The most important harvested organs of crop plants, such as seeds, tubers and fruits, are often described as assimilate sinks. They play little or no part in the fixation of carbon through the production of sugars through photosynthesis, or in the uptake of nitrogen and sulphur, but import these assimilated resources to support metabolism and to store them in the form of starch, oils and proteins. Wild plants store resources in seeds and tubers to later support an emergent young plant. Cultivated crops are effectively storing resources to provide us with food and many have been bred to accumulate much more than would be required otherwise. For example, approximately 80% of a cultivated potato plant's dry weight is contained in its tubers, ten times the proportion in the tubers of its wild relatives (Inoue & Tanaka 1978). Cultivation and breeding has brought about a shift in the partitioning of carbon and nitrogen assimilate between the organs of the plant.

The effect of drought stress in isolation, or in combination with beet yellows virus infection, on sugar beet growth was studied in the field and glasshouse. Drought reduced total plant weight by 26%, due to 20 and 29% reductions in foliage and storage root yields respectively. Sugar extraction efficiency was depressed by an increase in amino-nitrogen impurities. Drought did not limit water extraction depth, despite decreasing lateral root growth in proportion to total weight. During the field experiments, total crop cover was decreased in all the droughted treatments (halved in some cases) for at least part of the season. Consequently, these treatments intercepted 12% less light, which in combination with a 16% decrease in the dry matter/light conversion coefficient, led to the decrease in growth. The decrease in conversion coefficient was due to temporary closure of the stomata rather than a function of drought-induced damage to the photosynthetic mechanism. The absolute effect of drought remained the same irrespective of whether the plants were infected with beet yellows virus, i.e. there was no interaction between the two stresses. The reasons for this lack of interaction are discussed but it is likely that the stress effects were mediated at different times of the day and season.

In four field experiments, the effects of single nitrogen (N) applications at planting on yield and nitrogen uptake of potato (Solanum tuberosum L.) was compared with two or three split applications. The total amount of N applied was an experimental factor in three of the experiments. In two experiments, sequential observations were made during the growing season. Generally, splitting applications (up to 58 days after emergence) did not affect dry matter (DM) yield at maturity and tended to result in slightly lower DM concentration of tubers, whereas it slightly improved the utilization of nitrogen. Maximum haulm dry weight and N content were lower when less nitrogen was applied during the first 50 days after emergence (DAE). The crops absorbed little extra nitrogen after 60 DAE (except when three applications were given). Soil mineral N (0–60 cm) during the first month reflected the pattern of N application with values up to 27 g/m2 N. After 60 DAE, soil mineral N was always around 2–5 g/m2. The efficiency of N utilization, i.e. the ratio of the N content of the crop to total N available (initial soil mineral N+deposition+net mineralization) was 0·45 for unfertilized controls. The utilization of fertilizer N (i.e. the apparent N recovery) was generally somewhat improved by split applications, but declined with the total amount of N applied (range 0·48–0·72). N utilization and its complement, possible N loss, were similar for both experiments with sequential observations. Separate analysis of the movement of Br− indicated that some nitrate can be washed below 60 cm soil depth due to dispersion during rainfall. The current study showed that the time when N application can be adjusted to meet estimated requirements extends to (at least) 60 days after emergence. That period of time can be exploited to match the N application to the actual crop requirement as it changes during that period.

Transplant crops derived from in vitro produced plantlets of cultivars differing in earliness were grown in three experiments in three years in the Netherlands, during field periods of maximally 12 weeks. Seed tuber crops were included in the first year. Fresh tuber yield was analysed as the result of the radiation intercepted by the crop's canopy (AIR; accumulated intercepted radiation), the efficiency of conversion of intercepted radiation into dry matter (RCC; radiation conversion coefficient), the proportion of dry matter allocated to tubers (HI; harvest index), and the tuber dry-matter concentration (TDMC). Transplant crops had a lower AIR and lower yields than crops from seed tubers. Variables RCC, HI and TDMC were not affected by the type of propagule used. In transplant crops, yields from early cultivars could be extremely low when compared to later cultivars, due to a low AIR. This cultivar effect did not occur in crops from seed tubers. Among transplant crops, the lower AIR was the most consistent reason for lower tuber yields of earlier cultivars. It resulted from a slower increase in soil cover after transplanting and a lower maximum soil cover, both caused by a relatively high allocation of dry matter to tubers immediately after transplanting and resulting first in reduced haulm growth rates and subsequently also in reduced total growth rates. Senescence was not different. A higher HI partly compensated the lower AIR of the earliest cultivars. RCC and TDMC were not consistently affected by cultivar's earliness in the transplant crops.

Winter wheat (Triticum aestivum L., cv Mercia) was grown at Rothamsted over three seasons (1992/94/95) with plant-available soil P (Olsen-P) ranging from very deficient to superoptimal (1–52 mg P/kg; ADAS soil P Index 0–4). Critical Olsen-P for 95% maximum grain yield changed little between years and averaged 9 mg P/kg. Total-P and inorganic-P (Pi) were measured in whole shoots several times between late tillering and anthesis to see if Pi was a better indicator than total-P of the adequacy of P supply for maximum grain yield. Inorganic-P accumulated in shoot tissue water with increasing Olsen-P to a maximum concentration of c. 15 mM. Inorganic-P was a better indicator of crop P status as there was less variation in critical values between seasons and with crop growth stage. Expressing Pi concentrations on a tissue water basis (Piw) reduced the effect of differences in soil water supply and also offers the prospect of in situ testing. Over the three seasons, critical shoot Piw for 95% maximum grain yield averaged 4.7 mM (range 4·3–5·8 mM) during stem elongation.

Russian wheat aphid (RWA), Diuraphis noxia is a serious pest of cereals in various regions of the world. Seventy-six barley genotypes collected from different areas in Iran were subjected to an initial mass screening test and six susceptible and 17 apparently resistant genotypes were identified. To determine resistance mechanisms, antixenosis, antibiosis and tolerance tests were conducted and selected genotypes were ranked according to their plant resistance indices.

Four crosses were made between the two most resistant lines (Shz.B-106, and Shz.B-108) and two most susceptible lines (Shz.B-105 and Shz.B-107). F1, F2 and BCF1 (backcross to the resistant parent) seedlings were screened for RWA resistance in the greenhouse by artificial infection. Resistance in Shz.B-106 was apparently controlled by a semi-dominant gene, whereas resistance in Shz.B-108 was controlled by a dominant one.

The pollination requirements of West Indian cherry (Malpighia emarginata DC) were investigated in NE Brazil through observations of flower morphology, floral rewards, anthesis, anther dehiscence, stigma receptivity, pollen cross and self-compatibility and proportion of fruit set. Potential insect pollinators and their foraging behaviour were also studied, and the pollination efficiency of the most frequent flower visitor, the bee Centris tarsata Smith (Anthophoridae), was assessed using single visits to flowers. It was shown that the West Indian cherry flower has a short lifespan, thus requiring pollination on the day of anthesis, when both cross- and self-pollen grains set fruit. The flower can potentially be pollinated by an array of insect visitors, but only oil-collecting bees of the genus Centris find it very attractive. Centris tarsata appeared as the main pollinator of West Indian cherry in the area studied, and farmers are advised to encourage its presence in orchards. Despite high levels of natural pollination, only 30% of flowers set fruits. It is suggested that a high proportion of abnormal ovule development may be responsible for low fruit set where pollination is satisfactory, and that West Indian cherry varieties showing a lower percentage of such abnormalities should be selected for.

This study describes the development of a highly repeatable cold screening procedure for lentil (Lens culinaris Medik.) using controlled conditions which involve first, acclimation of the plants at the vegetative stage in a growth chamber and second, freeze testing in a freeze chamber. The seeds were first germinated in Petri dishes and then planted in styrofoam trays with individual cells. Initial growing temperatures in the growth chamber for two weeks were 25 °C day and 10 °C night with a 12 h photoperiod. In the third week the photoperiod was changed to 10 h and in the fourth the temperatures were changed to 10 °C day and 0 °C night to acclimate the plants. Using a modified freeze chamber (household deep freezer), a freeze test temperature of −15 °C, following a 6–8 weeks acclimation period (because both acclimation times had the same effect for cold tolerance of the genotypes), at 2 and 3 °C/h cooling rate, and an exposure time of 3 h at −15 °C were appropriate to detect significant (P<0·01) differences among several lines at a comparatively low degree of injury for the most cold-hardy genotypes.

Near-isogenic lines of mungbean (Vigna radiata) were produced from backcrossing two Thai recommended cultivars, KPS 1 and CN 36, with the multiple leaflet line, V 5926, until BC9. The isogenic lines were evaluated against the two recipients at Kasetsart University, Thailand, in the rainy season of 1996 and the dry season of 1997. The effect of the gene controlling the multiple leaflet on physiological and agronomical characters was investigated. Seed yield, plant height, and number of pods per plant were greater in the trifoliate leaflet cultivars than in the multiple leaflet near-isogenic lines, whereas the numbers of seeds per pod and 1000-seed weights were not significantly different. Light saturation and photosynthetic rate did not differ, while light interception and dry matter accumulation were greater in the trifoliate leaflet cultivars than in the multifoliate lines. The leaf area index (LAI) of the trifoliate parents was greater at the vegetative stage but lower at the flowering and young pod stages, due to the greater number of leaflets per plant of the multifoliate lines. After the beginning of the podding stage, the LAI of the multifoliate lines was less than that of the parents, as the number of leaflets per plant of these lines hardly changed while that of the trifoliate parents increased.

Nitrogen fertilization of okra (Abelmoschus esculentus) when intercropped with cassava (Manihot esculenta) and maize (Zea mays) was studied using three N rates (0, 60 and 120 kg/ha) in two field experiments in Nigeria between 1995 and 1997. The okra was grown between the cassava and maize rows. The effect of using two contrasting maize cvs. TZE Composite 3 Cl (less-tall, early-maturing with narrow leaves) and DMRSR-Y (tall, late-maturing with broad leaves) on N response of okra in the mixture was also investigated. Intercropping with maize or cassava and maize significantly increased stem elongation, delayed pod harvest, and decreased branch or pod formation and marketable pod yield in okra. In the cassava/maize/okra combination, maize had greater effect than cassava in determining most growth and pod yield attributes of the okra crop, regardless of N rate. However, the effect was relatively less severe in mixtures with early-maize cv. TZE Composite 3 Cl than with late-maize cv. DMRSR-Y. Although applied N at rates above 60 kg/ha in intercropping increased maize grain and cassava tuber yields, this did not result in a further significant gain in pod yield of the associated okra crop. The present recommendation of 60 kg/ha for N fertilization of cassava/maize intercrop was found to be applicable to okra grown between the cassava/maize rows using the less-tall, early-maize cultivar with relatively narrow leaves, and possibly reduced leaf area.

The relationship between variations in in vitro microbial biomass production and microbial protein supply to the host animal was examined in 13 Malawian goats fed on stover leaves from two varieties of maize at Bunda College, Malawi, in 1995. The in vitro parameters were analysed based on the concept of the partitioning factor (PF) which is obtained by the combination of gas volume measurements with determinations of the amount of substrate truly degraded. The PF reflects substrate-dependent variation in the in vitro partitioning of degraded substrate between short chain fatty acids (SCFA), gases and microbial biomass. The in vivo microbial protein synthesis was estimated by the urinary excretion of the purine derivatives (PD) allantoin, uric acid and xanthine + hypoxanthine. For the two types of stover leaves examined, the higher microbial efficiency of one variety in vitro was reflected by different PD excretions on practically identical digestible dry matter intakes in vivo. It is concluded that substrate-dependent variations in microbial efficiencies as detected by the PF are also evident and relevant for in vivo microbial protein synthesis.

This group, which is concerned with the applications of mathematics to agricultural science, was formed in 1970 and has since met at approximately yearly intervals in London for one-day meetings. The thirty-first meeting of the group, chaired by Professor M. Gill of NR International, Chatham, was held in the Kohn Centre at the Royal Society, 6 Carlton House Terrace, London on Friday, 16 April 1999 when the following papers were read.