Growth duration and above-ground dry-matter partitioning in oats

Duration of vegetative, generative, and grain-filling phases contribute to dry-matter accumulation and partitioning. Fourteen oat (Avena saliva L.) cultivars and six breeding lines were evaluated at the Viikki Experimental Farm, University of Helsinki, in 1988-1990.The following observations were made: (1) a short vegetative period accumulated less dry-matter into vegetative plant organs and resulted in higher grain yield and harvest index (HI), (2) a long period for maximum floret initiation yielded more grains per panicle and high panicle weight and (3) a short grain-filling period yielded high rates of panicle and grain filling associated with high HI. Hence, oat breeding and crop management should aim at improving the synchronization of the growth phases as shown in this study.


Introduction
A short growing season and the consequent de- mand for early maturing cultivars are the main factors limiting productivity of annual small grains grown in Finland.Long days enable cereals to rapidly reach and pass several developmental stages (DVS), especially at pre-anthesis (Peltonen-Sainio 1994).The growth period of cereals is divided into three phases: 1) the vegetative period from germination to double-ridge stage, 2) the generative period from initiation of double ridges to pollination and 3) the grain-filling period.The vegetative period of oats is completed by 140 dd °C (base temperature 5°C) and hence, generative growth begins already at the two-leaf stage, and grain filling starts at <5OO dd °C in Finland (Peltonen-Sainio and Pekkala 1993).The duration of growth phases is likely to con- tribute to both source and sink capacities, i.e. expansion, persistence and assimilate production of green area, set and assimilate demands of flo-rets and grains, and the reciprocal feed-back ef- fects between sources and sinks.The HI indi- cates the success of the synchronization between sources and economic sinks.Hence, this study evaluated the contribution of duration of vegeta- tive, generative and grain-filling phases to drymatter accumulation and partitioning in oats.
Early double-ridge stage (DVS 3.4) (Peltonen-Sainio and Pekkala 1993), the developmental stage when floret abortion begins (DVS 14.9) and pollination (DVS 25.3) were recorded from 10 randomly sampled main shoots, and were con- sidered to be reached when >60% of the plants were at that stage.Cumulated degree days (CDD, dd °C, base temperature 5°C) were calculated for: (1) the vegetative period (from sowing to DVS 3.4), (2) the period for production of maxi- mum floret number (from DVS 3.4 to DVS 14.9), and (3) the generative phase (from DVS 3.4 to DVS 25.3).

Results and discussion
The longer the growth period before heading, the higher was the grain yield, which resulted from increased weight of total phytomass and panicle (Table 1) associated with higher PGR.However, more dry-matter accumulated into above-ground vegetative plant organs in cultivars that were char- acterized by longer vegetative period (Fig. 1).Correlation between the length of the vegetative period and VGR (r=o.s2*)supported this finding.Production of high vegetative phytomass result- ed in smaller grain yield and HI, which indicated reduced ability to allocate photoassimilates into harvestable yield.This can be attributed to the negative correlation between length of vegetative period and generative phase.Each 10 dd °C increase in duration of vegetative period resulted in a 7-8 dd °C shorter generative phase and 1.5-2% units lower HI.Moreover, the less CDD from initiation of double ridges (DVS 3.4) to beginning of floret abortion (DVS 14.9), the less grains per panicle were produced, which possibly re- sulted from reduced floret set.Slafer and Miralles (1993) showed that a decreased floret set was associated with a decreased grain number in wheat.Hence, a shorter period for production of maximum floret number resulted in a lower panicle weight.
The results from this study indicated that cul- tivars that have a short grain-filling period had high rates of panicle and grain filling, which re- sulted in increased HI (Table 1) but had no effect on grain yield.An increase of one day in the grain-filling period reduced PER by 1.7 mg pani- cle 1 d -1 , GFR by 0.02 mg grain 1d ', and HI by 1% unit.Therefore, oat cultivars in which the relative length of the growth phases is synchronized as (1) short period for vegetative growth, (2) relatively long period for appearance of max- imum floret number and (3) moderate duration of grain filling are likely to be advantageous in the growing conditions prevailing in Finland and should therefore be aimed at by breeding and crop management.

Fig. I .
Fig.  I. Relationship between duration of growth phases and accumulation of dry-matter in1988-1990.

Table 1 .
Relationship between duration of pre-heading and grain-filling phases and morpho-physiological traits of oats in1988-1990.