope of the plot it is calculated that each extra unit of G is equivalent to an additional 5.2 g of grain per plant. For a density of 70000 plants ha-1, this means an extra 360-plus kg, around 5% of the 6-7 t ha-1 which constitutes a decent yield. A similar calculation using published figures for wheat (Borojevic et al 1980) reveals that each additional day of G yields an extra 26 kg ha-1. Further refinements to the precise formulation of foliar duration have been employed. Welbank et al (1966) found that G above the flag leaf node was more highly correlated than total D with wheat yield. Relationships have also been observed between grain yield and duration of the areas of specific leaves, both throughout the entire period of crop growth and in the interval from flowering onward (Thomas 1992). (Green) leaf area duration is a useful index of crop performance, and in some cases has been a target trait for crop improvement. D could be increased by increasing L, but in practice this is difficult for highly developed crop species. A simple calculation shows why. The photosynthetically-active photon flux density Q is related to L thus: F = ln(Q0/QF)/k where Q0 is the flux density at the canopy surface and QF the flux density at L = F. The factor k is the foliar absorption coefficient, which lies in the range 0.3 - 1.3 (Nobel and Long 1985). Taking Q at the photosynthetic compensation point to be approximately 10 μmol m-2 s-1 at 20° and ambient CO2, then at a Q0 of 2000 μmol m-2 s-1, which is maximal sunlight, k sets the absolute limits of L for maximal light interception at 4.1 to 17.7. In practice, values of maximal L are much lower than this. For example in potato, with planophile leaves and a k value in excess of 1.0, interception of solar radiation is maximal at an L of 3-3.5 (Firman and Allen 1989). L values considerably greater than this can result from high rates of N fertilisation but tuber yield is, not surprisingly, quite insensitive to the associated increases in D (Allen and Scott 1980). Since the L component of D is an unrewarding character to breed for, attention is focused on the time element. In practice this means growing and closing the canopy quickly and keeping it there for as long as possible before senescence kicks in at the end of the season. Breeding and crop management for fast establishment and delayed senescence, either explicitly or incidentally, have resulted in increases in yields, in some cases to within the range of the theoretical maximum (Thomas 1992, Thomas and Howarth 2000). References
Created by: system last modification: Monday 28 of April, 2008 [17:25:13 UTC] by Sid |
Login |
![[hide]](javascript:icntoggle('mod-mnu_application_menu','mo.png');)
