The C3–CAM epiphytic bromeliad Guzmania monostachia var. monostachia may be exposed to high incident photosynthetically active radiation (PAR) during the dry season in Trinidad, and resultant variations in photochemical efficiency have been investigated for ‘exposed’ (receiving ∼100% incident PAR), ‘semi‐exposed’ (∼60% PAR) and shaded populations under natural conditions. The more succulent leaves of the plants growing fully exposed within the canopy had higher overall CAM activity (measured as ΔH+, the dawn‐dusk titratable acidity), a smaller proportion of chlorenchyma and lower total chlorophyll content. There was a gradation of morphological and physiological characteristics between these and shaded leaves. Diurnal time‐courses of photosynthetic light responses (as O2 evolution) showed marked variations in apparent quantum yield (AQY) and light‐saturated rates for both exposed and semi‐exposed populations, dependent on incident PAR during the day. Similar measurements of photosystem II fluorescence characteristics showed that Fv/Fm declined from 0·70 to 0·42 at midday for exposed plants (on a day when total incident PAR was 44 mol photon m−2), indicating non‐photochemical quenching (qNP) of photosynthesis. However, in contrast to AQY determinations, Fv/Fm recovered during the afternoon. The decrease in Fv/Fm was reduced from 0·72 to 0·64 under 24 mol photon m−2 d−1. The long–term recovery of photo‐synthetic efficiency was assessed for exposed plants placed under three shading regimes (60, 30 and 3% of incident PAR) over a 17‐d period. During this time, total chlorophyll content increased from 228 to 515 and 585 μg g−1 fresh weight (for 3 and 30%, respectively) and chlorophyll a:b declined. While AQY recovery was much longer under the lowest PAR (17d), under 30% PAR both AQY and Fv/Fm had recovered after 2d shading. The differences between timing of recovery for Fv/Fm during diurnal time courses and in the long term suggest that, while quenching associated with PSII recovers rapidly, enzyme activation and/or protein synthesis of other photosynthetic components may be limiting under low PAR. However, it is suggested that the occurrence of qNP on a daily basis may preclude long‐term photoinhibitory damage under natural conditions during the dry season.

Summary: The longevity of buried Rottboellia cochinchinensis (Lour.) W.D. Clayton seed represents a major survival mechanism for the weed, enabling the persistence of a continuing source of weed seeds in crop land. The pattern of seed persistence and depletion of R. cochinchinensis in cultivated maize soils was investigated by means of (1) studies on the effect of depth and duration of burial on the viability of the weed seeds, (2) quantitative estimation of the seed population and viability in cultivated fields, and (3) the periodicity of emergence and effects of cultivation on seed germination both in the field and in the greenhouse. The results indicated that the mode of persistence was innate (8.5%) and enforced (35%) dormancy after 1 year of burial, and that the persistency component of the seed population on cultivated soils ranged from 40.60%. The weed was able to remain viable at depths of 45 cm, indicating an excellent mechanism of escaping the effects of most soil‐applied herbicides, and it was shown that tillage increases the depletion rate of the weed seed reserve by 32% per year.

Pigeonpea [Cajanus cajan (L.) Millsp.; cv. UW‐10], a crop being developed for use in low‐rainfall environments, was studied during development of water stress and recovery after rewatering to determine the treatment effects on photosynthesis and leaf diffusive conductance. Plants were grown in a greenhouse and leaf gas exchange was measured at various photosynthetic photon flux densities during an 8‐d period of water stress development and during an 18‐d period of recovery following rewatering. Predawn total water potential (ψw) declined by 1.0 MPa after 8 d of withholding water, whereas the pressure component of water potential (ψp) remained positive due a decrease in the solute component of water potential (ψp) from −1.2 to −1.7 MPa. During stress development, decreases in leaf diffusive conductance (gL) occurred earlier than decreases in photosynthetic CO2 exchange rate (CER), indicating that a water conservation mechanism was induced. Differences in gL between unwatered and control plants were first detected at 5 d after withholding water. Differences in CER between control and unwatered plants were not detected until Day 8 of the treatment. As a result of these CER and gL changes, the CO2 concentration in the intercellular air space (Ci) decreased during the period from 5 to 8 d after withholding water. After rewatering, CER and gL slowly recovered over an 18‐d period. It is concluded that pigeonpea responds to water stress by partially limiting the rate of water loss and maintaining a low CER, and it responds to rewatering by slowly recovering CER and gL.

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SUMMARYPruning the roots of light-grown pea seedlings every 4 days accelerated senescence of older leaves and reduced their rate of fixation of '*C02. Application of 6-benzylaminopurine to the shoot largely compensated for root removal and increased the shoot/root ratio of both pruned and intact plants. It is suggested that the supply of cytokinins from root to shoot may affect leaves by delaying senescence and maintaining tlieir capacity for photosynthesis; it may also alter the distribution of dry matter within the plant by influencing the proportion of assimilates and other metabolites retained by the shoot.